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

Abstract

We discuss the spectral energy distributions and physical properties of six galaxies whose photometric redshifts suggest they lie beyond a redshift $z\simeq$9. Each was selected on account of a prominent excess seen in the Spitzer/IRAC 4.5$\hat{I}¼$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 $z\simeq 10$. 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-$\hat{I}\pm$ or [OIII] 88 $\hat{I}¼$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.

updated version with correction of a numerical error on MACS1149-JD1 age estimate. The predicted earlier luminosity for this galaxy has been corrected on Figure 11