The headlight cloud in NGC 628: An extreme giant molecular cloud in a typical galaxy disk

Abstract

Context. Cloud-scale surveys of molecular gas reveal the link between giant molecular cloud properties and star formation across a range of galactic environments. Cloud populations in galaxy disks are considered to be representative of the 'normal' star formation process, while galaxy centers tend to harbour denser gas that exhibits more extreme star formation. At high resolution, however, molecular clouds with exceptional gas properties and star formation activity may also be observed in normal disk environments. In this paper, we study the brightest cloud traced in CO(2−1) emission in the disk of nearby spiral galaxy NGC 628. Aims. We characterize the properties of the molecular and ionized gas that is spatially coincident with an extremely bright H ii region in the context of the NGC 628 galactic environment. We investigate how feedback and large-scale processes influence the properties of the molecular gas in this region. Methods. High resolution ALMA observations of CO(2−1) and CO(1−0) emission are used to characterize the mass and dynamical state of the "headlight" molecular cloud. The characteristics of this cloud are compared to the typical properties of molecular clouds in NGC 628. A simple large velocity gradient (LVG) analysis incorporating additional ALMA observations of 13 CO(1−0), HCO + (1−0) and HCN(1−0) emission is used to constrain the beam-diluted density and temperature of the molecular gas. We analyze the MUSE spectrum using Starburst99 to characterize the young stellar population associated with the H ii region. Results. The unusually bright headlight cloud is massive (1 − 2 × 10 7 M ), with a beam-diluted density of n H 2 = 5 × 10 4 cm −3 based on LVG modeling. It has a low virial parameter, suggesting that the CO emission associated with this cloud may be overluminous due to heating by the H ii region. A young (2 − 4 Myr) stellar population with mass 3 × 10 5 M is associated. Conclusions. We argue that the headlight cloud is currently being destroyed by feedback from young massive stars. Due to the cloud's large mass, this phase of the cloud's evolution is long enough for the impact of feedback on the excitation of the gas to be observed. The high mass of the headlight cloud may be related to its location at a spiral co-rotation radius, where gas experiences reduced galactic shear compared to other regions of the disk, and receives a sustained inflow of gas that can promote the cloud's mass growth.

Giant molecular cloud catalogues for PHANGS-ALMA: methods and initial results

“…Both of these systems also have the pressures in their diffuse ISM peaking outside of their centres (e.g. Herrera et al 2020;Sun et al, in preparation), which is consistent with the internal pressure being closely linked to the diffuse ISM pressure.…”

“…In parallel, we reduced the total power data following the procedures described in Herrera et al (2020). We combined the total power data with the interferometric imaging using feathering.…”

Dynamical Equilibrium in the Molecular ISM in 28 Nearby Star-forming Galaxies

“…Our cross-spatial-scale analysis produces a rich multiwavelength database for every target in our sample. This database has been used in a previous publication (Herrera et al 2020), and is used in the current work as the foundation for carrying out the calculations presented in Section 4. The full database, as well as the correlations between the basic observables, will be published in a companion paper (J.…”

ALMA resolves giant molecular clouds in a tidal dwarf galaxy

“…We used the total power observations as model within tclean in CASA, so that the final cube recovers all the flux. The total power data were calibrated and imaged following the strategy presented in Herrera et al (2020), Appendix A. For the baseline correction, we fitted a polynomial of order 1 in a fixed velocity window (755 to 855 km s −1 and 1500 to 1600 km s −1 ).…”