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Clumping and the Interpretation of kpc-scale Maps of the Interstellar Medium: Smooth H I and Clumpy, Variable H_2 Surface Density

Leroy, Adam K. and Lee, Cheoljong and Schruba, Andreas and Bolatto, Alberto and Hughes, Annie and Pety, Jérôme and Sandstrom, Karin and Schinnerer, Eva and Walter, Fabian (2013) Clumping and the Interpretation of kpc-scale Maps of the Interstellar Medium: Smooth H I and Clumpy, Variable H_2 Surface Density. Astrophysical Journal Letters, 769 (1). Art. No. L12. ISSN 2041-8205. doi:10.1088/2041-8205/769/1/L12.

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Many recent models consider the structure of individual interstellar medium (ISM) clouds as a way to explain observations of large parts of galaxies. To compare such models to observations, one must understand how to translate between surface densities observed averaging over large (~kpc) scales and surface densities on the scale of individual clouds (~pc scale), which are treated by models. We define a "clumping factor" that captures this translation as the ratio of the mass-weighted surface density, which is often the quantity of physical interest, to the area-weighted surface density, which is observed. We use high spatial resolution (sub-kpc) maps of CO and H I emission from nearby galaxies to measure the clumping factor of both atomic and molecular gas. The molecular and atomic ISM exhibit dramatically different degrees of clumping. As a result, the ratio H_2/H I measured at ~kpc resolution cannot be trivially interpreted as a cloud-scale ratio of surface densities. H I emission appears very smooth, with a clumping factor of only ~1.3. Based on the scarce and heterogeneous high-resolution data available, CO emission is far more clumped with a widely variable clumping factor, median ~7 for our heterogeneous data. Our measurements do not provide evidence for a universal mass-weighted surface density of molecular gas, but also cannot conclusively rule out such a scenario. We suggest that a more sophisticated treatment of molecular ISM structure, one informed by high spatial resolution CO maps, is needed to link cloud-scale models to kpc-scale observations of galaxies.

Item Type:Article
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URLURL TypeDescription DOIArticle
Leroy, Adam K.0000-0002-2545-1700
Bolatto, Alberto0000-0002-5480-5686
Sandstrom, Karin0000-0002-4378-8534
Schinnerer, Eva0000-0002-3933-7677
Walter, Fabian0000-0003-4793-7880
Additional Information:© 2013 The American Astronomical Society. Received 2013 February 14; accepted 2013 March 25; published 2013 May 7. We thank the referee for a constructive report and Scott Schnee and Mark Krumholz for feedback on drafts. We acknowledge the BIMA SONG, LITTLE THINGS, and VLA ANGST collaborations for making their data public. We thank IRAM for making the moment 0 map of M31 public. This Letter makes use of the following ALMA data: ADS/JAO.ALMA#2011.0.00003.SV. ALMA is a partnership of ESO (representing its member states), NSF (USA), and NINS (Japan), together with NRC (Canada) and NSC and ASIAA (Taiwan), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO, and NAOJ. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. A.B. acknowledges partial support from grants NSF AST-0838178, NSF AST-0955836, and a Cottrell Scholar award from the Research Corporation for Science Advancement. K.S. is supported by a Marie Curie International Incoming fellowship. A.H. acknowledges funding from the Deutsche Forschungsgemeinschaft via grants SCHI 536/5-1 and SCHI 536/7-1 as part of the priority program SPP 1573 “ISM-SPP: Physics of the Interstellar Medium.” J.P. was partially funded by the grant ANR-09-BLAN-0231-01 from the French Agence Nationale de la Recherche as part of the SCHISM project.
Group:Infrared Processing and Analysis Center (IPAC)
Funding AgencyGrant Number
Cottrell Scholar of Research CorporationUNSPECIFIED
Marie Curie FellowshipUNSPECIFIED
Deutsche Forschungsgemeinschaft (DFG)SCHI 536/5-1
Deutsche Forschungsgemeinschaft (DFG)SCHI 536/7-1
Deutsche Forschungsgemeinschaft (DFG)SPP 1573
Agence Nationale de la Recherche (ANR)ANR-09-BLAN-0231-01
Subject Keywords:galaxies: ISM; ISM: structure; stars: formation
Issue or Number:1
Record Number:CaltechAUTHORS:20130716-110942499
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Official Citation:Clumping and the Interpretation of kpc-scale Maps of the Interstellar Medium: Smooth H I and Clumpy, Variable H2 Surface Density Adam K. Leroy et al. 2013 ApJ 769 L12
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:39390
Deposited By: Tony Diaz
Deposited On:17 Jul 2013 22:01
Last Modified:09 Nov 2021 23:44

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