ISM Studies of GRB 030329 with High-Resolution Spectroscopy

We present early Very Large Telescope UV-Visual Echelle Spectrograph (VLT UVES) high-resolution spectra of the afterglow of GRB 030329 at redshift z = 0.16867 ± 0.00001. In contrast to other spectra from this burst, both emission and absorption lines were detected. None of them showed any temporal evolution. From the emission lines, we determine the properties of the host galaxy, which has a star formation rate (SFR) of 0.198 M☉ yr-1 and a low metallicity of 0.17 Z☉. Given the low total stellar host mass log M = 7.75 ± 0.15 M☉ and an absolute luminosity mB = -16.29, we derive specific SFRs (SSFR) of log SFR = -8.5 yr-1 and SFR = 15.1 M☉ yr-1 L. This fits well into the picture of GRB hosts as being low-mass, low-metallicity, actively star-forming galaxies. The Mg II and Mg I absorption lines from the host show multiple narrow (Doppler width b = 5-12 km s-1) components spanning a range of v ~ 230 km s-1, mainly blueshifted compared to the redshift from the emission lines. These components are likely probing outflowing material of the host galaxy, which could arise from former galactic superwinds, driven by supernovae from star-forming regions. Similar features have been observed in QSO spectra. The outflowing material has high column densities of log NMg II = 13.99 ± 0.04 cm-2 and log NMg I = 12.39 ± 0.04 cm-2 and the nonvariability of the column densities implies a distance of at least 560 pc from the burst, further supporting an outflow scenario.

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