The role of quenching time in the evolution of the mass-size relation of passive galaxies from the WISP survey

TL;DR

This study investigates how the size and age of passive galaxies at z~1.5 relate, suggesting that minor mergers, not new quenched galaxies, drive the evolution of their mass-size relation over time.

Contribution

It provides observational evidence that minor mergers, rather than the emergence of new quenched galaxies, primarily explain the size evolution of passive galaxies.

Findings

No significant size difference between young and old galaxies.

The age-size relation slope is shallow, alpha ≥ -0.6.

Minor mergers likely dominate size evolution.

Abstract

We analyze how passive galaxies at z $\sim$ 1.5 populate the mass-size plane as a function of their stellar age, to understand if the observed size growth with time can be explained with the appearance of larger quenched galaxies at lower redshift. We use a sample of 32 passive galaxies extracted from the Wide Field Camera 3 Infrared Spectroscopic Parallel (WISP) survey with spectroscopic redshift 1.3 $\lesssim$ z $\lesssim$ 2.05, specific star-formation rates lower than 0.01 Gyr$^{-1}$, and stellar masses above 4.5 $\times$ 10$^{10}$ M$_\odot$. All galaxies have spectrally determined stellar ages from fitting of their rest-frame optical spectra and photometry with stellar population models. When dividing our sample into young (age $\leq$ 2.1 Gyr) and old (age $>$ 2.1 Gyr) galaxies we do not find a significant trend in the distributions of the difference between the observed radius and the one predicted by the mass-size relation. This result indicates that the relation between the galaxy age and its distance from the mass-size relation, if it exists, is rather shallow, with a slope alpha $\gtrsim$ -0.6. At face value, this finding suggests that multiple dry and/or wet minor mergers, rather than the appearance of newly quenched galaxies, are mainly responsible for the observed time evolution of the mass-size relation in passive galaxies.