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.
