%\documentclass[manuscript]{aastex} \documentclass[preprint]{aastex} \shortauthors{Nagar et al. 2000} \shorttitle{VLA survey of 48 LLAGN} \begin{document} \title{Radio Sources in Low-Luminosity Active Galactic Nuclei. I. VLA Detections of Compact, Flat-Spectrum Cores} \author{Neil M. Nagar\altaffilmark{1}, Heino Falcke\altaffilmark{2}, Andrew S. Wilson\altaffilmark{1,3}, Luis C. Ho\altaffilmark{4}} \altaffiltext{1}{Department of Astronomy, University of Maryland, College Park, MD 20742; neil@astro.umd.edu, wilson@astro.umd.edu} \altaffiltext{2}{Max-Planck-Institut f\"{u}r Radioastronomie, Auf dem H\"{u}gel 69, 53121 Bonn, Germany; hfalcke@mpifr-bonn.mpg.de} \altaffiltext{3}{Adjunct Astronomer, Space Telescope Science Institute} \altaffiltext{4}{Observatories of the Carnegie Institution of Washington, 813 Santa Barbara St., Pasadena, CA 91101; lho@ociw.edu} \received{} \accepted{} \notetoeditor{Footnote 1 (a footnote to the first sentence of the abstract) does not appear in the output file, perhaps because of the way latex handles affiltext 1 and footnote 1. Please ensure that footnote 1 appears in the final version.} \begin{abstract} We report a high resolution (0\farcs2), 15~GHz survey of a sample of 48 low-luminosity active galactic nuclei with the Very Large Array{\footnote{The VLA is operated by the National Radio Astronomy Observatory, a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.}}. Compact radio emission has been detected above a flux density of 1.1~mJy in 57\% (17 of 30) of low-ionization nuclear emission-line region (LINER) nuclei and low-luminosity Seyferts. The 2~cm radio power is significantly correlated with the emission-line ([O~I]~$\lambda$6300) luminosity for all objects in the sample. Using radio fluxes at other frequencies from the literature, we find that at least 15 of the 18 detected radio cores have a flat to inverted spectrum ($\alpha~\geq$ $-$0.3, S$_{\nu}~\propto~\nu^{\alpha}$). The presence of a compact, flat-spectrum radio core is consistent with current ideas involving accretion onto massive black holes in low-luminosity active galactic nuclei such as jet and advection-dominated accretion flows. While the present observations are consistent with the radio emission originating in star formation regions (the brightness temperatures are $\geq$~10$^{2.5-4.5}$~K for objects in the sample), higher resolution radio observations of 10 of the detected sources, reported in an accompanying paper \citep{falet00a}, show that the cores are very compact ($\lesssim$~pc), of high brightness temperature (T$_b~\gtrsim$ 10$^8$~K) and probably synchrotron self-absorbed, ruling out a starburst origin. Thus, our results suggest that at least 50\% of low-luminosity Seyferts and LINERs in the sample are accretion powered. We have detected only 1 of 18 ``transition'' (i.e. LINER + H~II) nuclei observed, so either their radio cores are significantly weaker than those of ``pure'' LINERs, or their central engines are intrinsically different from those of ``pure'' LINERs. Compact 2~cm radio cores are found in both type 1 (i.e. with broad H$\alpha$) and type 2 (without broad H$\alpha$) nuclei. There is weak evidence, limited in significance by small numbers, that low-luminosity active galactic nuclei with compact radio cores exhibit radio ejecta preferentially aligned along the rotation axis of the galaxy disk. If this result is confirmed by a larger sample, it would lend support to the idea that the misalignment of accretion disks with the galaxy stellar disk in more luminous Seyfert galaxies is a result of radiation pressure induced warping of their accretion disks. There is some support for a scenario in which most nuclei which exhibit a 2~cm compact core also have an unresolved nuclear UV source. Unresolved UV sources in nuclei with 2~cm compact cores are more prone to host galaxy obscuration than those in nuclei undetected at 2~cm. \end{abstract} \keywords{accretion, accretion disks --- galaxies: active --- galaxies: Seyfert --- galaxies: nuclei --- radio continuum: galaxies --- surveys} \section{Introduction} There is increasing evidence that a large fraction of the nuclei of nearby galaxies show many similarities with powerful active galactic nuclei (AGN); these objects are termed low-luminosity active galactic nuclei (LLAGN; active galaxies with nuclear L$_{H\alpha}~\leq$ 10$^{40}$ erg s$^{-1}$; Ho, Filippenko \& Sargent 1997a, hereafter H97a). These similarities include broad H$\alpha$ lines (Ho et al. 1997b), broader H$\alpha$ lines in polarized emission than in total emission \citep{baret99}, nuclear UV sources \citep{maoet95,baret98} and water vapor megamasers \citep{braet97}. However, the emission-line spectra of LLAGNs (i.e. low-luminosity Seyferts, LINERs, and ``transition'' nuclei [nuclei with spectra intermediate between those of LINERs and H~II regions]), can also be modeled in terms of photoionization by hot, young stars \citep{termel85,filter92,shi92}, by collisional ionization in shocks \citep{kosost76,foset78,hec80,teret92,dopsut95}, or by aging starbursts \citep{aloet99}. Thus, it is not clear that accretion onto a black hole powers all LLAGNs. How does one distinguish accretion-powered LLAGNs from LLAGNs powered by hot stars or supernova shocks? Broad H$\alpha$ lines and bright unresolved optical or UV sources are ambiguous indicators because they can all be produced in starburst models (see Terlevich et al. 1992), and a search for broader polarized H$\alpha$ emission is currently feasible in only a few of the brightest LLAGNs \citep{baret99}. Further, all these indicators are highly dependent on viewing geometry and obscuration and on the signal to noise of the observations, a problem exacerbated by the low optical and UV luminosities of LLAGNs. Their observation demands both high signal-to-noise spectra and high spatial resolution to separate weak nuclear emission lines from the starlight of the host galaxy. One well-known property of some powerful AGN is a compact, flat-spectrum nuclear radio source, usually interpreted as the synchrotron self-absorbed base of the jet which fuels larger-scale radio emission. Astrophysical jets are known to be produced in systems undergoing accretion onto a compact object \citep[e.g.][]{pri93,bla93}, so such compact radio sources in galactic nuclei may reasonably be considered a signature of an AGN. Much theoretical work \citep[e.g.][]{beget84,lovrom96,falbie99} has been devoted to this disk-jet relationship in the case of galactic nuclei. Nuclear radio emission with an inverted spectrum is also expected from an advection-dominated accretion flow \citep[ADAF; e.g.][]{naret98}, a possible form of accretion onto a black hole at low accretion rates \citep{reeet82}. From the observational perspective, \citet{hec80} showed that LINER nuclei tend to be associated with a compact radio source, and compact, flat-spectrum radio cores are known to be present in many `normal' E/S0 galaxies \citep{sadet89, wrohee91, sleet94}. Flat-spectrum radio cores are, however, uncommon in normal spirals or Seyfert galaxies \citep{ulvwil89,vilet90,sadet95}. Flat-spectrum radio sources can also result through thermal emission from optically-thin ionized gas or through free-free absorption of non-thermal radio emission, a process which probably occurs in compact nuclear starbursts \citep{conet91}. The brightness temperature, T$_b$, in such starbursts is limited to log~[T$_b~$(K)]~$\lesssim$ 5 \citep{conet91}. Thus it is necessary to show that T$_b$ exceeds this limit before accretion onto a black hole can be claimed as the power source. In this paper, we present a high-frequency (15~GHz), high-resolution ($\sim$~0{\farcs}15) survey of LLAGNs with the Very Large Array \citep[VLA,][]{thoet80}. This resolution is high enough to isolate nuclear emission from that of the host galaxy, and the radiation is unaffected by the obscuration present at UV or optical wavelengths, and less affected by free-free absorption than observations at longer cm wavelengths. Further, large samples can be quickly studied, as deep radio