Merger-induced Shocks in the Nearby LIRG VV 114 through Methanol Observations with ALMA

Abstract

We report the detection of two CH${}_3$OH lines (J${}_K$ = 2${}_K$-1${}_K$ and 3${}_K$-2${}_K$) between the progenitor's disks ("Overlap") of the mid-stage merging galaxy VV 114 obtained using the Atacama Large Millimeter/submillimeter Array (ALMA) Band 3 and Band 4. The detected CH${}_3$OH emission show an extended filamentary structure (~ 3 kpc) across the progenitor's disks with relatively large velocity width (FWZI ~ 150 km/s). The emission is only significant in the "overlap" and not detected in the two merging nuclei. Assuming optically-thin emission and local thermodynamic equilibrium (LTE), we found the CH${}_3$OH column density relative to H${}_2$ ($X_{\mathrm{C}{\mathrm{H}}_3\mathrm{OH}}$) peaks at the "Overlap" (~ 8 $\times$ 10${}^{-9}$), which is almost an order of magnitude larger than that at the eastern nucleus. We suggest that kpc-scale shocks driven by galaxy-galaxy collision may play an important role to enhance the CH${}_3$OH abundance at the "Overlap". This scenario is consistent with that shock-induced large velocity dispersion components of ionized gas have been detected in optical wavelength at the same region. Conversely, low $X_{\mathrm{C}{\mathrm{H}}_3\mathrm{OH}}$ at the nuclear regions might be attributed to the strong photodissociation by nuclear starbursts and/or putative active galactic nucleus (AGN), or inefficient production of CH${}_3$OH on dust grains due to initial high temperature conditions (i.e., desorption of the precursor molecule, CO, into gas-phase before forming CH${}_3$OH on dust grains). These ALMA observations demonstrate that CH${}_3$OH is a unique tool to address kpc-scale shock-induced gas dynamics and star formation in merging galaxies.

8 pages, 4 figures, 3 tables, accepted for publication in ApJ