UV CONTINUUM SLOPE AND DUST OBSCURATION FROM z ∼ 6 TO z ∼ 2: THE STAR FORMATION RATE DENSITY AT HIGH REDSHIFT

We provide a systematic measurement of the rest-frame UV continuum slope beta over a wide range in redshift (z similar to 2-6) and rest-frame UV luminosity (0.1 L-z=3* to 2 L-z=3*) to improve estimates of the star formation rate (SFR) density at high redshift. We utilize the deep optical and infrared data (Advanced Camera for Surveys/NICMOS) over the Chandra Deep Field-South and Hubble Deep Field-North Great Observatories Origins Deep Survey fields, as well as the UDF for our primary UBVi "dropout" Lyman Break Galaxy sample. We also use strong lensing clusters to identify a population of very low luminosity, high-redshift dropout galaxies. We correct the observed distributions for both selection biases and photometric scatter. We find that the UV-continuum slope of the most luminous galaxies is substantially redder at z similar to 2-4 than it is at z similar to 5-6 (from similar to-2.4 at z similar to 6 to similar to-1.5 at z similar to 2). Lower luminosity galaxies are also found to be bluer than higher luminosity galaxies at z similar to 2.5 and z similar to 4. We do not find a large number of galaxies with beta's as red as -1 in our dropout selections at z similar to 4, and particularly at z greater than or similar to 5, even though such sources could be readily selected from our data (and also from Balmer Break Galaxy searches at z similar to 4). This suggests that star-forming galaxies at z greater than or similar to 5 almost universally have very blue UV-continuum slopes, and that there are not likely to be a substantial number of dust-obscured galaxies at z greater than or similar to 5 that are missed in "dropout" searches. Using the same relation between UV- continuum slope and dust extinction as has been found to be appropriate at both z similar to 0 and z similar to 2, we estimate the average dust extinction of galaxies as a function of redshift and UV luminosity in a consistent way. As expected, we find that the estimated dust extinction increases substantially with cosmic time for the most UV luminous galaxies, but remains small (less than or similar to 2 times) at all times for lower luminosity galaxies. Because these same lower luminosity galaxies dominate the luminosity density in the UV continuum, the overall dust extinction correction remains modest at all redshifts and the evolution of this correction with redshift is only modest. We include the contribution from ultra-luminous IR galaxies in our SFR density estimates at z similar to 2-6, but find that they contribute only similar to 20% of the total at z similar to 2.5 and less than or similar to 10% at z greater than or similar to 4.