maps are achievable in as little as 15--20 min per object. Higher resolution follow-up observations are then required in order to eliminate the possibility that the radio cores are thermal in origin, as discussed above. We have therefore embarked on a program to observe a large number of LLAGNs at high resolution with the VLA and the Very Long Baseline Array \cite[VLBA,][]{napet94} in order to identify accretion-powered nuclei, to test the predictions of ADAF and jet models, and to characterize the presence and structure of radio jets on sub-pc to hundred-pc scales. In this paper, we report on the results of the first stage of this program --- VLA observations of 48 LLAGNs which have been extensively observed at other wavebands. Preliminary results of this project have been published in \citet{falet97,falet98,falet99} and \citet{naget99b}. \section{Sample, Observations and Data Reduction} A total of 48 galaxies were selected from the magnitude-limited Palomar spectroscopic survey of 486 nearby bright galaxies (Ho, Filippenko, \& Sargent 1995). All 48 galaxies were originally selected, using preliminary spectral classification, to have nuclei with ``pure'' LINER or ``transition object'' (composite LINER~+~H~II) characteristics. The spectral type of eight of the nuclei was somewhat uncertain, and these galaxies were later reclassified as Seyferts (H97a). Thus, our sample of 48 LLAGNs consists of 22 ``pure'' LINERs, 18 ``transition objects,'' and 8 low-luminosity Seyferts. The 48 objects were not selected by well-defined criteria; they were individually chosen for study at multiple wavelengths because they were particularly ``interesting'' LLAGNs. Our sample is the subject of a rigorous observational campaign involving radio observations at 2~cm (reported here), 3.6~cm and 6~cm \citep{hoet00}, {\it Hubble Space Telescope (HST)} optical \citep{penet98} and UV imaging \citep{maoet95,baret98} as well as spectroscopy \citep{maoet98}, and X-ray imaging by the {\it Chandra} telescope. All 48 LLAGNs in the sample, plus two additional objects -- NGC~4449 (whose nuclear spectrum is that of an H~II region) and NGC~5756 (a southern spiral not in the Palomar spectroscopic survey) -- were observed by the VLA over three runs on 1996 October 7, 11 and 18, while the array configuration was being changed from `D' to `A' (see Thompson et al. 1980 for a description of the VLA configurations). The weather was fair for all runs with clear skies and temperatures between 0 and 23{\arcdeg}C. Two 8 min observations of each galaxy were sandwiched between three 2 min observations of a nearby phase calibrator, so that the total time on source was typically 16 min. For the October 7 run, there were 9 antennas at A-configuration pads plus 18 antennas at D-configuration pads. Five D-configuration antennas were taken out of the array at various times during the run to be moved to their A-configuration pads; they did not start observing again until after our run was completed. A sixth D-configuration antenna was removed from the array for painting. Of the nine antennas at A-configuration pads, the data from the three most recently moved were unusable, since the antenna positions were poorly determined. For the October 11 run, 25 of the 27 antennas were on their A-configuration pads, but the data from the four most recently moved antennas were unusable as their positions were poorly determined. For the October 18 run, all except one of the antennas were at their A-configuration pads, and all had reasonably well-determined positions. The data were calibrated using the AIPS software, following the standard procedures outlined in the AIPS cookbook. Since positions for recently-moved antennas were not well determined at the time of observations, antenna positions listed in the data files were updated with the corrections to the position found by VLA operators over the few weeks after the observations. We found that using all position corrections up to those determined on November 23, 1996, led to the best calibrator phase solutions for most of the recently-moved antennas. Other bad data were removed before the phase solutions, derived from the phase calibrators, were applied to the target galaxies. On the October 7 and October 18 runs, the source 1328+307 (3C~286) was observed twice and used to set the flux-density scale to that of \citet{baaet77}. On the shorter October 11 run, an observation of 0404+768 was used to set the flux-density scale (we used a 2~cm flux-density of 1.46~Jy for this source). The phase calibrator flux-densities were bootstrapped to that of 0404+768, and the bootstrapped values were found to be in good agreement with those listed in the VLA flux-density database (part of the ``VLA calibrator manual''; available on-line at http://www.nrao.edu). The maps were made with AIPS task IMAGR. For the October 7 run, we imaged each object twice, once using only the antennas on D-configuration pads, which resulted in $\sim$5{\arcsec}-resolution maps, and once using only antennas on A-configuration pads, which resulted in $\sim$0{\farcs}15 resolution maps. For the other two runs, a single map was made for each source, using all antennas. For sources stronger than about 3 mJy, we were able to iteratively self-calibrate the data so as to increase the signal-to-noise ratio in the final map. Phase-only self-calibration was found to give good results, with little further improvement from a combined amplitude \& phase self-calibration. The typical r.m.s. noise in the maps prior to self-calibration was 0.1~mJy. Many of the maps of the undetected nuclei show 4 to 5~$\sigma$ peaks at various locations in the map. We therefore conservatively use 10 times the r.m.s. noise in the map as the upper limit for the flux of the undetected sources, so the sensitivity of our survey is about 1.1~mJy. On the October 11 run, NGC~266, NGC~404, and NGC~7217 were also observed at 0.7~cm and 3.6~cm, with the VLA split into two subarrays and each subarray observing at a different frequency, since not all antennas had 0.7~cm receivers. These data were reduced as described above. An observation of 0404+768 was used to set the flux-density scale at 0.7~cm. We set the 0.7~cm flux density of 0404+768 to 578.2~mJy for the AC IF's and 575.3 for the BD IF's, after finding, from the VLA flux density database, that these were the flux densities on 1997 February 24, the date closest to our run that the source was observed (the flux density stayed between 500 and 600~mJy for the following $\sim$300 days). The flux density of the phase calibrators, 0055+300 and 2201+315, were bootstrapped to that of 0404+768, and the results were consistent with the values listed in the flux density database. At 3.6~cm, the observation of 3C~48 could not be used to set the flux density scale, as the four innermost antennas (and almost every alternate antenna on each arm) were observing at 0.7~cm, so that none of the 3.6~cm interferometer baselines was small enough for an accurate flux calibration solution (the VLA calibrator manual specifies that A-configuration observations of this calibrator should be restricted to 0--40 k$\lambda$). We therefore set the flux density of 2201+315 (the phase calibrator for NGC~7217) to 2.51~Jy in both AC and BD IF's at 3.6~cm, as this was its measured value on 1997 January 17, the date closest to our run (after this date, the flux density of this source rose steadily to 3.5~Jy over the next $\sim$300 days and then stayed roughly constant). The flux density of 0055+300 (the phase calibrator for NGC~266 and NGC~404) was then bootstrapped from that of 2201+315. The result, S(0055+300)~= 0.82 Jy, is consistent with the values listed in the flux density database for this source. Nevertheless, the 3.6~cm and 0.7~cm flux densities of the three galaxies are subject to additional uncertainty. All data were calibrated and imaged independently by two of us (NMN and HF), yielding consistent results (all corrections for antenna positions were also independently done). We are therefore confident of the results, despite