Chemically primitive dwarf accretion reignites the inner disk assembly of Malin 1

TL;DR

This study reveals that Malin 1's inner disk has been recently assembled through accretion of primitive, dwarf galaxy gas, leading to localized star formation and complex kinematic features.

Contribution

It provides detailed kinematic and stellar population evidence for external gas accretion shaping Malin 1's inner disk, a novel insight into galaxy assembly processes.

Findings

Identification of four star-forming complexes with distinct velocities

Detection of a kinematically decoupled, metal-poor, young stellar clump

Evidence for recent accretion of primitive gas from a disrupted dwarf galaxy

Abstract

We present a detailed kinematic and stellar population analysis of the inner disk of Malin 1, a giant low surface brightness (GLSB) galaxy with a prominent SB0-type central morphology. AstroSat far-UV imaging reveals clumpy emission features indicating recent star formation. Using MUSE integral field spectroscopy, we identify four star-forming complexes (SFCs) within the inner 10 kpc, each associated with localized ionized gas emission in distinct H$\alpha$ velocity channels. Two of the SFCs, including a far-UV clump, appear on the blue-shifted side ($V_{H\alpha}=-230~\mathrm{kms^{-1}}$), while the other two are redshifted. The far-UV clump shows a strong velocity offset ($\sim150~\mathrm{kms^{-1}}$) and high gas dispersion ($\sim250~\mathrm{kms^{-1}}$), indicating that it is kinematically decoupled from the rotating disk. The spatial and velocity isolation of these features in the channel map confirms they do not follow regular disk rotation. The far-UV clump hosts young (250-500 Myr), extremely metal-poor ([M/H]$\simeq$ -1.69) and $\alpha$-enriched ($[\alpha/Fe] \sim 0.5$) stellar populations, sharply contrasting with the surrounding super-solar gas-phase metallicity. The young stellar populations in each SFC are chemically distinct (similar to the far-UV clump) from the enriched central ISM, indicating rapid, local star formation from primitive gas before efficient mixing with the enriched ISM. Their spatial and velocity segregation, age synchronicity, and chemical homogeneity suggest an origin of gas delivered by a disrupted, gas-rich dwarf on a high-inclination (off-plane) orbit. These results suggest that the central HSB within $\rm \sim 9^{\prime\prime} (14\ kpc)$ radius component of Malin 1 has grown through discrete, externally driven accretion, contributing to its complex, hybrid disk morphology.