Optical Emission Lines for Outflow Characterization and High-z ISM Properties

3 February 2022

Bright emission features at optical wavelengths (from hydrogen, oxygen, nitrogen, and sulfur) are commonly used tracers of galaxy activity (i.e., formation of new stars and the growth of supermassive black holes). These features are bright and so can be used to identify gas that is outflowing from galaxies due to the impact of star formation or black hole growth.

In “The BPT Diagram in Cosmological Galaxy Formation Simulations: Understanding the Physics Driving Offsets at High-Redshift” (Garg+), we used galaxy formation simulations and radiative transfer post-processing to study the locations of simulated redshift 2 galaxies in the “BPT” diagnostic diagram. This diagram is frequently used to discriminate between galaxies with active star formation and galaxies whose supermassive black hole is growing. However, in this diagram, “normal” galaxies at redshifts of ~2 are offset from the location of normal galaxies at redshifts ~0 (i.e., the present-day universe). Using the simulations we suggests that reducing the ionization parameter or increasing the relative N/O abundance can reproduce the redshift ~2 galaxy observations.

In “BASS XXXI: Outflow scaling relations in low redshift X-ray AGN host galaxies with MUSE” (Kakkad+) we use the emission feature from doubly ionized oxygen to identify outflowing gas in a sample of galaxies with active supermassive black holes. Observations with the “Multi-unit Spectroscopic Explorer” on the Very Large Telescope were used to map the outflows across the host galaxies and measure the outflowing mass rates. We compared the mass outflow rates with properties of the black holes and their growth rates; the mass outflow rates correlate with the energy output of the active galactic nuclei (AGN) and suggest that approximately 1% of the energy output of the AGN powers the outflowing wind.

Both papers have been accepted for publication (in the Astrophysical Journal and Monthly Notices of the Royal Astronomical Society, respectively). Preprints are available at the links above.