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

TL;DR

Using deep UV imaging and optical integral-field spectroscopy, the paper investigates the late-stage inner-disk growth of Malin 1, a giant GLSB with a chemically evolved central region. They identify four chemically primitive, kinematically decoupled star-forming complexes in the inner $\sim 10\ { m kpc}$, consistent with accretion from a disrupted dwarf on a high-inclination orbit delivering metal-poor gas that fuels localized star formation. The young stars in these clumps are extremely metal-poor and alpha-enhanced, while the central ISM remains metal-rich, indicating rapid local enrichment with limited mixing. A Monte Carlo analysis confirms a two-phase stellar population in the C1 clump, supporting an externally originated accretion scenario; overall, the results imply that Malin 1’s inner disk is actively assembled by minor mergers, challenging the view of GLSBs as passive systems.

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$α$ velocity channels. Two of the SFCs, including a far-UV clump, appear on the blue-shifted side ($V_{Hα}=-230~\mathrm{kms^{-1}}$), 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 $α$-enriched ($[α/Fe] \sim 0.5$) 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.

Figures (9)

• Figure 1: Left: CFHT (i+g+u) RGB image of Malin 1 overlaid with MUSE RGB, showing the spiral structure. The red square marks the MUSE cutout used for analysis.Right: Astrosat/UVIT F154W (FUV) and N263M (NUV) smoothed images. FUV reveals the central source (C0) and an offset clump (C1), while only C0 is seen in NUV. Black contours: HST/WFPC2 F814W stellar emission. Gray contours: $\rm H\alpha$ channel map contours of clumps detected at different velocities. Blue dashed contours (1, 3, and 15$\sigma$): $\rm H_{\alpha}$ emission at $\rm V_{H\alpha}=-230\ \rm kms^{-1}$. White circles ($r=0.6^{\prime\prime}$): MUSE spectral extraction regions. Pink dashed circle ($r=1.6^{\prime\prime}$): UVIT/F154W PSF FWHM; solid red circle: bar minor-axis extraction region. Black line: bar orientation from HST. Zoom-in highlights the clumpy FUV extended region, and asymmetric $\rm H\alpha$ emission relative to the bar.
• Figure 2: Kinematic maps of the central HSB region, from MUSE IFU data, of Malin 1 (red square in the left panel of Figure \ref{['fig:malin1_rgb_fuv']}). The solid black and dashed gray contours are HST/WFPC2 F814W and Astrosat/UVIT F154W ($2\sigma$), respectively. Top panel: stellar kinematics maps: left to right panel shows the velocity (V), dispersion ($\sigma$), and $|V|/\sigma$ maps. Bottom panel: ionized gas kinematics: left: gas velocity maps with dashed magenta circle ($r = 0.6^{\prime\prime}$) show the velocity enhanced region, the deepink dashed circle is centered at the location of C1 (see Figure \ref{['fig:malin1_rgb_fuv']}) with $r = 1.6^{\prime\prime}$ corresponds to the FWHM of the FUV. The velocity-enhanced region is highlighted in the zoom-in. Middle: Shows the velocity dispersion with the black dot-dashed circle showing the off-center compact source (OCCS, Johnston2024) and two distinct regions of high dispersion, right: shows the $\rm |V|/\sigma$ with a high $\rm |V|/\sigma \sim 4$ region in corresponding to SE arc (see Figure \ref{['fig:malin1_ha_channel']}).
• Figure 3: $\rm H\alpha$ channel maps from -250 to 250 $\rm kms^{-1}$ with $\rm 50\ kms^{-1}$ bins consisting of a single MUSE wavelength slice (except the central -50 to 50 $\rm kms^{-1}$) showing different features. The top middle panel (-200, -150) shows the clump C1 and SE arc clearly. The middle right panel (50, 100) shows an extended feature called the NW clump. The bottom panel shows the OCCS clump with the highest contrast, although it is visible in most of the channel maps (-50, 250).
• Figure 4: Top Panel: Resolved gas metallicity (left) and the extinction, E(B-V) (right). With gas metallicity being $\gtrsim$ solar metallicity (12 + log(O/H) = 8.69) in most parts and supersolar, especially at the center. Bottom Panel: The left column shows the distribution of metallicity in the stellar population of different features identified in the central HSB disk, the right column shows the alpha-enrichment $[\alpha/Fe]$ for the same stellar population. The Bar minor axis sps is added for the comparison, where there is no FUV emission.
• Figure 5: plot showing the corner plots of the Monte Carlo simulation of age, metallicity [M/H], and alpha enrichment $[\rm \alpha/Fe]$ resulting from 6240 realisations of the clump spectra. The red shows the parameter distribution of old stellar populations ($\rm age > 1Gyr$) and the blue one represents the younger population.