the uncertainties caused by the uncertain antenna positions. \section{Results} \subsection{Detection Rate} The results of the VLA observations are listed in Table 1, with columns explained in the footnotes. The radio positions for the 22 detected objects are limited by the positional accuracy of the phase calibrators, which is typically 2--10~milli-arcsec (mas), and by the accuracy of the Gaussian peak fitting, which depends on the signal to noise ratio of the nuclear source. The overall accuracy should typically be better than $\sim$50~mas for all target sources detected in A-configuration, and better than 500~mas for target sources detected in only D-configuration. We have compared the radio positions derived here with optical positions from Cotton, Condon, \& Arbizzani (2000), which were measured from the digital sky survey with typical 1$\sigma$ accuracy 1{\farcs}5--2{\farcs}5 in each of right ascension and declination. For all but two of the 2~cm detected nuclei, the radio and optical positions agree to within 1{\arcsec}--3{\farcs}5 (see Table 1). In NGC~185 and NGC~4550, the offsets between radio and optical positions are significant, and as discussed in Appendix A, it is possible that the radio sources we detected in these two galaxies are not related to the galaxy nucleus. Assuming that the radio sources towards NGC~185 and NGC~4550 are in fact associated with their nuclei, the detection rate of 2~cm radio cores in the A-configuration maps of our LLAGN sample is 37\% (18 of 48), at a 10$\sigma$ detection limit of 1.1 mJy. The rate is more striking if we consider the detection rates for low-luminosity Seyferts (75\%; 6 of 8) and for LINERs (50\%; 11 of 22). On the other hand, only one of the 18 observed `transition' nuclei was detected. The morphological type of the galaxy with the detected transition nucleus, NGC~5866, is one of the earliest of the transition objects observed. An additional four objects were detected in D- but not A-configuration maps; we do not consider these four as nuclei with compact 2~cm radio cores. Our detection rate is rather high. In comparison, \citet{wrohee91} detected 30\% (64 of 210) of all E/S0 galaxies with D $<$ 40 Mpc and m$_B$ $<$ 14 mag in a 6~cm VLA survey with resolution 5{\arcsec} and 10$\sigma$ detection limit 1 mJy; the lower resolution and lower frequency make their survey more prone to detecting emission from sources unrelated to the central engine. The radio luminosities of most of the detected 2~cm cores are between 10$^{19.3}$ and 10$^{22}$ W Hz$^{-1}$ (Fig.~1), similar to the luminosities seen in `normal' E/S0 galaxies \citep{sadet89}. It is notable, however, that a significant fraction of the detected 2~cm compact cores are in spiral galaxies (Fig.~1). The 0.7~cm maps of NGC~404, NGC~266, and NGC~7217 were, as expected, noisy, and all three galaxies were undetected at a 10$\sigma$ limit of 10~mJy. At 3.6~cm, NGC~266 and NGC~7217 were detected as unresolved sources with peak fluxes 3.1~mJy and 1.2~mJy, respectively, while NGC~404 was not detected at a 10$\sigma$ upper limit of 0.9~mJy. \subsection{Compact, Flat-Spectrum Cores} None of the objects show reliable extended structure in either A- or D-configuration maps. This is not surprising as the high resolution may resolve out most extended emission, which is expected to be weak at the high frequency observed. Gaussian fits reveal a source size slightly larger than the beam size (and a total flux density slightly larger than the peak flux) for a few sources; however, the deconvolved source sizes are much smaller than half the beam size, so we consider these sources as unresolved. Of the 8 objects which are detected in both our A- and D-configuration maps, NGC~2655 is the only one with significantly more flux at 5{\arcsec} resolution than at 0{\farcs}15 resolution. Thus, most of the detected 2~cm nuclear radio sources are compact at the 0{\farcs}15 resolution (typically 15--25~pc) of our survey. The implied 15~GHz brightness temperatures for the 2~cm compact nuclei are T$_b$~$\geq$ 10$^{3.0-4.0}$~K except for NGC~4278 and NGC~6500 which have T$_b$~$\geq$ 10$^{4.5}$~K, and NGC~185, NGC~4548, NGC~4550, NGC~4636, NGC~5033 which have T$_b$~$\geq$ 10$^{2.5-3.0}$~K. We have supplemented our 2~cm observations with radio data at other wavelengths from the literature, such as the 6~cm radio survey of \citet{wrohee91} and the 20~cm VLA FIRST survey catalog \citep[hereafter FIRST]{whiet97}, in order to derive non-simultaneous spectral indices for the 2~cm detected nuclei; details of the spectral index determination are listed in Appendix A for each galaxy detected at 2~cm. While the actual value of the spectral index is uncertain, given the resolution mismatch and the non-simultaneity of the observations, we can determine whether the core is flat spectrum ($\alpha \geq~-$0.3; S$_\nu \propto \nu^{\alpha}$), or steep spectrum ($\alpha <~-$0.3), as noted in column 12 of Table 1. Since compact flat-spectrum radio sources are often variable, the use of non-simultaneous data at two frequencies can cause some intrinsically flat-spectrum radio sources to be misclassified as steep-spectrum sources. However, since extended steep-spectrum radio sources are not variable, the use of non-simultaneous data at two frequencies should rarely cause intrinsically steep-spectrum radio sources to be misclassified as flat-spectrum sources. Also, since the resolution is better at the higher frequency, resolution effects will tend to steepen the measured spectrum. Thus we are confident of the reality of the flat spectra obtained. Fifteen of the 18 objects detected in our A-configuration 2~cm maps show a flat-spectrum radio core, and one has an undetermined radio spectrum. The remaining two objects detected in the A-configuration 2~cm maps, NGC~2655 and NGC~4636, show evidence for the presence of a steep-spectrum radio core, although NGC~4636 has ``jet''-like steep-spectrum nuclear radio extensions \citep{stawar86}, which may dominate over any flat-spectrum nuclear component within the beam of the radio maps. In the case of NGC~2655, the inferred steep-spectrum of the nucleus is consistent with the presence of extended 2~cm radio emission. The radio morphology of the LLAGNs detected at 2~cm is different from what is seen in ``classical'' Seyferts \citep{ulvwil89,naget99a}, where the nuclear radio emission is usually dominated by steep-spectrum ``jet'' or double radio components on tens to hundreds of pc scales, and any pc-scale radio core may potentially be hidden by free-free absorption at cm wavelengths \citep[e.g.,][]{ulvet81,galet99}. The high incidence of compact, flat-spectrum radio cores in the spiral galaxies in our sample is unusual \citep[e.g.,][]{vilet90,sadet95}. In fact, the high incidence of flat-spectrum radio cores in low-luminosity Seyferts is also unusual, given that flat-spectrum compact radio cores are found in only about 10\% of ``classical'' Seyferts \citep{debwil78,ulvwil89,sadet95}. \subsection{The Relationship Between Radio and Emission-line Luminosities} Correlations between the emission-line and radio luminosities of active galaxies are well known, and are found in both Seyfert \citep{debwil78} and radio galaxies \citep{bauhec89}. The [O~III]~$\lambda$5007 line is usually used in assessing these correlations, but this line is weak in LINERs and we use instead [O~I]~$\lambda$6300, which also has the advantage of being uncontaminated by emission from H~II regions. In our LLAGN sample, the 2~cm radio power appears closely correlated with the [O~I]~$\lambda$6300 luminosity (Fig.~2a and 2b). Testing the statistical significance of this apparent correlation is not straightforward as there exist both upper limits (line not detected) and highly uncertain values (non-photometric data) to the [O~I]~$\lambda$6300 fluxes for the nuclei in the sample, along with upper limits to the 2~cm radio powers. Eight of the LLAGNs in the sample have non-photometric [O~I]~$\lambda$6300 flux values. We treated these eight flux values as photometric measurements when using the bivariate tests from the ASURV statistical software package \citep[][hereafter ASURV]{lavet92}. This approximation should not bias the results significantly as the flux uncertainties are expected to be much less than an order of magnitude. When this is done, tests on the whole sample indicate a correlation between the radio and [O~I]~$\lambda$6300 luminosities at the 97\% significance level. Deleting NGC~185 (the point at the bottom left corner of Fig.