The Fingerprints of Black Holes
(Falcke H., Wilson A.S., Simpson C. 1998, ApJ 502, 199)

The typical signature for an active black hole in the center of a
galaxy is the presence of hot (several 10,000 Degrees) gas in its
nucleus which is heated by intense ionizing ultraviolet (UV) radiation
(the same radiation which is responsible for a sun tan).  This "Narrow
(Emission) Line Region" (NLR, because of its narrow emission-line
features in the optical spectrum) is so compact that it is often
barely resolved by ground based telescopes. The images obtained by HST
now give a detailed view of the structures and morphology of the
ionized gas in Seyfert galaxies, revealing a wealth of startling
structures and funny shapes: long strands and fingers which are 1000
lightyears long and only a few tens of parsecs wide, giant arches and
spokes similar of size, a huge `figure of eight' written into the gas
of one galaxy, etc. (first figure)

The hot gas in Seyfert galaxies was first discovered by Carl Seyfert
in 1943 through its strong emission lines in the optical
spectrum. Only much later it became clear that he had discovered the
first visible sign of super massive black holes. This class of
galaxies, which ever since bears his name, are believed to be the
nearby, albeit less powerful siblings of the luminous quasars
discovered many years later.

While the conventional view a few years ago had been that the NLR
consists of an irregular distribution of clouds rotating around the
black hole, the coherent, almost sculptured structures found by HST in
many Seyfert galaxies makes clear that another, invisible power is at
work in shaping the NLR. This invisible agent was found with a large
array of radio 28 telescopes in new Mexico (VLA, operated by NRAO,
known from the movie "Contact") which showed plasma beams of highly
relativistic electrons emanating from the nucleus and interacting with
the hot interstellar gas. (second figure)

Powerful radio jets are well known from radio galaxies, where they are
believed to be produced in the direct vicinity of black holes. Through
a still mysterious process, possibly involving magnetic fields, some
particles probably manage to extract energy from the rest of the
matter being swallowed by the black hole and escape in two oppositely
directed narrow beams from the nucleus, sometimes traveling Millions
of light years deep into intergalactic space.

Seyfert galaxies, however, are usually considered "radio-quiet" with
at best feeble radio jets that were often ignored. Yet, their relative
weakness probably holds the clue to the intriguing shapes found by
HST. In its desperate attempt to escape out of the galaxy the jets are
are continuously slowed down, wrestling with the interstellar material
by which they are finally overwhelmed. The trace of this struggle
leaves its visible imprints in the optically emitting gas showing
nevertheless that even "quiet" jets can be strong. Thus each black hole
leaves its individual finger prints in the gas of its host galaxy.


The two galaxies shown here (ESO 428-G14 D=; Mrk 573, D=) are part of a
larger program to study the central regions of Seyfert galaxies with
high quality HST and VLA (radio) observations. Most other galaxies in
this galaxies showed similar evidence for the importance of the
jet/gas interaction. 

ESO 428-G14 has a very asymmetric jet that produces a small `figure
of eight' in the hot gas one side of the galaxy and long irregular
strands on the other. The `figure of eight' might be the result of
helical motion in the boundary layer between the jet and the
interstellar material. Higher resolution images of the radio jet show
that also the radio structure similar yet not identical to the optical
structure.

The structure of Mrk 573 is dominated by a set of two arches on each
side of the nucleus with some faint, connecting spokes in between
which might be similar to the strands in ESO 428-G14. The inner set of
arches coincides with the location of two radio blobs (hot spots)
indicating that here the radio jet is stopped, driving a bow shock
into the interstellar medium. The outer set of arches might be the
remnants of an earlier radio outburst.

To image the ionized gas, and not the underlying stellar population,
of the galaxies we used the Linear Ramp Filters (LRFs) installed in
the WFPC2. This is an elaborated set of filters which transmit only
light of a well defined color where the transmitted color is changing
over the field of view of the camera.  The telescope then had to be
positioned such that the image of the galaxy was falling onto the
portion of the camera where the transmitted color (redshifted by the
cosmological expansion) corresponded exactly to strong emission lines
from ionized Hydrogen (H$\alpha$, coded red) and Oxygen ([OIII], coded
green, continuum emission was coded blue).

Using the light at these different wavelengths one can construct an
excitation (temperature) map (third figure) which, similar to the beam
of a lighthouse in dense fog, traces the ionizing radiation
responsible for the heating of the gas. The fan-shaped structure,
which nicely connects to very similar structures seen before on larger
scales, indicates that indeed a sharply defined UV beam is emitted
from the very nucleus of the galaxy - exactly where an accreting black
hole is suspected as the source for the UV radiation. Such excitation
cones also confirm so called unified schemes where one argues that the
same galaxy can have a very different optical appearance whether one
looks down the beam or side on.