The Hierarchical Dynamical State of Molecular Gas from 3 to 300 pc in NGC 253

Abstract Understanding how the dynamical state of the interstellar medium (ISM) changes across spatial scales can provide important insights into how the gas is organized and ultimately collapses to form stars. To this end, we present ALMA 12 CO(2–1) observations at 7 pc (0 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover> <mml:mo>.</mml:mo> <mml:mtext>″</mml:mtext> </mml:mover> </mml:math> 4) spatial resolution across a 1.4 kpc × 5.6 kpc ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mn>1</mml:mn> </mml:mrow> <mml:mover> <mml:mrow> <mml:mo>.</mml:mo> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">′</mml:mi> </mml:mrow> </mml:mover> <mml:mn>3</mml:mn> <mml:mo>×</mml:mo> <mml:mrow> <mml:mn>1</mml:mn> </mml:mrow> <mml:mover> <mml:mrow> <mml:mo>.</mml:mo> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">′</mml:mi> </mml:mrow> </mml:mover> <mml:mn>3</mml:mn> </mml:math> ) region located in the disk of the nearby ( D = 3.5 Mpc), massive, star-forming galaxy NGC 253. We decompose this emission with a hierarchical, multiscale dendrogram algorithm to identify 2463 structures with deconvolved sizes ranging from ∼3 to 300 pc, complete to a limiting mass of 10 4 M ⊙ . By comparing the virial parameter of these structures against physical properties including size, mass, surface density, velocity dispersion, and hierarchical position, we carry out a comprehensive search for a preferred scale at which gravitationally bound structures emerge. Ultimately, we do not identify evidence of an emergent scale for bound objects in our data, nor do we find a significant correlation between the virial parameter and structure sizes. These findings suggest that simple observational estimates of gravitational binding cannot be used to define molecular clouds and emphasize the need for multiscale approaches to characterize the ISM.