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J. R. Martin, R. W. O'Connell (UVa), J. E. Hibbard (NRAO), R. van der Marel, S. Ravindranath (STScI)
The hierarchical formation scenario implies that early-type galaxies have been built up over an extended period of time from the mergers of smaller systems. This process should leave long-lived signatures in their light profiles and colors detectable over a range of 1-5 Gyrs, though these will fade with time. If the mergers are gaseous, many ellipticals and S0 bulges should show evidence of multiple, discrete, intermediate age stellar populations. Gaseous merger models predict that the largest long-lived disturbances will be concentrated to the centers of merger remnants. Since the ultraviolet wavelength region is the most sensitive to variations in stellar ages, we are searching for central merger signatures in near-UV photometry of early-type galaxies.
We have observed the central regions of dust- and AGN-free early-type galaxies with the ACS HRC on HST in the F250W, F330W, and F555W bands. The primary sample of 6 galaxies has anomolous surface brightness profiles in the form of excess central stellar light above the smooth profile of the galaxy body previously detected in one of three monochromatic HST surveys in V, R, or H band. A control sample consists of 6 galaxies chosen to be similar to the first but without the anomalous profiles.
Half the sample shows distinctly bluer near-UV cores, but most of these would not have been detected at longer wavelengths or at ground-based resolution. There is no good correlation between the color anomalies and the structural anomalies. We are interpreting these results in conjunction with a large sample of multi-color photometry for 135 early-type galaxies in the HST data archive.
This work was supported in part by grants HST-AR-10306 and HST-GO-10435 from STScI.
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.