Program: GS-2020A-FT-102

Determining when galaxies first emerged from the dark ages of the Universe represents the “Holy Grail” of modern observational cosmology. Surveys of redshift z~7-9 galaxies have revealed several sources whose red Spitzer/IRAC colours may be due to the presence of a significant Balmer break, indicative of mature stellar populations that can shed light on the timing of the birth of the first stars and galaxies. Accurate redshifts are a prerequisite towards accurately modelling and quantifying the contribution of such stellar populations to the total light of the galaxy, and ultimately the timing of their emergence and Cosmic Dawn. The first and only example to date of such stellar populations was found in a redshift z=9.11, gravitationally-lensed galaxy, confirmed and modelled with data from the VLT and ALMA which indicated the onset of star formation at a redshift of z~15. However, the lensed nature of the galaxy lead to large uncertainties in the associated stellar mass and star formation activity of the object. Here, we propose to follow up the only unlensed z>9 galaxy with a red Spitzer/IRAC colour accessible with both FLAMINGOS2/Gemini and ALMA with the aim of a) determining whether such mature stellar populations are common among high redshift galaxies and b) place stronger constraints on the timing of Cosmic Dawn.