Unusual metallicity gradient and star formation efficiency in the giant low surface brightness galaxy Malin 1

Giant low-surface brightness (GLSB) galaxies are an extreme class of objects with very faint and extended gas-rich disks. Only about 100 GLSBs are currently known. Malin 1 is the largest GLSB galaxy known to date, but its formation is still poorly understood. We use VLT/MUSE IFU spectroscopic observations of Malin 1 to reveal, for the first time, the presence of H-alpha emission distributed across numerous regions along its disk, up to radial distances of about 100 kpc. We observe a steep decline in the star formation rate surface density within the inner 20 kpc, followed by a shallow decline in the extended disk. Similarly, the gas phase metallicity we estimated shows a steep gradient in the inner 20 kpc, followed by a flattening of the metallicity in the extended disk with a relatively high value of about 0.6 solar. Such a flattening provides clues on the formation of its disk. We found that the accretion of pre-enriched gas from a past merger is the most likely scenario for its formation. A comparison of the star formation rate surface density and gas surface density shows that Malin 1 exhibits a very low star formation efficiency (about 2 orders of magnitude lower than normal spirals), indicating a break in the well-known Kennicutt-Schmidt relationship at the LSB regime. Our findings contribute to a more detailed understanding of the formation of the giant disk of Malin 1 and also constrain possible proposed scenarios on the nature of GLSB galaxies in general. The upcoming surveys from LSST and SKA are expected to discover thousands of GLSBs, enabling us to study this population in detail for the first time.