ENVIRONMENTAL EFFECTS IN THE EVOLUTION OF GALACTIC BULGES

We investigate possible environmental trends in the evolution of galactic bulges over the redshift range 0 < z < 0.6. For this purpose, we construct the fundamental plane (FP) for cluster and field samples at redshifts (z) = 0.4 and (z) = 0.54 using surface photometry based on Hubble Space Telescope imaging and velocity dispersions based on Keck spectroscopy. As a reference point for our study, we include data for pure ellipticals, which we model as single-component Sersic profiles; whereas for multi-component galaxies we undertake decompositions using Sersic and exponential models for the bulge and disk, respectively. Although the FP for both distant cluster and field samples is offset from the local relation, consistent with evolutionary trends found in earlier studies, we detect significant differences in the zero point of ≅0.2 dex between the field and cluster samples at a given redshift. For both clusters, the environmentally dependent offset is in the sense expected for an accelerated evolution of bulges in dense environments. By matching the mass range of our samples, we confirm that this difference does not arise as a result of the mass-dependent downsizing effects seen in larger field samples. Our result is also consistent with the hypothesis that-at fixed mass and environment-the star formation histories of galactic bulges and pure spheroids are indistinguishable and difficult to reconcile with the picture whereby the majority of large bulges form primarily via secular processes within spiral galaxies.