%% Version 11/17/99 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %% Proceedings Sample, ProcChap.tex %% Kluwer Academic Press %% %% Prepared by Amy Hendrickson, TeXnology Inc., July 1999. %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %%%%% %% LaTeX2e %% Uncomment documentclass, \documentclass{KapProc} % Computer Modern font calls \usepackage{astrobib} \usepackage{psfig} \usepackage{astro} %% and, optionally, one or more %% of the \usepackage commands below: %%%%% %% If you use a font encoding package, please enter it here, i.e., %% \usepackage{T1enc} %%%%% % If you have MathTimes and MathTimesPlus fonts, you % may uncomment the line below and use them, but you are % not obligated to do so, and most authors do not have % these fonts. (You may need to edit m-times.sty to make the % font names match those on your system) % You must have the MathTimes fonts for this to work. They may be % purchased from the Y&Y company, http://www.YandY.com. % \usepackage[mtbold,noTS1]{m-times} %%%%% % PostScript font calls % % If you use the ProcPs PS font file, you may need to edit it % to make sure the font names match those on your system. See % the top of the ProcPs.sty file for more info. % \usepackage{ProcPs} %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %% LaTeX209, uncomment only one: %% (Make sure documentclass and usepackage commands above are commented out!) % % \documentstyle{KapProc} % Computer Modern fonts % \documentstyle[ProcPs]{KapProc} %For PostScript fonts % (MathTimes style is not available for authors using LaTeX2.09) %%%%%%% Formatting Commands You Can Set or Change ===>>> % optional, uncomment to make current time and `draft' appear at % bottom of page. %\draft %%%% To change footnotes to appear at bottom of page ==> %% (Default is endnotes that appear at the end of the chapter, above %% the references, or whereever \notes is written.) %% uncomment to make footnote appear at bottom of page: \let\footnote\savefootnote %% uncomment if you want footnotetext to appear at the bottom of the page: \let\footnotetext\savefootnotetext %% uncomment if you want a ruled line above the footnote: \let\footnoterule\savefootnoterule %%%% <== end footnote changes %% How many levels of section head would you like numbered? %% 0= no section numbers, 1= section, 2= subsection, 3= subsubsection %%==>> \setcounter{secnumdepth}{3} %% How many levels of section head would you like to appear in the %% Table of Contents? %% 0= chapter titles, 1= section titles, 2= subsection titles, %% 3= subsubsection titles. %%==>> \setcounter{tocdepth}{3} %%%%%%% Bibliography Style Settings ==>> %%% Uncomment one of the Following: \kluwerbib %\normallatexbib %%%%%%% % \kluwerbib will produce this kind of bibliography entry: % % Anderson, Terry L.,... % More bib entry here... % % \cite{xxx} will print without brackets around the citation. % % \bibliographystyle{apalike} should be use with \kluwerbib %%%%%%% % \normallatexbib will produce bibliography entries as shown in the % LaTeX book % % [1] Anderson, Terry L.,... % More bib entry here... % % \cite{xxx} will print with square brackets around the citation, [1]. % % Any \bibliographystyle{} may be used with \normallatexbib, but % you should check with your editor to find the style preferred for % the book you are contributing to. %%%%%%% To change brackets around citation ==>> % Default with \kluwerbib is no brackets around citation. % Default with \normallatexbib is square brackets around citation. %If you want parens, around citation, i.e., (citation), uncomment these lines: %\let\lcitebracket( %\let\rcitebracket) %%%%%%% <<== End Bibliography Style Settings %%%%%%% Author and Topic Indices %% If you want to have both an author and a topic index, uncomment this: %\startauthorindex %%%% <<== End Formatting Commands You Can Set or Change %%%%%%%%%%%%%%%%% %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %Citation commands from Bibtex %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \citation{ChurchwellWitzelPauliny-Toth1977} \citation{BraatzWilsonHenkel1994,BraatzWilsonHenkel1996} \citation{AntonucciMiller1985} \citation{Pogge1988} \citation{MiyoshiMoranHerrnstein1995,GreenhillJiangMoran1995} \citation{GreenhillMoranHerrnstein1997} \citation{TrotterGreenhillMoran1998} \citation{GreenhillGwinnAntonucci1996} \citation{NeufeldMaloney1995} \citation{BraatzWilsonHenkel1996} \citation{FalckeWilsonHenkel2000} \citation{CecilBland-HawthornVeilleux2001} \citation{MarconiOlivavanderWerf2000} \citation{FalckeWilsonHenkel2000} \citation{FerruitWilsonMulchaey2000} \citation{TrotterGreenhillMoran1998} \citation{BraatzWilsonHenkel1997} \citation{ClaussenDiamondBraatz1998} \citation{FalckeHenkelPeck2000} \bibstyle{aa} \bibdata{aamnemonic,../../Review/review} \bibcite{AntonucciMiller1985}{\citeauthoryear {{Antonucci} \& {Miller}}{{Antonucci} \& {Miller}}{1985}} \bibcite{BraatzWilsonHenkel1994}{\citeauthoryear {{Braatz}, {Wilson}, \& {Henkel}}{{Braatz} et\nobreakspace {}al.}{1994}} \bibcite{BraatzWilsonHenkel1996}{\citeauthoryear {{Braatz}, {Wilson}, \& {Henkel}}{{Braatz} et\nobreakspace {}al.}{1996}} \bibcite{BraatzWilsonHenkel1997}{\citeauthoryear {{Braatz}, {Wilson}, \& {Henkel}}{{Braatz} et\nobreakspace {}al.}{1997}} \bibcite{CecilBland-HawthornVeilleux2001}{\citeauthoryear {{Cecil} et\nobreakspace {}al.}{{Cecil} et\nobreakspace {}al.}{2001}} \bibcite{ChurchwellWitzelPauliny-Toth1977}{\citeauthoryear {{Churchwell} et\nobreakspace {}al.}{{Churchwell} et\nobreakspace {}al.}{1977}} \bibcite{ClaussenDiamondBraatz1998}{\citeauthoryear {{Claussen} et\nobreakspace {}al.}{{Claussen} et\nobreakspace {}al.}{1998}} \bibcite{FalckeHenkelPeck2000}{\citeauthoryear {{Falcke} et\nobreakspace {}al.}{{Falcke} et\nobreakspace {}al.}{2000a}} \bibcite{FalckeWilsonHenkel2000}{\citeauthoryear {{Falcke} et\nobreakspace {}al.}{{Falcke} et\nobreakspace {}al.}{2000b}} \bibcite{FerruitWilsonMulchaey2000}{\citeauthoryear {{Ferruit}, {Wilson}, \& {Mulchaey}}{{Ferruit} et\nobreakspace {}al.}{2000}} \bibcite{GreenhillGwinnAntonucci1996}{\citeauthoryear {{Greenhill} et\nobreakspace {}al.}{{Greenhill} et\nobreakspace {}al.}{1996}} \bibcite{GreenhillJiangMoran1995}{\citeauthoryear {{Greenhill} et\nobreakspace {}al.}{{Greenhill} et\nobreakspace {}al.}{1995}} \bibcite{GreenhillMoranHerrnstein1997}{\citeauthoryear {{Greenhill}, {Moran}, \& {Herrnstein}}{{Greenhill} et\nobreakspace {}al.}{1997}} \bibcite{MarconiOlivavanderWerf2000}{\citeauthoryear {{Marconi} et\nobreakspace {}al.}{{Marconi} et\nobreakspace {}al.}{2000}} \bibcite{MiyoshiMoranHerrnstein1995}{\citeauthoryear {{Miyoshi} et\nobreakspace {}al.}{{Miyoshi} et\nobreakspace {}al.}{1995}} \bibcite{NeufeldMaloney1995}{\citeauthoryear {{Neufeld} \& {Maloney}}{{Neufeld} \& {Maloney}}{1995}} \bibcite{Pogge1988}{\citeauthoryear {{Pogge}}{{Pogge}}{1988}} \bibcite{TrotterGreenhillMoran1998}{\citeauthoryear {{Trotter} et\nobreakspace {}al.}{{Trotter} et\nobreakspace {}al.}{1998} } %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \begin{document} \articletitle{Host galaxies and nuclear\\structure of AGN with \\H$_2$O megamasers as seen with HST} \chaptitlerunninghead{H$_2$O megamasers seen with HST} \author{Heino Falcke, Christian Henkel} \affil{Max-Planck-Institut f\"ur Radioastronomie, Auf dem H\"ugel 69, 53121 Bonn, Germany} \email{hfalcke@mpifr-bonn.mpg.de, chenkel@mpifr-bonn.mpg.de} \vskip-0.25cm \author{Andrew S. Wilson\footnote{Adjunct Astronomer, Space Telescope Science Institute}} \affil{Astronomy Department, University of Maryland, College