The Atacama Cosmology Telescope: DR5 maps of 18,000 square degrees of
  the microwave sky from ACT 2008-2018 data
  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 $> 5\sigma$, 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 are available on LAMBDA at
https://lambda.gsfc.nasa.gov/product/act/actpol_prod_table.cfm. There is also a
web atlas at https://phy-act1.princeton.edu/public/snaess/actpol/dr5/atlas.