~2a), for which the association of the radio source with the nucleus is uncertain (Sec. 3.1), lowers the significance level by only 1\%. \subsection{The 2~cm Non-detections} Do the LLAGNs undetected at 2~cm truly lack radio cores, or do a significant fraction of them have radio cores which are weaker than those in the detected LLAGNs because of, for example, a lower bulge luminosity \citep{sadet89} or a later morphological type? The distributions of host galaxy bulge luminosity and distance (as listed in H97a) are more or less similar for the detected and undetected LINERs. The 2~cm compact core LINERs have a median host galaxy morphological type (T$_{median}$= $-$1.5) which is earlier than, and a median [O~I]~$\lambda$6300 luminosity (Log~L$_{[O~I]}$(median)~= 31.5 Watts) which is higher than, that for LINERs without compact 2~cm cores (T$_{median}$= 2.0, Log~L$_{[O~I]}$(median)~= 31.1 Watts), though the difference is not statistically significant. Given the correlation between emission-line and radio luminosity, it is possible that deeper radio imaging will find compact radio cores in the 2~cm undetected LINERs. The host galaxies of the two undetected Seyferts have bulge luminosities in the B band among the lowest of the eight Seyferts in our sample. There is no clear difference between the morphological types of the detected and undetected Seyferts. Seventeen of the non-detected nuclei are ``transition objects.'' Statistical tests from the ASURV package indicate that the distributions of the 2~cm radio power of LINERs and transition objects in the sample are different at a confidence level of 99.3--99.5\%. While the numerical value of the significance level is questionable given that there is only one detection among the transition objects, arbitrarily changing the four transition objects with the lowest radio flux upper limits into radio detections still results in a difference between transition object and LINER radio powers at the 92--98\% significance level. While the transition nuclei have somewhat lower median bulge luminosities and generally later morphological types compared to the detected LINERs, the statistical significance of these differences is not high. Not unexpectedly, the transition objects in our sample do appear to have weaker [O~I]~$\lambda$6300 luminosities as compared to the LINERs and Seyferts (Fig.~2). If the non-photometric [O~I]~$\lambda$6300 flux measurements are considered to be accurate, statistical tests from the ASURV package indicate that in our sample the [O~I]~$\lambda$6300 luminosities of the transition objects are lower than that of LINERs and Seyferts at the $\sim$99.8\% significance level. It remains to be seen whether this luminosity difference is large enough to explain the striking difference in the detection rates. Surprisingly, the one transition nucleus detected at 2~cm, NGC~5866 (the rightmost circle in Fig.~2b), has one of the lowest [O~I]~$\lambda$6300 luminosities among the transition nuclei. \subsection{Other AGN indicators} We find compact 2~cm radio cores in both type 1 and type 2 nuclei, that is, those with and without visible broad H$\alpha$ emission. Furthermore, some type 1 nuclei in our sample (two Seyferts and four LINERs) do not have a compact radio core at our detection limits. Among LINERs, 64\% (7 of 11) of type 1 nuclei host compact radio cores, as compared to only 36\% (4 of 11) of type 2 nuclei. Additional evidence supports the accretion-powered nature of the nuclei with compact, flat-spectrum 2~cm cores. NGC~4579 is one of the very few LLAGNs with a detected Fe~K$\alpha$ line (Terashima et al. 1998, 2000); such lines are believed to originate through fluorescence in an accretion disk. The optical--UV spectrum of NGC~4579 has been studied in detail by \citet{baret96}, who show that the broad-line region in this object is detectable in a number of permitted transitions. The broad-band spectral energy distribution of NGC 4579 \citep{ho99} has been interpreted in the context of an ADAF model \citep{quaet99}. NGC~4278 and NGC~6500 are both known to harbor pc-scale radio cores studied with Very Long Baseline Interferometry (VLBI) techniques \citep{jon84,jonet81,falet00a}, and NGC~4278 and NGC~4636 show kinematic evidence for massive dark objects at their centers \citep{maget98}. The lack of clear evidence for X-ray emission from an AGN in NGC~404, NGC~4111, NGC~4192, and NGC~4569 \citep{teret99}, galaxies undetected at 2~cm, is consistent with an absence of an accretion-powered source. Also, as mentioned in the next section, \citet{maoet98} find that the nuclei of NGC~404, NGC~4569, and NGC~5055, all undetected at 2~cm, contain massive stars which probably generate sufficient ionizing photons to power the optical emission lines, so it is not necessary to invoke an accretion-powered source. \subsection{Nuclear UV Sources and Compact Radio Cores} A large number of LLAGNs have been imaged in the UV by {\it HST} using the FOC \citep{maoet95,maoet96} and WFPC2 \citep{baret98} detectors. Compact, nuclear UV continuum sources were detected in about 25\% of these LLAGNs. \citet{baret98} used their LLAGN sample and that of \citet{maoet96} to show that LLAGNs with detected UV emission have significantly lower host galaxy inclinations and lower Balmer decrements (the ratio of observed H$\alpha$ to H$\beta$ emission-line fluxes) than those which are not detected in the UV. The same result is obtained if only the LINERs in their sample are considered. These results strongly suggest that the host galaxy is responsible for obscuring the nuclear UV source in a significant fraction of the LLAGNs undetected in the UV \citep{baret98}, a conclusion also reached by the recent work of \citet{poget99}. UV spectroscopy of the brightest candidates were analyzed by \citet{maoet98}. They found that NGC~404, NGC~4569, and NGC~5055 show clear absorption-line signatures of massive stars, which probably generate sufficient ionizing photons to power the optical emission lines. Consistent with this result, all three are undetected in our 2~cm observations. Three of the other objects -- M~81, NGC~4594, and NGC~6500 (and perhaps NGC~4579) -- were found to exhibit a severe ``deficit'' in ionizing photons (i.e. apparently insufficient photons shortward of the Lyman limit, as inferred through extrapolation of the observed UV spectrum, to power the optical and UV emission lines), but their X-ray/UV ratios are two orders of magnitude higher than those of the three stellar-powered LINERs. \citet{maoet98} concluded that, in these four objects a component that emits primarily in the extreme-UV may be the main photoionizing source. We observed and strongly detected two of these nuclei (NGC~4579 and NGC~6500) at 2~cm, and the other two are known to have pc-scale radio cores \citep{baret82,graet81}. The combined \citet{maoet96} and \citet{baret98} UV imaged samples contain 71 galaxies (16 with unresolved UV sources at their nucleus) from the Palomar spectroscopic survey \citep{hoet95}, though some of the UV-imaged nuclei, e.g. NGC~1275 and NGC~4151, are too luminous to count as LLAGNs. Of the 71 galaxies, 27 LLAGNs are in our 2~cm radio sample, which allows us to investigate the correspondence between the incidence of a compact 2~cm radio core and the incidence of an unresolved nuclear UV source in LLAGNs. To improve our statistics, we use the results of further 2~cm radio observations of LLAGNs with similar resolutions and sensitivities to those reported here. These additional observations \citep{falet00b} add four objects (NGC~3368, NGC~3486, NGC~4486 and NGC~4736) with a compact 2~cm core and nine objects (NGC~3627, NGC~3718, NGC~4438, NGC~4725, NGC~4762, NGC~5194, NGC~5195, NGC~6503, NGC~7331) with no detected 2~cm core at a 10$\sigma$ upper limit of $\sim$~1~mJy. The total subset of LLAGNs used for the results in this section therefore includes 11 nuclei with a detected unresolved nuclear UV source, and 29 nuclei with no detected unresolved nuclear UV source. Histograms of the distribution of host galaxy inclination and Balmer decrement are shown for all 40 LLAGNs (Fig.