Population-based identification of H{\alpha}-excess sources in the Gaia
  DR2 and IPHAS catalogues
  We present a catalogue of point-like H{\alpha}-excess sources in the Northern
Galactic Plane. Our catalogue is created using a new technique that leverages
astrometric and photomeric information from Gaia to select H{\alpha}-bright
outliers in the INT Photometric H{\alpha} Survey of the Northern Galactic Plane
(IPHAS), across the colour-absolute magnitude diagram. To mitigate the
selection biases due to stellar population mixing and to extinction, the
investigated objects are first partitioned with respect to their positions in
the Gaia colour-absolute magnitude space, and in the Galactic coordinates
space, respectively. The selection is then performed on both partition types
independently. Two significance parameters are assigned to each target, one for
each partition type. These represent a quantitative degree of confidence that
the given source is a reliable H{\alpha}-excess candidate, with reference to
the other objects in the corresponding partition. Our catalogue provides two
flags for each source, both indicating the significance level of the
H{\alpha}-excess. By analysing their intensity in the H{\alpha} narrow band,
28,496 objects out of 7,474,835 are identified as H{\alpha}-excess candidates
with a significance higher than 3. The completeness fraction of the H{\alpha}
outliers selection is between 3% and 5%. The suggested 5{\sigma} conservative
cut yields a purity fraction of 81.9%.
