Physical Origins of Outflowing Cold Clouds in Local Star-forming Galaxies
We study the physical origins of outflowing cold clouds in a sample of 14 low-redshift galaxies from the COS Legacy Archive Spectroscopic SurveY (CLASSY) using Keck/ESI data. Outflows are traced by broad (FWHM ~ 260 km/s) and very-broad (VB; FWHM ~ 1000 km/s) velocity components in strong emission lines like [OIII] 5007 and H-alpha. Broad components exhibit outflow velocities and scaling relations with galaxy observables like SFR, similar to UV absorption lines yet distinct from VB components. By quantitatively modeling the surface brightness and velocity widths of [OIII] 5007 using single-phase and multiphase supernova-driven galactic wind models, we find that (1) either model better reproduces the velocity widths of VB components compared to broad components, (2) the multiphase model cannot explain the observed velocity difference between broad and VB components if they represent different cloud populations entrained within the same winds, and (3) both models underestimate broad and VB components’ surface brightness by at least one dex. Excitation diagnostic diagrams indicate that stellar photoionization contributes to most of the luminosity of both broad and VB components. Two-zone photoionization models featuring density-bounded channels are required to explain the low-ionization lines ([SII] and [OI]) of broad components, suggesting the anisotropic leakage of Lyman continuum photons in an inhomogeneous interstellar medium. We conclude that VB components originate from galactic winds. In contrast, broad components likely trace expanding superbubble shells.