We present a characterization of the close environment of GRB980425 based on 5-160mic spectro-imaging obtained with Spitzer. The Gamma-Ray Burst GRB980425 occurred in a nearby (z=0.0085) SBc-type dwarf galaxy, at a projected distance of 900pc from an HII 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 25-30mic. This atypical property is rarely observed among morphologically-relaxed dwarves, suggesting a strong causal link with the GRB event. The luminosity of the WR region (L_8-1000mic=4.6x10^8 Lsol), the peak of its spectral energy distribution at <~100mic and the presence of highly-ionized emission lines (e.g., [NeIII]) also reveal extremely young (<5Myr) star-forming activity, with a typical time-scale of only 47Myr 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 (tau_9.7mic ~ 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.
It is now widely believed that Long Gamma-Ray Bursts (LGRBs 11 ) originate from the cataclismic collapse of massive stars (e.g., Woosley 1993;Galama et al. 1998;MacFadyen & Woosley 1999;Daigne & Mochkovitch 2000;Heger et al. 2003;Stanek et al. 2003;Meynet & Maeder 2005) in star-forming regions of distant galaxies (e.g., Fruchter 1999;Bloom et al. 2002;Hjorth et al. 2002;Fruchter et al. 2006). They can be observed at extremely high redshift (z > 6, Greiner et al. 2009) and because of their association with young and short-lived stars, they are invaluable tools for sign-posting the activity of star formation back to the earliest epochs of cosmic history and galaxy evolution. Furthermore, their optical and near-infrared afterglows can be used as transient background sources for probing the intergalactic space and the interstellar medium of galaxies up to very large cosmological distances (e.g., Vreeswijk et al. 2001;Totani et al. 2006). As a result they might eventually provide unique constraints on the era of reionization and the formation of the very first structures in the Universe (Lamb & Reichart 2000).
However, it appears that LGRBs may only trace regions of star-forming activity in a biased way, in the sense that their formation is more likely to occur in low-mass and sub-L * galaxies with low-metallicity environments (Sokolov et al. 2001;Fynbo et al. 2003;Courty et al. 2004Courty et al. , 2007;;Kistler et al. 2009;Kocevski et al. 2009;Han et al. 2010;Niino et al. 2011). In the latest stages of their evolution, massive stars with metal-poor envelopes retain a higher angular momentum and they are less subject to mass loss. After the final collapse these physical conditions lead to the formation of fast-rotating black holes and the subsequent accretion of material thus favors the formation of GRBs (e.g., MacFadyen & Woosley 1999;Hirschi et al. 2005). In fact, observational evidence for a possible relationship between LGRBs and chemicallyyoung galaxies has already emerged from a number of studies. On average the hosts of LGRBs are characterized by blue colors, rather low extinction and modest bolometric luminosities, as well as diffuse and irregular morphologies (e.g., Bloom et al. 2002;Le Floc’h et al. 2003;Le Floc’h et al. 2006;Fruchter et al. 2006). They also exhibit higher Lyα emission and larger specific star formation rates (SSFR) with respect to other galaxies at similar redshifts (Fynbo et al. 2003;Christensen et al. 2004), and their oxygen abundance is lower compared to field star-forming sources with similar masses and luminosities (Modjaz et al. 2008;Han et al. 2010). Furthermore, the connection between GRBs and Type Ic supernovae has led to the idea that the progenitors of long GRBs should more likely originate from binary systems (Podsiadlowski et al. 2004), which complicates further the link that exists between LGRBs and the cosmic history of structure formation. Better understanding the physical conditions and the properties of the environments favoring the formation of these cosmic explosions appears, therefore, as a critical step before allowing a full exploitation of their hosts as cosmological tracers of galaxy evolution.
Follow-up of Gamma-Ray Bursts has shown that highredshift LGRB hosts are challenging to characterize in detail, since they tend to be low-mass and sub-luminous systems (e.g., Savaglio et al. 2009). Host galaxies at very low redshift may thus provide us with some of the best cases to explore the intimate connection between the occurence of GRBs and the properties of their very close environment. For example, the now-popular GRB 980425 was identified at z = 0.0085 from its apparent association with the hypernova SN 1998bw (Galama et al. 1998). With a luminosity distance of only 36 Mpc it is currently the closest GRB known with a confirmed redshift in the local Universe. At this distance an angular separation of 1 on the sky corresponds to 170 pc. It provides therefore an excellent prototype for studying the immediate surrounding of LGRBs within their host galaxies.
The host of GRB 980425 is identified as ESO 184-G82 in the survey of the southern sky that was performed by Holmberg et al. (1977). Observations with the Hubble Space Telescope (HST) showed that it is an isolated and barred SBc-type sub-luminous galaxy (L B = 0.02 L * B ) harboring a large number of active star-forming regions (Fynbo et al. 2000). While the surface brightness of the area where GRB 980425 was observed is not particularly large, a remarkable result is that the GRB occurred at a projected distance of only ∼ 900 pc from a quite luminous HII region characterized by very distinct properties compared to the rest of the system. This region is the brightest complex of star formation in the host and although it only makes a few percent of the total luminosity at optical wavelengths, infrared (IR) imaging obtained with the Spitzer Space Telescope revealed that its 5-25 µm spectral energy distribution (
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