The merger rates and sizes of galaxies across the peak epoch of star formation from the HiZELS survey

TL;DR

This study investigates galaxy merger rates, sizes, and morphologies across redshifts 0.4 to 2.23, revealing that merger activity and galaxy sizes remain relatively constant despite rising star formation rates, challenging previous assumptions.

Contribution

It provides new insights into the constancy of galaxy sizes and merger rates over a significant cosmic epoch, highlighting secular processes over mergers as drivers of star formation.

Findings

No increase in merger rate with redshift after sSFR normalization.

Approximately 40-50% of starburst galaxies are major mergers, independent of redshift.

Galaxy sizes at a given mass do not evolve across the studied redshift range.

Abstract

We use the HiZELS narrow-band H-alpha survey in combination with CANDELS, UKIDSS and WIRDS near-infrared imaging, to investigate the morphologies, merger rates and sizes of a sample of H-alpha emitting galaxies in the redshift range z=0.40 - 2.23, an epoch encompassing the rise to the peak of the star formation rate density. Merger rates are estimated from space- and ground-based imaging using the M20 coefficient. To account for the increase in the specific star-formation rate (sSFR) of the star forming `main-sequence' with redshift, we normalise the star-formation rate of galaxies at each epoch to the typical value derived from the H-alpha luminosity function. Once this trend in sSFR is removed we see no evidence for an increase in the number density of star-forming galaxies or the merger rate with redshift. We thus conclude that neither is the main driver of the enhanced star-formation rate density at z=1-2, with secular processes such as instabilities within efficiently fuelled, gas-rich discs or multiple minor mergers the most likely alternatives. However, we find that 40-50% of starburst galaxies, those with enhanced specific star formation at their epoch, are major mergers and this fraction is redshift independent. Finally, we find the surprising result that the typical size of a star-forming galaxy of a given mass does not evolve across the redshift range considered, suggesting a universal size-mass relation. Taken in combination, these results indicate a star-forming galaxy population that is statistically similar in physical size, merger rate and mass over the 6 Gyr covered in this study, despite the increase in typical sSFR.