# The abundance of compact quiescent galaxies since z ~ 0.6

## TL;DR

This study measures the number density of compact quiescent galaxies at intermediate redshifts, revealing a milder decline over time than previously thought, and highlighting the importance of definition and cosmic variance in such analyses.

## Contribution

It provides a comprehensive analysis of the abundance of compact quiescent galaxies at 0.2<z<0.6 using large survey data, reducing cosmic variance effects and clarifying size evolution.

## Key findings

- Number density of compact galaxies depends heavily on the definition used.
- The decrease in number density from z~1.5 to z~0.2 is smaller than previously reported.
- Most compact galaxies show little size evolution, supporting progenitor bias.

## Abstract

We set out to quantify the number density of quiescent massive compact galaxies at intermediate redshifts. We determine structural parameters based on i-band imaging using the CFHT equatorial SDSS Stripe 82 (CS82) survey (~170 sq. degrees) taking advantage of an exquisite median seeing of ~0.6''. We select compact massive (M > 5x10^10 M_sun) galaxies within the redshift range of 0.2<z<0.6. The large volume sampled allows to decrease the effect of cosmic variance that has hampered the calculation of the number density for this enigmatic population in many previous studies. We undertake an exhaustive analysis in an effort to untangle the various findings inherent to the diverse definition of compactness present in the literature. We find that the absolute number of compact galaxies is very dependent on the adopted definition and can change up to a factor of >10. We systematically measure a factor of ~5 more compacts at the same redshift than what was previously reported on smaller fields with HST imaging, which are more affected by cosmic variance. This means that the decrease in number density from z ~ 1.5 to z ~ 0.2 might be only of a factor of ~2-5, significantly smaller than what previously reported. This supports progenitor bias as the main contributor to the size evolution. This milder decrease is roughly compatible with the predictions from recent numerical simulations. Only the most extreme compact galaxies, with Reff < 1.5x( M/10^11 M_sun)^0.75 and M > 10^10.7 M_sun, appear to drop in number by a factor of ~20 and hence likely experience a noticeable size evolution.