Understanding star formation in molecular clouds

II. Signatures of gravitational collapse of IRDCs^⋆

¹ Université Bordeaux, LAB, UMR 5804, 33270 Floirac, France
e-mail: nschneid@cea.fr
² CNRS, LAB, UMR 5804, 33270 Floirac, France
³ Max-Planck Institut für Radioastronomie, Auf dem Hügel, 53121 Bonn, Germany
⁴ Zentrum für Astronomie der Universität Heidelberg, Inst. für Theor. Astrophysik, Albert-Ueberle Str. 2, 69120 Heidelberg, Germany
⁵ Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95064, USA
⁶ Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA
⁷ Astrophysics Group, University of Exeter, EX4 4 QL Exeter, UK
⁸ Maison de la Simulation, CEA-CNRS-INRIA-UPS-UVSQ, USR 3441, CEA Saclay, 91191 Gif-sur-Yvette, France
⁹ I. Physikalisches Institut, Universität zu Köln, Zülpicher Straße 77, 50937 Köln, Germany
¹⁰ School of Physics and Astronomy, Cardiff University, Queens Buildings, Cardiff CF24 3AA, UK17
¹¹ Monash Centre for Astrophysics, School of Mathematical Sciences, Monash University, VIC 3800, Australia
¹² Research School of Astronomy&Astrophysics, The Australian National University, Canberra, ACT 2611, Australia

Received: 11 June 2014
Accepted: 11 March 2015

Abstract

We analyse column density and temperature maps derived from Herschel dust continuum observations of a sample of prominent, massive infrared dark clouds (IRDCs) i.e. G11.11-0.12, G18.82-0.28, G28.37+0.07, and G28.53-0.25. We disentangle the velocity structure of the clouds using ¹³CO 1→0 and ¹²CO 3→2 data, showing that these IRDCs are the densest regions in massive giant molecular clouds (GMCs) and not isolated features. The probability distribution function (PDF) of column densities for all clouds have a power-law distribution over all (high) column densities, regardless of the evolutionary stage of the cloud: G11.11-0.12, G18.82-0.28, and G28.37+0.07 contain (proto)-stars, while G28.53-0.25 shows no signs of star formation. This is in contrast to the purely log-normal PDFs reported for near and/or mid-IR extinction maps. We only find a log-normal distribution for lower column densities, if we perform PDFs of the column density maps of the whole GMC in which the IRDCs are embedded. By comparing the PDF slope and the radial column density profile of three of our clouds, we attribute the power law to the effect of large-scale gravitational collapse and to local free-fall collapse of pre- and protostellar cores for the highest column densities. A significant impact on the cloud properties from radiative feedback is unlikely because the clouds are mostly devoid of star formation. Independent from the PDF analysis, we find infall signatures in the spectral profiles of ¹²CO for G28.37+0.07 and G11.11-0.12, supporting the scenario of gravitational collapse. Our results are in line with earlier interpretations that see massive IRDCs as the densest regions within GMCs, which may be the progenitors of massive stars or clusters. At least some of the IRDCs are probably the same features as ridges (high column density regions with N> 10²³ cm^-2 over small areas), which were defined for nearby IR-bright GMCs. Because IRDCs are only confined to the densest (gravity dominated) cloud regions, the PDF constructed from this kind of a clipped image does not represent the (turbulence dominated) low column density regime of the cloud.