Probing cosmic dawn: Ages and star formation histories of candidate z ≥ 9 galaxies

We discuss the spectral energy distributions and physical properties of six galaxies whose photometric redshifts suggest they lie beyond a redshift z similar or equal to 9. Each was selected on account of a prominent excess seen in the Spitzer/IRAC 4.5m 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 similar or equal to 10. 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] 88m 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 similar or equal to 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 similar or equal to 10. We use our best-fitting 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.

Automated summary

This study discusses the spectral energy distributions and physical properties of six galaxies with photometric redshifts suggesting they are at a redshift z similar or equal to 9. Analysis of excess seen in the Spitzer/IRAC 4.5m band indicates the presence of a stellar component that formed earlier than a redshift z similar or equal to 10. Spectroscopic follow-up was done using various telescopes, confirming the redshift of two galaxies. The study supports a cosmic star formation history extending beyond redshifts z similar or equal to 10.