Uncovering the formation of the counter-rotating stellar discs in SDSS J074834.64+444117.8

Abstract

ABSTRACT Using the integral field spectroscopic data from Mapping Nearby Galaxies at Apache Point Observatory survey, we study the kinematics and stellar population properties of the two counter-rotating stellar discs in a nearby galaxy SDSS J074834.64+444117.8. We disentangle the two stellar discs by three methods, including Ca ii λ8542 double Gaussian fit, penalized pixel fitting (pPXF) spectral decomposition, and orbit-based dynamical model. These three different methods give consistent stellar kinematics. The pPXF spectral decomposition provides the spectra of two stellar discs, with one being more luminous across the whole galaxy named primary disc, and the other named secondary disc. The primary disc is counter-rotating with ionized gas, while the secondary disc is co-rotating with ionized gas. The secondary disc has younger stellar population and poorer stellar metallicity than the primary disc. We estimate the stellar mass ratio between the primary and secondary discs to be ∼5.2. The DESI g, r, z colour image does not show any merger remnant feature in this galaxy. These findings support a scenario that the counter-rotating stellar discs in SDSS J074834.64+444117.8 formed through gas accretion from the cosmic web or a gas-rich companion.