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A High-dispersion Molecular Gas Component in Nearby Galaxies

Caldú-Primo, Anahi and Schruba, Andreas and Walter, Fabian and Leroy, Adam and Sandstrom, Karin and de Blok, W. J. G. and Ianjamasimanana, R. and Mogotsi, K. M. (2013) A High-dispersion Molecular Gas Component in Nearby Galaxies. Astronomical Journal, 146 (6). Art. No. 150. ISSN 0004-6256. doi:10.1088/0004-6256/146/6/150.

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We present a comprehensive study of the velocity dispersion of the atomic (HI) and molecular (H2) gas components in the disks (R≾R_(25)) of a sample of 12 nearby spiral galaxies with moderate inclinations. Our analysis is based on sensitive high-resolution data from the THINGS (atomic gas) and HERACLES (molecular gas) surveys. To obtain reliable measurements of the velocity dispersion, we stack regions several kiloparsecs in size, after accounting for intrinsic velocity shifts due to galactic rotation and large-scale motions. We stack using various parameters: the galactocentric distance, star formation rate surface density, HI surface density, H_2 surface density, and total gas surface density. We fit single Gaussian components to the stacked spectra and measure median velocity dispersions for HI of 11.9 ± 3.1 km s^(–1) and for CO of 12.0 ± 3.9 km s^(–1). The CO velocity dispersions are thus, surprisingly, very similar to the corresponding ones of HI, with an average ratio of σ_(HI)/σ_(CO)= 1.0 ± 0.2 irrespective of the stacking parameter. The measured CO velocity dispersions are significantly higher (factor of ~2) than the traditional picture of a cold molecular gas disk associated with star formation. The high dispersion implies an additional thick molecular gas disk (possibly as thick as the H I disk). Our finding is in agreement with recent sensitive measurements in individual edge-on and face-on galaxies and points toward the general existence of a thick disk of molecular gas, in addition to the well-known thin disk in nearby spiral galaxies.

Item Type:Article
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URLURL TypeDescription Paper
Walter, Fabian0000-0003-4793-7880
Leroy, Adam0000-0002-2545-1700
Sandstrom, Karin0000-0002-4378-8534
Additional Information:© 2013 American Astronomical Society. Received 2013 March 24; accepted 2013 September 16; published 2013 November 7. We thank the anonymous referee for very favorable and thoughtful comments. A.C.P. acknowledges support from the DFG priority program 1573, "The physics of the interstellar medium," from the IMPRS for Astronomy & Cosmic Physics at the University of Heidelberg, and from the Mexican National Council for Science and Technology (CONACYT). A.S. and F.B. thank Mark Krumholz for a stimulating discussion on the implications for models of H_2 formation.
Funding AgencyGrant Number
Deutsche Forschungsgemeinschaft (DFG)1573
University of Heidelberg IMPRS for Astronomy & Cosmic PhysicsUNSPECIFIED
Consejo Nacional de Ciencia y Tecnología (CONACYT)UNSPECIFIED
Subject Keywords: galaxies: ISM; ISM: molecules; radio lines: galaxies
Issue or Number:6
Record Number:CaltechAUTHORS:20140110-105132281
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Official Citation:A High-dispersion Molecular Gas Component in Nearby Galaxies Anahi Caldú-Primo et al. 2013 The Astronomical Journal 146 150
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:43310
Deposited By: Ruth Sustaita
Deposited On:10 Jan 2014 19:33
Last Modified:10 Nov 2021 16:36

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