Le Floc'h, Emeric and Charmandaris, Vassilis and Gordon, Karl and Forrest, William J. and Brandl, Bernhard and Schaerer, Daniel and Dessauges-Zavadsky, Miroslava and Armus, Lee (2012) The First Infrared Study of the Close Environment of a Long Gamma-Ray Burst. Astrophysical Journal, 746 (1). Art. No. 7. ISSN 0004-637X http://resolver.caltech.edu/CaltechAUTHORS:20120524-073829775
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We present a characterization of the close environment of GRB 980425 based on 5-160 μm spectro-imaging obtained with Spitzer. The gamma-ray burst GRB 980425 occurred in a nearby (z = 0.0085) SBc-type dwarf galaxy at a projected distance of 900 pc from an H II region with strong signatures of Wolf-Rayet (WR) stars. While this "WR region" produces less than 5% of the B-band emission of the host, we find that it is responsible for 45% ± 10% of the total infrared luminosity, with a maximum contribution reaching 75% at 25-30 μm. This atypical property is rarely observed among morphologically relaxed dwarfs, suggesting a strong causal link with the gamma-ray burst (GRB) event. The luminosity of the WR region (L_(8-1000 μm) = 4.6 × 10^8 L_☉), the peak of its spectral energy distribution at ≾100 μm, and the presence of highly ionized emission lines (e.g., [Ne III]) also reveal extremely young (<5 Myr) star-forming activity, with a typical timescale of only 47 Myr to double the stellar mass already built. Finally, the mid-IR over B-band luminosity ratio in this region is substantially higher than in star-forming galaxies with similar L_(IR), but it is lower than in young dust-enshrouded stellar clusters. Considering the modest obscuration measured from the silicate features (τ_(9.7 μm) ~ 0.015), this suggests that the WR region is dominated by one or several star clusters that have either partly escaped or cleared out their parent molecular cloud. Combined with the properties characterizing the whole population of GRB hosts, our results reinforce the idea that long GRBs mostly happen within or in the vicinity of relatively unobscured galactic regions harboring very recent star formation.
|Additional Information:||© 2012 American Astronomical Society. Received 2011 August 24; accepted 2011 November 3; published 2012 January 18. Based on observations made with Spitzer, operated by the Jet Propulsion Laboratory under NASA contract 1407. This work was enabled based on the funding from the IRS and the MIPS projects which are supported by NASA through the Jet Propulsion Laboratory (subcontracts 1257184 and 960785), and thanks to the efficient technical support provided by the Spitzer Science Center. We thank our referee for relevant suggestions and a careful review of the manuscript as well as Michal Michałowski for useful comments on our work. We are particularly grateful to Vianney Lebouteiller for fruitful discussions and for providing us with updated IRS data reduction prior to the publication of the Cornell AtlaS of Spitzer IRS Sources (CASSIS; Lebouteiller et al. 2011a) and to Ho Seong Hwang for sharing his results on the SDSS data. We also acknowledge Yanling Wu for her help in the IRS data reduction as well as Jason Marshall for providing some of his IR SED modeling and J.D. Smith for making publicly available his PAHFIT routines. We greatly appreciated the help from Stephane Arnouts and Olivier Ilbert when using the code LePhare. We finally thank Tanio Díaz-Santos, Frédéric Galliano, David Elbaz, Suzanne Madden, and Marc Sauvage for useful discussions.|
|Subject Keywords:||galaxies: individual (GRB980425); infrared: galaxies|
|Official Citation:||The First Infrared Study of the Close Environment of a Long Gamma-Ray Burst Emeric Le Floc'h et al. 2012 ApJ 746 7|
|Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Ruth Sustaita|
|Deposited On:||24 May 2012 15:01|
|Last Modified:||26 Dec 2012 15:15|
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