Interacting galaxies: SFR & Metallicity

4 sets of interacting galaxies from the Sloan Digital Sky Survey Data Release 7.  These galaxies are part of the sample of Scudder et al. (2012b).

4 sets of interacting galaxies from the Sloan Digital Sky Survey Data Release 7.  These galaxies are part of the sample of Scudder et al. (2012b).

By studying the star formation rates and gas phase metallicities of a set of galaxies which appear to be in close pairs, we can observe the response of a galaxy to an interaction.  The gas phase metallicity traces the flow of gas towards the centre of a galaxy, due to the gravitational disturbance caused by a nearby companion galaxy.  If enough gas falls to the centre of the galaxy, that new collection of gas serves as a perfect reservoir for the production of new stars.  The more extreme the gas flow to the center of the galaxy (and by extension, the more extreme the gravitational perturbation), the more unusual the galaxy's star formation rates and metallicities will appear.  By using the projected separations of the galaxies as a proxy for time (which we can only explore through simulations), we can gain a sense of how long an interaction is likely to affect a galaxy.

In collaboration with theoretical studies of galaxy interactions, we determine that galaxies show significant changes in their star formation rates and gas phase metallicities (when compared to a carefully matched, non-interacting, control sample) over long timescales.  We found that galaxies can still have strongly boosted star formation rates at separations of at least 80 kpc.  This was later extended by Patton et al. (2013) to determine that this boosting signal dwindles to insignificance at 150 kpc separations.

Figure 3 of Scudder et al. (2012b). The upper panel shows the SFR response of the pairs sample as a function of projected separation, relative to a control sample of non-interacting pairs, matched in stellar mass, local environment, and redshift.  The lower panel shows the same, but for gas phase metallicity.  In both panels, the horizontal dotted line indicates the controls.  Star formation rates are consistently enhanced across all projected separation ranges probed in this sample.

Figure 3 of Scudder et al. (2012b). The upper panel shows the SFR response of the pairs sample as a function of projected separation, relative to a control sample of non-interacting pairs, matched in stellar mass, local environment, and redshift.  The lower panel shows the same, but for gas phase metallicity.  In both panels, the horizontal dotted line indicates the controls.  Star formation rates are consistently enhanced across all projected separation ranges probed in this sample.