Exploring the Galaxy Mass-Metallicity Relations at z ~ 3-5

Long-duration gamma-ray bursts (GRBs) provide a premier tool for studying high-redshift star-forming galaxies thanks to their extreme brightness and association with massive stars. Here we use GRBs to study the galaxy stellar mass-metallicity (M_*-Z) relation at z ~ 3-5, where conventional direct metallicity measurements are extremely challenging. We use the interstellar medium metallicities of long GRB hosts derived from afterglow absorption spectroscopy, in conjunction with host galaxy stellar masses determined from deep Spitzer 3.6 μm observations of 20 GRB hosts. We detect about 1/4 of the hosts with M_(AB)(I) ≈ –21.5 to –22.5 mag and place a limit of M_(AB)(I) ≳ –19 mag on the remaining hosts from a stacking analysis. Using these observations, we present the first rest-frame optical luminosity distribution of long GRB hosts at z ≳ 3 and find that it is similar to the distribution of long GRB hosts at z ~ 1. In comparison to Lyman-break galaxies at the same redshift, GRB hosts are generally fainter, but the sample is too small to rule out an overall similar luminosity function. On the other hand, the GRB hosts appear to be more luminous than the population of Lyα emitters at z ~ 3-4. Using a conservative range of mass-to-light ratios for simple stellar populations (with ages of 70 Myr to ~2 Gyr), we infer the host stellar masses and present mass-metallicity measurements at z ~ 3-5 ((z) ≈ 3.5). We find that the detected GRB hosts, with M_* ≈ 2 × 10^(10) M_☉, display a wide range of metallicities, but that the mean metallicity at this mass scale, Z ≈ 0.3 Z_☉, is lower than measurements at z ≾ 3. Combined with stacking of the non-detected hosts with M_* ≾ 3 × 10^9 M_☉ and Z ≾ 0.1 Z_☉, we find tentative evidence for the existence of an M_*-Z relation at z ~ 3.5 and continued evolution of this relation to systematically lower metallicities from z ~ 2

Radio Linear Polarization of GRB Afterglows:Instrumental Systematics in ALMA Observations of GRB 171205A

Polarization measurements of gamma-ray burst (GRB) afterglows are a promising means of probing the structure, geometry, and magnetic composition of relativistic GRB jets. However, a precise treatment of instrumental calibration is vital for a robust physical interpretation of polarization data, requiring tests of and validations against potential instrumental systematics. We illustrate this with ALMA Band 3 (97.5 GHz) observations of GRB 171205A taken $\approx5.19$ days after the burst, where a detection of linear polarization was recently claimed. We describe a series of tests for evaluating the stability of polarization measurements with ALMA. Using these tests to re-analyze and evaluate the archival ALMA data, we uncover systematics in the polarization calibration at the $\approx0.09\%$ level. We derive a 3$\sigma$ upper limit on the linearly polarized intensity of $P<97.2~\mu$Jy, corresponding to an upper limit on the linear fractional polarization of $\Pi_{\rm L}<0.30\%$, in contrast to the previously claimed detection. Our upper limit improves upon existing constraints on the intrinsic polarization of GRB radio afterglows by a factor of 3. We discuss this measurement in the context of constraints on the jet magnetic field geometry. We present a compilation of polarization observations of GRB radio afterglows, and demonstrate that a significant improvement in sensitivity is desirable for eventually detecting signals polarized at the $\approx0.1\%$ level from typical radio afterglows.Comment: 10 pages, 8 figures. Accepted for publication in Ap

A VLA Study of High-redshift GRBs I - Multi-wavelength Observations and Modeling of GRB 140311A

We present the first results from a recently concluded study of GRBs at $z\gtrsim5$ with the Karl G. Jansky Very Large Array (VLA). Spanning $1$ to $85.5$ GHz and 7 epochs from 1.5 to 82.3 d, our observations of GRB 140311A are the most detailed joint radio and millimeter observations of a GRB afterglow at $z\gtrsim5$ to date. In conjunction with optical/near-IR and X-ray data, the observations can be understood in the framework of radiation from a single blast wave shock with energy $E_{\rm K,iso}\approx8.5\times10^{53}$ erg expanding into a constant density environment with density, $n_0\approx8\,{\rm cm}^{-3}$. The X-ray and radio observations require a jet break at $t_{\rm jet}\approx0.6$ d, yielding an opening angle of $\theta_{\rm jet}\approx4^{\circ}$ and a beaming-corrected blast wave kinetic energy of $E_{\rm K}\approx2.2\times10^{50}$ erg. The results from our radio follow-up and multi-wavelength modeling lend credence to the hypothesis that detected high-redshift GRBs may be more tightly beamed than events at lower redshift. We do not find compelling evidence for reverse shock emission, which may be related to fast cooling driven by the moderately high circumburst density.Comment: 16 pages, 13 figures, submitted to Ap

Dust sputtering by Reverse Shocks in Supernova Remnants

We consider sputtering of dust grains, believed to be formed in cooling supernovae ejecta, under the influence of reverse shocks. In the regime of self-similar evolution of reverse shocks, we can follow the evolution of ejecta density and temperature analytically as a function of time in different parts of the ejecta, and calculate the sputtering rate of graphite and silicate grains embedded in the ejecta as they encounter the reverse shock. Through analytic (1D) calculations, we find that a fraction of dust mass ($1\hbox{--}20$% for silicates and %$\le 5$% for graphites) can be sputtered by reverse shocks, the fraction varying with the grain size distribution and the steepness of the density profile of the ejecta mass. It is expected that many more grains will get sputtered in the region between the forward and reverse shocks, so that our analytical results provide a lower limit to the destroyed fraction of dust mass.Comment: 10 pages, 6 figures, uses emulateapj5.sty, accepted in Ap