Confirming the Calibration of ALMA Using Planck Observations

Abstract

We test the accuracy of ALMA flux density calibration by comparing ALMA flux density measurements of extragalactic sources to measurements made by the Planck mission; Planck is absolutely calibrated to subpercent precision using the dipole signal induced by the satellite's orbit around the solar system barycenter. Planck observations ended before ALMA began systematic observations, however, and many of the sources are variable, so we employ measurements by the Atacama Cosmology Telescope (ACT) to bridge the two epochs. We compare ACT observations at 93 and ∼145 GHz to Planck measurements at 100 and 143 GHz and to ALMA measurements made at 91.5 and 103.5 GHz in Band 3. For both comparisons, flux density measurements were corrected to account for the small differences in frequency using the best available spectral index for each source. We find the ALMA flux density scale (based on observations of Uranus) is consistent with Planck. All methods used to make the comparison are consistent with ALMA flux densities in Band 3 averaging 0.99 times those measured by Planck. One specific test gives ALMA/Planck = 0.996 ± 0.024. We also test the absolute calibration of both ACT at 93 and ∼145 GHz and the South Pole Telescope (SPT) at 97.43, 152.9, and 215.8 GHz, again with reference to Planck measurements at 100, 143, and 217 GHz, as well as the internal consistency of measurements of compact sources made by all three instruments. © 2021. The American Astronomical Society. All rights reserved.