Probing Cosmic Dawn : Ages and Star Formation Histories of Candidate $z\geq$9 Galaxies

TL;DR

This study investigates the ages and star formation histories of candidate galaxies beyond redshift 9, using spectral energy distributions, stellar population models, and follow-up spectroscopy to understand early cosmic star formation.

Contribution

It provides new spectroscopic confirmations and detailed modeling of galaxies at redshifts above 9, extending knowledge of early star formation and galaxy evolution.

Findings

Confirmed redshifts of z=8.78 and z=9.28 for two galaxies.

Supported the existence of star formation activity beyond z=10.

Reconstructed past UV luminosities to trace early galaxy progenitors.

Abstract

We discuss the spectral energy distributions and physical properties of six galaxies whose photometric redshifts suggest they lie beyond a redshift $z\simeq$9. Each was selected on account of a prominent excess seen in the Spitzer/IRAC 4.5$\mu$m band which, for a redshift above $z=9.0$, likely indicates the presence of a rest-frame Balmer break and a stellar component that formed earlier than a redshift $z\simeq10$. In addition to constraining the earlier star formation activity on the basis of fits using stellar population models with BAGPIPES, we have undertaken the necessary, but challenging, follow-up spectroscopy for each candidate using various combinations of Keck/MOSFIRE, VLT/X-shooter, Gemini/FLAMINGOS2 and ALMA. Based on either Lyman-$\alpha$ or [OIII] 88 $\mu$m emission, we determine a convincing redshift of $z$=8.78 for GN-z-10-3 and a likely redshift of $z$=9.28 for the lensed galaxy MACS0416-JD. For GN-z9-1, we conclude the case remains promising for a source beyond $z\simeq$9. Together with earlier spectroscopic data for MACS1149-JD1, our analysis of this enlarged sample provides further support for a cosmic star formation history extending beyond redshifts $z\simeq$10. We use our best-fit stellar population models to reconstruct the past rest-frame UV luminosities of our sources and discuss the implications for tracing earlier progenitors of such systems with the James Webb Space Telescope.