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Open Access Publications from the University of California

The Origin and Impact of Broadened Emission Lines in Star-Forming Galaxies at 1.4

  • Author(s): Freeman, William Robert
  • Advisor(s): Siana, Brian
  • et al.
Creative Commons Attribution 4.0 International Public License

We present results from the MOSFIRE Deep Evolution Field (MOSDEF) survey on broad emission from the nebular emission lines \Ha, \NII, \OIII, \Hb, and \SII. After removing known AGN, the sample consists of 127 galaxies with $1.37 < z < 2.61$ and 84 galaxies with $2.95 < z < 3.80$. We study broad flux by decomposing the emission lines using narrow and broad components for individual galaxies and stacks. For the z $\sim2$ sample, the broad flux accounts for 20-50\% of the flux in nebular emission lines and in the z $\sim3.3$ sample the broad component comprises 30-60\% of the flux.

We calculate \SII/\Ha, \NII/\Ha, and \OIII/\Hb \ line ratios for the narrow components and broad components and compare these to a single Gaussian fit. When placed on the BPT diagram (\OIII/\Hb \ vs. \NII/\Ha) the broad components are shifted towards the higher \OIII/\Hb \ and \NII/\Ha \ ratios. This is likely the results of partial ionization from shocks or low luminosity AGN. The narrow component line ratios are closer to local measurements than other studies at z $\sim2$ but are still slightly offset.

Assuming the broad component is an outflow we estimate the mass loading factor ($\eta=$mass outflow rate/SFR) as a function of mass and find generally good agreement with previous studies. We find our galaxies are only compatible with simulations (which predict $\eta$ should decrease as a function of mass) only if a large fraction of the outflows are below 300 km s$^{-1}$ for galaxies below $10^{10}$ stellar mass.

Outflows and shocks may significantly impact measurements from outflows from the \Ha \ line, change line ratios, and impact single line measurements such as calculating star formation rate from \Ha. We find that partially shocked flux from outflows could account for some of the offset seen in the BPT diagram from z $\sim0$ to z $\sim2$.

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