Hubble Space TelescopeWFPC2 Morphologies ofK‐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 (F814WÀK 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 arcmin 2 , for a morphological study of the ERO galaxy population. The survey covered a total of $409 arcmin 2 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% AE 5% of our (F814WÀK s ) ! 4 selected sample have morphologies consistent with a pure bulge or bulge-dominated galaxy (equivalent to E/S0), while disks comprise 64% AE 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% AE 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 (IÀK) versus (RÀK) 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.

Overdensities of Extremely Red Objects in the Fields of High-Redshift Radio-Loud Quasars

“…Note that we have not compared with field counts derived from other colors (e.g., IÀK or IÀH ), since they may bias toward later type galaxies with prolonged star formation (McCarthy et al 2001;Yan & Thompson 2003 The literature surveys cover wide, random areas and should therefore be representative of the ERO counts in the general field. The field counts agree well with each other, except for the counts in the Chandra Deep Field South by Scodeggio & Silva (2000), which are consistently lower than the other.…”

An overdensity of extremely red objects around faint mid-IR galaxies

“…At brighter levels, K < 17.5, we find a smaller percentage <20% of dusty EROs (Väisänen & Johansson 2004, hereafter Paper I). Recent HST morphological studies have complicated the picture somewhat by finding large fractions of disk galaxy EROs (Yan & Thompson 2003;Moustakas et al 2004;Gilbank et al 2003). In addition to morphological, spectroscopic, or colour property methods, the separation of dusty and early type EROs can also be done using detections at longer wavelengths, where ellipticals are not expected to radiate much.…”

Surface density of extremely red objects with R - J ≥ 5

“…Strategies to distinguish the fraction of different extragalactic ERO types include near-infrared photometric classification (Pozzetti & Mannucci 2000;Mannucci et al 2002;Martini 2001;Hempel et al 2003), morphological tests (Yan et al 2000;Yan & Thompson 2003;Gilbank et al 2003;Moriondo et al 2000), and an increasing number of spectroscopic discriminators (Cimatti et al 1999(Cimatti et al , 2002aSmith et al 2001). Near-infrared spectroscopy of 9 EROs (R − K > 5 and K < 19.0) by Cimatti et al (1999) showed neither strong emission lines nor continuum breaks.…”