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Recycling intergalactic and interstellar matter
IAU Symposium Series, Vol. 217, 2004
Pierre­Alain Duc, Jonathan Braine and Elias Brinks, eds.
HST Observations of the Toomre Sequence of Merging
Galaxies
J˜orn Rossa 1 , Roeland P. van der Marel 1 , Torsten B˜oker 1 , Seppo Laine 2 ,
J. Christopher Mihos 3 , John E. Hibbard 4 & Ann I. Zabludo# 5
1 Space Telescope Science Institute, Baltimore, MD 21218, USA
2 SIRTF Science Center, CALTECH, Pasadena, CA 91125, USA
3 Case Western Reserve University, Cleveland, OH 44106, USA
4 NRAO, Charlottesville, VA 22903­2475, USA
5 Steward Observatory, University of Arizona, Tucson, AZ 85721, USA
Abstract.
We discuss our ongoing multi­instrument HST investigation of the
nuclear regions of the 11 interacting and merging galaxies in the Toomre
Sequence. We are studying the nuclear kinematics using STIS (G750M)
spectra, the nuclear stellar populations using STIS (G430L) spectra, and
the nuclear morphology using NICMOS and WFPC2 images. The results
will provide new insight into the physical processes that operate during
galaxy interactions.
1. Introduction
It is a generally accepted fact that galaxy interactions can trigger a burst of
intense star formation. Various evolutionary aspects can be studied best in
merging galaxies. Many galaxies of the Toomre Sequence (Toomre 1977) feature
prominent tidal tails and complex structures on large scales. However, physical
processes that operate during galaxy interactions often leave their most impor­
tant signatures in the very central region. It is in these violent regimes where the
recycling processes of the ISM can be investigated in great detail. The Toomre
galaxies show abundantly distributed ionized gas which is the result of strong
star formation activity. In turn, the star formation is due to gas flows to the cen­
ter triggered by the merging process [cf. Schweizer (1998) for an overview]. HST
allows us to investigate and quantify these processes at high spatial resolution.
2. Observations and results
We have used the WFPC2 for optical broad­band and emission­line imaging,
and we see a slight trend of increasing luminosity density towards the latest
merger stages within 100pc and 1 kpc (Laine et al. 2003). However, NIR images
are required to peer through the dust that is present in many of the galaxies.
NICMOS K­band images reveal both nuclei of NGC 520, whereas optical images
1

2 J. Rossa et al.
Figure 1. Nuclear H#+[N ii] rotation curves of four Toomre Sequence galaxies,
derived from our STIS G750M observations. The solid horizontal line marks the H i
systemic velocity. Note, that only NGC 7592 is an early­type merger.
only show one. In NGC7764A we detect a double nucleus in the NIR. The STIS
kinematics reveal for late­type mergers a more or less regular rotation curve (see
Fig. 1), and generally a line broadening towards the central regions is found
(velocity dispersions of # # 250 - 300 km s -1 are observed). Low­resolution
STIS spectra are being fitted with stellar population synthesis models to study
stellar population characteristics and ages. The results will be quantified both
as a function of nuclear radial distance and merger stage within the Toomre
Sequence. The combined data set will allow us to assess whether the Toomre
Sequence forms an evolutionary sequence, and to study in detail the physical
processes involved.
Acknowledgments. This contribution was partly supported by a travel grant from the AAS.
The project is based on observations made with the NASA/ESA Hubble Space Telescope,
obtained [from the Data Archive] at the Space Telescope Science Institute, which is operated
by the Association of Universities for Research in Astronomy, Inc., under NASA contract
NAS 5­26555, and was supported by NASA/STScI grants for programs GO­8669 and GO­9402.
References
Laine, S., van der Marel, R. P., Rossa, J., B˜oker, T., Mihos, J. C., Hibbard, J.
E., & Zabludo# A. I. 2003, AJ, in press [astro­ph/0309377]
Schweizer, F. 1998, in: Galaxies: Interactions and Induced Star Formation, Saas
Fee Advanced Course 26, ed. D. Friedli, L. Martinet & D. Pfenniger,
(Berlin: Springer Verlag), 105
Toomre, A. 1977, in: The Evolution of Galaxies and Stellar Populations, ed.
B.M. Tinsley & R.B. Larson (New Haven: Yale Univ.), 401