Red Nugget Watch

from Saracco, Longhetti, & Andreon, http://arxiv.org/abs/0810.2795

This paper presents another analysis of the sizes and surface densities of early-type galaxies at z=1-2; here they use a sample of 32 spectroscopically-confirmed galaxies with a mean redshift of 1.45 from several different surveys. Masses and ages are determined via SED fitting to the photometry. As has been reported before, these galaxies lie well off the z=0 size-luminosity relation. By evolving the galaxies (assuming pure luminosity evolution) from their measured  redshifts to the present, the authors find that some galaxies would actually evolve to the z=0 relation in this manner, while some would not. These galaxies appear to be "young" and "old" respectively, and the authors appear to claim a bimodal age distribution in ETGs at this redshift, with typical ages ~1 Gyr and 3.5 Gyr. Young ETGs follow the local size-mass relation, old ETGs do not.

The authors conclude that the young objects have more or less completed their evolution (except for luminosity evolution), while the old galaxies still need some process to increase their effective radii. Dry merging cannot do this because it would create too many high-mass galaxies, so some other process must be at work. Much of this rests on the assumption that the relative ages can be accurately determined through photometry, of course.

From the paper 'Red Nuggets at z ∼ 1.5: Compact passive galaxies and the
formation of the Kormendy Relation' by Damjanov et al (0807.1744) I chose these two plots.

We had discussions before about these tiny galaxies that Mariska and Pieter investigated. These authors do sort of the same job, but at slightly lower redshift (1.5-ish). The left plot shows the effective radius - stellar mass plane, with the dots and contours being local SDSS red galaxies. The bigger points with error bars are their (and some other) red galaxies at higher redshift, which appear to small. I show this plot, because the arrows indicate the approximate track of evolution due to three different processes: dry mergers, pure stellar mass growth without changing size and adiabatic expansion (stellar mass loss makes the systems less bound). All three processes seem incapable of transforming the galaxies towards the low - z counterparts.

The right plot shows the galaxies in the stellar mass density - effective radius plane (Kormendy relation). Here they are all on the same trend, with the high redshift galaxies smaller and denser than their local red SDSS partners. Color coding here is redshift, which appears to hint at some evolution: the higher the redshift of the galaxy, the smaller and denser it is. The main part of the evolution takes place at 1.1 < z < 1.5.