Galaxy Star-formation as a Function of Environment in the Early Data Release of the Sloan Digital Sky Survey1

We present in this paper a detailed analysis of the effect of environment on the star–formation activity of galaxies within the Early Data Release (EDR) of the Sloan Digital Sky Survey (SDSS). We have used the Hα 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 strongly correlated with the local (projected) galaxy density and thus we present here the 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, the overall distribution of SFRs is shifted to lower values in dense environments compared with the field population. Second, the effect is most noticeable for the strongly star–forming galaxies (Hα EW > 5Å) in the 75th percentile of the SFR distribution. Third, there is a “break” (or characteristic density) in the density–SFR relation at a local galaxy density of ∼ 1h−2 75 Mpc −2. To understand this break further, we have studied the SFR of galaxies as a function of clustercentric radius from 17 clusters and groups objectively selected from the SDSS EDR data. The distribution of SFRs of cluster galaxies begins to change, compared with the field population, at a clustercentric radius of 3–4 virial radii (at the > 1σ statistical significance), which is consistent with the characteristic break in density that we observe in the density–SFR relation. This effect with clustercentric radius is again most noticeable for the most strongly star–forming galaxies. Our tests suggest that the density-morphology relation alone is unlikely to explain the density–SFR relation we observe. For example, we have used the (inverse) concentration index of SDSS galaxies to classify late–type galaxies and show that the distribution of the star–forming (EW Hα > 5Å) late–type galaxies is different in dense regions (within 2 virial radii) compared with similar galaxies in the field. However, at present, we are unable to make definitive statements about the independence of the density–morphology and density–SFR relation. We have tested our work against potential systematic uncertainties including stellar absorption, reddening, SDSS survey strategy, SDSS analysis pipelines and aperture bias. Our observations are in qualitative agreement with recent simulations of hierarchical galaxy formation that predict a decrease in the SFR of galaxies within the virial radius. Our results are in agreement with recent 2dF Galaxy Redshift Survey results as well as consistent with previous observations of a decrease in the SFR of galaxies in the cores of distant clusters. Taken all together, these works demonstrate that the decrease in SFR of galaxies in dense environments is a universal phenomenon over a wide range in density (from 0.08 to 10h−2 75 Mpc −2) and redshift (out to z ≃ 0.5). Subject headings: galaxies: clusters: general – galaxies: evolution – stars: formation – galaxies: stellar content – surveys