Braving the Storm: Quantifying Disk-wide Ionized Outflows in the Large Magellanic Cloud with ULLYSES

Abstract

The Large Magellanic Cloud (LMC) is home to many H ii regions, which may lead to significant outflows. We examine the LMC’s multiphase gas ( T ∼10 ^4-5 K) in H i , S ii , Si iv , and C iv using 110 stellar sight lines from the Hubble Space Telescope’s Ultraviolet Legacy Library of Young Stars as Essential Standards program. We develop a continuum fitting algorithm based on the concept of Gaussian process regression and identify reliable LMC interstellar absorption over v _helio = 175–375 km s ^−1 . Our analyses show disk-wide ionized outflows in Si iv and C iv across the LMC with bulk velocities of ∣ v _out, bulk ∣ ∼ 20–60 km s ^−1 , which indicates that most of the outflowing mass is gravitationally bound. The outflows’ column densities correlate with the LMC’s star formation rate surface densities (Σ _SFR ), and the outflows with higher Σ _SFR tend to be more ionized. Considering outflows from both sides of the LMC as traced by C iv , we conservatively estimate a total outflow rate of ${\dot{M}}_{\mathrm{out}}\gtrsim 0.03\,{M}_{\odot }{\mathrm{yr}}^{-1}$ and a mass-loading factor of η ≳ 0.15. We compare the LMC’s outflows with those detected in starburst galaxies and simulation predictions, and find a universal scaling relation of $| {v}_{\mathrm{out},\ \mathrm{bulk}}| \propto {{\rm{\Sigma }}}_{\mathrm{SFR}}^{0.23}$ over a wide range of star-forming conditions (Σ _SFR ∼ 10 ^−4.5 –10 ^2 M _⊙ yr ^−1 kpc ^−2 ). Lastly, we find that the outflows are corotating with the LMC’s young stellar disk and the velocity field does not seem to be significantly impacted by external forces; we thus speculate on the existence of a bow shock leading the LMC, which may have shielded the outflows from ram pressure as the LMC orbits the Milky Way.

Keywords

• Large Magellanic Cloud
• Stellar feedback
• Metal line absorbers
• Galaxy evolution
• Interstellar medium