Physical Origins of Outflowing Cold Clouds in Local Star-forming Dwarf Galaxies

Abstract We study the physical origins of outflowing cold clouds in a sample of 14 low-redshift dwarf ( M * ≲ 10 10 M ⊙ ) galaxies from the Cosmic Origins Spectrograph Legacy Archive Spectroscopic SurveY (CLASSY) using Keck/ESI data. Outflows are traced by broad (FWHM ∼260 km s −1 ) and very-broad (VB; FWHM ∼1200 km s −1 ) velocity components in strong emission lines like [O iii ] λ 5007 and H α . The maximum velocities ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>v</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">max</mml:mi> </mml:mrow> </mml:msub> </mml:math> ) of broad components correlate positively with star formation rate, unlike the anticorrelation observed for VB components, and are consistent with superbubble models. In contrast, supernova-driven galactic wind models better reproduce the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>v</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">max</mml:mi> </mml:mrow> </mml:msub> </mml:math> of VB components. Direct radiative cooling from a hot wind significantly underestimates the luminosities of both broad and VB components. A multiphase wind model with turbulent radiative mixing reduces this discrepancy to at least 1 dex for most VB components. Stellar photoionization likely provides additional energy since broad components lie in the starburst locus of excitation diagnostic diagrams. We propose a novel interpretation of outflow origins in star-forming dwarf galaxies—broad components trace expanding superbubble shells, while VB components originate from galactic winds. One-zone photoionization models fail to explain the low-ionization lines ([S ii ] and [O i ]) of broad components near the maximal starburst regime, which two-zone photoionization models with density-bounded channels instead reproduce. These two-zone models indicate anisotropic leakage of Lyman continuum photons through low-density channels formed by expanding superbubbles. Our study highlights extreme outflows ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>v</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">max</mml:mi> </mml:mrow> </mml:msub> <mml:mo>≳</mml:mo> <mml:mn>1000</mml:mn> <mml:mspace width="1em"/> <mml:mi mathvariant="normal">km</mml:mi> <mml:mspace width="1em"/> <mml:msup> <mml:mrow> <mml:mi mathvariant="normal">s</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> </mml:math> ) in nine out of 14 star-forming dwarf galaxies, comparable to active galactic nucleus–driven winds.