Cold molecular gas distribution and kinematics in the low-metallicity dusty starburst of Mrk 996 resolved with ALMA

TL;DR

This study uses ALMA to map cold molecular gas in the low-metallicity galaxy Mrk 996, revealing compact CO clouds, complex kinematics, and a significant CO-dark H2 component, advancing understanding of molecular gas in metal-poor starbursts.

Contribution

First high-resolution ALMA observations of molecular gas in Mrk 996, showing its distribution, kinematics, and the presence of CO-dark H2 in a low-metallicity starburst galaxy.

Findings

Detected compact CO clouds offset from starburst core.

Molecular gas exhibits narrow, supersonic lines with mild blueshift.

Significant CO-dark H2 component inferred from low CO-to-H2 ratio.

Abstract

Detecting cold molecular gas in metal-poor starbursts remains a major challenge. Low carbon and oxygen abundances hinder CO formation, while low dust content reduces shielding against UV photodissociation. Consequently, CO, the main tracer of molecular gas, becomes faint or undetectable. We study the spatial distribution and kinematics of cold molecular gas in Mrk 996, a nearby low-mass Wolf-Rayet galaxy hosting a dense, low-metallicity (about 1/5 solar) and nitrogen-enriched nuclear starburst with complex ionized gas kinematics. Using ALMA observations of CO(1-0) and CO(2-1), we map the morphology and kinematics of the molecular gas and compare them with optical and UV data, tracing the ionized gas and young stellar populations. We detect compact CO clouds within 800 pc of the starburst, spatially offset from the nuclear super star cluster (SSC) and the most highly ionized regions. The CO lines are narrow and supersonic, exhibiting velocity gradients with a mild global blueshift, indicating dynamically perturbed gas without evidence for fast outflows, in contrast with the highly ionized phase. The global CO(2-1)/CO(1-0) ratio is low (R21 ~ 0.3), consistent with subthermal excitation. The millimeter continuum peaks at the SSC, while CO emission is displaced toward obscured regions, suggesting it traces dense shielded clumps. ALMA recovers about half of the single-dish flux, indicating the presence of extended, low-surface-brightness molecular gas. Using a metallicity-dependent CO-to-H2 conversion factor, we infer a molecular gas mass of a few 10^7 solar masses. The molecular gas is only weakly coupled to the stellar feedback that dominates the ionized phase. Our results support a multiphase scenario in which dense molecular clumps survive in shielded regions, while CO is photodissociated in their envelopes, leaving a significant CO-dark H2 component (Abridged).