# Star formation in galaxies at z~4-5 from the SMUVS survey: a clear starburst/main-sequence bimodality for Halpha emitters on the SFR-M* plane

## TL;DR

This study uses the SMUVS survey to identify Halpha emitters at z~4-5, revealing a bimodal distribution in the SFR-M* plane with implications for galaxy evolution and star formation modes in the early universe.

## Contribution

It demonstrates, for the first time, the clear bimodality of star-forming galaxies at high redshift on the SFR-M* plane using Halpha emission indicators.

## Key findings

- Halpha excess sources constitute 37-40% of galaxies at M*~10^9-10^10 Msun
- Starburst and main-sequence galaxies form a bimodal distribution in SFR-M* plane
- Starburst galaxies account for over 50% of cosmic SFR density at z~4

## Abstract

We study a large galaxy sample from the Spitzer Matching Survey of the UltraVISTA ultra-deep Stripes (SMUVS) to search for sources with enhanced 3.6 micron fluxes indicative of strong Halpha emission at z=3.9-4.9. We find that the percentage of "Halpha excess" sources reaches 37-40% for galaxies with stellar masses log10(M*/Msun) ~ 9-10, and decreases to <20% at log10(M*/Msun) ~ 10.7. At higher stellar masses, however, the trend reverses, although this is likely due to AGN contamination. We derive star formation rates (SFR) and specific SFR (sSFR) from the inferred Halpha equivalent widths (EW) of our "Halpha excess" galaxies. We show, for the first time, that the "Halpha excess" galaxies clearly have a bimodal distribution on the SFR-M* plane: they lie on the main sequence of star formation (with log10(sSFR/yr^{-1})<-8.05) or in a starburst cloud (with log10(sSFR/yr^{-1}) >-7.60). The latter contains ~15% of all the objects in our sample and accounts for >50% of the cosmic SFR density at z=3.9-4.9, for which we derive a robust lower limit of 0.066 Msun yr^{-1} Mpc^{-3}. Finally, we identify an unusual >50sigma overdensity of z=3.9-4.9 galaxies within a 0.20 x 0.20 sq. arcmin region. We conclude that the SMUVS unique combination of area and depth at mid-IR wavelengths provides an unprecedented level of statistics and dynamic range which are fundamental to reveal new aspects of galaxy evolution in the young Universe.