CLASSY III: The Properties of Starburst-Driven Warm Ionized Outflows

TL;DR

This study analyzes galactic outflows in 50 low-redshift starburst galaxies using high-quality spectroscopic data, revealing correlations with galaxy properties and quantifying outflow rates and efficiencies, providing new constraints for wind models.

Contribution

It offers a detailed analysis of warm ionized outflows in starburst galaxies, including measurements of velocities, mass-loading factors, and energy transfer, advancing understanding of galactic wind properties.

Findings

Outflow velocities correlate with star-formation rate and galaxy mass.

Only about 20% of silicon from supernovae is in the observed warm phase.

Mass-loading factor increases steeply with decreasing galaxy mass.

Abstract

We report the results of analyses of galactic outflows in a sample of 45 low-redshift starburst galaxies in the COS Legacy Archive Spectroscopic SurveY (CLASSY), augmented by five additional similar starbursts with COS data. The outflows are traced by blueshifted absorption-lines of metals spanning a wide range of ionization potential. The high quality and broad spectral coverage of CLASSY data enable us to disentangle the absorption due to the static ISM from that due to outflows. We further use different line multiplets and doublets to determine the covering fraction, column density, and ionization state as a function of velocity for each outflow. We measure the outflow's mean velocity and velocity width, and find that both correlate in a highly significant way with the star-formation rate, galaxy mass, and circular velocity over ranges of four orders-of-magnitude for the first two properties. We also estimate outflow rates of metals, mass, momentum, and kinetic energy. We find that, at most, only about 20% of silicon created and ejected by supernovae in the starburst is carried in the warm phase we observe. The outflows' mass-loading factor increases steeply and inversely with both circular and outflow velocity (log-log slope $\sim$ -1.6), and reaches $\sim 10$ for dwarf galaxies. We find that the outflows typically carry about 10 to 100% of the momentum injected by massive stars and about 1 to 20% of the kinetic energy. We show that these results place interesting constraints on, and new insights into, models and simulations of galactic winds.