A New Multi-Wavelength Census of Blazars
  Context:Blazars are the rarest and most powerful active galactic nuclei,
playing a crucial and growing role in today multi-frequency and multi-messenger
astrophysics. Current blazar catalogs, however, are incomplete and particularly
depleted at low Galactic latitudes. Aims: We aim at augmenting the current
blazar census to build a catalog of blazar candidates with homogeneous sky
coverage that can provide candidate counterparts to unassociated gamma-ray
sources, sources of high-energy neutrino emission, and ultra-high energy cosmic
rays. Methods: Starting from the ALMA Calibrator Catalog we built a catalog of
1580 blazar candidates (ALMA Blazar Candidates, ABC) for which we collect
multi-wavelength information. We also compared ABC sources with existing blazar
catalogs. Results: The ABC catalogue fills the lack of low Galactic latitude
sources in current blazar catalogues. ABC sources are significantly dimmer than
known blazars in Gaia g band, and they appear bluer in SDSS and WISE colors.
The majority of ABC sources (~ 90%) have optical spectra that classify them as
QSO, while the remaining sources resulted galactic objects. ABC sources are
similar in X-rays to known blazar, while in gamma-rays they are on average
dimmer and softer, indicating a significant contribution of FSRQ sources.
Making use of WISE colours, we classified 715 ABC sources as candidate
gamma-ray blazar of different classes. Conclusions: We built a new catalogue of
1580 candidate blazars with a rich multi-wavelength data-set, filling the lack
of low Galactic latitude sources in current blazar catalogues. This will be
particularly important to identify the source population of high energy
neutrinos or ultra-high energy cosmic rays. The data collected by the upcoming
LSST surveys will provide a key tool to investigate the possible blazar nature
of these sources.
