Two Orders of Magnitude Variation in the Star Formation Efficiency Across the Pre-Merger Galaxy NGC 2276

Abstract

We present the first spatially resolved (~0.5 kpc) measurements of the molecular gas depletion time $τ_{depl}$ across the disk of the interacting spiral galaxy NGC\,2276, a system with an asymmetric morphology in various SFR tracers. To estimate $τ_{depl}$, we use new NOEMA observations of the $^{12}$CO(1-0) emission tracing the bulk molecular gas reservoir in NGC 2276, and extinction-corrected H$α$ measurements obtained with the PMAS/PPaK integral field unit for robust estimates of the SFR. We find a systematic decrease in $τ_{depl}$ of 1-1.5 dex across the disk of NGC 2276, with a further, abrupt drop in $τ_{depl}$ of ~1 dex along the galaxy's western edge. The global $τ_{depl}$ in NGC 2776 is $τ_{depl}=0.55$ Gyr, insistent with literature measurements for the nearby galaxy population. Such a large range in $τ_{depl}$ on sub-kpc scales has never previously been observed within an individual isolated or pre-merger system. When using a metallicity-dependent molecular gas conversion factor X$\rm_{CO}$ the variation decreases by 0.5 dex. We attribute the variation in $τ_{depl}$ to the influence of galactic-scale tidal forces and ram pressure on NGC 2276's molecular interstellar medium (ISM). Our observations add to the growing body of numerical and observational evidence that galaxy-galaxy interactions significantly modify the molecular gas properties and star-forming activity within galactic disks throughout the interaction, and not just during the final merger phase.