Virial Clumps in Central Molecular Zone Clouds

TL;DR

This study analyzes 755 molecular cloud clumps in the Central Molecular Zone using SMA data, revealing most are unbound and star formation may be suppressed due to a lack of gravitationally bound clumps.

Contribution

It provides a detailed virial and dynamical analysis of CMZ clumps, highlighting their bound/unbound status and implications for star formation suppression.

Findings

Most clumps follow a pressure-bounded column-density-mass trend

Majority of clumps are unbound with high virial parameters

Star formation in the CMZ may be suppressed due to few bound clumps

Abstract

CMZoom survey observations with the Submillimeter Array are analyzed to describe the virial equilibrium (VE) and star-forming potential of 755 clumps in 22 clouds in the Central Molecular Zone (CMZ) of the Milky Way. In each cloud, nearly all clumps follow the column-density-mass trend N~M^s, where s = 0.38 +- 0.03 is near the pressure-bounded limit s=1/3. This trend is expected when gravitationally unbound clumps in VE have similar velocity dispersion and external pressure. Nine of these clouds also harbor one or two distinctly more massive clumps. These properties allow a VE model of bound and unbound clumps in each cloud, where the most massive clump has the VE critical mass. These models indicate that 213 clumps have velocity dispersion 1-2 km s^(-1), mean external pressure 0.5-4 x 10^8 cm^(-3) K, bound clump fraction 0.06, and typical virial parameter alpha=4-15. These mostly unbound clumps may be in VE with their turbulent cloud pressure, possibly driven by inflow from the galactic bar. In contrast, most Sgr B2 clumps are bound according to their associated sources and N-M trends. When the CMZ clumps are combined into mass distributions, their typical power-law slope is analyzed with a stopped accretion model. It also indicates that most clumps are unbound and cannot grow significantly, due to their similar time scales of accretion and dispersal, ~0.2 Myr. Thus, virial and dynamical analyses of the most extensive clump census available indicate that star formation in the CMZ may be suppressed by a significant deficit of gravitationally bound clumps.