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

Abstract

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.

Keywords

• Compact galaxies
• Galaxy evolution
• Starburst galaxies
• Scaling relations