HERACLES: The HERA CO-Line Extragalactic Survey
  We present the HERA CO-Line Extragalactic Survey (HERACLES), an atlas of CO
emission from 18 nearby galaxies that are also part of The HI Nearby Galaxy
Survey (THINGS) and the Spitzer Infrared Nearby Galaxies Survey (SINGS). We
used the HERA multi-pixel receiver on the IRAM 30-m telescope to map the CO
J=2-1 line over the full optical disk (defined by the isophotal radius r_25) of
each target, at 13" angular resolution and 2.6 km/s velocity resolution. Here
we describe the observations and reduction of the data and show channel maps,
azimuthally averaged profiles, integrated intensity maps, and peak intensity
maps. The implied H2 masses range from 7 \times 10^6 to 6 \times 10^9 M_sun,
with four low metallicity dwarf irregular galaxies yielding only upper limits.
In the cases where CO is detected, the integrated H2-to-HI ratios range from
0.02 - 1.13 and H2-to-stellar mass ratios from 0.01 to 0.25. Exponential scale
lengths of the CO emission for our targets are in the range 0.8 - 3.2 kpc, or
0.2 \pm 0.05 r_25. The intensity-weighted mean velocity of CO matches that of
HI very well, with a 1\sigma scatter of only 6 km/s. The CO J=2-1 / J=1-0 line
ratio varies over a range similar to that found in the Milky Way and other
nearby galaxies, \sim 0.6 - 1.0, with higher values found in the centers of
galaxies. The typical line ratio, \sim 0.8, could be produced by optically
thick gas with an excitation temperature of \sim 10 K.
