First spectroscopic study of ionised gas emission lines in the extreme low surface brightness galaxy Malin 1

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Context. Malin 1 is the largest known low surface brightness (LSB) galaxy, the archetype of so-called giant LSB galaxies. The structure and origin of such galaxies are still poorly understood, especially because of the lack of high-resolution kinematics and spectroscopic data. Aims. We use emission lines from spectroscopic observations of Malin 1 aiming to bring new constraints on the internal dynamics and star formation history of Malin 1. Methods. We extracted a total of 16 spectra from different regions of Malin 1 and calculated the rotational velocities of these regions from the wavelength shifts and star formation rates from the observed Hα emission line fluxes. We compared our data with existing data and models for Malin 1. Results. For the first time we present the inner rotation curve of Malin 1, characterised in the radial range r < 10 kpc by a steep rise in the rotational velocity up to at least ∼350 km s^(−1) (with a large dispersion), which had not been observed previously. We used these data to study a suite of new mass models for Malin 1. We show that in the inner regions dynamics may be dominated by the stars (although none of our models can explain the highest velocities measured) but that at large radii a massive dark matter halo remains necessary. The Hα fluxes derived star formation rates are consistent with an early-type disc for the inner region and with the level found in extended UV galaxies for the outer parts of the giant disc of Malin 1. We also find signs of high metallicity but low dust content for the inner regions.
© Junais et al. 2020. We acknowledge the support by the Programme National Cosmology et Galaxies (PNCG) of CNRS/INSU with INP and IN2P3, co-funded by CEA and CNES. We thank M. Fossati for providing some plotting codes for the line diagnostic diagram and S. Arnouts for his help on the environment of Malin 1. This work also utilised some data from the SDSS DR12 Science Archive Server (SAS) for comparison purposes. Funding for SDSS-III has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, and the US Department of Energy Office of Science. The SDSS-III web site is
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