~3a to 3d), with the shaded area representing nuclei which have unresolved UV sources. As in \citet{baret98}, the histograms of inclination include only galaxies with morphological type later than T~= $-$3. All values are as listed in H97a except for: 1) the disk inclinations of NGC~4565 and NGC~4762, which are not listed in H97a and for which we use 89{\arcdeg} and 87{\arcdeg}, respectively \citep[from the Uppsala General Catalogue of Galaxies, hereafter UGC,][]{nil73}; 2) the Balmer decrement of NGC~4579, which is not listed in H97a and for which we use a value of 3.2 \citep{delper96}. The Balmer decrement lower limit in Figure 3c is that of NGC~4762; we could not find a more reliable value in the literature as the H$\beta$ emission is contaminated by H$\beta$ absorption \citep{fiset96}. While the number of objects is small, two interesting results are worth mentioning. 1)~For LLAGNs \textit{with} detected 2~cm compact cores, it appears that the nuclei with unresolved UV sources (shaded area of the histogram; Fig.~3a) are at distinctly lower host galaxy inclinations than nuclei without unresolved UV sources. ASURV statistical tests support this difference in inclination distributions at a significance of 99\%, though of course this numerical value is not reliable given the small number of objects (in fact, the Kolmogorov-Smirnov test does not support a difference in inclination distributions at a significant level). Keeping in mind the large uncertainties of the measured values of the Balmer decrements (H97a), a similar result appears to hold for the distribution of Balmer decrements (Fig.~3c). ASURV statistical tests support this difference in Balmer decrement distributions at a 98\% significance. If accretion-powered, compact UV cores and compact radio sources are associated, this result supports the conclusion of \citet{baret98} that compact UV sources in highly inclined galaxies are present but obscured. 2)~For LLAGNs \textit{without} detected 2~cm compact cores, there is a considerable overlap between the inclination distributions (Fig~3b), and the Balmer decrement distributions (Fig~3d), of UV detected and UV undetected nuclei. That is, for these nuclei, galaxy inclination does not play as strong a role in the obscuration of an unresolved nuclear UV source as is apparently the case for 2~cm detected nuclei. We speculate that in LLAGNs without detected 2~cm cores, the UV source is extended (as would be the case if it is starlight), though still unresolved at \textit{HST} resolution, and less easily obscured than a point-like nuclear UV source. Admittedly, the reliability of these results is limited because deeper radio imaging may find compact radio cores in some of the LLAGNs currently undetected at 2~cm. \subsection{Minor-axis Jets or Outflows} A significant number of the LLAGNs with 2~cm compact radio cores in our sample show extended 1.4~GHz radio emission in 5{\arcsec} resolution FIRST maps, and/or 1{\farcs}5--4{\arcsec} maps from other sources (see Appendix A). We therefore compared the P.A. of this radio extension, when available, with that of the host galaxy major axis for all LLAGNs in our sample which host 2~cm compact radio cores. The relevant P.A.'s are listed in Table 2, and a histogram of the difference in the two P.A.'s, $\delta$, is shown in Fig.~4. The single transition object (shown in grey) shows extended emission along the host galaxy major axis, so it is likely that the radio emission is from the galaxy disk. On the other hand, LINERs and low-luminosity Seyferts with compact 2~cm radio cores appear to favor extended radio emission along the galaxy disk minor axis, although the small number of objects prohibits any meaningful statistical tests. The extent of this minor-axis radio emission is typically only 0{\farcs}5 to 3{\arcsec} (50--350~pc for most galaxies in the sample), and could trace the inner parts of wide-angle, minor-axis outflows, as seen in classical Seyferts \citep{colet96}, or low-luminosity collimated ``jets'' from the central engine. \section{Discussion} We have detected unresolved 2~cm nuclear radio emission in 75\% of low-luminosity Seyferts and 50\% of LINERs. At least 83\% of these nuclear radio sources have flat radio spectra. At this point it is not clear whether the 2~cm non-detected LINERs and low-luminosity Seyferts really do not have compact radio cores, or whether their compact cores simply have lower radio luminosities or are hidden by, for example, free-free absorption. In powerful AGNs, compact flat-spectrum nuclear radio cores are widely accepted as the signature of synchrotron emission from the base of a synchrotron self-absorbed jet (see the review by Zensus 1997). The compact, flat-spectrum radio cores we detect in LLAGNs could be scaled-down versions of powerful AGNs \cite[e.g.][]{falet99}, or could trace emission from an ADAF \citep{naret98}, or material undergoing spherical accretion \citep{mel92}. Alternatively, the radio cores could be produced by thermal bremsstrahlung from ionized gas or by synchrotron radiation which has suffered free-free absorption. Such emission could originate from an active nucleus or nuclear star forming regions. The lower limits to the brightness temperatures of the 2~cm compact core objects in our sample are 10$^{2.5-4.5}$~K, so the VLA observations alone cannot rule out the notion that the radio emission originates in star formation regions. However, most objects have brightness temperatures too high to be optically-thin thermal emission from a gas at 10$^4$~K. The expected value of T$_b$ is $\sim$ 10$^4$~K ($\nu/\nu_{\tau})^{-2.1}$, where $\nu$ is the observing frequency and $\nu_{\tau}$ is the frequency at which the source becomes optically thick. If $\nu_{\tau}$~$<$ 1.4~GHz, as is the case for normal spiral galaxies and extended starbursts \citep{con83}, a thermal source with a flat spectrum between 2~cm and 20~cm has T$^{(2cm)}_b < 10^4 (20/2)^{-2.1}$ K i.e. $<$~90~K, lower than all of our measurements. Further, higher resolution observations of 10 of the detected objects in this sample reveal brightness temperatures $\gtrsim$~10$^8$~K, strongly suggesting that synchrotron self-absorption is responsible for the flat spectra and arguing against a thermal origin \citep{falet00a}. Thus, it is very likely that at least 50\% of the LINERs and low-luminosity Seyferts in our sample contain accretion-powered nuclei. Several independent factors, described in Section 3.4, support our proposition that the presence of a compact, flat-spectrum (as determined from VLA observations) radio core is related to the presence of accretion onto a massive black hole in LLAGNs. The detection rate of radio cores in transition objects in our sample is dramatically low (1 of 18). The significant difference between transition and ``pure'' LINER detection rates, combined with the lack of any clear difference between their host galaxy properties, suggests one of the following. 1) The central engines of transition objects are intrinsically different from those of ``pure'' LINERs, or 2) Transition objects are truly composite nuclei with a LINER and an H~II component \citep{hoet93}, in which case the LINER component is much weaker than that in ``pure'' LINERs, so that the radio cores in transition objects are weaker than those in ``pure'' LINERs. Using the luminosity in the [O~I]~$\lambda$6300 line as an indicator of the luminosity of the LINER component, this second premise is supported by the results that the [O~I]~$\lambda$6300 and 2~cm luminosities are correlated at the $\sim$97\% significance level, and that the [O~I]~$\lambda$6300 luminosity of the transition objects in the sample is lower than that of the LINERs and Seyferts in the sample at the $\sim$99.8\% significance level (Fig.