THE UV CONTINUUM OF z > 1 STAR-FORMING GALAXIES IN THE HUBBLE ULTRAVIOLET ULTRADEEP FIELD

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(similar to 0.006L(z=1)(*); 0.02L(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 theirmean (median) values (beta)= -1.382(-1.830)+/- 0.002 (random)+/- 0.1 (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 +/- 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) similar to 0.1, and a comparable increase in the dispersion of dust reddening atz < 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 +/- 0.01 for low luminosity (0.05 L*z=3), and d beta/dz = -0.06 +/- 0.01 for medium luminosity (0.25 L-z=3(*)) galaxies.