Population-based identification of H{\alpha}-excess sources in the Gaia DR2 and IPHAS catalogues

TL;DR

This paper introduces a new Gaia-based method to identify Hα-excess sources in the Northern Galactic Plane, resulting in a catalogue of over 28,000 candidates with high confidence levels, aiding stellar population studies.

Contribution

The study develops a novel technique combining Gaia astrometry and photometry with IPHAS data to reliably detect Hα-bright outliers, reducing biases from stellar population mixing and extinction.

Findings

Identified 28,496 Hα-excess candidates with >3 significance.

Achieved a purity of 81.9% using a 5σ conservative cut.

Completeness of detection is between 3% and 5%.

Abstract

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%.