Deep imaging of the galaxy Malin 2 shows new faint structures and a candidate satellite dwarf galaxy

Abstract

Giant Low Surface Brightness (GLSB) galaxies are extreme disk systems with exceptionally large sizes and low stellar densities. Their formation and evolution remain poorly constrained due to the challenges of detecting their faint disks. We present deep, multi-band optical imaging of Malin 2, a prototypical GLSB galaxy, with the newly commissioned Two-meter Twin Telescope (TTT) at the Teide Observatory. Our $g$, $r$, and $i$-band data reach surface brightness depths of 30.3, 29.5, and 28.2 mag arcsec$^{-2}$ (3$\sigma$, $10^{\prime\prime} \times 10^{\prime\prime}$), tracing the stellar disk of Malin 2 to $\sim$110 kpc. We detect new diffuse structures, including a prominent emission in the northwest coincident with the HI distribution, and a faint spiral arm-like feature in the southeast. We also identify a very faint dwarf galaxy, TTT-d1 ($\mu_{0,g} \sim 26$ mag arcsec$^{-2}$), about 130 kpc southeast of Malin 2, possibly its first known satellite ultra-diffuse galaxy. A multi-directional wedge photometric analysis of Malin 2 shows strong azimuthal variations in its stellar disk. Compared with nearby spirals and other GLSBs, Malin 2 lies at the extreme end in radial extent and stellar mass surface density. The overlap between the asymmetric stellar emission and a lopsided HI distribution suggests contributions from tidal interactions in the formation of the giant disk of Malin 2. Our results highlight the importance of ultra-deep, wide-field imaging in understanding the structural complexity of giant LSB galaxies. Upcoming surveys such as LSST will be crucial to determine whether the features we observe in Malin 2 are common to other giant LSB disk galaxies.