~2). While the presence of an unresolved nuclear UV source does not necessarily imply the presence of a compact 2~cm radio core, the data are consistent with a scenario in which {\it{most}} 2~cm compact core LLAGNs have an unresolved nuclear UV source, and this nuclear source is obscured by the host galaxy (or a nuclear structure aligned with the host galaxy disk) in more edge-on host galaxies (see Fig.~3a and c), consistent with the findings of \citet{baret98} and \citet{poget99}. For galaxies without detected 2~cm compact radio cores, the role of the host galaxy in obscuring any nuclear UV structure is not as dramatic as in 2~cm compact core LLAGNs (Fig~3b and d). Of the 6 LLAGNs which do not host a 2~cm compact radio core, but do have an unresolved nuclear UV source, the UV emission in three (NGC~404, NGC~4569, NGC~5055) originates from hot stars \citep{maoet98}. The origin of the UV source in the other three, NGC~3368, NGC~3486, NGC~3642, is unknown. Perhaps the unresolved UV emission in the nuclei without 2~cm compact radio cores is dominated by stars and therefore more extended than in the case of UV emission from an accreting black hole. If so, it will be less susceptible to being completely hidden by obscuration. Overall, the radio-UV relationship investigated here is consistent with the idea that the 2~cm detected and 2~cm undetected nuclei represent two separate classes of objects. The former are dominated by accreting black holes, while the latter have weaker, or absent, accreting black holes, but can have nuclear UV emission which originates from hot stars. Admittedly, deeper radio imaging may result in some of the objects included in Figs. 3b and 3d moving to Figs. 3a and 3c. Finally, if the extended 1.4~GHz radio emission, which appears preferentially aligned with the host galaxy minor axis in a subset of 2~cm compact core LLAGNs (Fig.~4), does trace jets from the central engine, there are interesting implications for the orientation of the central engines of AGN. Similar analyses of ``classical'' Seyferts \citep{ulvwil84,schet97,claet98,nagwil99} have shown that the distribution of P.A.$_{radio}$ -- P.A.$_{galaxy~major~axis}$ is more or less uniform between 0{\arcdeg} and 90{\arcdeg}. One explanation for this uniform distribution is that radiation from the central engine illuminates the inner accretion disk inducing a ``radiative instability'' \citep{pri97} which causes large warps in the inner disk. In this case, even if the outer accretion disk shares the same axis of rotation as the host galaxy disk, the jet (presumably launched along the axis of the inner accretion disk) may then be at a random angle with respect to the axis of the galaxy disk. If the radiation instability is indeed the correct explanation, then in LLAGNs, where the radiation field of the central engine is much weaker, the disk is expected to be less prone to the radiation instability (see e.g. equation 3.2 and Sec. 3.2 of Pringle 1997), so that the jet axis is more likely to be along the host galaxy rotation axis. This is precisely what we find. Alternately, the jet power in LLAGNs may be low enough that buoyancy dominates jet expansion at large scales, which redirects the jet along the minor axis. Our further surveys for 2~cm radio cores in LLAGNs and our scheduled observations of 17 nearby bright LLAGNs at 0{\farcs}3 resolution with the Multi-Element Radio Linked Interferometer Network (MERLIN) at 1.4~GHz will potentially reveal more on this subject. \section{Conclusions} Our detection of compact, flat-spectrum radio cores in about 50\% of low-luminosity Seyferts and LINERs in a sample of 48 low-luminosity AGNs, when combined with VLBA observations \citep{falet00a}, suggests that at least half of all low-luminosity Seyferts and LINERs are accretion powered. The moderate sensitivity-limit of our survey and the lack of any clear difference in other properties between detected and undetected LINERs and low-luminosity Seyferts suggest that the true incidence of radio cores is likely to be higher. The 2~cm radio power is significantly correlated with the [O~I]~$\lambda$6300 luminosity for LLAGNs in our sample. The lower detection rate (at $\geq$92\% significance) of compact radio cores in ``transition'' nuclei suggests that either these nuclei are intrinsically different from ``pure'' LINERs, or their ``pure'' LINER component is of lower luminosity, with lower luminosity radio cores. The latter interpretation is favored by the correlation between the radio and [O~I]~$\lambda$6300 luminosity coupled with the lower [O~I]~$\lambda$6300 luminosities of the ``transition'' nuclei as compared to the LINERs. The presence or absence of a detected broad H$\alpha$ line is not a good indicator of the presence or absence of a compact, flat-spectrum 2~cm radio core. Investigation of the correlation between the incidence of a 2~cm compact radio core and an unresolved (at {\it HST} resolution) nuclear UV source, suggests that most LLAGNs with a 2~cm compact radio core also contain an unresolved nuclear UV source, and that this UV source is obscured for more inclined galaxy disks. For LLAGNs without a 2~cm compact core, the role of the host galaxy in obscuring the nuclear UV source is not as clear -- perhaps the UV emission in these nuclei is more spatially extended and therefore less prone to complete obscuration. A significant number of the LINERs and low-luminosity Seyferts which contain 2~cm compact radio cores show evidence, at low resolution (1{\arcsec}--5{\arcsec}) and frequency (1.4~GHz), for extended radio emission along the galaxy disk minor axis. This radio emission may trace a wide-angle outflow or a weak, highly-collimated jet along the disk rotation axis. If the latter is true, it lends support to the idea that it is the ``radiative instability'' which causes warps in the nuclear accretion disks of more luminous Seyfert galaxies. Finally, we note that the data presented here are the initial results of a larger program to study a well-defined sample of LLAGNs at high resolution with the VLA and VLBA; results of these will appear in future papers in this series. \acknowledgements This research has been supported by NSF grant AST~9527289 and by NASA grant NAG~81027. The work of HF is funded by DFG grant Fa 358/1-1 \& 2. The work of L.~C.~H. is partly funded by NASA grant NAG 5-3556, and by NASA grants GO-06837.01-95A and AR-07527.02-96A from the Space Telescope Science Institute (operated by AURA, Inc., under NASA contract NAS5-26555). \appendix \section{Notes on Individual Objects} Specific notes on all galaxies detected at 2~cm and on some undetected galaxies are listed here. \paragraph{NGC~185} The 2~cm source we detect is offset 14{\farcs}5 west and 4{\arcsec} north of the optical position determined by \citet{cotet00} from digital sky survey plates. The 1{$\sigma$} error of the optical position determination is estimated to be 2{\farcs}7 in each of right ascension and declination. The galaxy is diffuse, $\sim$ 12{\arcmin} x 10{\arcmin} in extent, and has a prominent dust lane to the north-west which may bias the position determination. Overlaying the optical and radio images shows that the 2~cm radio source lies at the south-west tip of the dust lane, and the offset from the optical center does appear to be more than can be explained by dust lane obscuration. Further, the radio source has a very low luminosity (10$^{16.7}$ Watts Hz$^{-1}$) which can be explained by a source in the galaxy stellar disk. \citet{hum80} lists a 5$\sigma$ upper limit of 10~mJy in a 23{\arcsec} resolution map at 1.4~GHz, and we find a 2~cm flux of 0.8 mJy in our 0{\farcs}15 map. It is therefore possible that the source is flat spectrum, but a measurement at a resolution closer to that of the 2~cm map is required for a definitive result, and for now we consider this source to have an