After the interaction: an efficiently star-forming molecular disk in NGC 5195

Creators: Alatalo, Katherine; Aladro, Rebeca; Nyland, Kristina; Aalto, Susanne; Bitsakis, Theodoros; Gallagher, John S.; Lanz, Lauranne

Abstract

We present new molecular gas maps of NGC 5195 (alternatively known as M51b) from the Combined Array for Research in Millimeter Astronomy, including ^(12)CO(1–0), ^(13)CO(1–0), CN(1_(0,2)–0_(0,1)), CS(2–1), and 3 mm continuum. We also detected HCN(1–0) and HCO^+(1–0) using the Onsala Space Observatory. NGC 5195 has a ^(12)CO/^(13)CO ratio (R_(12/13) = 11.4 ± 0.5) consistent with normal star-forming galaxies. The CN(1–0) intensity is higher than is seen in an average star-forming galaxy, possibly enhanced in the diffuse gas in photo-dissociation regions. Stellar template fitting of the nuclear spectrum of NGC 5195 shows two stellar populations: an 80% mass fraction of old (≳10 Gyr) and a 20% mass fraction of intermediate-aged (≈1 Gyr) stellar populations. This provides a constraint on the timescale over which NGC 5195 experienced enhanced star formation during its interaction with M51a. The average molecular gas depletion timescale in NGC 5195 is τ_(dep) = 3.08 Gyr, a factor of ≈2 larger than the depletion timescales in nearby star-forming galaxies, but consistent with the depletion seen in CO-detected early-type galaxies. While radio continuum emission at centimeter and millimeter wavelengths is present in the vicinity of the nucleus of NGC 5195, we find it is most likely associated with nuclear star formation rather than radio-loud AGN activity. Thus, despite having a substantial interaction with M51a ~1/2 Gyr ago, the molecular gas in NGC 5195 has resettled and is currently forming stars at an efficiency consistent with settled early-type galaxies.

Additional Information

© 2016 American Astronomical Society. Received 2016 April 27; revised 2016 July 22; accepted 2016 August 3; published 2016 October 18. The authors thank Dr Henrik Olofsson for carrying out the observations with the 20 m telescope for them. KA thanks the anonymous referee for an insightful report that has markedly improved the manuscript, and M. Daprà for gildas advice. Support for KA is provided by NASA through the Hubble Fellowship Grant #HST-HF2-51352.001 awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS5-26555. SA acknowledges support from the Swedish National Science Council Grant 621-2011-4143. LL acknowledges support for this work provided by NASA through an award issued by JPL/Caltech. KN acknowledges support from NASA through the Spitzer Space Telescope. 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. The 20 m telescope is operated by Onsala Space Observatory (OSO), the Swedish National Facility for Radio Astronomy. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. 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 National Aeronautics and Space Administration. This work is based [in part] on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA. Funding for the Sloan Digital Sky Survey IV has been provided by the Alfred P. Sloan Foundation, the US Department of Energy Office of Science, and the Participating Institutions. SDSS-IV acknowledges support and resources from the Center for High-Performance Computing at the University of Utah. The SDSS web site is www.sdss.org. Facilities: CARMA - Combined Array for Research in Millimeter-Wave Astronomy, Herschel - , OSO: 20m - , SDSS - , Spitzer. -

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