Shocks and Star Formation in Stephan's Quintet. I. Gemini Spectroscopy of Hα-bright Knots

Creators: Konstantopoulos, I. S.; Appleton, P. N.; Guillard, P.; Trancho, G.; Cluver, M. E.; Bastian, N.; Charlton, J. C.; Fedotov, K.; Gallaher, S. C.; Smith, L. J.; Struck, C. J.

Abstract

We present a Gemini-GMOS spectroscopic study of Hubble Space Telescope (HST)-selected Hα-emitting regions in Stephan's Quintet (HCG 92), a nearby compact galaxy group, with the aim of disentangling the processes of shock-induced heating and star formation in its intra-group medium. The ≈40 sources are distributed across the system, but most densely concentrated in the ~kiloparsec-long shock region. Their spectra neatly divide them into narrow- and broad-line emitters, and we decompose the latter into three or more emission peaks corresponding to spatial elements discernible in HST imaging. The emission-line ratios of the two populations of Hα-emitters confirm their nature as H II regions (90% of the sample) or molecular gas heated by a shock front propagating at ≾300 km s^(–1). Their redshift distribution reveals interesting three-dimensional structure with respect to gas-phase baryons, with no H II regions associated with shocked gas, no shocked regions in the intruder galaxy NGC 7318B, and a sharp boundary between shocks and star formation. We conclude that star formation is inhibited substantially, if not entirely, in the shock region. Attributing those H II regions projected against the shock to the intruder, we find a lopsided distribution of star formation in this galaxy, reminiscent of pileup regions in models of interacting galaxies. The Hα luminosities imply mass outputs, star formation rates, and efficiencies similar to nearby star-forming regions. Two large knots are an exception to this, being comparable in stellar output to the prolific 30 Doradus region. We also examine Stephan's Quintet in the context of compact galaxy group evolution, as a paradigm for intermittent star formation histories in the presence of a rich, X-ray-emitting intra-group medium. All spectra are provided as supplemental materials.

Additional Information

© 2014 American Astronomical Society. Received 2013 October 16; accepted 2013 November 27; published 2014 February 25. We thank the anonymous referee for helping us reinforce some of the analysis. Based on observations obtained at the Gemini Observatory (program ID GN-2010B-Q-56), which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the National Research Council (Canada), CONICYT (Chile), the Australian Research Council (Australia), Ministério da Ciência, Tecnologia e Inovação (Brazil), and Ministerio de Ciencia, Tecnología e Innovaciόn Productiva (Argentina). Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Science Institute. STScI is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract NAS 5-26555. I.S.K. acknowledges support from grant no. 1439092 from the Jet Propulsion Laboratory. I.S.K. is also grateful for the hospitality of the staff at the NASA Herschel Science Center, the Giant Magellan Telescope Organisation, and Carnegie Observatories, all in Pasadena, CA, where much of this work was undertaken. S.C.G. and K.F. thank the Canadian Natural Science and Engineering Research Council and the Ontario Early Researcher Award Program for support.

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