First Detection of Molecular Gas in the Giant Low Surface Brightness Galaxy Malin 1

TL;DR

This study reports the first detection of molecular gas in the giant low surface brightness galaxy Malin 1 using ALMA, revealing very low molecular content despite its large size, and positioning it below the main sequence in the Kennicutt-Schmidt diagram.

Contribution

First detection of molecular gas in Malin 1, providing new estimates of its molecular mass and surface density, and placing it on the K-S diagram for the first time.

Findings

Molecular gas emission detected in Malin 1's center and disc regions.

Molecular-to-atomic gas mass ratio is very low.

Malin 1 falls below the main sequence in the K-S diagram.

Abstract

After over three decades of unsuccessful attempts, we report the first detection of molecular gas emission in Malin 1, the largest spiral galaxy observed to date, and one of the most iconic giant low surface brightness galaxies. Using ALMA, we detect significant $^{12}$CO(J=1-0) emission in the galaxy's central region and tentatively identify CO emission across three regions on the disc. These observations allow for a better estimate of the H$_2$ mass and molecular gas mass surface density, both of which are remarkably low given the galaxy's scale. By integrating data on its HI mass, we derive a very low molecular-to-atomic gas mass ratio. Overall, our results highlight the minimal presence of molecular gas in Malin 1, contrasting sharply with its extensive, homogeneous atomic gas reservoir. For the first time, we position Malin 1 on the Kennicutt-Schmidt (K-S) diagram, where it falls below the main sequence for normal spirals, consistent with previous upper limits but now with more accurate figures. These findings are crucial for constraining our understanding of star formation processes in environments characterized by extremely low molecular gas densities and for refining models of galaxy formation, thereby improving predictions concerning the formation, evolution, and distribution of these giant, elusive galaxies.