IGAPS: the merged IPHAS and UVEX optical surveys of the northern Galactic plane*

The INT Galactic Plane Survey (IGAPS) is the merger of the optical photometric surveys, IPHAS and UVEX, based on data from the Isaac Newton Telescope (INT) obtained between 2003 and 2018. Here, we present the IGAPS point source catalogue. It contains 295.4 million rows providing photometry in the filters, i, r, narrow-band H alpha, g, and U-RGO. The IGAPS footprint fills the Galactic coordinate range, |b| < 5 degrees and 30 degrees< l < 215 degrees. A uniform calibration, referred to as the Pan-STARRS system, is applied to g, r, and i, while the H alpha calibration is linked to r and then is reconciled via field overlaps. The astrometry in all five bands has been recalculated in the reference frame of Gaia Data Release 2. Down to i similar to 20 mag (Vega system), most stars are also detected in g, r, and H alpha. As exposures in the r band were obtained in both the IPHAS and UVEX surveys, typically a few years apart, the catalogue includes two distinct r measures, r(I) and r(U). The r 10 sigma limiting magnitude is approximately 21, with median seeing of 1.1 arcsec. Between approximately 13th and 19th mag in all bands, the photometry is internally reproducible to within 0.02 mag. Stars brighter than r=19.5 mag are tested for narrow-band H alpha excess signalling line emission, and for variation exceeding |r(I)-r(U)| = 0.2 mag. We find and flag 8292 candidate emission line stars and over 53 000 variables (both at > 5 sigma confidence).