[Paper Review] Gas inflow and outflow in an interacting high-redshift galaxy: The remarkable host environment of GRB 080810 at $z=3.35$

P. Wiseman, D. A. Perley+6 more|Max Planck Digital Library|May 3, 2017

Gamma-ray bursts and supernovae123 references7 citations

TL;DR

This study uses high-resolution Keck/HIRES spectroscopy of the GRB 080810 afterglow to reveal a complex, interacting high-redshift galaxy system at z ≈ 3.35, detecting multiple gas components including a low-neutral-H column density (log NH I ≤ 18.35 cm⁻²), a metal-rich outflow at −200 km s⁻¹, and a candidate metal-poor inflow, offering one of the clearest views of gas inflow and outflow in a star-forming galaxy at high redshift.

ABSTRACT

We reveal multiple components of an interacting galaxy system at $z\approx3.35$ through a detailed analysis of the exquisite high-resolution Keck/HIRES spectrum of the afterglow of a gamma-ray burst (GRB). Through Voigt-profile fitting of absorption lines from the Lyman-series, we constrain the neutral hydrogen column density to $N_{\mathrm{HI}} \leq 10^{18.35}$ cm$^{-2}$ for the densest of four distinct systems at the host redshift of GRB~080810, among the lowest $N_{\mathrm{HI}}$ ever observed in a GRB host, despite the line of sight passing within a projected 5 kpc of the galaxy centres. By detailed analysis of the corresponding metal absorption lines, we derive chemical, ionic and kinematic properties of the individual absorbing systems, and thus build a picture of the host as a whole. Striking differences between the systems imply that the line of sight passes through several phases of gas: the star-forming regions of the GRB host; enriched material in the form of a galactic outflow; the hot and ionised halo of a second, interacting galaxy falling towards the host at a line-of-sight velocity of 700 km s$^{-1}$; and a cool, metal-poor cloud which may represent one of the best candidates yet for the inflow of metal-poor gas from the intergalactic medium.

Motivation & Objective

• To investigate the multi-phase gas environment of a high-redshift gamma-ray burst host galaxy at z ≈ 3.35.
• To determine the kinematic, chemical, and ionization structure of absorbing gas components along the line of sight to GRB 080810.
• To identify and characterize gas inflow and outflow processes in a star-forming, interacting galaxy system at high redshift.
• To assess the role of metal-poor gas accretion and metal-enriched outflows in regulating star formation in early galaxies.
• To test the hypothesis that the observed low neutral hydrogen column density is consistent with a dense, star-forming host environment despite close proximity to galactic centers.

Proposed method

• Conducted high-resolution spectroscopy using Keck/HIRES on the afterglow of GRB 080810 to obtain detailed absorption line profiles.
• Performed Voigt-profile fitting on Lyman-series absorption lines to measure neutral hydrogen column densities (NH I) and redshifts.
• Analyzed metal absorption lines (e.g., Si II, C II, Mg II) to derive ionization states, metallicities, and kinematics of individual gas components.
• Used redshift and velocity offsets to distinguish between components associated with the GRB host, a foreground galaxy, and potential intergalactic gas.
• Applied cosmological models and radial velocity corrections to interpret the physical location and motion of absorbing systems.
• Evaluated the significance of low NH I values in the context of GRB host environments and compared with Damped Lyman-Alpha Absorbers (DLAs) and Lyman Limit Systems (LLS).

Experimental results

Research questions

• RQ1 What is the nature and origin of the low neutral hydrogen column density (log NH I ≤ 18.35 cm⁻²) observed in the GRB 080810 host environment despite the line of sight passing within 5 kpc of the galactic centers?
• RQ2 What evidence supports the presence of a galactic outflow in the GRB host system, and what is its ionization state and velocity relative to the star-forming region?
• RQ3 Can a metal-poor gas component detected in the spectrum be identified as a candidate for inflowing, metal-poor gas from the intergalactic medium (IGM)?
• RQ4 How do the kinematic and chemical properties of the four main absorbing systems at z ≈ 3.35 relate to the overall interaction and star formation in the host galaxy system?
• RQ5 What is the significance of the 700 km s⁻¹ line-of-sight velocity difference between the GRB host and a second, interacting galaxy, and how does this affect the interpretation of the gas components?

Key findings

• The GRB 080810 host system is part of an interacting pair of galaxies at z ≈ 3.35, with the burst occurring in the more distant, star-forming component, forming stars at ∼100 M☉ yr⁻¹.
• The neutral hydrogen column density in the densest component of the host system is log NH I ≤ 18.35 cm⁻², among the lowest ever measured in a GRB host, despite the line of sight passing within ∼5 kpc of the galactic centers.
• A metal-rich outflowing component is detected at a velocity of ∼−200 km s⁻¹ relative to the densest gas, indicating gas expulsion from the ISM into the hot halo at galactocentric radii ≤5 kpc.
• A candidate metal-poor gas component (system D) shows only marginal metal detections, suggesting it may represent inflowing, pristine gas from the IGM, potentially within 1.5 Mpc of the host complex.
• The interacting system is closing at a relative velocity of ∼700 km s⁻¹, with the second galaxy approaching along the line of sight, contributing to the complex gas environment.
• The detection of a low-NH I system in a star-forming environment challenges the assumption that high neutral column densities are typical in GRB hosts, highlighting the diversity of high-redshift galaxy ISM conditions.