The Galactic disk structure revealed by Gaia XP spectra

The spatial structures of different chemical populations in the Galactic disk provide important constraints on disk evolution and underlying physical processes. Gaia XP spectra provide one of the largest spectroscopic samples currently available, offering a new opportunity to investigate Galactic disk structure. We use Gaia XP spectra to study the structure of mono-abundance populations (MAPs) of the Galactic disk. In particular, we measure their scale heights as a function of radius as well as radial surface-mass-density profiles. We constructed a red giant branch (RGB) sample from Gaia XP spectra and divided it into MAPs according to α/M and M/H . The working sample covers a 6, region centered on the Sun in the Galactic plane and extends to |Z|<3.7, kpc kpc in the vertical direction. We estimated the XP completeness using a Gaia photometric parent sample. We then combined PARSEC isochrones and a Kroupa initial mass function (IMF) to calculate the selection function and convert the observed stellar counts into intrinsic stellar mass densities. The vertical density profiles were fitted with an exponential model to derive the scale heights, and the corresponding surface mass densities were obtained by integrating the fitted vertical profiles. The scale height and surface-mass-density profiles derived from Gaia XP are generally consistent with those obtained from APOGEE. All MAPs show flaring, with the high-α population exhibiting the strongest flaring and the metal-poor and sub-solar low-α populations showing the weakest flaring. The surface-mass-density profiles also differ significantly among MAPs. The high-α and metal-rich low-α populations show approximately exponential declines, whereas the other low-α populations exhibit plateau-like features, indicating that the Galactic disk cannot be described by a simple single-exponential profile. The differential structures of MAPs suggest a complex assembly history of the Galactic disk likely regulated by birth conditions, secular dynamical heating and radial migration, and external perturbations.