Euclid preparation: XVIII. Cosmic Dawn Survey. Spitzer observations of
  the Euclid deep fields and calibration fields
  We present a new infrared survey covering the three Euclid deep fields and
four other Euclid calibration fields using Spitzer's Infrared Array Camera
(IRAC). We have combined these new observations with all relevant IRAC archival
data of these fields in order to produce the deepest possible mosaics of these
regions. In total, these observations represent nearly 11% of the total Spitzer
mission time. The resulting mosaics cover a total of approximately 71.5deg$^2$
in the 3.6 and 4.5um bands, and approximately 21.8deg$^2$ in the 5.8 and 8um
bands. They reach at least 24 AB magnitude (measured to sigma, in a 2.5 arcsec
aperture) in the 3.6um band and up to ~ 5 mag deeper in the deepest regions.
The astrometry is tied to the Gaia astrometric reference system, and the
typical astrometric uncertainty for sources with 16<[3.6]<19 is <0.15 arcsec.
The photometric calibration is in excellent agreement with previous WISE
measurements. We have extracted source number counts from the 3.6um band
mosaics and they are in excellent agreement with previous measurements. Given
that the Spitzer Space Telescope has now been decommissioned these mosaics are
likely to be the definitive reduction of these IRAC data. This survey therefore
represents an essential first step in assembling multi-wavelength data on the
Euclid deep fields which are set to become some of the premier fields for
extragalactic astronomy in the 2020s.
