New identifications and multi-wavelength properties of extragalactic Fermi Gamma-Ray sources in the SPT-SZ survey field

The fourth Fermi Large Area Telescope (LAT) catalog (4FGL) contains 5064 gamma-ray sources detected at high significance, but 26% of them still lack associations at other wavelengths. Identifying these unknown gamma-ray sources is one of the major scientific goals in high-energy astronomy. The South Pole Telescope (SPT) has surveyed 2500 square degrees of the southern extragalactic sky in the millimeter (mm) band, and detected roughly 5000 point sources. In this study, we introduce a new cross-matching scheme to search for multi-wavelength counterparts of extragalactic gamma-ray sources using a mm catalog. We evaluate the rate of spurious false associations and compare the multi-wavelength associations from the radio, mm, near-infrared, and X-ray with gamma-ray sources in the 4FGL catalog. In the SPT-SZ survey fields, 85% of 4FGL sources are associated with mm counterparts. These mm sources include 94% of previously associated 4FGL sources and 56% of previously unassociated 4FGL sources. The latter group contains 40 4FGL sources for which SPT has provided the first identified counterparts. Nearly all of the SPT-associated 4FGL sources can be described as flat-spectrum active galactic nuclei (AGN) or blazars. We find that the mm band is the most efficient wavelength for detecting gamma-ray blazars when considering both completeness and purity. We also demonstrate that the mm-wave band correlates better to the gamma-ray band than the radio or X-ray bands. With the next-generation of CMB experiments this technique can be extended to greater sensitivities and more sky area to further complete the identifications of the remaining unknown gamma-ray blazars.