Quantifying the UV continuum slopes of galaxies to z~10 using deep Hubble and Spitzer/IRAC observations

Measurements of the UV-continuum slopes provide valuable information on the physical properties of galaxies forming in the early universe, probing the dust reddening, age, metal content, and even the escape fraction. While constraints on these slopes generally become more challenging at higher redshifts as the UV continuum shifts out of the Hubble Space Telescope bands (particularly at z>7), such a characterisation actually becomes abruptly easier for galaxies in the redshift window z=9.5-10.5 due to the Spitzer/IRAC 3.6um-band probing the rest-UV continuum and the long wavelength baseline between this Spitzer band and the Hubble H-band. Higher S/N constraints on the UV slope are possible at z~10 than at z=8. Here we take advantage of this opportunity and five recently discovered bright z=9.5-10.5 galaxies to present the first measurements of the mean slope for a multi-object sample of galaxy candidates at z~10. We find the measured observed slopes of these candidates are $-2.1\pm0.3\pm0.2$ (random and systematic), only slightly bluer than the measured slopes at 3.5<z<7.5 for galaxies of similar luminosities. Small increases in the stellar ages, metallicities, and dust content of the galaxy population from z~10 to z~7 could easily explain the apparent evolution in slopes.