Toward Identifying First-Generation Elliptical Galaxies in the SDSS
American Astronomical Society, AAS Meeting #221, #313.01
Published in Jan 2013
The build up of massive non-star-forming galaxies (red ellipticals) over cosmic time is observationally well established. What remains unclear in detail is the relative importance of different mass assembly processes. Most growth routes involve a pre-existing red elliptical adding stars and/or gas through merging or late infall. Only the merger hypothesis predicts the formation of new "1st-generation" ellipticals through the major merging of gas-rich disks, which evolve rapidly from very disturbed systems of enhanced star formation and black hole growth activity, to quenched and relaxed ellipticals that transition from bluer to red color. To better constrain the portion of 1st-generation ellipticals added by such mergers at late cosmic times, we use the SDSS to analyze massive (>1e10Msun) spheroid-dominated galaxies with blue colors. We visually identify 1350 normal ellipticals, 100 peculiar ellipticals with a variety of morphological asymmetries (e.g., loops, shells), and 100 highly-disturbed spheroidal post-mergers. These galaxies typically have `green-valley' colors consistent with transforming from star-forming to quenched. Whether peculiar or not, bluer-than-normal ellipticals have similar sizes, densities and axial ratios as red ellipticals of the same mass; i.e., they have spheroidal structure suggesting a major merger origin. Yet, closer scrutiny of the core colors and central spectra reveals that only a fraction are strong 1st-generation candidates with young cores that appear quenched or are likely quenching owing to an AGN. The majority have old-color cores and are inconsistent with a recent major gas-rich merger. Thus, the use of core color improves identification of plausible 1st-generation ellipticals which aids in constraining the growth of massive red galaxies at the expense of blue systems. We plan to apply this technique to high-z samples to better map the formation of new ellipticals at different epochs.
(c) 2013: American Astronomical Society