The giant low surface brightness galaxy Malin 1: new constraints for its molecular gas mass from GBT/ARGUS observations

TL;DR

Deep GBT/ARGUS observations of Malin 1 set new upper limits on its molecular gas content, revealing very low molecular gas surface density and suggesting different interstellar medium conditions compared to typical high surface brightness galaxies.

Contribution

This study provides the most sensitive CO observations of Malin 1 to date, establishing new upper limits on its molecular gas mass and surface density, and highlighting its distinct interstellar medium properties.

Findings

No CO(1-0) emission detected despite deep observations.

Upper limit for molecular gas mass is approximately 7.4 x 10^9 solar masses.

Molecular gas surface density is lower than 0.3 solar masses per square parsec.

Abstract

We report on results from GBT/ARGUS $^{12}$CO(1-0) observations for the giant low surface brightness galaxy Malin 1, which allow us to determine an upper limit for its CO mass, and hence its molecular gas mass and molecular gas mass surface density $\Sigma_{H_2}$. Although we performed very deep observations through 17 hours on source integration time, reaching a noise level of $\sim 0.2$ mK (T$^{*}_{A}$) with a corresponding extended source CO limit (3$\sigma$) of 0.09 K km s$^{-1}$, 19 times more sensitive than previous works, we do not detect the $^{12}$CO(1-0) emission line. However, the observations allow us to estimate an upper limit (3$\sigma$) for the CO mass of about $7.4 \times 10^9$ M$_\odot$ for the extended emission, and $1.4 \times 10^8$ M$_\odot$ for the central part of the galaxy. With these figures we conclude that the molecular gas surface density is lower than 0.3 M$_\odot$ pc$^{-2}$, and the corresponding molecular to atomic gas mass ratio is lower than 0.13. The evidence suggests a quite different physical conditions for the interstellar medium in Malin 1 compared to that of normal, high surface brightness spirals. This, in one way to another, keeps an usual molecular gas tracer as CO hidden from our observations, in spite of the diverse stellar and structural properties of Malin 1 observed by several authors since more than 30 years.