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

TL;DR

This study provides a primer on uniformly-calibrated VPHAS+ photometry in the southern Galactic plane's third quadrant, using Q3-bright data to evaluate the interim gri calibration against PS1, DECaPS-2, Skymapper, and IGAPS. By deriving area-averaged zeropoint corrections and applying synthetic photometry, the authors fix small gri offsets and substantial $u$ and $H\alpha$ scales, enabling robust cross-survey compatibility. They demonstrate practical applications, including extinction mapping with A stars and multi-class stellar selection (sub-luminous, luminous red, YSOs, emission-line, and OB stars) via cross-matches with Gaia DR3, SIMBAD, and other catalogs. The results reveal a largely uniform gri calibration across Q3 with residual systematics at the 0.01–0.02 mag level, identify a significant OB-star concentration around $(l,b) \approx (212^{\circ}, -0.6^{\circ})$, and illustrate how dust and Galactic warp shape stellar distributions, informing future full-VPHAS+ analyses.

Abstract

The southern Galactic plane has been mapped at optical wavelengths and at under one-arcsecond angular resolution by the VST Photometric Ha 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+ ugriHa photometry of r < 19 mag. point sources in the third Galactic quadrant (longitudes 210^o to 260^o). 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 Ha and u magnitude scales are determined via comparison with synthetic photometry. VPHAS+ and its northern counterpart, the INT Galactic Plane Survey (IGAPS), are shown to closely align, where they overlap across the celestial equator. Aided by Gaia Data Release 3, SIMBAD, and specialist catalogues, we present selections of: A stars; sub-luminous 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 - Ha) excess. It is argued the (r - i, r - Ha) plane is the better choice for this selection than (g - i, r - Ha). 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 ~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 (l,b) = (212.0, -0.6).

Figures (26)

• Figure 1: Section of the VPHAS+ footprint examined in this study, shown as a stellar density map. The grayscale applied is shaded according to the square root of the density. The darker areas pick out the parts of the region with lower stellar density, where there is a greater accumulation of extinction. The level of contrast present is of the order of a factor of 10. In the fourth and first quadrants of the Galactic plane, density variations are much more pronounced.
• Figure 2: Magnitude distributions in the $g$, $r$ and $i$ bands. The primary selection of point sources for inclusion in this study requires $r = 0.5(r_r + r_b) < 19.0$ mag., good point-source morphology, and little/no photometric impact from blending with neighbours in $g$, $r$ or $i$ (Q3-bright). The resultant $r$ AB-magnitude distribution is in solid red. The $g$ and $i$ magnitude distributions are outlined by the green and yellow lines, respectively. The solid grey distribution (on top of the red) is the $r$ distribution after imposing the further constraints that $g$, $r$ and $i$ pipeline errors are less than 0.01. The vertical scale is linear source counts.
• Figure 3: Uncertainty in $r$ magnitude over the range $13 < r < 19$ mag. It is presented in two ways: as pipeline-reported random error (distribution shown in a density grayscale, with the median in red), and as $|r_r - r_b|$ -- a more directly empirical measure that brings into account systematic effects as well. The latter is represented by the 68th and 95th percentiles of the distribution, respectively in black and lighter grey.
• Figure 4: Median pipeline errors for all bands as a function of magnitude. The curves are colour-coded as follows: NB659/H$\alpha$ -- magenta; $i$ -- deep yellow; red $r$ -- red; blue $r$ -- pink, $g$ -- green; $u$ -- cyan. For the $u$ band the full distribution of errors is shown as a grayscale density plot.
• Figure 5: Colour-colour $gri$ diagrams obtained from the VPHAS+, DECaPS-2, Pan-STARRS 1 and Skymapper DR4 survey catalogues, for the sky area $240^{\circ} < \ell < 245^{\circ}$, $-5^{\circ} < b < +5^{\circ}$. The distributions are coloured according to the logarithm of point density (low density, red; high density, violet). The linked black data points mark the AB colours of M dwarfs as determined from the Sloan Digital Sky Survey by West et al. (2011). The vertical dashed line at $r - i = 0.8$ is for reference against the $r - i$ axis. The VPHAS+, DECaPS-2 and PS1 datasets have been cleaned by requiring magnitude errors $< 0.01$ (leaving of order 1 million stars in each survey excerpt). For Skymapper, this error limit was relaxed to $< 0.02$ (641k stars).