Cold Molecular Gas in Merger Remnants: I. Formation of Molecular Gas Disks

TL;DR

This study uses high-resolution CO imaging to analyze molecular gas disks in 37 local merger remnants, revealing that most host rotating gas disks of varying sizes, which are common in late-stage mergers and may relate to galaxy evolution.

Contribution

It provides new high-resolution observations of molecular gas disks in merger remnants, showing their prevalence and characteristics, supporting merger-driven formation of such disks.

Findings

80% show rotating molecular gas disks

Disks range from 1.1 to 9.3 kpc in size

Over half have more compact disks than stellar components

Abstract

We present < 1 kpc resolution CO imaging study of 37 optically-selected local merger remnants using new and archival interferometric maps obtained with ALMA, CARMA, SMA and PdBI. We supplement a sub-sample with single-dish measurements obtained at the NRO 45 m telescope for estimating the molecular gas mass (10^7 - 10^11 M_sun), and evaluating the missing flux of the interferometric measurements. Among the sources with robust CO detections, we find that 80 % (24/30) of the sample show kinematical signatures of rotating molecular gas disks (including nuclear rings) in their velocity fields, and the sizes of these disks vary significantly from 1.1 kpc to 9.3 kpc. The size of the molecular gas disks in 54 % of the sources is more compact than the K-band effective radius. These small gas disks may have formed from a past gas inflow that was triggered by a dynamical instability during a potential merging event. On the other hand, the rest (46 %) of the sources have gas disks which are extended relative to the stellar component, possibly forming a late-type galaxy with a central stellar bulge. Our new compilation of observational data suggests that nuclear and extended molecular gas disks are common in the final stages of mergers. This finding is consistent with recent major-merger simulations of gas rich progenitor disks. Finally, we suggest that some of the rotation-supported turbulent disks observed at high redshifts may result from galaxies that have experienced a recent major merger.