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Register a new userGRB 980425 host: [CII], [OI] and CO lines reveal recent enhancement of star formation due to atomic gas inflow
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Micha{\\l} Jerzy Micha{\\l}owski
Publication date
2016
fields
Physics
and research's language is
English
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We have recently suggested that gas accretion can be studied using host galaxies of gamma-ray bursts (GRBs). We obtained the first ever far-infrared (FIR) line observations of a GRB host, namely Herschel/PACS resolved [CII] 158 um and [OI] 63 um spectroscopy, as well as APEX CO(2-1) and ALMA CO(1-0) observations of the GRB 980425 host. It has elevated [CII]/FIR and [OI]/FIR ratios and higher values of star formation rate (SFR) derived from line ([CII], [OI], Ha) than from continuum (UV, IR, radio) indicators. [CII] emission exhibits a normal morphology, peaking at the galaxy center, whereas [OI] is concentrated close to the GRB position and the nearby Wolf-Rayet region. The high [OI] flux indicates high radiation field and gas density. The [CII]/CO luminosity ratio of the GRB 980425 host is close to the highest values found for local star-forming galaxies. Its CO-derived molecular gas mass is low given its SFR and metallicity, but the [CII]-derived molecular gas mass is close to the expected value. The [OI] and HI concentrations, and the high radiation field and density are consistent with the hypothesis of a very recent (at most a few tens of Myr ago) inflow of atomic gas triggering star formation. Dust has not had time to build up (explaining high line-to-continuum ratios). Such a recent enhancement of star-formation would indeed manifest itself in high SFR_line/SFR_continuum ratios, because the line indicators are sensitive only to recent (<10 Myr) activity, whereas the continuum indicators measure the SFR averaged over much longer periods (~100 Myr). Other GRB hosts exhibit a mean SFR_line/SFR_continuum of 1.74+-0.32. This is consistent with a very recent enhancement of star formation being common among GRB hosts, so galaxies which have recently experienced inflow of gas may preferentially host stars exploding as GRBs. Hence GRB hosts may be used to investigate recent gas accretion.
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We present a pilot search of CO emission in three H$_2$-absorbing, long-duration gamma-ray burst (GRB) host galaxies at z~2-3. We used the Atacama Large Millimeter/sub-millimeter Array (ALMA) to target the CO(3-2) emission line and report non-detections for all three hosts. These are used to place limits on the host molecular gas masses, assuming a metallicity-dependent CO-to-H$_2$ conversion factor ($alpha_{rm CO}$). We find, $M_{rm mol} < 3.5times 10^{10},M_{odot}$ (GRB,080607), $M_{rm mol} < 4.7times 10^{11},M_{odot}$ (GRB,120815A), and $M_{rm mol} < 8.9times 10^{11},M_{odot}$ (GRB,181020A). The high limits on the molecular gas mass for the latter two cases are a consequence of their low stellar masses $M_star$ ($M_star lesssim 10^{8},M_{odot}$) and low gas-phase metallicities ($Zsim 0.03,Z_{odot}$). The limit on the $M_{rm mol}/M_star$ ratio derived for GRB,080607, however, is consistent with the average population of star-forming galaxies at similar redshifts and stellar masses. We discuss the broader implications for a metallicity-dependent CO-to-H$_2$ conversion factor, and demonstrate that the canonical Galactic $alpha_{rm CO}$, will severely underestimate the actual molecular gas mass for all galaxies at $z>1$ with $M_star < 10^{10},M_odot$. To better quantify this we develop a simple approach to estimate the relevant $alpha_{rm CO}$ factor based only on the redshift and stellar mass of individual galaxies. The elevated conversion factors will make these galaxies appear CO-dark and difficult to detect in emission, as is the case for the majority of GRB hosts. GRB spectroscopy thus offers a complementary approach to identify low-metallicity, star-forming galaxies with abundant molecular gas reservoirs at high redshifts that are otherwise missed by current ALMA surveys.
We report Giant Metrewave Radio Telescope (GMRT) , Very Large Telescope (VLT) and Spitzer Space Telescope observations of ESO 184$-$G82, the host galaxy of GRB 980425/SN 1998bw, that yield evidence of a companion dwarf galaxy at a projected distance of 13 kpc. The companion, hereafter GALJ193510-524947, is a gas-rich, star-forming galaxy with a star formation rate of $rm0.004,M_{odot}, yr^{-1}$, a gas mass of $10^{7.1pm0.1} M_{odot}$, and a stellar mass of $10^{7.0pm0.3} M_{odot}$. The interaction between ESO 184$-$G82 and GALJ193510-524947 is evident from the extended gaseous structure between the two galaxies in the GMRT HI 21 cm map. We find a ring of high column density HI gas, passing through the actively star forming regions of ESO 184$-$G82 and the GRB location. This ring lends support to the picture in which ESO 184$-$G82 is interacting with GALJ193510-524947. The massive stars in GALJ193510-524947 have similar ages to those in star-forming regions in ESO 184$-$G82, also suggesting that the interaction may have triggered star formation in both galaxies. The gas and star formation properties of ESO 184$-$G82 favour a head-on collision with GALJ193510-524947 rather than a classical tidal interaction. We perform state-of-the art simulations of dwarf--dwarf mergers and confirm that the observed properties of ESO 184$-$G82 can be reproduced by collision with a small companion galaxy. This is a very clear case of interaction in a gamma ray burst host galaxy, and of interaction-driven star formation giving rise to a gamma ray burst in a dense environment.
