Peak 12CO(2−1) temperature of the zoom field along each line-of-sight pixel, with the full-galaxy context shown by a Spitzer image in gray. The sub-cloud scale resolution over such a large field allows us to compare structures across 2.5 orders of magnitude in size.
A dendrogram decomposition of our field reveals the complex hierarchy of 2463 structures. We calculate the virial parameter, a proxy for gravitational boundedness, for 0.1 K contours within each structure. Boundedness clearly increases (αvir decreases) moving up the hierarchy and towards higher intensity.
The virial parameter is remarkably constant with radius, from R ~ 2 to R ~ 230 pc. This suggests that dynamical physical conditions, not size, likely drive the bound state of objects. In the lower panel, the ratio of child-to-parent αvir shows that the virial parameter typically decreases (gravitational boundedness increases) as we move up the hierarchy.
The NGC 253 zoom field
- Data: 12m + 7m + TP ALMA 12CO(2−1)
- Field: 1.4 × 1.4 kpc2
- Spatial resolution: 0.3'' ~ 5 pc
- Max. recoverable scale: 32'' ~ 570 pc
- Velocity resolution: 1 km/s
Are giant molecular clouds real?
Stars form from gravitationally bound, collapsing molecular gas in the interstellar medium. On larger scales of 10–100 pc, clumps of gas known as giant molecular clouds swirl, form, and disperse under the influence of gravity, stellar feedback, and galactic dynamics. However, whether these structures are well-defined or gravitationally coherent is unclear. With an unprecedented combination of resolution and size offered by the NGC 253 zoom field, we seek to answer: Is there a physical scale at which cloud-like structures emerge from the dynamic, hierarchical ISM?