XP-TEAL: Gaia XP Tool for Emission and Absorption Lines II. Gaia-HELIX catalogue of H alpha emitters in Gaia BP/RP spectra

Context: With Gaia's third Data Release (DR3), low-resolution BP/RP (XP) spectra became available for more than 219 million sources. In previous work, we developed a fast method for detecting absorption and emission lines while measuring their equivalent widths directly from the XP spectral coefficients. Aims: Our aim is to conduct a systematic search for H alpha emitters in the Gaia DR3 XP spectra. Methods: We measured the equivalent width of the Balmer H alpha line for all sources with Gaia XP spectra, and selected an initial sample of 556100 sources depicting an H alpha emission-like feature at 2 sigma level or above. Using a semi-supervised classifier complemented by manual spectral labelling, we removed sources that merely masqueraded as emission-line stars. Results: We find that 95% of the initial candidates are most likely cool M-type stars, where a molecular absorption band gives rise to a local maximum that can mimic H alpha emission. Subsequently, 28394 sources are flagged as bona-fide H alpha emitters using the Gaia DR3 XP spectra. The majority are Be-like candidates (26287), active M-type stars (717), Herbig Ae/Be candidates (525), quasi-stellar objects (204), Wolf-Rayet stars (177), cataclysmic variables (51), and carbon stars (18), among other sources. Our equivalent widths and stellar classifications are in good agreement with results from higher-resolution studies and recent Gaia XP spectra-based catalogues. Ground-based spectroscopic follow-up for some of the newly found emitters reinforces our confidence in the catalogue.