Abstract
We estimate the UV continuum slope, $\beta$, for 923 galaxies in the range $1
< z < 8$ in the Hubble Ultradeep Field (HUDF). These data include 460 galaxies
at $1<z<2$ down to an absolute magnitude $M_UV = -14~(0.006~L^*_z=1;
0.02~L^*_z=0$), comparable to dwarf galaxies in the local universe. We
combine deep HST/UVIS photometry in F225W, F275W, F336W wavebands (UVUDF) with
recent data from HST/WFC3/IR (HUDF12). Galaxies in the range $1<z<2$ are
significantly bluer than local dwarf galaxies. We find their mean (median)
values $łeft<\right> = -1.382~(-1.830)\pm0.002$ (random) $\pm0.1$
(systematic). We find comparable scatter in $\beta$ (standard deviation = 0.43)
to local dwarf galaxies and 30% larger scatter than $z>2$ galaxies. We study
the trends of $\beta$ with redshift and absolute magnitude for binned
sub-samples and find a modest color-magnitude relation, $d\beta/dM = -0.11 \pm
0.01$ and no evolution in $d\beta/dM$ with redshift. A modest increase in dust
reddening with redshift and luminosity, $\Delta E(B-V) 0.1$, and a
comparable increase in the dispersion of dust reddening at $z<2$, appears
likely to explain the observed trends. At $z>2$, we find trends that are
consistent with previous works; combining our data with the literature in the
range $1<z<8$, we find a color evolution with redshift, $d\beta/dz =
-0.09\pm0.01$ for low luminosity (0.05 $L^*_z=3$), and $d\beta/dz =
-0.06\pm0.01$ for medium luminosity (0.25 $L^*_z=3$) galaxies.
Description
[1407.3680] The UV Continuum of $z>1$ Star-forming Galaxies in the Hubble Ultraviolet UltraDeep Field
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