Spatially Resolved Galaxy Star Formation and Its Environmental Dependence. I.

Abstract

The role of star formation in galaxiesis clearly a fundamental component of their evolution, although itis becoming clear that galaxy environments may also play a significantrole. To explore the relationship between environment and star formationin galaxies, I use the photometric information contained in individualpixels of 44,964 galaxies (volume-limited) fromthe Fourth Data Release of the Sloan Digital Sky Survey. Iuse the pixel-z technique, which combines stellar population synthesis modelswith photometric redshift template fitting on the scale of individualpixels in galaxy images. Spectral energy distributions are constructed, sampling a wide range ofproperties such as age, star formation rate (SFR), dust obscuration andmetallicity. By summing the SFRs in the pixels, I show that, as found in other studies, thedistribution of total galaxy SFR shifts to lower values as the local densityof surrounding galaxies increases. The effectis 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 asa function of radius. I find that, on average, the mean SFR is dominatedby star formation in the central regions of galaxies and it is this centralstar formation that is suppressed in high density environments. The mean SFR in the outskirts of galaxies is found to be largely independentof environmental effects. These trends are shared by galaxieswhich 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 accountfor 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.