undetermined spectral index. \paragraph{NGC~266} Our simultaneous 3.6~cm and 2~cm observations on 1996 October 11, with resolutions 0{\farcs}29 x 0{\farcs}24, and 0{\farcs}15 x 0{\farcs}13, detected an unresolved nuclear source with peak flux 3.1~mJy and 4.1~mJy, respectively. The nucleus therefore has an inverted radio spectrum, $\alpha^{3.6}_{2}~\geq$ 0.5. This source was, however, not detected at a 10$\sigma$ upper limit of 10~mJy in our simultaneous 0.7~cm observation. NGC~266 has been identified with source NVSS~J004947+321637 by the NASA Extragalactic Database \citep[NED;][]{helet91} i.e. it has been detected in the 1.4~GHz NRAO VLA sky survey (NVSS), with a peak flux of 8.9~mJy at a resolution of 45{\arcsec}, though the 1.4~GHz position is offset 44{\arcsec} from our 2~cm position. In any case, 8.9~mJy may be taken as an upper limit to the 1.4~GHz flux. \paragraph{NGC~404} Our simultaneous 3.6~cm, 2~cm, and 0.7~cm observations on 1996 October 11 did not detect the nucleus at 10$\sigma$ upper limits of 0.9~mJy, 1.3~mJy, and 10~mJy, respectively. \paragraph{NGC~2655} Comprehensive radio continuum and optical emission-line imaging, as well as optical spectra, are presented by \citet{keehum88}. Their VLA 6~cm map shows a roughly E-W symmetric extension at 0{\farcs}5 resolution (with a peak flux of 36~mJy from a component with half-power size 0{\farcs}4 x 0{\farcs}5). Combining this peak flux with our 2~cm peak fluxes of 6~mJy and 13.6~mJy at resolutions of 0{\farcs}15 and 5{\arcsec}, respectively, results in a non-simultaneous spectral index, $\alpha^{6}_{2}$~= $-$0.7 to $-$1.6, for the radio emission within 0{\farcs}5 of the nucleus. \citet{keehum88} find S$_{\nu} \propto \nu^{-0.67}$ for the nucleus, from 4{\arcsec} resolution non-simultaneous VLA maps at 6 and 20~cm. They also find a secondary 6~cm source in P.A. 117{\arcdeg}, with a diffuse bridge of radio emission joining it to the nuclear component. The morphology, kinematics, and line ratios of the surrounding emission-line gas, suggest that this secondary 6~cm source and bridge are related to nuclear ejecta \citep{keehum88}. Clearly, most of the radio emission in this object is extended, and if the emission from the unresolved radio nucleus is related to an accreting black hole, then its steep radio spectrum suggests that it is more likely that the emission is from synchrotron-emitting jets, rather than from a compact radio core. The host galaxy P.A. is not defined for this galaxy in the UGC or the Third Reference Catalogue of Bright Galaxies (de Vaucouleurs et al. 1991, hereafter RC3) as the galaxy is nearly round. \paragraph{NGC~2681} FIRST lists a 1.4~GHz flux of 3.8 mJy at 5{\arcsec} resolution, with extended emission in P.A. 35{\arcdeg}. We did not detect this object at 2~cm. The UGC and RC3 do not list a P.A. for this galaxy as it is almost round. \paragraph{NGC~2787} \citet{hecet80} find 1.4~GHz and 4.9~GHz fluxes of $<$9 mJy and 9 mJy, at resolutions of 4{\arcsec} and 1{\farcs}7, respectively, so the nucleus has a flat or inverted spectrum between 20~cm and 6~cm, $\alpha^{20}_{6} \geq$ 0. Combining the 4.9~GHz peak flux with our 2~cm peak fluxes of 7~mJy and 8.9~mJy at resolutions of 0{\farcs}15 and 5{\arcsec}, respectively, results in a non-simultaneous spectral index, $\alpha^{6}_{2}$ between $-$0.2 and 0, for all emission within the central 1{\farcs}7. \paragraph{NGC~3147} \citet{vilet90} find S$_{20cm}^{peak}$ = 10.6 mJy and S$_{6cm}^{peak}$ = 8.55 mJy at resolutions of $\sim$1{\farcs}2 and $\sim$1{\farcs}5, respectively, with both maps showing only a compact core. Consistent with this, \citet{lauet97} find a core peak flux of 9 mJy at 6~cm at a resolution of 0{\farcs}4. Combining this 6~cm flux with our 2~cm peak flux (8~mJy), the non-simultaneous spectral index of the emission in the central 0{\farcs}4 is flat, $\alpha^{6}_{2}~\geq$~0. \paragraph{NGC~3169} \citet{humet87} find a core flux of 8.2 mJy in the central $\leq$2{\arcsec} of their 1{\farcs}2 resolution, 1.4~GHz map, and also extended emission in P.A. 120{\arcdeg}, more or less along the minor axis of the host galaxy. Combining their 1.4~GHz peak flux with our measured 2~cm peak fluxes of 6.8~mJy and 10~mJy at resolutions of 0{\farcs}15 and 5{\arcsec}, respectively, results in a non-simultaneous spectral index, $\alpha^{20}_{2}$, between $-$0.1 and 0.1, for the radio emission within the central 2{\arcsec}. \paragraph{NGC~4111} FIRST lists this object's peak 1.4~GHz flux as 5.8 mJy at 5{\arcsec} resolution. The emission is extended in P.A. 70{\arcdeg}, along the minor axis of the host galaxy. We did not detect this object at 2~cm. \paragraph{NGC~4143} - \citet{wrohee91} find a 5~GHz flux of 6.7 mJy at 5{\arcsec} resolution. The similar resolution (5{\arcsec}) FIRST map shows a peak flux of 5 mJy at 1.4~GHz, with extended emission in P.A. 10{\arcdeg}, which is not along the minor axis of the host galaxy. Combining the FIRST flux with our 2~cm peak fluxes of 3.3~mJy and 5.3~mJy at resolutions of 0{\farcs}15 and 5{\arcsec}, respectively, results in a non-simultaneous spectral index, $\alpha^{20}_{2}$ between $-$0.2 and 0, for all emission within the central 5{\arcsec}. \citet{hoet97b} note that broad H$\beta$ may also be present in this object. \paragraph{NGC~4203} \citet{wro91} derived a 4.9~GHz flux of 12.5~mJy at $\sim$5{\arcsec} resolution, and FIRST lists a peak 1.4~GHz flux of 6.9~mJy (at a resolution of 5{\arcsec}), implying that the core has a highly inverted spectrum between 20~cm and 6~cm, $\alpha^{20}_{6}~\sim$ 0.5. Our D-configuration 2~cm flux of 12.3~mJy, suggests a non-simultaneous spectral index $\alpha^{6}_{2}~\sim$ 0. The extended emission in the FIRST map is in P.A. 90{\arcdeg}, more or less along the minor axis of the host galaxy. \paragraph{NGC~4216} \citet{humet87} observed this object with 1{\farcs}3 resolution at 1.4~GHz and did not detect any emission in the central 2{\arcsec} of the galaxy, at a 3$\sigma$ upper limit of 0.5 mJy. \paragraph{NGC~4278} \citet{jon84} have observed this object using VLBI and find the core flux is 180 mJy and 190~mJy at 18~cm and 6~cm, on size scales $<$5~mas and $<$1~mas, respectively. \citet{wilet98} find a 3.6~cm peak flux of 153~mJy at 200 mas resolution, and we find a 2~cm peak flux of 88.3~mJy at 150~mas resolution, so the core may be flat between 18 and 6~cm and then steeper down to 2~cm. Alternatively, the lower flux at 2~cm may be due to source variability. The 1.4~GHz emission detected in FIRST is extended in P.A. 166{\arcdeg}, but the UGC and RC3 do not list a host galaxy P.A. as the galaxy is almost round. \paragraph{NGC~4419} \citet{humet87} find a core flux of 32~mJy in the central $\leq$2{\arcsec} of their 1{\farcs}3 resolution 1.4~GHz map, and extended emission in P.A. 77{\arcdeg}. Combining this flux with our 2~cm D-configuration peak flux of 5.4~mJy at 5{\arcsec} resolution, results in a non-simultaneous spectral index, $\alpha^{20}_{2}~\leq -$0.8. This object was not detected in our 0{\farcs}15 resolution, 2~cm A-configuration map. \paragraph{NGC~4449} FIRST lists a 1.4~GHz peak flux of 1.7~mJy at 5{\arcsec} resolution, with extended emission in P.A. 65{\arcdeg}, more or less along the host galaxy major axis. We did not detect this object at 2~cm. \paragraph{NGC~4527} \citet{vilet90} find S$_{20cm}^{peak}$ = 4.3 mJy and S$_{6cm}^{peak}$ = 1.3 mJy at resolutions of $\sim$1{\farcs}3 and $\sim$1{\farcs}4, respectively (a spectral index, $\alpha^{20}_{6}$, of about $-$1.1). The 20~cm and 6~cm maps are both highly extended (P.A. 65{\arcdeg}) along the major axis of the galaxy, with an edge brightened morphology, suggestive of an annulus \citep{vilet90}. We did not detect this galaxy at 2~cm. \paragraph{NGC~4548} \citet{humet87} find a 3$\sigma$ upper limit of 2.5~mJy at 1.4~GHz, over the central 2{\arcsec} of their 1{\farcs}3 resolution VLA map. Since we detect a flux of 1.2 mJy at 2~cm, the non-simultaneous spectral index of the core is $\alpha^{20}_{2} \geq$ $-$0.3. \paragraph{NGC~4550} The 2~cm source we detect is offset 9{\farcs}3 west and 5{\farcs}5 