Measurements of the UV-continuum slopes β 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
characterization actually becomes abruptly easier for galaxies in the
redshift window z = 9.5-10.5 due to the Spitzer/Infrared Array Camera
3.6 μm-band probing the rest-UV continuum and the long wavelength
baseline between this Spitzer band and the Hubble Hf160w
band. Higher S/N constraints on β 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 β for a multi-object sample of galaxy
candidates at z ˜ 10. We find the measured βobs's
of these candidates are -2.1 ± 0.3 ± 0.2 (random and
systematic), only slightly bluer than the measured β's
(βobs ≈ -1.7) 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 β.