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Session 21 - Luminous Infrared Galaxies.
Display session, Wednesday, January 07
Exhibit Hall,

[21.02] New Results from HST WFPC2 Images of Ultraluminous IR Galaxies

K. D. Borne (Hughes STX and NASA/GSFC), H. Bushouse, L. Colina, R. A. Lucas (STScI)

We have received nearly 120 I-band images of ultraluminous IR galaxies (ULIRGs) from HST-WFPC2. Numerous discoveries and exciting results have followed from this survey. We will report on some of the new results. (a) In the majority of the galaxies, there is evidence for star formation on all scales, indicating that the starburst event is the dominant heating source for the majority of ULIRGs. (b) A ``stellar'' (AGN-like) nucleus is seen in 10% of the ULIRGs, for which the dominant heating source may be a dust-enshrouded active nucleus. (c) Several of the galaxies show clear evidence for a circumnuclear (starburst?) ring (with a diameter of a few hundred kpc), which may be related to a dusty torus surrounding an AGN. (d) There is evidence for at least one classical collisional ring galaxy in the sample. (e) Some of the galaxies previously classified as non-interacting show clear evidence for a secondary nucleus (merger remnant?) or other evidence for interaction (e.g., tidal tails) on small angular scales. (f) Some of the ULIRGs should be re-classified as compact groups of galaxies. In many instances, there are a significant number of small companion galaxies in the same CCD chip as the ULIRG, with fewer companions in the other WFPC2 CCD chips, suggesting that the small, fainter companions are physically associated with the ULIRG. Very few large companions are seen, though there are a few notable examples of this. This research was supported in part by NASA through grant number GO-06346.01-95A from the STScI.

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