north of the optical position determined by \citet{cotet00} from digital sky survey plates. The 1{$\sigma$} error of the optical position determination is estimated to be 2{\farcs}3 in each of the R.A. and Dec. The galaxy does not show any obvious sign of a peculiar morphology, and the east-west galaxy extent is $\sim$1{\arcmin}, so the radio-optical offset does appear significant. A $\sim$3{\arcsec} resolution 2.7~GHz map \citep{haysra80} shows an unresolved radio core at a position between our radio detection and the optical nucleus. \citet{wrohee91} find the 4.9~GHz emission over the central 5{\arcsec} is $<$~1~mJy. We measure a 2~cm peak flux of 0.7~mJy, so the source is probably flat spectrum, $\alpha^{6}_{2} \geq$ $-$0.3. \paragraph{NGC~4565} \citet{humet87} find a core 1.4~GHz flux of 1.2~mJy in the central $\leq$2{\arcsec} of their 1{\farcs}3 resolution VLA map. FIRST lists a peak flux of 1.5 mJy at 1.4~GHz in a 5{\arcsec} resolution map, with extended emission in P.A. 36{\arcdeg}, along the minor axis of the host galaxy. Combined with our 2~cm peak flux of 3.7~mJy at 0{\farcs}15 resolution, the non-simultaneous spectral index is $\alpha^{20}_{2}~\geq$ 0.5. This galaxy has the lowest broad H$\alpha$ luminosity found in any known active nucleus (Ho et al. 1997b). \paragraph{NGC~4579} \citet{humet87} find a core flux of 25.1~mJy in the central $\leq$2{\arcsec} of their 1{\farcs}3 resolution, 1.4~GHz VLA map, with extended emission in P.A. $\sim$135{\arcdeg}. The radio core was found to have an inverted spectrum between 2.3 and 8.4~GHz with the Parkes-Tidbinbilla 275 km interferometer (S$_{\nu}~\propto$ $\nu^{0.2}$; Sadler et al. 1995). Van der Hulst, Crane \& Keel (1981) found the 5~GHz emission to be extended in P.A. 134{\arcdeg}, with extent 0{\farcs}4$\pm$0{\farcs}2 and flux 39.9 mJy. \paragraph{NGC~4636} We derive a peak 2~cm flux of 1.6~mJy for this galaxy. \citet{stawar86} have imaged this object at 4{\arcsec} resolution at 4.9~GHz and 1.4~GHz. Both maps show extended jet-like structure in P.A. $\sim$~45{\arcdeg}, more or less along the minor axis of the host galaxy. They found a peak 1.4~GHz flux of 11.4~mJy, and a steep spectrum ($\alpha$ = $-$0.6) over the jets and at the core. The steep-spectrum jets may dominate the core emission within the central 4{\arcsec} beam, so the core could potentially have a flat spectrum at higher resolution. In the absence of higher resolution maps, we list this object as a steep spectrum source in Table 1. \paragraph{NGC~5005} \citet{vilet90} find S$_{20cm}^{peak}$ = 14.6 mJy and S$_{6cm}^{peak}$ = 2.7 mJy at resolutions of $\sim$1{\farcs}1 and $\sim$1{\farcs}0, respectively (a spectral index of $\alpha^{20}_{6}$~= $-$1.8). The source is resolved at both frequencies, in P.A. $\sim$~135{\arcdeg} on a $\sim$5{\arcsec} scale, again, more or less along the minor axis of the host galaxy. \citet{humet85} find a 1.4~GHz peak flux of 70 mJy at 14{\arcsec} resolution, with extended emission (about 2{\arcmin} extent) in P.A. 67{\arcdeg}, along the major axis of the galaxy. FIRST also lists this source with a peak 20~cm flux of 35.9 mJy (at 5{\arcsec} resolution), with extended emission in P.A. 124{\arcdeg}. The relatively steep spectral slope between 20 and 6~cm, and the extended radio morphology at these frequencies, is consistent with our 2~cm non-detection. \paragraph{NGC~5033} FIRST lists a peak flux of 1.4~mJy at 1.4~GHz at a resolution of 5{\arcsec}. The inner radio emission in the FIRST map is extended in P.A. 91{\arcdeg}, along the minor axis of the host galaxy, and there is an outer diffuse component which is more aligned with the host galaxy major axis. Combining the FIRST peak flux with our 2~cm peak flux (1.4~mJy) results in a non-simultaneous spectral index $\alpha^{20}_{2}~\geq$ 0. \paragraph{NGC~5055} The 1.4~GHz FIRST map has peak flux 1.5~mJy, and shows extended emission in P.A. 42{\arcdeg}. We did not detect this galaxy at 2~cm. \paragraph{NGC~5273} FIRST lists a peak flux of 2.6~mJy at 1.4~GHz; the extended radio emission (P.A. 180{\arcdeg}) is along the major axis of the host galaxy disk. We did not detect this galaxy at 2~cm. \paragraph{NGC~5866} \citet{conet90} find a peak 1.4~GHz flux of 9.5~mJy in a 5{\arcsec} resolution map made with the VLA. This is somewhat different from the value listed in FIRST - 1.4~GHz peak flux of 15.4~mJy at a resolution of 5{\arcsec}. \citet{wrohee91} derived a flux of 7.4~mJy from their 5{\arcsec} resolution 4.9~GHz map. We measured a peak 2~cm flux of 7.1~mJy at 0{\farcs}15 resolution, so the core has a spectral index, $\alpha^{6}_{2}~\geq$ 0. The extended flux in the FIRST map is in P.A. 132{\arcdeg}, along the major axis of the host galaxy, not unexpected in this transition object. This is the only transition object in the sample in which we detected a 2~cm radio core. \paragraph{NGC~6500} The spectrum of the core of this spiral is known to be flat on scales of $\sim$20 mas \citep{jonet81}. The 1{\farcs}2 resolution 1.4~GHz map of \citet{unget89} shows extended emission in P.A. 140{\arcdeg}, along the minor axis of the galaxy, suggestive of an outflow. Their higher resolution (0{\farcs}3) 1.4~GHz MERLIN map shows an extension in P.A. $\sim$70{\arcdeg}. \paragraph{NGC~7217} Our simultaneous 3.6~cm observation on 1996 October 11, with resolution 0{\farcs}27 x 0{\farcs}25, detected an unresolved nuclear source with flux 1.2~mJy. 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S. 1991, \aj, 101, 148 \bibitem[Zensus(1997)]{zen97} Zensus, A. 1997, \araa, 35, 607 \end{thebibliography} \pagestyle{empty} \clearpage \begin{figure} \vspace{-2in} \figurenum{1} \epsscale{1.0} \plotone{f1.ps} \vspace{-4in} \caption{ Distribution of the 2~cm nuclear power for all 48 LLAGNs in the sample, as a function of host galaxy morphological type. Detections are plotted with filled circles while non-detections are plotted with crosses. Note the large number of spiral galaxies detected. } \end{figure} \clearpage \begin{figure} \figurenum{2} \epsscale{1.0} %\plotfiddle{f2a.ps}{4in}{0}{50}{50}{-300}{70} %\plotfiddle{f2b.ps}{4in}{0}{50}{50}{-300}{400} \plotone{f2a.ps} \vspace{-6in} \plotone{f2b.ps} \vspace{-5in} \caption{ (a)~Relationship between [O~I]~$\lambda$6300{\AA} luminosity and 2~cm radio power for Seyfert and LINER nuclei in our sample. Objects with non-photometric [O~I]~$\lambda$6300{\AA} flux measurements are shown as open circles; (b)~same as (a) but for transition nuclei in our sample. For clarity, lower limit signs for the 2~cm power are not shown for the transition objects; the single transition object with a detected 2~cm core is the rightmost object in the figure. } \end{figure} \begin{figure} \figurenum{3} \epsscale{1.0} \plotone{f3.ps} \vspace{-3in} \caption{ (a) to (d): Histograms of the distribution of host galaxy inclination and Balmer decrement for all LLAGNs observed in both the UV and in our 2~cm radio surveys. Inclinations are not plotted for early-type (T~$<$ $-$3) galaxies. The histograms for LLAGNs with detected 2~cm compact radio cores are shown in the left panels, and those for LLAGNs without detected 2~cm compact cores are shown in the right panels. The upper line of the histogram is for all nuclei, while the shaded area is for those nuclei which exhibit unresolved nuclear UV sources (as observed with \textit{HST} -- see text). } \end{figure} \begin{figure} \figurenum{4} \epsscale{1.0} \plotone{f4.ps} \vspace{-3in} \caption{ Histogram of the distribution of the difference between the position angle of the 1.4~GHz radio emission at 1{\arcsec}--5{\arcsec} resolution and the position angle of the host galaxy major axis, as listed in the UGC, for all LLAGNs in our sample which contain a 2~cm compact core and have extended nuclear radio emission. The single transition object is shown in grey. } \end{figure} \clearpage \hoffset=-1.0in \voffset=-2in \begin{figure} \epsscale{1.35} \plotone{p1tab1.ps} \end{figure} \clearpage %\hoffset=-2in %\voffset=-3in \begin{figure} \epsscale{1.5} \plotone{p1tab2.ps} \end{figure} \end{document} \singlespace \end{document}