Environmental Effects in the Evolution of Galactic Bulges

TL;DR

This study examines how environment influences the evolution of galactic bulges by comparing properties of bulges in cluster and field galaxies over redshifts 0 to 0.6, revealing environment-dependent evolutionary differences.

Contribution

It provides the first evidence of significant environmental effects on the evolution of galactic bulges using the Fundamental Plane at intermediate redshifts.

Findings

Cluster bulges evolve faster than field bulges in dense environments.

The observed differences are not due to mass-dependent effects.

Star formation histories of bulges and pure spheroids are similar at fixed mass and environment.

Abstract

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 HST 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 are 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.