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Register a new userThe Luminous Infrared Host Galaxy of Short-Duration GRB 100206A
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The known host galaxies of short-hard gamma-ray bursts (GRBs) to date are characterized by low to moderate star-formation rates and a broad range of stellar masses. In this paper, we positionally associate the recent unambiguously short-hard Swift GRB 100206A with a disk galaxy at redshift z=0.4068 that is rapidly forming stars at a rate of ~30 M_sun/yr, almost an order of magnitude higher than any previously identified short GRB host. Using photometry from Gemini, Keck, PAIRITEL, and WISE, we show that the galaxy is very red (g-K = 4.3 AB mag), heavily obscured (A_V ~ 2 mag), and has the highest metallicity of any GRB host to date (12 + log[O/H]_KD02 = 9.2): it is a classical luminous infrared galaxy (LIRG), with L_IR ~ 4 x 10^11 L_sun. While these properties could be interpreted to support an association of this GRB with very recent star formation, modeling of the broadband spectral energy distribution also indicates that a substantial stellar mass of mostly older stars is present. The current specific star-formation rate is modest (specific SFR ~ 0.5 Gyr^-1), the current star-formation rate is not substantially elevated above its long-term average, and the host morphology shows no sign of recent merger activity. Our observations are therefore equally consistent with an older progenitor, similar to what is inferred for other short-hard GRBs. Given the precedent established by previous short GRB hosts and the significant fraction of the Universes stellar mass in LIRG-like systems at z >~0.3, an older progenitor represents the most likely origin of this event.
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We report optical and near-infrared broad band observations of the short-duration GRB 050724 host galaxy, used to construct its spectral energy distribution (SED). Unlike the hosts of long-duration gamma-ray bursts (GRBs), which show younger stellar populations, the SED of the GRB 050724 host galaxy is optimally fitted with a synthetic elliptical galaxy template based on an evolved stellar population (age ~2.6 Gyr). The SED of the host is difficult to reproduce with non-evolving metallicity templates. In contrast, if the short GRB host galaxy metallicity enrichment is considered, the synthetic templates fit the observed SED satisfactorily. The internal host extinction is low (A_v ~< 0.4 mag) so it cannot explain the faintness of the afterglow. This short GRB host galaxy is more massive (~5x10^10 Mo) and luminous (~1.1 L*) than most of the long-duration GRB hosts. A statistical comparison based on the ages of short- and long-duration GRB host galaxies strongly suggests that short-duration GRB hosts contain, on average, older progenitors. These findings support a different origin for short- and long-duration GRBs.
We present the discovery of short GRB 080905A, its optical afterglow and host galaxy. Initially discovered by Swift, our deep optical observations enabled the identification of a faint optical afterglow, and subsequently a face-on spiral host galaxy underlying the GRB position, with a chance alignment probability of <1%. There is no supernova component present in the afterglow to deep limits. Spectroscopy of the galaxy provides a redshift of z=0.1218, the lowest redshift yet observed for a short GRB. The GRB lies offset from the host galaxy centre by ~18.5 kpc, in the northern spiral arm which exhibits an older stellar population than the southern arm. No emission lines are visible directly under the burst position, implying little ongoing star formation at the burst location. These properties would naturally be explained were the progenitor of GRB 080905A a compact binary merger.
We report on radio continuum observations of the host galaxy of the short gamma-ray burst 071227 (z=0.381) with the Australia Telescope Compact Array (ATCA). We detect the galaxy in the 5.5 GHz band with an integrated flux density of Fnu = 43 +/- 11 microJy, corresponding to an unobscured star-formation rate (SFR) of about 24 Msun/yr, forty times higher than what was found from optical emission lines. Among the ~30 well-identified and studied host galaxies of short bursts this is the third case where the host is found to undergo an episode of intense star formation. This suggests that a fraction of all short-burst progenitors hosted in star-forming galaxies could be physically related to recent star formation activity, implying a relatively short merger time scale.
GRB 020903 is a long-duration gamma ray burst (LGRB) with a host galaxy close enough and extended enough for spatially-resolved observations, making it one of less than a dozen GRBs where such host studies are possible. GRB 020903 lies in a galaxy host complex that appears to consist of four interacting components. Here we present the results of spatially-resolved spectroscopic observations of the GRB 020903 host. By taking observations at two different position angles we were able to obtain optical spectra (3600-9000{AA}) of multiple regions in the galaxy. We confirm redshifts for three regions of the host galaxy that match that of GRB 020903. We measure metallicity of these regions, and find that the explosion site and the nearby star-forming regions both have comparable sub-solar metallicities. We conclude that, in agreement with past spatially-resolved studies of GRBs, the GRB explosion site is representative of the host galaxy as a whole rather than localized in a metal-poor region of the galaxy.
We present the optical discovery and sub-arcsecond optical and X-ray localization of the afterglow of the short GRB 120804A, as well as optical, near-IR, and radio detections of its host galaxy. X-ray observations with Swift/XRT, Chandra, and XMM-Newton to ~19 d reveal a single power law decline. The optical afterglow is faint, and comparison to the X-ray flux indicates that GRB 120804A is dark, with a rest-frame extinction of A_V~2.5 mag (at z~1.3). The intrinsic neutral hydrogen column density inferred from the X-ray spectrum, N_H~2x10^22 cm^-2, is commensurate with the large extinction. The host galaxy exhibits red optical/near-IR colors. Equally important, JVLA observations at 0.9-11 d reveal a constant 5.8 GHz flux density and an optically-thin spectrum, unprecedented for GRB afterglows, but suggestive instead of emission from the host galaxy. The optical/near-IR and radio fluxes are well fit with the scaled spectral energy distribution of the local ultra-luminous infrared galaxy (ULIRG) Arp 220 at z~1.3, with a resulting star formation rate of ~300 Msun/yr. The inferred extinction and small projected offset (2.2+/-1.2 kpc) are also consistent with the ULIRG scenario, as is the presence of a companion galaxy at a separation of about 11 kpc. The limits on radio afterglow emission, in conjunction with the observed X-ray and optical emission, require a circumburst density of ~10^-3 cm^-3 an isotropic-equivalent energy scale of E_gamma,iso ~ E_K,iso ~ 7x10^51 erg, and a jet opening angle of >8 deg. The expected fraction of luminous infrared galaxies in the short GRB host sample is ~0.01-0.3 (for pure stellar mass and star formation weighting, respectively). Thus, the observed fraction of 2 events in about 25 hosts (GRBs 120804A and 100206A), provides additional support to our previous conclusion that short GRBs track both stellar mass and star formation activity.
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