Indermuehle, Balthasar; O'Dougherty, Stefan N.; Lowe,Vicki; Cunningham, Maria; Hernandez,Audra K.; Fuller, Gary A. Nguyễn, Hans; Nguyễn Lu'o'ng, Quang; Martin, Peter G.;Lo, Nadia; Motte, Frédérique

We present ThrUMMS - The Three-millimetre Ultimate Mopra Milkyway Survey - A complete survey of molecular mass within the Galactic Plane's 4th quadrant: 300-360 degrees longitude, and ±1 degree in latitude, simultaneously in four transitions: 12CO(1-0), 13CO(1-0), C18O(1-0), and CN(1-0). Obtained with the Mopra telescope in beam-sampling mode (~1 arcmin), these wide-field and high-resolution data (improving on extant published surveys by a factor of ~50 in SDR) are freely available for community use as processed or raw data. The product dataset samples approximately one third of the CO luminosity of our Galaxy, is already employed in recent studies of the Galactic molecular population, and is part of several international partner collaborations (using Nobeyama, APEX and JCMT, and in a different capacity; ALMA) designed to explore more fully the nature of molecular material within and throughout our Galaxy. A radiative transfer analysis applied to the ThrUMMS data already demonstrates the wide diversity of temperatures, opacities, and column densities of the Galactic molecular cloud population, and truly highlights regions of the Galaxy that are most massive/warm/opaque within the ensemble. Most interestingly though, we find the measured CO mass in our Galaxy should be corrected upwards by a factor of 2-3, an effect that seems to have gone unnoticed previously in lower resolution, single transition, and/or targeted surveys. Our new conversion law should find wide applicability in both Galactic and extragalactic studies, and suggests a much longer gas-depletion timescale in disk galaxies like the Milky Way, as well as radical revisions to Kennicutt-Schmidt type scaling laws in such disks.