Star formation in the extended gaseous disk of the isolated galaxy CIG 96

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We study the Kennicutt-Schmidt star formation law and efficiency in the gaseous disk of the isolated galaxy CIG 96 (NGC 864), with special emphasis on its unusually large atomic gas (H I) disk (r_ H mathsci/r_ 25 = 3.5, r_ 25 = 1 85). We present deep Galaxy Evolution Explorer near- and far-UV observations, used as a recent star formation tracer, and we compare them with new, high-resolution (16''or 1.6 kpc) Very Large Array H I observations. The UV and H I maps show good spatial correlation outside the inner 1', where the H I phase dominates over H_2. Star-forming regions in the extended gaseous disk are mainly located along the enhanced H I emission within two (relatively) symmetric, giant gaseous spiral arm-like features, which emulate an H I pseudo-ring at r≃3'. Inside this structure, two smaller gaseous spiral arms extend from the northeast and southwest of the optical disk and connect to the previously mentioned H I pseudo-ring. Interestingly, we find that the (atomic) Kennicutt-Schmidt power-law index systematically decreases with radius, from N ≃3.0 ± 0.3 in the inner disk (0 8-1 7) to N = 1.6 ± 0.5 in the outskirts of the gaseous disk (3 3-4 2). Although the star formation efficiency (SFE), the star formation rate per unit of gas, decreases with radius where the H I component dominates as is common in galaxies, we find that there is a break of the correlation at r = 1.5r_25. At radii 1.5r_ 25 < r < 3.5r_25, mostly within the H I pseudo-ring structure, regions exist whose SFE remains nearly constant, SFE ≃10^–11 yr^–1. We discuss possible mechanisms that might be triggering the star formation in the outskirts of this galaxy, and we suggest that the constant SFE for such large radii (r > 2r_25) and at such low surface densities might be a common characteristic in extended UV disk galaxies.
© 2011. The American Astronomical Society. All rights reserved. We thank the anonymous referee for a careful reading and very detailed report, which helped to improve this paper significantly. D. E., A.G.d.P., and L. V.-M. acknowledge support for this work from the GALEX Guest Investigator program under NASA grant NNX09AQ356. D. E. was supported by a Marie Curie International Fellowship within the Sixth European Community Framework Programme (MOIF-CT-2006-40298). D. E., J.S., and L. V.-M. were partially supported by the DGI Grant AYA2008-06181-C02 and the Junta de Andalucía (Spain) P08-FQM-4205. S. V. was partially supported by Junta de Andalucía Grant FQM108, Spanish MEC Grant AYA-2007-67625-C02-02, and a Juan de la Cierva fellowship. A.G.d.P. and J.C.M.-M. are partially financed by the Spanish Programa Nacional de Astronomía y Astrofísica under grants AyA2006-02358 and AyA2009-10368 and the Consolider-GTC project. A.G.d.P. is also financed by the Spanish Ramón y Cajal program. J.C.M.-M. acknowledges financial support from NASA JPL/Spitzer grant RSA 1374189; he also acknowledges support from the National Radio Astronomy Observatory, which is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. GALEX is operated for NASA by the California Institute of Technology under NASA contract NAS5-98034. The Spitzer Space Telescope is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. Funding for the SDSS and SDSS-II was provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, the U.S. Department of Energy, the National Aeronautics and Space Administration, the Japanese Monbukagakusho, the Max Planck Society, and the Higher Education Funding Council for England. This research has made use of the NASA/IPAC Extragalactic Database, which is operated by JPL/Caltech, under contract with NASA. We acknowledge the usage of the HyperLeda database (
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