Does gas content control the starburst within an interaction?

From Figure A6 of Scudder et al. 2015. The two galaxies in this interaction have star formation rates which differ by a factor of two. Could the difference be driven by a difference in gas content?

From Figure A6 of Scudder et al. 2015. The two galaxies in this interaction have star formation rates which differ by a factor of two. Could the difference be driven by a difference in gas content?

In Scudder et al. 2012b, we determined that galaxies which are found in roughly equal mass encounters which are also within 30 kpc separations of each other are the most likely galaxies to show significant changes to their star formation rates (SFRs).  However, it soon became clear that simply selecting on these parameters did not select a clean sample of strong starbursts.  This unclean selection indicates that there is at least one additional parameter which controls the strength of the starburst.

One parameter which has been suggested by theoretical works is the gas content of the galaxy.  In order to test whether this parameter is a major contributor to the starburst strength, we selected a sample of 34 star forming galaxies in 17 pairs, which were in nearly equal mass encounters and close to each other in projected separation.  These 34 galaxies were observed with the Jansky Very Large Array (VLA) in New Mexico to obtain HI gas masses for these galaxies.  We could then plot the HI masses against the starburst strength measured from the SDSS DR7, and found a positive correlation between HI gas fraction and enhancement to SFR, at 2.5 sigma.

Figure 2 of Scudder et al. 2015.  The log of the HI gas fraction for all galaxies detected at > 3 sigma, as a function of SFR enhancement.  The two grey lines are fits to the data: dashed is including all data points, and solid excludes the point at SFR enhancement = -1.0 (marked with a kappa).  Spearman Rank Correlation tests indicate a positive correlation at 2.5 sigma.

Figure 2 of Scudder et al. 2015.  The log of the HI gas fraction for all galaxies detected at > 3 sigma, as a function of SFR enhancement.  The two grey lines are fits to the data: dashed is including all data points, and solid excludes the point at SFR enhancement = -1.0 (marked with a kappa).  Spearman Rank Correlation tests indicate a positive correlation at 2.5 sigma.

In interpreting this correlation, we made use of theoretical galaxy interactions, which led us to the conclusion that the positive direction of this correlation is due to an underlying strong correlation between the initial gas fraction of the galaxy and the strength of star formation (independent of the interaction), which is much stronger than the response of the galaxy as a result of the interaction. In manipulating the simulations in the same way as the observations, we are able to reproduce the same qualitative trend between gas fraction and starburst strength.

Figure 6 of Scudder et al. 2015.  A suite of simulations run with varying gas fractions (indicated by colour in the legend).  Their instananeous gas fractions are plotted against their SFR enhancements, where the SFR enhancements are calculated in an identical way to the observations.  The observations are matched to an average gas fraction control sample, so the simulations are compared to the Mgas/M* = 0.32 run. 

Figure 6 of Scudder et al. 2015.  A suite of simulations run with varying gas fractions (indicated by colour in the legend).  Their instananeous gas fractions are plotted against their SFR enhancements, where the SFR enhancements are calculated in an identical way to the observations.  The observations are matched to an average gas fraction control sample, so the simulations are compared to the Mgas/M* = 0.32 run.