HOPS + MALT90 + Hi-GAL: Probing star formation on a Galactic scale through mm molecular line and far-IR continuum Galactic plane surveys

Longmore, Steven N.; Rathborne, Jill; Bastian, Nate; Alves, Joao; Ascenso, Joana; Bally, John; Testi, Leonardo; Longmore, Andy; Battersby, Cara; Bressert, Eli; Purcell, Cormac; Walsh, Andrew; Jackson, James; Foster, Jonathan; Molinari, Sergio; Meingast, Stefan; Amorim, A.; Lima, J.; Marques, R.; Moitinho, A.; Pinhao, J.; Rebordao, J.; Santos, F. D.
Proceedings of the symposium "From Atoms to Pebbles: Herschel's view of Star and Planet Formation", held in Grenoble, France, March 20-23, Eds.: J.-C. Augereau (2012).


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 the rate of star formation per unit mass of dense gas in the CMZ may be 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 interpretating 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.