We present the first spatially resolved study of molecular gas in the vicinity of a Gamma Ray Burst, using CO(2-1) emission line observations with the Atacama Large Millimetre Array (ALMA) at ~50 pc scales. The host galaxy of GRB 980425 contains a ring of high column density HI gas which is likely to have formed due to a collision between the GRB host and its companion galaxy, within which the GRB is located. We detect eleven molecular gas clumps in the galaxy, seven of which are within the gas ring. The clump closest to the GRB position is at a projected separation of ~280 pc. Although it is plausible that the GRB progenitor was ejected from clusters formed in this clump, we argue that the in situ formation of the GRB progenitor is the most likely scenario. We measure the molecular gas masses of the clumps and find them to be sufficient for forming massive star clusters. The molecular gas depletion times of the clumps show a variation of ~2 dex, comparable with the large variation in depletion times found in starburst galaxies in the nearby Universe. This demonstrates the presence of starburst modes of star formation on local scales in the galaxy, even while the galaxy as a whole cannot be categorised as a starburst based on its global properties. Our findings suggest that the progenitor of GRB 9802425 was originated in a young massive star cluster formed in the starburst mode of star formation.
Molecular species, most frequently H_2, are present in a small, but growing, number of gamma-ray burst (GRB) afterglow spectra at redshifts z~2-3, detected through their rest-frame UV absorption lines. In rare cases, lines of vibrationally excited states of H_2 can be detected in the same spectra. The connection between afterglow line-of-sight absorption properties of molecular (and atomic) gas, and the observed behaviour in emission of similar sources at low redshift, is an important test of the suitability of GRB afterglows as general probes of conditions in star formation regions at high redshift. Recently, emission lines of carbon monoxide have been detected in a small sample of GRB host galaxies, at sub-mm wavelengths, but no searches for H_2 in emission have been reported yet. In this paper we perform an exploratory search for rest-frame K band rotation-vibrational transitions of H_2 in emission, observable only in the lowest redshift GRB hosts (z<0.22). Searching the data of four host galaxies, we detect a single significant rotation-vibrational H_2 line candidate, in the host of GRB 031203. Re-analysis of Spitzer mid-infrared spectra of the same GRB host gives a single low significance rotational line candidate. The (limits on) line flux ratios are consistent with those of blue compact dwarf galaxies in the literature. New instrumentation, in particular on the JWST and the ELT, can facilitate a major increase in our understanding of the H_2 properties of nearby GRB hosts, and the relation to H_2 absorption in GRBs at higher redshift.
Event rate of long Gamma-Ray Bursts (GRBs) is expected to be an useful tracer of the cosmic star-formation history. For this purpose, it is necessary to understand what kind of star formations/galaxies are traced by GRBs. Here we report rest-frame far-infrared (FIR) continuum detections of GRB 070521 and 080207 host galaxies at $zsim2$ with ALMA band 8 and 9. The FIR photometries provide the reliable star-formation rates (SFRs), because FIR emission is free from dust extinction and possible radio contamination from long-lived afterglows of GRBs. The spectral energy distribution fittings indicate 49.85$^{+72.33}_{-2.86}$ and 123.4$^{+25.19}_{-21.78}$ $M_{odot}$ yr$^{-1}$ for the 070521 and 080207 hosts, respectively. The derived SFRs place them on the lq lq main sequencerq rq of normal star-forming galaxies at $zsim2$. The derived SFRs are significantly lower than that of radio observations. It is inferred that the observed radio fluxes in a previous study are contaminated by the afterglows. ALMA marginally detected [C~{sc ii}],158,$mu$m emission line from the GRB 080207 host galaxy with S/N $sim$ 4. This is the first detection of [C~{sc ii}],158,$mu$m of a GRB host at $z>2$, and the second detection among known GRBs. The luminosity ratio of [C~{sc~ii}],158$mu$m to FIR is 7.5$times 10^{-4}$, which is one of the smallest values among galaxies at $zsim 1-2$ with the same FIR luminosity. The lq lq [C~{sc ii}] deficitrq rq could be a new physical property to characterise GRB hosts at $zsim1-2$. Possible parameters controlling the deficit include the metallicity, initial mass function, and gas density.
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