The Atacama Cosmology Telescope: arcminute-resolution maps of 18,000 square degrees of the microwave sky from ACT 2008-2018 data combined with Planck

This paper presents a maximum-likelihood algorithm for combining sky maps
with disparate sky coverage, angular resolution and spatially varying anisotropic noise into
a single map of the sky. We use this to merge hundreds of individual maps covering the
2008–2018 ACT observing seasons, resulting in by far the deepest ACT maps released so
far. We also combine the maps with the full Planck maps, resulting in maps that have
the best features of both Planck and ACT: Planck’s nearly white noise on intermediate
and large angular scales and ACT’s high-resolution and sensitivity on small angular scales.
The maps cover over 18 000 square degrees, nearly half the full sky, at 100, 150 and 220
GHz. They reveal 4 000 optically-confirmed clusters through the Sunyaev Zel’dovich effect
(SZ) and 18 500 point source candidates at > 5σ, the largest single collection of SZ clusters
and millimeter wave sources to date. The multi-frequency maps provide millimeter images
of nearby galaxies and individual Milky Way nebulae, and even clear detections of several
nearby stars. Other anticipated uses of these maps include, for example, thermal SZ and
kinematic SZ cluster stacking, CMB cluster lensing and galactic dust science. The method
itself has negligible bias. However, due to the preliminary nature of some of the component
data sets, we caution that these maps should not be used for precision cosmological analysis.
The maps are part of ACT DR5, and will be made available on LAMBDA no later than three
months after the journal publication of this article, along with an interactive sky atlas.