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

TL;DR

This study uses high-resolution ALMA observations to analyze the hierarchical dynamical states of molecular gas in NGC 253 across scales from 3 to 300 pc, revealing no preferred scale for gravitationally bound structures.

Contribution

It introduces a multiscale dendrogram analysis of molecular gas structures in NGC 253, showing the absence of a characteristic scale for gravitational binding in the ISM.

Findings

No evidence of a preferred scale for bound structures.

Virial parameter shows no significant correlation with size.

Multiscale approaches are essential for ISM characterization.

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 $^{12}\mathrm{CO}(2-1)$ observations at $7$ pc ($0''.4$) spatial resolution across a $1.4~\mathrm{kpc}\times5.6~\mathrm{kpc}$ ($1'.3\times1'.3$) 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 $\sim3$ to $300$ pc, complete to a limiting mass of $10^4~M_\odot$. 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.