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

Abstract

We provide a systematic measurement of the rest-frame UV continuum slope beta\nover a wide range in redshift (z~2-6) and rest-frame UV luminosity (0.1-2L*) to\nimprove estimates of the SFR density at high redshift. We utilize the deep\noptical and infrared data (ACS/NICMOS) over the CDF-S and HDF-N GOODS fields,\nas well as the UDF for our primary UBVi "dropout" sample. We correct the\nobserved distributions for selection biases and photometric scatter. We find\nthat the UV-continuum slope of the most luminous galaxies is substantially\nredder at z~2-4 than it is at z~5-6. Lower luminosity galaxies are also found\nto be bluer than higher luminosity galaxies at z~2.5 and z~4. We do not find a\nlarge number of galaxies with beta's as red as -1 in our dropout selections at\nz~4, and particularly at z&gt;~5, even though such sources could be readily\nselected from our data. This suggests that star-forming galaxies at z&gt;~5 almost\nuniversally have very blue UV-continuum slopes, and that there are not likely\nto be a substantial number of dust-obscured galaxies at z&gt;~5 that are missed in\n"dropout" searches. Using the same relation between UV-continuum slope and dust\nextinction as found to be appropriate at z~0 and z~2, we estimate the average\ndust extinction of galaxies as a function of redshift and UV luminosity in a\nconsistent way. We find that the estimated dust extinction increases\nsubstantially with cosmic time for the most UV luminous galaxies, but remains\nsmall (&lt;~2x) at all times for lower luminosity galaxies. Because these same\nlower luminosity galaxies dominate the luminosity density in the UV, the\noverall dust extinction correction remains modest at all redshifts. We include\nthe contribution from ULIRGs in our SFR density estimates at z~2-6, but find\nthat they contribute only ~20% of the total at z~2.5 and &lt;~10% at z&gt;~4.\n