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Cloud Structure of Galactic Ob Cluster-Forming Regions

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Cloud Structure of Galactic Ob Cluster-Forming Regions The Astrophysical Journal, 828:32 (50pp), 2016 September 1 doi:10.3847/0004-637X/828/1/32 © 2016. The American Astronomical Society. All rights reserved. CLOUD STRUCTURE OF GALACTIC OB CLUSTER-FORMING REGIONS FROM COMBINING GROUND- AND SPACE-BASED BOLOMETRIC OBSERVATIONS Yuxin Lin1, Hauyu Baobab Liu2,DiLi1,3, Zhi-Yu Zhang4,2, Adam Ginsburg2, Jaime E. Pineda5, Lei Qian1, Roberto Galván-Madrid6, Anna Faye McLeod2, Erik Rosolowsky7, James E. Dale8,9, Katharina Immer2, Eric Koch7, Steve Longmore10, Daniel Walker10, and Leonardo Testi2 1 National Astronomical Observatories, Chinese Academy of Sciences, China; [email protected] 2 European Southern Observatory (ESO), Karl-Schwarzschild-Str. 2, D-85748 Garching, Germany 3 Key Laboratory of Radio Astronomy, Chinese Academy of Sciences, China 4 Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK 5 Max-Planck-Institut für Extraterrestrische Physik, Gießenbach-str 1, D-85748, Garching bei München, Germany 6 Instituto de Radioastronomía y Astrofísica, UNAM, Apdo. Postal 3-72 (Xangari), 58089 Morelia, Michoacán, México 7 Department of Physics, University of Alberta, 4-181 CCIS, Edmonton, AB T6G 2E1, Canada 8 Universitäts–Sternwarte München, Scheinerstr. 1, D-81679 München, Germany 9 Excellence Cluster “Universe,” Boltzmannstr. 2, D-85748 Garching, Germany 10 Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF, UK Received 2016 March 31; revised 2016 June 22; accepted 2016 June 23; published 2016 August 25 ABSTRACT We have developed an iterative procedure to systematically combine the millimeter and submillimeter images of OB cluster-forming molecular clouds, which were taken by ground-based (CSO, JCMT, APEX, and IRAM-30 m) 6 and space telescopes (Herschel and Planck ). For the seven luminous (L > 10 Le) Galactic OB cluster-forming molecular clouds selected for our analyses, namely W49A, W43-Main, W43-South, W33, G10.6-0.4, G10.2-0.3, and G10.3-0.1, we have performed single-component, modified blackbody fits to each pixel of the combined (sub) millimeter images, and the Herschel PACS and SPIRE images at shorter wavelengths. The ∼10″ resolution dust column density and temperature maps of these sources revealed dramatically different morphologies, indicating very different modes of OB cluster-formation, or parent molecular cloud structures in different evolutionary stages. The molecular clouds W49A, W33, and G10.6-0.4 show centrally concentrated massive molecular clumps that are connected with approximately radially orientated molecular gas filaments. The W43-Main and W43-South molecular cloud complexes, which are located at the intersection of the Galactic near 3 kpc (or Scutum) arm and the Galactic bar, show a widely scattered distribution of dense molecular clumps/cores over the observed ∼10 pc spatial scale. The relatively evolved sources G10.2-0.3 and G10.3-0.1 appear to be affected by stellar feedback, and show a complicated cloud morphology embedded with abundant dense molecular clumps/cores. We find that with the high angular resolution we achieved, our visual classification of cloud morphology can be linked to the systematically derived statistical quantities (i.e., the enclosed mass profile, the column density probability distribution function (N-PDF), the two-point correlation function of column density, and the probability distribution function of clump/core separations). In particular, the massive molecular gas clumps located at the center of G10.6-0.4 and W49A, which contribute to a considerable fraction of their overall cloud masses, may be special OB cluster-forming environments as a direct consequence of global cloud collapse. These centralized massive molecular gas clumps also uniquely occupy much higher column densities than what is determined by the overall fit of power-law N-PDF. We have made efforts to archive the derived statistical quantities of individual target sources, to permit comparisons with theoretical frameworks, numerical simulations, and other observations in the future. Key words: ISM: structure – methods: statistical – stars: formation – submillimeter: ISM – techniques: image processing 1. INTRODUCTION and 160 μm (∼6″ and 12″) bands can help identify heating sources (normally young stars) in the molecular clouds The Herschel far-infrared and submillimeter imaging ( ) observations on the Galactic plane have significantly advanced Churchwell et al. 2009; Stutz et al. 2013 , which help our our knowledge about the morphology, as well as the understanding of their star formation history. Studies of the ( ) temperature and density distributions, of molecular clouds parent molecular cloud properties require observations at