Park, MD 20742-2421, USA} \email{wilson@astro.umd.edu} \vskip-0.25cm \author{James A. Braatz} \affil{National Radio Astronomy Observatory, P.O. Box 2, Green Bank, WV 24944, USA} \email{jbraatz@nrao.edu} \medskip \begin{center}{ to appear in: ``QSO hosts and Their Environments'', Granada, January 2001, Ed. I. M\'arquez P\'erez et al., Kluwer Academic Press} \end{center} \vskip-0.4cm \begin{abstract} We present results of an HST survey in H$\alpha$ and continuum filters of a sample of H$_2$O megamaser galaxies compiled by Braatz et al., all of which contain AGN. These observations allow us to study the AGN/host-galaxy connection, e.g. study the relation between the parsec scale masing disk/torus, bipolar outflows, and large scale properties of the galaxies such as dust lanes, signs for interaction, and galaxy types. A number of galaxies indeed show large-scale bi-polar H$\alpha$ structures which, however, are more reminiscent of outflows then excitation cones. Most megamaser galaxies are found in spiral galaxies. Only one galaxy in the original sample is known to be an elliptical and one galaxy imaged by us shows clear signs of interactions. In all cases we see evidence for obscuration of the nucleus (e.g. dust-lanes) in our color maps and the disk galaxies are preferentially edge on. This suggests that the nuclear masing disk has some relation to the large scale properties of the galaxy and the dust distribution. \end{abstract} %\begin{keywords} %\end{keywords} \section{Introduction} After the discovery of extragalactic water vapor masers in star-forming regions \cite{ChurchwellWitzelPauliny-Toth1977} a search started for other possible extragalactic H$_2$O masers. In the late seventies and early eighties, this work lead to to the discovery of five so-called megamasers in the nuclei of NGC 4945, the Circinus galaxy, NGC 1068, NGC 4258, and NGC 3079 which are $10^6$ times more powerful than the commonly observed galactic masers. Only much later, after an intense search of a distance limited galaxy sample, Braatz, Wilson, \& Henkel (1994, 1996) were able to increase the number of known megamasers by a factor of three, thus making for the first time a statistical analysis of this rare phenomenon possible. \nocite{BraatzWilsonHenkel1994,BraatzWilsonHenkel1996} It is obvious that the H$_2$O megamaser phenomenon must have something to do with nuclear activity as all such megamaser sources are either in Seyfert or Liner galaxies and the emission is always concentrated at the nucleus. Interestingly, Seyferts of type 1 are absent from this group. The standard interpretation for the difference between Sy 1 and Sy 2 is that a torus surrounds the nucleus which obscures the central engine (black hole and accretion disk) for large inclination angles. It therefore appears reasonable to infer that the masers trace molecular material associated with this torus or an accretion disk that feeds the nucleus. This, and the small number of H$_2$O megamasers known, suggests that the maser emission occurs for certain, very restricted, viewing angles, perhaps where the line of sight is along the plane of the molecular disk or torus. This notion was recently confirmed in great detail by VLBI observations of the megamaser in NGC 4258 \cite{MiyoshiMoranHerrnstein1995,GreenhillJiangMoran1995}. The positions and velocities of the central, the red- and the blue-shifted H$_2$O maser lines show that the masing region is in a thin disk structure, rotating on Keplerian orbits around a central mass of $3.6\cdot10^7M_\odot$ at a distance of 0.13 pc from the nucleus. This observations proved for the first time the existence of a small scale molecular disk surrounding an active nucleus. Recently, VLBI maps have been obtained for even more maser