HOPS + MALT90 + Hi-GAL: Probing star formation on a Galactic scale through mm molecular line and far-IR continuum Galactic plane surveys Steven Longmore ESO, Munich, Germany
With the HOPS and MALT90 Galactic plane surveys we are mapping a significant fraction of the dense molecular gas in the Galaxy in over 20 dense-gas-tracing transitions (e.g. from H2O, NH3, HC3N, HC5N, N2H+, HCN, HNC, HCO+, CH3CN, SiO, C2H, ...). Combining this with the far-IR continuum emission from Hi-GAL we can derive the physical/chemical/kinematic properties and evolutionary state of much of the molecular gas in the Galaxy destined to form stars. I will present results from three science projects based on this combined dataset, namely: i) looking for variations in the star formation rate across the Galaxy as a function of environment, in particular, comparing the CMZ with the rest of the Galactic disk -- we find evidence the rate of star formation per unit mass of dense gas in the CMZ may be at least an order of magnitude lower than that in the disk; ii) seeing if Galactic dense molecular clouds follow the empirical relations observed in extragalactic systems (e.g. the Kennicutt-Schmidt and Gao & Solomon relations) and what this implies for interpreting the extragalactic relations; iii) searching for molecular cloud progenitors of the most extreme (massive and dense) stellar clusters. I will present one cloud we have studied as part of project iii) which lies close to the Galactic center and which is clearly extreme compared to the rest of the Galactic molecular cloud population. With a mass of 10^5 Msun, a radius of only ~3pc and almost no signs of star formation it appears to be the progenitor of an Arches-like stellar cluster. As such, we speculate this molecular cloud may be a local-universe-analogue of the initial conditions of a super star cluster or potentially even a small globular cluster. From our Galactic plane survey data this object appears to be unique in the Galaxy, making it extremely important for testing massive cluster formation models. We have been awarded 6 hours of ALMA Cycle 0 observing time to study this object in detail and I hope to show preliminary results from this data at the meeting.
Caption: Radius versus mass for Galactic dense, cluster-forming molecular clouds. Plus symbols show ammonia clouds detected in HOPS (Walsh et al. 2011) (blue/brown denote an assumed near/far kinematic distance, respectively). Green crosses show infrared dark clouds (IRDCs) from the survey of Rathborne et al. (2006). The hatched rectangles show the mass-radius range of different stellar clusters (Portegies Zwart et al. 2010). The black dots show Galactic young massive clusters. With the exception of a few clouds which may form small YMCs, assuming a reasonable star formation effciency, most of the observed molecular clouds seem destined to form open clusters. G0:253 + 0:016 is marked with a red star and clearly stands out as unique. It has a mass and radius that would be expected of a molecular cloud progenitor of a large YMC or a globular cluster. The dashed lines show constant density and free-fall time.
Collaborators: Jill Rathborne, CASS, Sydney, Australia Nate Bastian, Excellence Cluster Universe, Garching, Germany Joao Alves, University of Vienna, Vienna, Austria Joana Ascenso, ESO, Garching, Germany John Bally, University of Colorado, Boulder, USA Leonardo Testi, ESO, Garching, Germany Andy Longmore, ITC (ROE), Edinburgh, UK Cara Battersby, University of Colorado, Boulder, USA Eli Bressert1, ESO, Garching, Germany Cormac Purcell, University of Sydney, Australia Andrew Walsh, James Cook University, Townesville, Australia James Jackson, Boston University, U.S.A Jonathan Foster, Boston University, U.S.A Sergio Molinari, INAF, Rome, Italy