CoRGI in MaNGA - II. NGC 5717: cold-gas accretion and disk growth in a BGG

Large-scale stellar counter-rotation, in which two coplanar stellar components rotate in opposite directions, is usually interpreted as a signature of external gas accretion. Such systems are most often found in relatively sparse environments, where cold gas can be acquired and remain dynamically undisturbed long enough to settle into a disk. It is less clear whether this channel can operate in massive central galaxies, whose group or cluster environments can heat, strip, or perturb cold gas. Here we present NGC 5717, one of the most massive galaxies known to host a counter-rotating stellar disk and the first identified case in a brightest group galaxy. Using integral-field spectroscopy from SDSS-IV Mapping Nearby Galaxies at Apache Point Observatory (MaNGA), together with CFHT imaging, ancillary HI data, and SDSS/DESI spectroscopy of the surrounding group, we show that the galaxy contains a dynamically cold counter-rotating disk embedded in an old dynamically hot elliptical component. The disk is young (about 1.3 Gyr), metal-poor ([Fe/H] about-0.7 dex), extended (h about 2.4 kpc), and massive (M/M about 9.9); the associated ionized gas co-rotates with it, while the available HI data indicate no significant remaining reservoir of neutral atomic gas (M HI/M 9). These properties support an in situ origin of the secondary component from externally accreted gas. The chemical evolution model favors gas accretion from a cosmic filament, which is consistent with the surrounding cosmic-web geometry, and indicates highly efficient burst-like star formation with substantial metal-enriched outflows. NGC 5717 may therefore represent a rare example of disk growth in a massive central galaxy, enabled by external cold-gas accretion during an earlier phase of the group assembly history.