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 HII regions, which may lead to significant outflows. We examine the LMC's multiphase gas ($T\sim 10^{4-5}$ K) in HI, SII, SiIV, and CIV using 110 stellar sight lines from the HST's Ultraviolet Legacy Library of Young Stars as Essential Standards (ULLYSES) program. We develop a continuum fitting algorithm based on the concept of Gaussian Process regression and identify reliable LMC interstellar absorption over $v_{\mathrm{helio}}=175-375$ km s${}^{-1}$. Our analyses show disk-wide ionized outflows in SiIV and CIV across the LMC with bulk velocities of $|v_{\mathrm{out},\mathrm{bulk}}|\sim 20-60$ 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 (${\Sigma }_{\mathrm{SFR}}$), and the outflows with higher ${\Sigma }_{\mathrm{SFR}}$ tend to be more ionized. Considering outflows from both sides of the LMC as traced by CIV, we conservatively estimate a total outflow rate of ${\dot{M}}_{\mathrm{out}}\gtrsim 0.03M_{\odot }{\mathrm{yr}}^{-1}$ and a mass loading factor of $\eta \gtrsim 0.15$. We compare the LMC's outflows with those detected in starburst galaxies and simulation predictions, and find a universal scaling relation of $|v_{\mathrm{out},\mathrm{bulk}}|\propto {\Sigma }_{\mathrm{SFR}}^{0.23}$ over a wide range of star-forming conditions (${\Sigma }_{\mathrm{SFR}}\sim 10^{-4.5}-10^2{M}_{\odot }{\mathrm{yr}}^{-1}{\mathrm{kpc}}^{-2}$). Lastly, we find that the outflows are co-rotating 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.

Accepted for publication in ApJ. Key findings can be found in Figures 9-12. Main updates from the previous version include new estimates on star formation rate surface densities. Normalized SII, SiIV, and CIV line spectra derived for this work are published as a High Level Science Product called LMC-FLOWS (doi: 10.17909/hz0m-np43), available on website: https://archive.stsci.edu/hlsp/lmc-flows

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