sub (André et al. 2010; Molinari et al. 2010), including those of OB millimeter wavelengths, which are typically more sensitive to cluster forming regions (Motte et al. 2010). Recent Planck the thermal emission of the 10–30 K interstellar medium from images at (sub)millimeter and millimeter wavelengths further the optically thin part of the spectrum. The quantitative revealed an extremely cold population of dense molecular analyses, including the spectral energy distribution (SED) cores, namely the Planck Cold Clumps (Planck Collaboration fitting, need to be adjusted to the angular resolution of the 2011, 2015). These observations have provided a large sample longest wavelength observations (e.g., 37″ for Herschel of star-forming molecular clumps/cores in a broad range of observations at 500 μm wavelength). For cold molecular evolutionary stages. In addition, the better angular resolution of clumps, which typically have spatial scales of ∼0.5 pc, the the Spitzer near-infrared (∼2″ at 3.6–8 μm) and Herschel 70 population will be resolved for molecular clouds within 1 The Astrophysical Journal, 828:32 (50pp), 2016 September 1 Lin et al. d∼3 kpc, such as the Rosette and Carina molecular clouds large-scale heating due to, e.g., illumination from the (e.g., Schneider et al. 2012; Rebolledo et al. 2016). However, embedded OB-stars or shocks caused by supernovae or the majority of the distant population, e.g., ∼17″ at d ~ 6 kpc cloud–cloud collisions. We also investigate the column density (Bergin & Tafalla 2007) represents the initial state of star distribution function, density profiles, and the spatial correla- formation in giant molecular clouds (GMCs) and is therefore tion of dense structures. These quantities are fundamental to easily missed in the derived smeared and poor angular our understanding of massive star cluster formation, since the resolution temperature and column density maps. The contrast structures of molecular clouds can reflect their initial condition of the localized heating sources can also be significantly and their potential subsequent evolution due to gravitational suppressed due to this spatial smearing. contraction (Vázquez-Semadeni et al. 1995, 2007, 2009; Lada Ground-based millimeter and submillimeter bolometric & Lada 2003; Li et al. 2016). Therefore, they may be linked to observations can probe long wavelength emission of cold the origin of different modes of star formation, and may clumps at high angular resolution compared to space telescopes additionally reveal signatures of interaction between newly (Schuller et al. 2009; Aguirre et al. 2011; Ginsburg et al. 2013; formed stars with their natal clouds. We update the identifica- Merello et al. 2015). However, ground-based observations are tion of dense cores/clumps in these regions, and derive the often subject to extended and strong atmospheric emission at mass, averaged temperature, and bolometric luminosity of these wavelengths, which is not always distinguishable from cores and clumps. Finally, we investigate the spatial distribu- / / the extended emission of the molecular clouds. The atmo- tion segregation of the cold and hot cores clumps, which may spheric foreground subtraction procedures may lead to give clues on the fragmentation processes of these molecular significant missing flux from the molecular cloud, which will clouds. bias the observed fluxes of bright sources, and may leave the Our observations and data analysis procedures are outlined adjacent fainter sources partially or fully immersed in regions in Section 2. Our results are provided in Section 3. Comparison of negative brightness, known as negative “bowls.” The details among the observed star-forming regions is given in Section 4. of how the extended structures will be missed in the ground- Our conclusion and ending remarks are in Section 5. based bolometric imaging will also depend on the sky condition during the observations, which cannot be easily predicted with accuracy. Therefore, it is not trivial to correctly 2. OBSERVATIONS fit the SED solely with the ground-based bolometric images for the longer wavelengths, and such difficulties may prevent the We provide a brief description of our target sources in Section 2.1. Details of our Caltech Submillimeter Observatory identification and quantitative studies of some faint structures. (CSO) Submillimetre High Angular Resolution Camera II Sadavoy et al. (2013) have proposed that artificially filtering (SHARC2) 350 μm observations and data reductions are given out the extended structures from both the ground-based (sub) in Section 2.2. Section 2.3 outlines the archival data we millimeter bolometric images and from the shorter wavelength included for the SED analysis. Our procedures for producing images taken by the space telescopes can alleviate the bias
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