sources: while NGC 4945 \cite{GreenhillMoranHerrnstein1997} and NGC 3079 \cite{TrotterGreenhillMoran1998} also show a structure interpreted as a disk, the masers in NGC1068 show weak emission from a region presumably shocked by the nuclear jet at a projected galactocentric distance of $\sim$30 pc from the nucleus \cite{GallimoreBaumHenkel2001} in addition to the masers in the disk \cite{GreenhillGwinnAntonucci1996}. Although plausible scenarios for the megamaser phenomenon exist (e.g.,~\citeNP{NeufeldMaloney1995}) it is by no means clear how the obscuring torus and the masing disk are related. The masing disk may be the innermost part of a molecular accretion disk adjacent to the torus or just be the thin, central plane of the torus in which the column density is high enough for strong amplification. Alternatively, it might also be dynamically independent from ``the torus'' with a strong misalignment between their axes. In the most simple-minded picture, however, one would hope to find that masing disk, obscuring torus and the extended molecular cloud distribution form one coherent accretion structure feeding the central engine. We have therefore investigated a sample of megamaser host galaxies with the Hubble-Space-Telescope (HST) to look for large scale evidence of the molecular material, some of which we describe in the following. \section{Observations} The sample was selected from the list of 16 known megamasers known in 1996 and published by \citeN{BraatzWilsonHenkel1996}. We observed all galaxies that had not been previously observed with HST, which included: NGC449 (Mrk 1), NGC1386, NGC2639, NGC3079, IC2560, NGC4945, IRAS18333-6528, TXS~2226-184, and IC1481. Observations were made with WFPC2 to cover a red and a green continuum filter, as well as the H$\alpha$+[\ion{N}{2}]$\lambda\lambda$6548,6583 emission lines. The filters used were F814W (red continuum), F547M (green continuum), F673N and the Linear-Ramp-Filters (LRF) for galaxies at higher redshifts. The pixel scales were either 0\farcs1/pixel or 0\farcs0455/pixel depending on whether LRF usage required imaging on the Wide Field or Planetary Camera. All observations were made within one orbit. \begin{figure}[ht] \centerline{\hbox{\psfig{figure=falcke_fig1a.ps,height=0.3\textheight}\hspace{0.5cm}\psfig{figure=falcke_fig1b.ps,height=0.3\textheight}}} \caption[]{Gray-scale representation of three-color HST images of the megamaser galaxies NGC1386 (left) and NGC2639 (right). North is up and East to the left. The images are 18\farcs25 and 32\farcs35 on each side respectively.} \end{figure} The images were processed through the standard Wide-Field and Planetary Camera 2 (WFPC2) pipeline data reduction at the Space Telescope Science Institute. Further data reduction was done in IRAF and included: cosmic ray rejection, flux calibration, and rotation to the cardinal orientation. The galaxy continuum near the H$\alpha$+[\ion{N}{2}] line was determined by combining the red and green continuum images, scaled to the filter width of the narrow-band filter and weighted by the relative offset of their mean wavelengths from the redshifted H$\alpha$+[\ion{N}{2}] emission. From the two broad-band images and the emission-line image, we constructed three-color (RGB) maps, where, for better contrast, the H$\alpha$+[\ion{N}{2}] image was assigned the green channel, and the F814W and F547M filter images were assigned the red and blue channels respectively. Galaxy profile fits were made to the red continuum images. More details and first results for one galaxy (TXS~2226--184) are given in \citeN{FalckeWilsonHenkel2000}. \section{Results} \begin{figure}[ht] \centerline{\hbox{\psfig{figure=falcke_fig2a.ps,width=0.3\textheight,bblly=3.7cm,bbllx=3.5cm,bburx=17cm,bbury=16.5cm,clip=}\hspace{0.5cm}\psfig{figure=falcke_fig2b.ps,width=0.3\textheight,bblly=3.2cm,bbllx=2.4cm,bburx=18.9cm,bbury=18.6cm}}} \caption[]{Left: The distribution of host galaxy types for all megamasers in the Braatz et al.