MUSCLE W49: A MULTI-SCALE CONTINUUM AND LINE EXPLORATION OF THE MOST LUMINOUS STAR FORMATION REGION IN THE MILKY WAY. I. DATA AND THE MASS STRUCTURE OF THE GIANT MOLECULAR CLOUD

The Multi-scale Continuum and Line Exploration of W49 is a comprehensive gas and dust survey of the giant molecular cloud (GMC) of W49A, the most luminous star-formation region in the Milky Way. The project covers, for the first time, the entire GMC at different scales and angular resolutions. In this paper, we present (1) an all-configuration Submillimeter Array mosaic in the 230 GHz (1.3 mm) band covering the central similar to 3' x 3' (similar to 10 pc, known as W49N), where most of the embedded massive stars reside and (2) Purple Mountain Observatory 14 m telescope observations in the 90 GHz band, covering the entire GMC with maps of up to similar to 35' x 35' in size, or similar to 113 pc. We also make use of archival data from the Very Large Array, JCMT-SCUBA, the IRAM 30 m telescope, and the Caltech Submillimeter Observatory BOLOCAM Galactic Plane Survey. We derive the basic physical parameters of the GMC at all scales. Our main findings are as follows. (1) The W49 GMC is one of the most massive in the Galaxy, with a total mass M-gas similar to 1.1 x 10(6) M-circle dot within a radius of 60 pc. Within a radius of 6 pc, the total gas mass is M-gas similar to 2 x 10(5) M-circle dot. At these scales, only similar to 1% of the material is photoionized. The mass reservoir is sufficient to form several young massive clusters (YMCs) as massive as a globular cluster. (2) The mass of the GMC is distributed in a hierarchical network of filaments. At scales <10 pc, a triple, centrally condensed structure peaks toward the ring of HC H II regions in W49N. This structure extends to scales from similar to 10 to 100 pc through filaments that radially converge toward W49N and its less-prominent neighbor W49S. The W49A starburst most likely formed from global gravitational contraction with localized collapse in a hub-filament geometry. (3) Currently, feedback from the central YMCs (with a present mass M-cl greater than or similar to 5 x 10(4) M-circle dot) is still not enough to entirely disrupt the GMC, but further stellar mass growth could be enough to allow radiation pressure to clear the cloud and halt star formation. (4) The resulting stellar content will probably remain as a gravitationally bound massive star cluster or a small system of bound clusters.