Low-ionization Line Emission from Starburst Galaxies: A New Probe of Galactic-Scale Outflows

TL;DR

This study uses low-ionization emission lines in a starburst galaxy at z=0.69 to directly observe and spatially resolve galactic outflows, providing new insights into their extent and impact on galaxy evolution.

Contribution

It presents the first direct measurement of the spatial extent and morphology of galactic outflows using MgII and FeII* emission lines in a distant starburst galaxy.

Findings

MgII emission extends beyond 7 kpc from the galaxy center.

Detected FeII* emission lines indicate radiative excitation from the ground state.

Observed P Cygni-like line profiles suggest photon scattering in outflows.

Abstract

We study the kinematically narrow, low-ionization line emission from a bright, starburst galaxy at z = 0.69 using slit spectroscopy obtained with Keck/LRIS. The spectrum reveals strong absorption in MgII and FeII resonance transitions with Doppler shifts of -200 to -300 km/s, indicating a cool gas outflow. Emission in MgII near and redward of systemic velocity, in concert with the observed absorption, yields a P Cygni-like line profile similar to those observed in the Ly alpha transition in Lyman Break Galaxies. Further, the MgII emission is spatially resolved, and extends significantly beyond the emission from stars and HII regions within the galaxy. Assuming the emission has a simple, symmetric surface brightness profile, we find that the gas extends to distances > ~7 kpc. We also detect several narrow FeII* fine-structure lines in emission near the systemic velocity, arising from energy levels which are radiatively excited directly from the ground state. We suggest that the MgII and FeII* emission is generated by photon scattering in the observed outflow, and emphasize that this emission is a generic prediction of outflows. These observations provide the first direct constraints on the minimum spatial extent and morphology of the wind from a distant galaxy. Estimates of these parameters are crucial for understanding the impact of outflows in driving galaxy evolution.