Hubble Space Telescope WFPC2 Morphologies of K-selected Extremely Red Galaxies

Abstract

We selected 115 extremely red objects (EROs) from deep Hubble Space Telescope (HST) WFPC2 archive data combined with ground-based K-band images, with (F814W-K_s) >= 4, K-band signal-to-noise ratio >=5, and a median limiting K_s magnitude of ~18.7, over a corresponding area of 228 arcmin², for a morphological study of the ERO galaxy population. The survey covered a total of ~409 arcmin² over 77 separate WFPC2 fields. This is the first complete sample of bright EROs with high-resolution HST morphologies. From a visual morphological classification, we find that 30%+/-5% of our (F814W-K_s) >= 4 selected sample have morphologies consistent with a pure bulge or bulge-dominated galaxy (equivalent to E/S0), while disks comprise 64%+/-7% of the sample. Only 6% of the EROs remained unclassifiable. Mergers or strongly interacting systems, which includes sources from both classes, make up 17%+/-4% of the full sample. The quantitative Medium Deep Survey profile fitting is consistent with these results. These results highlight the complex nature of optical/near-IR color selected EROs. The dominant component of our sample is composed of disks, not spheroids or strongly interacting systems like HR 10. Using Bruzual & Charlot spectral energy distribution models, we investigated population differences in EROs selected by their (I-K) versus (R-K) colors and found that I-band based surveys preferentially select systems with prolonged star formation. Real differences in the surface densities of EROs in R-band and I-band based survey may reflect this color selection effect, complicating the comparisons between and interpretations on the nature of the ERO population. We conclude that only a small fraction of EROs at z~1-2 could be passively evolving elliptical galaxies formed at high redshift through a ``monolithic collapse'' mechanism. For the majority of EROs, even if most of their stellar mass is already in place at z~1, interaction with the environment and accretion of gas still play important roles in their continuing evolution.