The impact of local vs. large-scale environment

Figure A2 of Scudder et al. (2012a). Four isolated compact groups of galaxies, from the Sloan Digital Sky Survey Data Release 7.  The SDSS object ID for the galaxy in the exact centre of the image is marked in the bottom left hand corner of each image.

Figure A2 of Scudder et al. (2012a). Four isolated compact groups of galaxies, from the Sloan Digital Sky Survey Data Release 7.  The SDSS object ID for the galaxy in the exact centre of the image is marked in the bottom left hand corner of each image.

Galaxies can be found in a number of local "environments"- a term used to describe the number of galaxies in the same region of space. Galaxies can be found in very empty regions of space, termed "isolated environments", or in very densely populated regions, such as clusters.  Very densely populated regions of space often cause the galaxies within them to be subjected to a number of additional forces- the large number of galaxies causes frequent high speed interactions between galaxies; the high temperatures created in these environments can strip away the gas from within a galaxy, and prevent additional gas from cooling.

To help disentangle the effects of close galaxy interactions from the high temperature effects of large clusters, I examined a sample of galaxies found in very dense groups, which held only 4-6 galaxies, and were otherwise isolated from large structure.  However, approximately half of these dense groups were found within 1 Mpc of a large cluster, so these dense groups were termed 'embedded' within a larger cluster, whereas those further than 1 Mpc were termed 'isolated' groups.

We found that while isolated groups did show signatures of enhanced star formation (among the star forming population of galaxies in these dense environments), the embedded groups showed no detectable signs of a similar enhancement in their star forming properties.  We interpreted this as the signature of a large-scale impact due to the dense groups being embedded within a cluster; this indicates that the cluster environment has an impact on galaxies beyond just the high galaxy density, and the high temperatures found in the densest environments.

Figure 4 of Scudder et al. (2012a).  Changes in star formation rate (SFR) relative to a control sample matched in stellar mass and redshift.  The blue histogram indicates the SFR changes in star forming galaxies found within dense groups that are further than 1 Mpc from a cluster environment, and its median change in SFR is indicated by the vertical solid line.  The changes to SFR in star forming galaxies in embedded clusters are histogrammed in the red dashed line, and their median is shown as the dashed vertical line. The galaxies found within isolated groups show statistically significant enhancements to their SFR, whereas the galaxies in embedded groups do not.  The difference in median SFR offset indicates that there is some additional impact of the cluster environment upon a dense group.

Figure 4 of Scudder et al. (2012a).  Changes in star formation rate (SFR) relative to a control sample matched in stellar mass and redshift.  The blue histogram indicates the SFR changes in star forming galaxies found within dense groups that are further than 1 Mpc from a cluster environment, and its median change in SFR is indicated by the vertical solid line.  The changes to SFR in star forming galaxies in embedded clusters are histogrammed in the red dashed line, and their median is shown as the dashed vertical line. The galaxies found within isolated groups show statistically significant enhancements to their SFR, whereas the galaxies in embedded groups do not.  The difference in median SFR offset indicates that there is some additional impact of the cluster environment upon a dense group.