Evidence of boosted ^(13)CO/^(12)CO ratio in early-type galaxies in dense environments

Abstract

We present observations of ^(13)CO(1-0) in 17 Combined Array for Research in Millimeter Astronomy (CARMA) Atlas^(3D) early-type galaxies (ETGs), obtained simultaneously with ^(12)CO(1-0) observations. The ^(13)CO in six ETGs is sufficiently bright to create images. In these 6 sources, we do not detect any significant radial gradient in the ^(13)CO/^(12)CO ratio between the nucleus and the outlying molecular gas. Using the ^(12)CO channel maps as 3D masks to stack the ^(13)CO emission, we are able to detect 15/17 galaxies to >3σ (and 12/17 to at least 5σ) significance in a spatially integrated manner. Overall, ETGs show a wide distribution of ^(13)CO/^(12)CO ratios, but Virgo cluster and group galaxies preferentially show a ^(13)CO/^(12)CO ratio about 2 times larger than field galaxies, although this could also be due to a mass dependence, or the CO spatial extent (R_(CO)/R_e). ETGs whose gas has a morphologically-settled appearance also show boosted ^(13)CO/^(12)CO ratios. We hypothesize that this variation could be caused by (i) the extra enrichment of gas from molecular reprocessing occurring in low-mass stars (boosting the abundance of ^(13)C to ^(12)C in the absence of external gas accretion), (ii) much higher pressure being exerted on the midplane gas (by the intracluster medium) in the cluster environment than in isolated galaxies, or (iii) all but the densest molecular gas clumps being stripped as the galaxies fall into the cluster. Further observations of ^(13)CO in dense environments, particularly of spirals, as well as studies of other isotopologues, should be able to distinguish between these hypotheses.

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© 2015 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. Accepted 2015 April 2. Received 2015 March 27. In original form 2014 October 8. First published online May 19, 2015. KA thanks Erik Rosolowsky, Mark Lacy, Francoise Combes, Adam Leroy, Asunción Fuente and Jeff Kenney for insightful conversations that have improved this work, Theodoros Bitsakis for Herschel-related questions and Selcuk Topal for a detailed investigation of the single-dish data from NGC 5866. KA would also like to thank the anonymous referee for insightful comments and thoughtful recommendations. KA is supported by funding through Herschel, a European Space Agency Cornerstone Mission with significant participation by National Aeronautics and Space Administration (NASA), through an award issued by JPL/Caltech. MB is supported by the rolling grants 'Astrophysics at Oxford' PP/E001114/1 and ST/H002456/1 from the UK Research Councils. LMY is supported by the National Science Foundation under Grant No. NSF AST-1109803. Support for CARMA construction was derived from the Gordon and Betty Moore Foundation, the Kenneth T. and Eileen L. Norris Foundation, the James S. McDonnell Foundation, the Associates of the California Institute of Technology, the University of Chicago, the states of California, Illinois and Maryland, and the National Science Foundation. Ongoing CARMA development and operations are supported by the National Science Foundation under a cooperative agreement, and by the CARMA partner universities. This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the NASA.

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Submitted - 1504.02095v2.pdf

Published - MNRAS-2015-Alatalo-3874-85.pdf

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