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Register a new userLong Gamma-Ray Bursts and Their Host Galaxies at High Redshift
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Motivated by the recent observational and theoretical evidence that long Gamma-Ray Bursts (GRBs) are likely associated with low metallicity, rapidly rotating massive stars, we examine the cosmological star formation rate (SFR) below a critical metallicity Z_crit Z_sun/10 - Z_sun/5, to estimate the event rate of high-redshift long GRB progenitors. To this purpose, we exploit a galaxy formation scenario already successfully tested on a wealth of observational data on (proto)spheroids, Lyman break galaxies, Lyman alpha emitters, submm galaxies, quasars, and local early-type galaxies. We find that the predicted rate of long GRBs amounts to about 300 events/yr/sr, of which about 30 per cent occur at z>~6. Correspondingly, the GRB number counts well agree with the bright SWIFT data, without the need for an intrinsic luminosity evolution. Moreover, the above framework enables us to predict properties of the GRB host galaxies. Most GRBs are associated with low mass galaxy halos M_H<~10^11 M_sun, and effectively trace the formation of small galaxies in such halos. The hosts are young, with age smaller than 5*10^7 yr, gas rich, but poorly extincted (A_V<~0.1) because of their chemical immaturity; this also implies high specific SFR and quite extreme alpha-enhancement. Only the minority of hosts residing in large halos with M_H>~10^12 M_sun have larger extinction (A_V~0.7-1), SFRs exceeding 100 M_sun/yr and can be detected at submm wavelengths. Most of the hosts have UV magnitudes in the range -20 <~M_1350<~ -16, and Lyman alpha luminosity in the range 2*10^40 <~L_Lya<~2*10^42 erg/s. GRB hosts are thus tracing the faint end of the luminosity function of Lyman break galaxies and Lyman alpha emitters.
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The galaxies hosting the most energetic explosions in the universe, the gamma-ray bursts (GRBs), are generally found to be low-mass, metal poor, blue and star forming galaxies. However, the majority of the targets investigated so far (less than 100) are at relatively low redshift, z < 2. We know that at low redshift, the cosmic star formation is predominantly in small galaxies. Therefore, at low redshift, long-duration GRBs, which are associated with massive stars, are expected to be in small galaxies. Preliminary investigations of the stellar mass function of z < 1.5 GRB hosts does not indicate that these galaxies are different from the general population of nearby star-forming galaxies. At high-z, it is still unclear whether GRB hosts are different. Recent results indicate that a fraction of them might be associated with dusty regions in massive galaxies. Remarkable is the a super-solar metallicity measured in the interstellar medium of a z = 3.57 GRB host.
We use numerical simulations of large scale structure formation to explore the cosmological properties of Gamma-Ray Burst (GRB) host galaxies. Among the different sub-populations found in the simulations, we identify the host galaxies as the most efficient star-forming objects, i.e. galaxies with high specific star formation rates. We find that the host candidates are low-mass, young galaxies with low to moderate star formation rate. These properties are consistent with those observed in GRB hosts, most of which are sub-luminous, blue galaxies. Assuming that host candidates are galaxies with high star formation rates would have given conclusions inconsistent with the observations. The specific star formation rate, given a galaxy mass, is shown to increase as the redshift increases. The low mass of the putative hosts makes them difficult to detect with present day telescopes and the probability density function of the specific star formation rate is predicted to change depending on whether or not these galaxies are observed.
Long-duration gamma-ray bursts (LGRBs) are the signatures of extraordinarily high-energy events occurring in our universe. Since their discovery, we have determined that these events are produced during the core-collapse deaths of rare young massive stars. The host galaxies of LGRBs are an excellent means of probing the environments and populations that produce their unusual progenitors. In addition, these same young stellar progenitors makes LGRBs and their host galaxies valuable potentially powerful tracers of star formation and metallicity at high redshifts. However, properly utilizing LGRBs as probes of the early universe requires a thorough understanding of their formation and the host environments that they sample. This review looks back at some of the recent work on LGRB host galaxies that has advanced our understanding of these events and their cosmological applications, and considers the many new questions that we are poised to pursue in the coming years.
Using multiwavelength observations of radio afterglows, we confirm the hypothesis that the flux density of gamma-ray bursts (GRBs) at a fixed observing frequency is invariable when the distance of the GRBs increases, which means the detection rate will be approximately independent of redshift. We study this behavior theoretically and find that it can be well explained by the standard forward shock model involving a thin shell expanding in either a homogeneous interstellar medium (ISM) or a wind environment. We also found that short GRBs and supernova-associated GRBs, which are at relatively smaller distances, marginally match the flux-redshift relationship and they could be outliers. We rule out the assumption that the medium density evolves with redshift as $npropto(1+z)^4$ from the current measurements of $n$ and $z$ for short and long GRBs. In addition, the possible dependence of host flux on the redshift is also investigated. We find that a similar redshift independence of the flux exists for host galaxies as well, which implies that the detection rate of radio hosts might also be independent of the redshift. It is also hinted that most radio hosts have the spectral indices ranging from $beta_hsimeq-1$ to 2.5 in statistics. Finally, we predict the detection rates of radio afterglows by the next-generation radio telescopes such as the Five-hundred meter Aperture Spherical Telescope (FAST) and the Square Kilometer Array (SKA).
Broad-line Ic supernovae (SNe Ic-BL) are a very rare class of core-collapse supernovae exhibiting high ejecta velocities and high kinetic energies. They are the only type of SNe that accompany long gamma-ray burst (GRB) explosions. Systematic differences found in the spectra of SNe Ic-BL with and without GRBs (GRB-SNe and SNe Ic-BL, respectively) suggest that either the progenitor or/and explosion mechanism of SNe Ic-BL with and without a GRB differ, or the difference could be only due to the viewing angle of the observer with respect to the orientation of the collimated explosion. We present the systematic comparison of the host galaxies of broad-lined SNe Ic with and without a detected GRB, the latter being detected in untargeted surveys, with the aim to find out whether there are any systematic differences between the environments in which these two classes of SNe preferentially explode. We study photometric properties of the host galaxies of a sample of 8 GRB-SNe and a sample of 28 SNe Ic-BL at z < 0.2. The two galaxy samples have indistinguishable luminosity and proper size distribution. We find indications that GRB-SNe on average occur closer to the centres of their host galaxies, i.e. the samples have a different distribution of projected offsets, normalized by the galaxy sizes. In addition we compare gas-phase metallicities of the GRB-SNe and SNe Ic-BL host samples and find that a larger fraction of super-solar metallicity hosts are found among the SNe Ic-BL without a GRB. Our results are indicative of a genuine difference between the two types of explosions and suggest that the viewing angle is not the main source of difference in the spectra of the two classes. We discuss the implications our results have on our understanding of progenitors of SNe Ic-BL with and without a GRB.
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