The Kinematics of Extended Lyman-alpha Emission in a Low Mass, Low Metallicity Galaxy at z = 2.3

TL;DR

This study investigates the spatial extent and kinematic properties of Ly$ ext{alpha}$ emission in a low-mass, low-metallicity galaxy at redshift 2.3, revealing complex gas dynamics and the importance of spatially resolved radiative transfer models.

Contribution

First spatially resolved analysis of Ly$ ext{alpha}$ emission and kinematics in a low-mass, low-metallicity galaxy at high redshift, highlighting variations in gas outflows.

Findings

Ly$ ext{alpha}$ halo extends to 23 kpc with a 6 kpc exponential scale length.

Significant variation in Ly$ ext{alpha}$ peak ratio and separation across the halo.

Red and blue peaks are roughly equal in the outskirts, indicating complex gas motions.

Abstract

Lyman-$\alpha$ photons are resonantly scattered by neutral hydrogen, and may therefore trace both the spatial extent and the kinematics of the gas surrounding galaxies. We present new observations of the extended Ly$\alpha$ halo of Q2343-BX418, a low mass ($M_{\star} = 5\times10^8$ M$_{\odot}$), low metallicity ($Z\approx 0.25$ Z$_{\odot}$) star-forming galaxy at $z = 2.3$. Using the Keck Cosmic Web Imager, the blue-sensitive optical integral field spectrograph recently installed on the Keck II telescope, we detect Ly$\alpha$ in emission to a radius of 23 kpc, and measure an exponential scale length of 6 kpc in the outer region of the extended halo. We study the double-peaked spectroscopic Ly$\alpha$ profile in individual spectral pixels ("spaxels") over a $\sim 25\times30$ kpc region, finding significant variations in the peak ratio and peak separation. The profile is dominated by the red peak in the central regions, while in the outskirts of the extended halo the red and blue peak strengths are roughly equal; these observations are consistent with a model in which the peak ratio is largely determined by the radial component of the outflow velocity. We find a gradient of 300 km s$^{-1}$ in the Ly$\alpha$ peak separation across the extended halo, indicating variations in the column density, covering fraction or velocity range of the gas. These new observations emphasize the need for realistic, spatially resolved models of Ly$\alpha$ radiative transfer in the halos of galaxies.