Overview

The role of star formation in galaxies is clearly a fundamental component of their evolution, although it is becoming clear that galaxy environments may also play a significant role. To explore the relationship between environment and star formation in galaxies, I use the photometric information contained in individual pixels of 44964 galaxies (volume-limited) from the Fourth Data Release of the Sloan Digital Sky Survey. I use the pixel-z technique, which combines stellar population synthesis models with photometric redshift template fitting on the scale of individual pixels in galaxy images. Spectral energy distributions are constructed, sampling a wide range of properties such as age, star formation rate (SFR), dust obscuration and metallicity. By summing the SFRs in the pixels, I show that, as found in other studies, the distribution of total galaxy SFR shifts to lower values as the local density of surrounding galaxies increases. The effect is most prominent in the galaxies with the highest SFR. Since the method enables an estimate to be made of the spatial distribution of star formation within galaxies, the mean SFR of each galaxy is then calculated as a function of radius. I find that, on average, the mean SFR is dominated by star formation in the central regions of galaxies and it is this central star formation that is suppressed in high density environments. The mean SFR in the outskirts of galaxies is found to be largely independent of environmental effects. These trends are shared by galaxies which are highly star forming. I also investigate the impact of the density-morphology relation of galaxies on the observed trends. Early-type and late-type galaxies exhibit distinct radial SFR distributions. A suppression of star formation in the highest density environments is still found in the highest star forming galaxies within each type. I show that the density-morphology relation alone cannot account for this observed suppression. This points to a mechanism by which the environment governs the evolution of galaxies, affecting the star formation in the innermost regions in both early and late-type galaxies. I suggest that this is a natural consequence of downsizing in galaxies. Keywords. galaxies: structure---galaxies: statistics---galaxies: distances and redshifts---galaxies: evolution---galaxies: formation.

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9781243601728

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