MUSE observations of the giant low surface brightness galaxy Malin 1: Numerous HII regions, star formation rate, metallicity, and dust attenuation

Context. Giant low surface brightness (GLSB) galaxies are an extreme class of objects with very faint and extended gas-rich disks. Malin 1 is the largest GLSB galaxy known to date and one of the largest individual spiral galaxies observed so far, but the properties and formation mechanisms of its giant disk are still poorly understood.Aims. We used VLT/MUSE IFU spectroscopic observations of Malin 1 to measure the star formation rate (SFR), dust attenuation, and gas metallicity within this intriguing galaxy.Methods. We performed a penalized pixel fitting modeling to extract emission line fluxes such as H alpha, H beta, [N II](6583) and [O III](5007) along the central region as well as from the extended disk of Malin 1.Results. Our observations reveal for the first time strong H alpha emission distributed across numerous regions throughout the extended disk of Malin 1. The emission extends to radial distances of similar to 100 kpc, which indicates recent star formation activity. We made an estimate of the dust attenuation in the disk of Malin 1 using the Balmer decrement and found that Malin 1 has a mean H alpha attenuation of 0.36 mag. We observe a steep decline in the radial distribution of the SFR surface density (Sigma(SFR)) within the inner 20 kpc, followed by a shallow decline in the extended disk. We estimated the gas phase metallicity in Malin 1. We also found for the first time that the metallicity shows a steep gradient from solar metallicity to subsolar values in the inner 20 kpc of the galaxy, followed by a flattening of the metallicity in the extended disk with a relatively high value of similar to 0.6 Z(circle dot). We found that the normalized abundance gradient of the inner disk of Malin 1 is similar to the values found in normal galaxies. However, the normalized gradient observed in the outer disk can be considered extreme when compared to other disk galaxies. A comparison of the SFR surface density and gas surface density shows that unlike normal disk galaxies or other low surface brightness galaxies, the outer disk of Malin 1 exhibits a relatively low star formation efficiency based on atomic gas-mass estimates, which may be mildly exacerbated by the vanishing upper molecular gas-mass limits found by recent CO studies.Conclusions. With the detection of emission lines in a large part of the extended disk of Malin 1, this work sheds light on the star formation processes in this unique galaxy, highlighting its extended star-forming disk, dust attenuation, almost flat metallicity distribution in the outer disk, and exceptionally low star formation efficiency. Together with previous results, our findings contribute to a more detailed understanding of the formation of the giant disk of Malin 1, and they also constrain possible proposed scenarios of the nature of GLSB galaxies in general.