Flare in the Galactic stellar outer disc detected in SDSS-SEGUE data

Abstract

Aims. We explore the outer Galactic disc up to a Galactocentric distance of 30 kpc to derive its parameters and measure the magnitude of its flare. Methods. We obtained the 3D density of stars of type F8V-G5V with a colour selection from extinction-corrected photometric data of the Sloan Digital Sky Survey - Sloan Extension for Galactic Understanding and Exploration (SDSS-SEGUE) over 1,400 deg^2 in off-plane low Galactic latitude regions and fitted it to a model of flared thin+thick disc. Results. The best-fit parameters are a thin-disc scale length of 2.0 kpc, a thin-disc scale height at solar Galactocentric distance of 0.24 kpc, a thick-disc scale length of 2.5 kpc, and a thick-disc scale height at solar Galactocentric distance of 0.71 kpc. We derive a flaring in both discs that causes the scale height of the average disc to be multiplied with respect to the solar neighbourhood value by a factor of 3.3^{+2.2}_{-1.6} at R=15 kpc and by a factor of 12^{+20}_{-7} at R=25 kpc. Conclusions. The flare is quite prominent at large R and its presence explains the apparent depletion of in-plane stars that are often confused with a cut-off at R>15 kpc. Indeed, our Galactic disc does not present a truncation or abrupt fall-off there, but the stars are spread in off-plane regions, even at z of several kpc for R>20 kpc. Moreover, the smoothness of the observed stellar distribution also suggests that there is a continuous structure and not a combination of a Galactic disc plus some other substructure or extragalactic component: the hypothesis to interpret the Monoceros ring in terms of a tidal stream of a putative accreted dwarf galaxy is not only unnecessary because the observed flare explains the overdensity in the Monoceros ring observed in SDSS fields, but it appears to be inappropriate.