High-redshift elliptical galaxies: are they (all) really compact?

TL;DR

This study challenges previous claims by showing that most high-redshift elliptical galaxies are not as compact as thought, suggesting a mass-dependent formation timeline and highlighting the importance of observational noise in size measurements.

Contribution

The paper provides evidence that high-redshift ETGs are generally larger than previously reported, emphasizing the role of observational noise and surface brightness halos in size estimation.

Findings

Most high-redshift ETGs have sizes comparable to local ellipticals.

Compactness in some galaxies can be explained by noise effects and missing low surface brightness halos.

Results support a downsizing scenario where massive ETGs form earlier and faster.

Abstract

We investigate the properties of 12 ultra-massive passively evolving early type galaxies (ETGs) at z_phot>1.4 in the COSMOS 2 deg^2 field. These 12 ETGs were selected as pBzKs, have accurate 1.4<= z_phot <=1.7,high Sersic index profiles typical of ellipticals, no detection at 24 micron, resulting in a complete ETG sample at M*>2.5x10^11 M_sun (Chabrier IMF). Contrary to previous claims, the half light radii estimated in very high S/N imaging data from HST+ACS are found to be large for most of the sample, consistent with local ellipticals. If the high redshift ETGs with M*<2.5x10^11 M_sun are really small in size and compact as reported in previous studies, our result may suggest a "downsizing" scenario, whereby the most massive ETGs reach their final structure earlier and faster than lower mass ones. However, simulating galaxies with morphological properties fixed to those of local ETGs with the same stellar mass show that the few compact galaxies that we still recover in our sample can be understood in term of fluctuations due to noise preventing the recovery of the extended low surface brightness halos in the light profile. Such halos, typical of Sersic profiles, extending even up to 40 kpc, are indeed seen in our sample.