The INT photometric H-alpha survey of the northern galactic plane

The Isaac Newton Telescope (INT) Photometric Halpha Survey of the Northern Galactic Plane (IPHAS) is a 1800-deg 2 CCD survey of the northern Milky Way spanning the latitude range -5° < b < + 5° and reaching down to r'~= 20 (10sigma). Representative observations and an assessment of point-source data from IPHAS, now underway, are presented. The data obtained are Wide Field Camera images in the Halpha narrow-band, and Sloan r' and i' broad-band filters. We simulate IPHAS (r'-Halpha,r'-i') point-source colours using a spectrophotometric library of stellar spectra and available filter transmission profiles: this defines the expected colour properties of (i) solar metallicity stars, without Halpha emission, and (ii) emission-line stars. Comparisons with observations of fields in Aquila show that the simulations of normal star colours reproduce the observations well for all spectral types earlier than M. A further comparison between colours synthesized from long-slit flux-calibrated spectra and IPHAS photometry for six objects in a Taurus field confirms the reliability of the pipeline calibration. Spectroscopic follow-up of a field in Cepheus shows that sources lying above the main stellar locus in the (r'- Halpha,r'-i') plane are confirmed to be emission-line objects with very few failures. In this same field, examples of Halpha deficit objects (a white dwarf and a carbon star) are shown to be readily distinguished by their IPHAS colours. The role IPHAS can play in studies of spatially resolved northern Galactic nebulae is discussed briefly and illustrated by a continuum-subtracted mosaic image of Shajn 147 (a supernova remnant, 3° in diameter). The final catalogue of IPHAS point sources will contain photometry on about 80 million objects. Used on its own, or in combination with near-infrared photometric catalogues, IPHAS is a major resource for the study of stellar populations making up the disc of the Milky Way. The eventual yield of new northern emission-line objects from IPHAS is likely to be an order of magnitude increase on the number already known.