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$_2$ ($X_{\rm CH_3OH}$) 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 $X_{\rm CH_3OH}$ 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.