Kinematics, Structure, and Mass Outflow Rates of Extreme Starburst Galactic Outflows

We present results on the properties of extreme gas outflows in massive ($\rm M_* \sim$10$^{11} \ \rm M_{\odot}$), compact, starburst ($\rm SFR \sim$$200 \, \rm M_{\odot} \ yr^{-1}$) galaxies at z = $0.4-0.7$ with very high star formation surface densities ($\rm \Sigma_{SFR} \sim$$2000 \,\rm M_{\odot} \ yr^{-1} \ kpc^{-2}$). Using optical Keck/HIRES spectroscopy of 14 HizEA starburst galaxies we identify outflows with maximum velocities of$820 - 2860 \kmps. High-resolution spectroscopy allows us to measure precise column densities and covering fractions as a function of outflow velocity and characterize the kinematics and structure of the cool gas outflow phase (T \sim$10$^4 K). We find substantial variation in the absorption profiles, which likely reflects the complex morphology of inhomogeneously-distributed, clumpy gas and the intricacy of the turbulent mixing layers between the cold and hot outflow phases. There is not a straightforward correlation between the bursts in the galaxies' star formation histories and their wind absorption line profiles, as might naively be expected for starburst-driven winds. The lack of strong \mgii \ absorption at the systemic velocity is likely an orientation effect, where the observations are down the axis of a blowout. We infer high mass outflow rates of \rm \sim$50$-$2200$\rm M_{\odot} \, yr^{-1}$, assuming a fiducial outflow size of 5 kpc, and mass loading factors of$\eta\sim$5 for most of the sample.$\eta\sim$20 for two galaxies. While these values have high uncertainties, they suggest that starburst galaxies are capable of ejecting very large amounts of cool gas that will substantially impact their future evolution.Comment: Accepted for publication in The Astrophysical Journa

Physical Properties of Massive Compact Starburst Galaxies with Extreme Outflows

© 2021. The Author(s). Published by the American Astronomical Society. This is an Open Access article, distributed under the terms of the Creative Commons Attribution 4.0 licence. https://creativecommons.org/licenses/by/4.0/We present results on the nature of extreme ejective feedback episodes and the physical conditions of a population of massive (M * ∼ 1011 M ⊙), compact starburst galaxies at z = 0.4–0.7. We use data from Keck/NIRSPEC, SDSS, Gemini/GMOS, MMT, and Magellan/MagE to measure rest-frame optical and near-IR spectra of 14 starburst galaxies with extremely high star formation rate surface densities (mean ΣSFR ∼ 2000 M ⊙ yr−1 kpc−2) and powerful galactic outflows (maximum speeds v 98 ∼ 1000–3000 km s−1). Our unique data set includes an ensemble of both emission ([O ii] λλ3726,3729, Hβ, [O iii] λλ4959,5007, Hα, [N ii] λλ6549,6585, and [S ii] λλ6716,6731) and absorption (Mg ii λλ2796,2803, and Fe ii λ2586) lines that allow us to investigate the kinematics of the cool gas phase (T ∼ 104 K) in the outflows. Employing a suite of line ratio diagnostic diagrams, we find that the central starbursts are characterized by high electron densities (median n e ∼ 530 cm−3), and high metallicity (solar or supersolar). We show that the outflows are most likely driven by stellar feedback emerging from the extreme central starburst, rather than by an AGN. We also present multiple intriguing observational signatures suggesting that these galaxies may have substantial Lyman continuum (LyC) photon leakage, including weak [S ii] nebular emission lines. Our results imply that these galaxies may be captured in a short-lived phase of extreme star formation and feedback where much of their gas is violently blown out by powerful outflows that open up channels for LyC photons to escape.Peer reviewedFinal Published versio

Extending the Dynamic Range of Galaxy Outflow Scaling Relations: Massive Compact Galaxies with Extreme Outflows

We investigate galactic winds in the HizEA galaxies, a collection of 46 late-stage galaxy mergers at z = 0.4–0.8, with stellar masses of \mathrm{log}({M}_{* }/{M}_{\odot })=10.4\mbox{--}11.5 , star formation rates (SFRs) of 20–500 M _⊙ yr ^−1 , and ultra-compact (a few 100 pc) central star-forming regions. We measure their gas kinematics using the Mg ii λ λ 2796,2803 absorption lines in optical spectra from MMT, Magellan, and Keck. We find evidence of outflows in 90% of targets, with maximum outflow velocities of 550–3200 km s ^−1 . We combine these data with ten samples from the literature to construct scaling relations for outflow velocity versus SFR, star formation surface density (Σ _SFR ), M _* , and SFR/ M _* . The HizEA galaxies extend the dynamic range of the scaling relations by a factor of ∼2–4 in outflow velocity and an order of magnitude in SFR and Σ _SFR . The ensemble scaling relations exhibit strong correlations between outflow velocity, SFR, SFR/ R , and Σ _SFR , and weaker correlations with M _* and SFR/ M _* . The HizEA galaxies are mild outliers on the SFR and M _* scaling relations, but they connect smoothly with more typical star-forming galaxies on plots of outflow velocity versus SFR/ R and Σ _SFR . These results provide further evidence that the HizEA galaxies’ exceptional outflow velocities are a consequence of their extreme star formation conditions rather than hidden black hole activity, and they strengthen previous claims that Σ _SFR is one of the most important properties governing the velocities of galactic winds