We selected 115 extremely red objects ( EROs ) from deep HST WFPC2 archive data combined with ground-based K-band images , with ( F 814 W - K _ { s } ) \geq 4 , K-band SNR \geq 5 , and a median limiting K _ { s } magnitude of \sim 18.7 , over a corresponding area of 228 sq .
arcminutes , for a morphological study of the ERO galaxy population .
The survey covered a total of \sim 409 sq .
arcminutes 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 \pm 5 % of our ( F 814 W - K _ { s } ) \geq 4 selected sample have morphologies consistent with a pure bulge or bulge-dominated galaxy ( equivalent to E/S0 ) , while disks comprise 64 \pm 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 \pm 4 % of the full sample .
The quantitative MDS 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 comprised of disks , not spheroids or strongly interacting systems like HR 10 .
Using Bruzual & Charlot SED models , we investigated population differences in EROs selected by their ( I - K ) vs. ( 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 \sim 1 - 2 could be passively evolving ellipticals 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 \sim 1 , interaction with the environment and accretion of gas still play important roles in their continuing evolution .