Heracles: The HERA CO Line Extragalactic Survey

We present the Heterodyne Receiver Array CO Line Extragalactic Survey, an atlas of CO emission from 18 nearby galaxies that are also part of The H I Nearby Galaxy Survey and the Spitzer Infrared Nearby Galaxies Survey. We used the HERA multipixel 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–1 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 × 106 to 6 × 109 M , with four low metallicity dwarf irregular galaxies yielding only upper limits. In the cases where CO is detected, the integrated H2-to-H I ratios range from 0.02 to 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 ± 0.05r 25. The intensity-weighted mean velocity of CO matches that of H I very well, with a 1σ scatter of only 6 km s–1. 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, ~0.6-1.0, with higher values found in the centers of galaxies. The typical line ratio, ~0.8, could be produced by optically thick gas with an excitation temperature of ~10 K.