Galaxy Star Formation as a Function of Environment in the Early Data Release of the Sloan Digital Sky Survey

(Abridged) We present in this paper a detailed analysis of the effect ofenvironment on the star-formation activity of galaxies within the EDR of theSDSS. We have used the Halpha emission line to derive the star-formation rate(SFR) for each galaxy within a volume-limited sample of 8598 galaxies with 0.05< z < 0.095 and M(r)<= -20.45. We find that the SFR of galaxies is stronglycorrelated with the local (projected) galaxy density and thus we present herethe density-SFR relation that is analogous to the density-morphology relation.The effect of density on the SFR of galaxies is seen in three ways. First, theoverall distribution of SFRs is shifted to lower values in dense environmentscompared with the field population. Second, the effect is most noticeable forthe strongly star-forming galaxies in the 75th percentile of the SFRdistribution. Third, there is a ``break'' (or characteristic density) in thedensity-SFR relation at a local galaxy density of 1h-2 Mpc-2. To understandthis break further, we have studied the SFR of galaxies as a function ofclustercentric radius from 17 clusters and groups objectively selected from theSDSS EDR data. The distribution of SFRs of cluster galaxies begins to change,compared with the field population, at a clustercentric radius of 3-4 virialradii, which is consistent with the characteristic break in density that weobserve in the density-SFR relation. Our tests suggest that thedensity-morphology relation alone is unlikely to explain the density-SFRrelation we observe. Taken all together, these works demonstrate that thedecrease in SFR of galaxies in dense environments is a universal phenomenonover a wide range in density (from 0.08 to 10h-2 Mpc-2) and redshift (out to z= 0.5).Comment: Accepted for publication in ApJ, 24 pages in emulateapj.st