Uniformly calibrated VPHAS+ photometry in the third quadrant of the Galactic plane

ABSTRACT The southern Galactic plane has been mapped at optical wavelengths and at under 1-arcsec angular resolution by the VST Photometric H$\alpha$ Survey of the Galactic plane and bulge (VPHAS+). Anticipating the release of a uniform photometric calibration of the entire survey, we examine the properties of VPHAS+ $ugri\mathrm{ H}\alpha$ photometry of $r < 19$ mag point sources in the third Galactic quadrant (longitudes $210^{\circ } \lesssim \ell < 260^{\circ }$). We compare our interim calibration in $gri$ with that of Pan-STARRS, the DECam Plane Survey (DECaPS-2) and Skymapper. We use the comparisons to identify small $gri$ photometric offsets. Corrections to the $\mathrm{ H}\alpha$ and u magnitude scales are determined via comparison with synthetic photometry. VPHAS+ and its northern counterpart, the Isaac Newton Telescope Galactic Plane Survey (IGAPS), are shown to closely align, where they overlap across the celestial equator. Aided by Gaia Data Release 3, the SIMBAD database, and specialist catalogues, we present selections of: A stars; subluminous stars; intrinsically red luminous stars; young stellar objects; emission-line and OB stars. Attention is drawn to stellar variability as a contaminant in selecting emission-line objects via $(r - \mathrm{ H}\alpha)$ excess. It is argued the $(r - i, r - \mathrm{ H}\alpha)$ plane is the better choice for this selection than $(g - i, r - \mathrm{ H}\alpha)$. Using A stars to map extinction, we trace the main run of dust obscuration, situated at mainly negative Galactic latitudes. Like the dust, OB and emission-line stars are more frequent below the Galactic equator: at heliocentric distances of up to $\sim$7 kpc, these stars’ distribution fit in with the known warping of the Galactic plane. An overdensity of B stars, several degrees across and potentially in the Outer Arm, is found around $(\ell ,b) = (212{{_{.}^{\circ}} }0,-0.6^{\circ })$.