Classification of Extremely Red Objects in the COSMOS Field

TL;DR

This study classifies extremely red objects at z ~1 in the COSMOS field into old galaxies and dusty starbursts using multi-wavelength imaging and morphological analysis, revealing their comparable proportions and dependencies on observational properties.

Contribution

It introduces a combined morphological and SED fitting approach to classify EROs, demonstrating the effectiveness of principal component analysis in segregating galaxy types.

Findings

Approximately 48% are old galaxies and 52% are dusty starbursts.

Morphology and SED parameters effectively distinguish galaxy types.

Dusty starbursts are more prevalent at fainter magnitudes and higher redshifts.

Abstract

We present a study of the classification of z ~1 extremely red objects (EROs), using a combination of HST/ACS, Spitzer/IRAC, and ground-based images of the COSMOS field. Our sample includes about 5300 EROs with i-Ks>2.45 (AB, equivalently I-Ks=4 in Vega) and Ks<=21.1 (AB). For EROs in our sample, we compute, using the ACS F814W images, their concentration, asymmetry, as well as their Gini coefficient and the second moment of the brightest 20% of their light. Using those morphology parameters and the Spitzer/IRAC [3.6]-[8.0] color, the spectral energy distribution (SED) fitting method, we classify EROs into two classes: old galaxies (OGs) and young, dusty starburst galaxies (DGs). We found that the fraction of OGs and DGs in our sample is similar, about 48 percentages of EROs in our sample are OGs, and 52 percentages of them are DGs. To reduce the redundancy of these three different classification methods, we performed a principal component analysis on the measurements of EROs, and find that morphology parameters and SEDs are efficient in segregating OGs and DGs. The [3.6]-[8.0] color, which depends on reddening, redshift, and photometric accuracy, is difficult to separate EROs around the discriminating line between starburst and elliptical. We investigate the dependence of the fraction of EROs on their observational properties, and the results suggest that DGs become increasingly important at fainter magnitudes, redder colors, and higher redshifts.