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∼104-5 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 (ΣSFR), and the outflows with higher ΣSFR 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 M ̇ out ≳ 0.03 M ⊙ 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 out , bulk ∣ ∝ Σ SFR 0.23 over a wide range of star-forming conditions (ΣSFR ∼ 10−4.5–102 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.