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 75 th percentile of the SFR distribution. Third, there is a "break" (or characteristic density) in the density-SFR relation at a local galaxy density of ∼ 1 h −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 10 h −2 75 Mpc −2 ) and redshift (out to z ≃ 0.5).

The Sloan Digital Sky Survey (SDSS) first data release provides a database of ≈ 106000 unique galaxies in the main galaxy sample with measured spectra. A sample of star-forming (SF) galaxies are identified from among the 3079 of these having 1.4 GHz luminosities from FIRST, by using optical spectral diagnostics. Using 1.4 GHz luminosities as a reference star formation rate (SFR) estimator insensitive to obscuration effects, the SFRs derived from the measured SDSS Hα, [Oii] and u-band luminosities, as well as far-infrared luminosities from IRAS, are compared. It is established that straightforward corrections for obscuration and aperture effects reliably bring the SDSS emission line and photometric SFR estimates into agreement with those at 1.4 GHz, although considerable scatter (≈ 60%) remains in the relations. It thus appears feasible to perform detailed investigations of star formation for large and varied samples of SF galaxies through the available spectroscopic and photometric measurements from the SDSS. We provide herein exact prescriptions for determining the SFR for SDSS galaxies. The expected strong correlation between [Oii] and Hα line fluxes for SF galaxies is seen, but with a median line flux ratio F [OII] /F Hα = 0.23, about a factor of two smaller than that found in the sample of Kennicutt (1992). This correlation, used in deriving the [Oii] SFRs, is consistent with the luminosity-dependent relation found by Jansen et al. (2001). The median obscuration for the SDSS SF systems is found to be A Hα = 1.2 mag, while for the radio detected sample the median obscuration is notably higher, 1.6 mag, and with a broader distribution.

We analyse the observed correlation between galaxy environment and Hα emission‐line strength, using volume‐limited samples and group catalogues of 24 968 galaxies at 0.05 < z < 0.095, drawn from the 2dF Galaxy Redshift Survey (MbJ< −19.5) and the Sloan Digital Sky Survey (Mr < −20.6). We characterize the environment by: (1) Σ5, the surface number density of galaxies determined by the projected distance to the fifth nearest neighbour; and (2) ρ1.1 and ρ5.5, three‐dimensional density estimates obtained by convolving the galaxy distribution with Gaussian kernels of dispersion 1.1 and 5.5 Mpc, respectively. We find that star‐forming and quiescent galaxies form two distinct populations, as characterized by their Hα equivalent width, W0(Hα). The relative numbers of star‐forming and quiescent galaxies vary strongly and continuously with local density. However, the distribution of W0(Hα) amongst the star‐forming population is independent of environment. The fraction of star‐forming galaxies shows strong sensitivity to the density on large scales, ρ5.5, which is likely independent of the trend with local density, ρ1.1. We use two differently selected group catalogues to demonstrate that the correlation with galaxy density is approximately independent of group velocity dispersion, for σ= 200–1000 km s‐1. Even in the lowest‐density environments, no more than ∼70 per cent of galaxies show significant Hα emission. Based on these results, we conclude that the present‐day correlation between star formation rate and environment is a result of short‐time‐scale mechanisms that take place preferentially at high redshift, such as starbursts induced by galaxy–galaxy interactions.

We have studied the morphology–density relation and morphology–cluster‐centric‐radius relation using a volume‐limited sample (0.05 < z < 0.1, Mr* < −20.5) of the Sloan Digital Sky Survey (SDSS) data. Major improvements compared with previous work are: (i) automated galaxy morphology classification capable of separating galaxies into four types; (ii) three‐dimensional local galaxy density estimation; and (iii) the extension of the morphology–density relation into the field region. We found that the morphology–density and morphology–cluster‐centric‐radius relation in the SDSS data for both of our automated morphological classifiers, Cin and Tauto, as fractions of early‐type galaxies increase and late‐type galaxies decrease toward increasing local galaxy density. In addition, we found that there are two characteristic changes in both the morphology–density and the morphology–radius relations, suggesting that two different mechanisms are responsible for the relations. In the sparsest regions (below 1 Mpc−2 or outside of 1 virial radius), both relations become less noticeable, suggesting that the physical mechanisms responsible for galaxy morphological change require a denser environment. In the intermediate‐density regions (density between 1 and 6 Mpc−2 or virial radius between 0.3 and 1), intermediate‐type fractions increase toward denser regions, whereas late‐disc fractions decrease. Considering that the median size of intermediate‐type galaxies is smaller than that of late‐disc galaxies, we propose that the mechanism is likely to stop star formation in late‐disc galaxies, eventually turning them into intermediate‐type galaxies after their outer discs and spiral arms become invisible as stars die. For example, ram‐pressure stripping is one of the candidate mechanisms. In the densest regions (above 6 Mpc−2 or inside 0.3 virial radii), intermediate‐type fractions decrease radically and early‐type fractions increase in turn. This is a contrasting result to that in intermediate regions and it suggests that yet another mechanism is more responsible for the morphological change in these regions. We also compared the morphology–density relation from the SDSS (0.01 < z < 0.054) with that of the MORPHS data (z ∼ 0.5). Two relations lie on top of each other, suggesting that the morphology–density relation was already established at z∼ 0.5 as in the present Universe. A slight sign of an excess elliptical/S0 fraction in the SDSS data in dense regions might suggest the additional formation of elliptical/S0 galaxies in the cluster core regions between z= 0.5 and 0.05.

We present the observed fraction of galaxies with an active galactic nucleus (AGN) as a function of environment in the early data release of the Sloan Digital Sky Survey (SDSS). Using 4921 galaxies in the redshift range 0:05 z 0:095 and brighter than M r à ð Þ ¼ À20:0 (or M à þ 1:45), we find at least $20% of these galaxies possess an unambiguous detection of an AGN, but this fraction could be as high as '40% after we model the ambiguous emission-line galaxies in our sample. We have studied the environmental dependence of galaxies, using the local galaxy density as determined from the distance to the 10th nearest neighbor. As expected, we observe that the fraction of star-forming galaxies decreases with density, while the fraction of passive galaxies (no emission lines) increases with density. In contrast, the fraction of galaxies with an AGN remains constant from the cores of galaxy clusters to the rarefied field population. We conclude that the presence of an AGN is independent of the disk component of a galaxy. We have extensively tested our results, and they are robust against measurement error, definition of an AGN, aperture bias, stellar absorption, survey geometry, and signal-to-noise ratio. Our observations are consistent with the hypothesis that a supermassive black hole resides in the bulge of all massive galaxies, and '40% of these black holes are seen as AGNs in our sample. A high fraction of local galaxies with an AGN suggests either that the mean lifetime of these AGNs is longer than previously thought (i.e., !10 8 yr) or that the AGNs burst more often than expected: $40 times over the redshift range of our sample.