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Astronomical Data Analysis Software and Systems VII
ASP Conference Series, Vol. 145, 1998
R. Albrecht, R. N. Hook and H. A. Bushouse, e
Ö Copyright 1998 Astronomical Society of the Pacific. All rights reserved.
ds.
The Co­Addition Technique Applied to Images of Galaxy
Cores
W. W. Zeilinger
Institut f˜ur Astronomie, University of Vienna, T˜urkenschanzstraúe 17,
A­1180 Wien, Austria
P. Crane, P. GrosbÜl and A. Renzini
European Southern Observatory, Karl­Schwarzschild Straúe 2, D­85748
Garching, Germany
Abstract. This is a preliminary report on an application of the co­
addition technique based upon the Richardson­Lucy algorithm. The com­
bination of high signal­to­noise ground­based images obtained with the
ESO NTT using SUSI with high angular resolution HST FOC f/96 images
gives new insights on the properties of galaxy cores and their potential
time variability.
1. Introduction
Cores of galaxies are one of the main thrusts of present extragalactic research.
Answers to questions like the possible presence of massive central dark objects
(black holes), central gas/dust disks, nuclear subcomponents and so on are essen­
tial to our understanding of the nature and evolution of galactic nuclei and their
interrelation with the host galaxy. Many ellipticals and bulges of spiral galaxies
show central, unresolved sources which are particularly bright in the UV and
significantly less conspicuous or even completely absent in the corresponding
visual images. The double nucleus of M31 is such a case: King, Stanford &
Crane (1995) found that the brightest peak in the UV is not coincident with the
brightest peak in the visual. Attesting to the value of UV observations, we note
that all the galaxies observed by Crane et al. (1993) which have blue nuclei are
also radio sources.
An important discovery was reported by Renzini et al. (1995) The core of
the elliptical NGC 4552 contains a variable UV source. The galaxy was imaged
in the UV using the HST FOC in 1991 and 1993. Over this period the central
spike, which is only visible in the UV, brightened by a factor of at least 7.
Subsequent observations have revealed broad emission features associated with
the flaring region. The UV--bright flare is attributed to tidal stripping of a star
during a close encounter with a central black hole. The frequency of such an
event is estimated to be of the order of once per 10 4 years in giant ellipticals
(Rees 1990). Assuming the complete disruption of the star, the flare is expected
to be visible for several years and is predicted to be even brighter than the event
observed in NGC 4552. Alternative explanations include the ideas of a collision
86

The Co­Addition Technique Applied to Images of Galaxy Cores 87
between two stars, gravitational lensing and the hypothesis of a recent accretion
of a gas rich dwarf galaxy whose material settled in the nuclear region. There
is some circumstantial evidence to support the latter scenario because extended
H# emission is detected in the central 2 ## region and because there is a nuclear
dust lane. However, based upon only one observed case, all the models are
unfortunately not well constrained.
2. Observations and Data Analysis
The most complete and homogeneous survey of galaxy cores in the UV has
been carried out by Maoz et al. (1995) who obtained a data sample using the
pre­COSTAR FOC in the f/96 mode. The analysis of the de­archived images
revealed that the majority of the bulge dominated galaxies contain a central
UV bright compact source. In those cases where the signal­to­noise ratio was
adequate, the central region was further analyzed and the point source nature
established. Nuclear subcomponents such as central dust lanes or ring­like struc­
tures were identified in some cases.
If the interpretation of Rees (1990) holds true, the brightness variations
caused by the flare event in the galaxy nucleus are expected to be observable
in a time scale of less then 2 months. A sample of suitable candidate galaxies
is now being monitored in the U band using SUSI at the ESO NTT in order
to search for UV variability of the central sources similar to that detected in
NGC 4552.
The archival FOC f/96 F220W images from the Maoz et al. (1995) survey
serve as reference for deconvolving the core regions of the SUSI images using the
Richardson­Lucy technique (Hook & Lucy 1994) as also described by Zeilinger
(1994). It has been demonstrated that the co­adding technique not only con­
solidates the signal of images with di#erent resolutions but also conserves the
resolution of the sharpest one. One may therefore take full advantage of the high
resolution FOC images by co­adding them with (high signal­to­noise) ground­
based data.
Nevertheless the ground­based observing programme carried out at the
NTT is clearly seeing dependent: In order to derive meaningful core fluxes in
combination with the high­resolution FOC images, the seeing conditions should
not be much worse then 0.7 ## FWHM and the variation of the seeing for each
galaxy data set should not exceed 0.15 ## .
The ground­based image is usually aligned with the FOC image in order to
leave the image with the weaker signal­to­noise ratio as unprocessed as possible.
Because of missing field stars as reference points in the FOC image, the align­
ment is carried out using the galaxy nucleus itself. An accuracy of typically one
FOC pixel (0.044 ## pixel -1 ) can be achieved with this method. The selection of
an appropriate PSF for the co­addition is a crucial part. For the ground­based
images well exposed stellar images are almost always available in the galaxy
field to determine the shape of the PSF. In the case of the FOC images one is
constrained to know the shape of the PSF in the position of the galaxy nucleus
on the detector. Therefore stellar images repeating the exact instrument confi­
guration are almost never available. The software tool Tiny Tim (Krist & Hook
1997) is a solution to this problem. An ``artificial'' PSF for a given observing

88 Zeilinger, Crane, GrosbÜl and Renzini
date and instrument configuration can be calculated. However, one has to be
aware that Tiny Tim is only a tool based upon current best fits of aberration
values for the various mirror positions and current best estimates of the obscu­
ration positions and sizes which may still not describe the HST PSF exactly (see
also Krist 1995).
Typically 100 to 300 iterations of the Richardson­Lucy algorithm are then
applied, depending on the signal­to­noise ratios of the input images and the
resolution (seeing) of the ground­based image. The latter turns out to be the
most serious constraint.
3. Results
We have obtained images of 6 galaxies so far of which one has been observed
at two epochs. The images were obtained in service mode under less than ideal
conditions. We are proceeding with the analysis.
There are other obvious applications of this technique such as the monitor­
ing variable stars in galaxies.
Acknowledgments. WWZ acknowledges the support by the Jubil˜aumsfonds
der Oesterreichischen Nationalbank (grant 6323).
References
Crane, P. et al. 1993, AJ, 106, 1371
Hook, R. & Lucy, L. B. 1994, ST­ECF Newsletter, 17, 10
King, I. R. Stanford, S. A. & Crane, P. 1995, AJ, 109, 164
Krist, J. 1995, in ASP Conf. Ser., Vol. 77, Astronomical Data Analysis Software
and Systems IV, ed. R. A. Shaw, H. E. Payne & J. J. E. Hayes (San
Francisco: ASP), 349
Krist, J. & Hook, R. 1997, The Tiny Tim User's Manual
Maoz, D. et al. 1995, ApJ, 440, 91
Rees, M. 1990, Science, 247, 817
Renzini, A. et al. 1995, Nature, 378, 39
Zeilinger, W. W. 1994, ST­ECF Newsletter, 21, 29