~(1996) sample determined from our HST observations and the literature. Right: Distribution of host galaxy inclination. The majority of megamaser galaxies are edge-on spirals.} \end{figure} The HST images contain a wealth of information. In many cases dust lanes and spiral arms are clearly visible and in basically all galaxies we resolve the narrow-line region (NLR). The most spectacular structures are the well-known H$\alpha$-bubble in NGC3079 (see also \citeNP{CecilBland-HawthornVeilleux2001}), a fan-shape region in NGC4945 \cite{MarconiOlivavanderWerf2000}, and linear jet-like features in TXS~2226-184 \cite{FalckeWilsonHenkel2000} and NGC~1386 \cite{FerruitWilsonMulchaey2000}. In these cases the structure might be mainly related to an outflow rather than an excitation cone. The water vapor maser emission in these galaxies has been imaged with VLBI (either published or private communication: L. Greenhill and J. Braatz). In those four cases the presumed axis of the nuclear disk (even though the disk interpretation is not secure in all cases) seems to be perpendicular within less than 30$^\circ$ to the closest nuclear dust lane, as would be the case if the maser disks and the dust lanes tend to align. On the other hand the large scale radio or H$\alpha$ axis is not always perpendicular to the disk axis (see, e.g.,~NGC3079, \citeNP{TrotterGreenhillMoran1998}). In all galaxies of our sample, we see clear evidence for the presence of large scale dust obscuration. In most, i.e. seven, cases we directly see relatively well-defined narrow dust-lanes passing through the nucleus; in the remaining two cases we see an elongated nucleus and reddening on one side of the inner galaxy. The profile fitting of the continuum images yielded a significant preference for disk models over bulge-only/elliptical models. We also see clear signs of spiral arms in a number of the disk galaxies. Only for one galaxy, IC1481, do we obtain inconclusive results. The optical appearance is very irregular suggesting that this might be the site of an ongoing galaxy merger. One galaxy, TXS~2226-184, was re-classified as a spiral (see \citeNP{FalckeWilsonHenkel2000} -- it was originally thought to be an elliptical). If we supplement our sample with literature data we find that only one out of 16 galaxies is an elliptical and one is irregular (see Fig.~2a) -- the vast majority of megamaser host galaxies are indeed spirals. From the observed ellipticity we can derive inclinations for the spiral galaxies in our sample. This shows that the majority of the galaxies are highly inclined with only a few face-on odd-balls (Fig.~2b). \section{Conclusions} \citeN{BraatzWilsonHenkel1997} suggested that the nuclear properties of AGN with megamasers might be connected with their large scale structure. Our investigation seems to confirm this trend. Megamaser galaxies seem to be preferentially edge-on and in spiral galaxies. After TXS~2228-184 was reclassified there is only one elliptical galaxy left (NGC~1052), which, however, has masers along the jet \cite{ClaussenDiamondBraatz1998} and hence may be different from the majority of galaxies which have been interpreted in terms of masers from molecular disks\footnote{The same may be true for the recently discovered megamaser in Mrk~348 \cite{FalckeHenkelPeck2000}, the galactic disk of which is very much face-on. VLBA observations of this galaxy show the masers to be at the tip of an expanding jet (Peck et al., in preparation).