The UV Continuum of z Greater Than 1 Star-Forming Galaxies in the Hubble Ultraviolet Ultradeep Field

We estimate the UV continuum slope, beta, for 923 galaxies in the range 1 less than z less than 8 in the Hubble Ultradeep Field (HUDF). These data include 460 galaxies at 1 less than z less than 2 down to an absolute magnitude M(sub UV) = 14(approx. 0.006 stellar luminosity (sub z=1); 0.02 stellar luminosity (sub 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 less than z less than 2 are significantly bluer than local dwarf galaxies.We find their mean (median) values = 1.382(1.830) plus or minus 0.002 (random) plus or minus 0.1 (systematic).We find comparable scatter in beta (standard deviation= 0.43) to local dwarf galaxies and 30% larger scatter than z greater than 2 galaxies.We study the trends of beta with redshift and absolute magnitude for binned sub-samples and find a modest color-magnitude relation, ((delta beta)/(delta M)) = 0.11 plus or minus 0.01, and no evolution in ((delta beta)/(delta M)) with redshift. A modest increase in dust reddening with redshift and luminosity, delta E(B V ) approx. 0.1, and a comparable increase in the dispersion of dust reddening at z less than 2, appears likely to explain the observed trends. At z greater than 2, we find trends that are consistent with previous works; combining our data with the literature in the range 1 less than z less than 8, we find a color evolution with redshift, ((delta beta)/(delta z)) = 0.09 plus or minus 0.01 for low luminosity (0.05 stellar luminosity (sub z=3), and ((delta beta)/(delta z)) = 0.06 plus or minus 0.01 for medium luminosity (0.25 stellar luminosity (sub z=3) galaxies.