The Recent Burstiness of Star Formation in Galaxies at z~4.5 from H$\alpha$ Measurements

TL;DR

This study investigates the bursty star formation in galaxies at z~4.5 using Hα and UV data, revealing significant variability and recent starburst activity, with implications for galaxy evolution models.

Contribution

It introduces a novel method to derive Hα properties from broad-band colors, providing new insights into the bursty star formation history of high-redshift galaxies.

Findings

Half of the galaxies show excess Hα indicating recent starbursts.

Most galaxies experienced a starburst within the last 50 Myrs.

Massive galaxies may be transitioning to quiescence by z=4.

Abstract

The redshift range z=4-6 marks a transition phase between primordial and mature galaxy formation in which galaxies considerably increase their stellar mass, metallicity, and dust content. The study of galaxies in this redshift range is therefore important to understand early galaxy formation and the fate of galaxies at later times. Here, we investigate the burstiness of the recent star-formation history (SFH) of 221 $z\sim4.5$ main-sequence galaxies at log(M) > 9.7 by comparing their ultra-violet (UV) continuum, H$\alpha$ luminosity, and H$\alpha$ equivalent-width (EW). The H$\alpha$ properties are derived from the Spitzer [3.6$\mu$m]-[4.5$\mu$m] broad-band color, thereby properly taking into account model and photometric uncertainties. We find a significant scatter between H$\alpha$ and UV-derived luminosities and star-formation rates (SFRs). About half of the galaxies show a significant excess in H$\alpha$ compared to expectations from a constant smooth SFH. We also find a tentative anti-correlation between H$\alpha$ EW and stellar mass, ranging from 1000$\r{A}$ at log(M) < 10 to below 100$\r{A}$ at log(M) > 11. Consulting models suggests that most $z\sim4.5$ galaxies had a burst of star-formation within the last 50 Myrs, increasing their SFRs by a factor of > 5. The most massive galaxies on the other hand might decrease their SFRs, and may be transitioning to a quiescent stage by z=4. We identify differential dust attenuation (f) between stars and nebular regions as the main contributor to the uncertainty. With local galaxies selected by increasing H$\alpha$ EW (reaching values similar to high-z galaxies), we predict that f approaches unity at $z>4$ consistent with the extrapolation of measurements out to z=2.