Observing the Effects of Galaxy Interactions on the Circumgalactic Medium

We continue our empirical study of the emission line flux originating in the cool (T ∼ 104 K) gas that populates the halos of galaxies and their environments. Specifically, we present results obtained for a sample of galaxy pairs with a range of projected separations, $10\lt {S}_{p}/\mathrm{kpc}\lt 200$, and mass ratios <1:5, intersected by 5443 Sloan Digital Sky Survey lines of sight at projected radii of 10–50 kpc from either or both of the two galaxies. We find significant enhancement in Hα emission and a moderate enhancement in [N ii]6583 emission for low-mass pairs (mean stellar mass per galaxy, ${\overline{M}}_{* },\lt {10}^{10.4}{M}_{\odot }$) relative to the results from a control sample. This enhanced Hα emission comes almost entirely from sight lines located between the galaxies, consistent with a short-term, interaction-driven origin for the enhancement. We find no enhancement in Hα emission, but significant enhancement in [N ii]6583 emission for high-mass (${\overline{M}}_{* }\gt {10}^{10.4}{M}_{\odot }$) pairs. Furthermore, we find a dependence of the emission line properties on the galaxy pair mass ratio such that those with a mass ratio below 1:2.5 have enhanced [N ii]6583 and those with a mass ratio between 1:2.5 and 1:5 do not. In all cases, departures from the control sample are only detected for close pairs (Sp < 100 kpc). Attributing an elevated [N ii]6583/Hα ratio to shocks, we infer that shocks play a role in determining the circumgalactic medium properties for close pairs that are among the more massive and have mass ratios closer to 1:1.