}. The fact that we always see some obscuring material in the HST images directly on scales of several tens to hundreds of parsecs could indicate that the masing structure (the `torus') on the milli-arcsecond (i.e., sub-pc) scale is related to large scale dust lanes and is thus not an isolated nuclear feature. Confirmation of this hypothesis may come from further VLBA-observations which could reveal the orientation and geometry of the nuclear disk in relation to the dust lanes seen with HST. \begin{acknowledgments} This work was supported by DFG grant 358/1-1 \& 2, NASA grants NAG8-1027 and HST GO 7278, and NATO grant SA.5-2-05 (GRG 960086)318/96. \end{acknowledgments} %\begin{thebibliography}{} \section*{References} \bibitem[\protect\citeauthoryear{{Braatz}, {Wilson}, \& {Henkel}}{{Braatz} et~al.}{1994}]{BraatzWilsonHenkel1994} {Braatz} J.~A., {Wilson} A.~S., {Henkel} C., 1994, \apjl, 437, L99 \bibitem[\protect\citeauthoryear{{Braatz}, {Wilson}, \& {Henkel}}{{Braatz} et~al.}{1996}]{BraatzWilsonHenkel1996} {Braatz} J.~A., {Wilson} A.~S., {Henkel} C., 1996, \apjs, 106, 51 \bibitem[\protect\citeauthoryear{{Braatz}, {Wilson}, \& {Henkel}}{{Braatz} et~al.}{1997}]{BraatzWilsonHenkel1997} {Braatz} J.~A., {Wilson} A.~S., {Henkel} C., 1997, \apjs, 110, 321 \bibitem[\protect\citeauthoryear{{Cecil} et~al.}{{Cecil} et~al.}{2001}]{CecilBland-HawthornVeilleux2001} {Cecil} G., {Bland-Hawthorn} J., {Veilleux} S., {Filippenko} A.~V., 2001, \apj, in press \bibitem[\protect\citeauthoryear{{Churchwell} et~al.}{{Churchwell} et~al.}{1977}]{ChurchwellWitzelPauliny-Toth1977} {Churchwell} E., {Witzel} A., {Pauliny-Toth} I., et~al., 1977, \aap, 54, 969 \bibitem[\protect\citeauthoryear{{Claussen} et~al.}{{Claussen} et~al.}{1998}]{ClaussenDiamondBraatz1998} {Claussen} M.~J., et al., 1998, \apjl, 500, L129 \bibitem[\protect\citeauthoryear{{Falcke} et~al.}{{Falcke} et~al.}{2000a}]{FalckeHenkelPeck2000} {Falcke} H., {Henkel} C., {Peck} A.~B., et~al., 2000a, \aap, 358, L17 \bibitem[\protect\citeauthoryear{{Falcke} et~al.}{{Falcke} et~al.}{2000b}]{FalckeWilsonHenkel2000} {Falcke} H., {Wilson} A.~S., {Henkel} C., {Brunthaler} A., {Braatz} J.~A., 2000b, \apjl, 530, L13 \bibitem[\protect\citeauthoryear{{Ferruit}, {Wilson}, \& {Mulchaey}}{{Ferruit} et~al.}{2000}]{FerruitWilsonMulchaey2000} {Ferruit} P., {Wilson} A.~S., {Mulchaey} J., 2000, \apjs, 128, 139 \bibitem[\protect\citeauthoryear{{Gallimore} et~al.}{{Gallimore} et~al.}{2001}]{GallimoreBaumHenkel2001} {Gallimore} J., {Baum} S., {Henkel} C., {et al.} , 2001, \apj, submitted \bibitem[\protect\citeauthoryear{{Greenhill} et~al.}{{Greenhill} et~al.}{1996}]{GreenhillGwinnAntonucci1996} {Greenhill} L.~J., {Gwinn} C.~R., {Antonucci} R., {Barvainis} R., 1996, \apjl, 472, L21 \bibitem[\protect\citeauthoryear{{Greenhill} et~al.}{{Greenhill} et~al.}{1995}]{GreenhillJiangMoran1995} {Greenhill} L.~J., {Jiang} D.~R., {Moran} J.~M., et~al., 1995, \apj, 440, 619 \bibitem[\protect\citeauthoryear{{Greenhill}, {Moran}, \& {Herrnstein}}{{Greenhill} et~al.}{1997}]{GreenhillMoranHerrnstein1997} {Greenhill} L.~J., {Moran} J.~M., {Herrnstein} J.~R., 1997, \apjl, 481, L23 \bibitem[\protect\citeauthoryear{{Marconi} et~al.}{{Marconi} et~al.}{2000}]{MarconiOlivavanderWerf2000} {Marconi} A., {Oliva} E., {van der Werf} P.~P., et~al., 2000, \aap, 357, 24 \bibitem[\protect\citeauthoryear{{Miyoshi} et~al.}{{Miyoshi} et~al.}{1995}]{MiyoshiMoranHerrnstein1995} {Miyoshi} M., {Moran} J., {Herrnstein} J., et~al., 1995, \nat, 373, 127 \bibitem[\protect\citeauthoryear{{Neufeld} \& {Maloney}}{{Neufeld} \& {Maloney}}{1995}]{NeufeldMaloney1995} {Neufeld} D.~A., {Maloney} P.~R., 1995, \apjl, 447, L17 \bibitem[\protect\citeauthoryear{{Trotter} et~al.}{{Trotter} et~al.}{1998}]{TrotterGreenhillMoran1998} {Trotter} A.~S., et~al., 1998, \apj, 495, 740 %\end{thebibliography} \end{document}