The Starburst-Driven Molecular Wind in NGC 253 and the Suppression of Star Formation (1307.6259v2)

Abstract: The under-abundance of very massive galaxies in the universe is frequently attributed to the effect of galactic winds. Although ionized galactic winds are readily observable most of the expelled mass is likely in cooler atomic and molecular phases. Expanding molecular shells observed in starburst systems such as NGC 253 and M 82 may facilitate the entrainment of molecular gas in the wind. While shell properties are well constrained, determining the amount of outflowing gas emerging from such shells and the connection between this gas and the ionized wind requires spatial resolution <100 pc coupled with sensitivity to a wide range of spatial scales, hitherto not available. Here we report observations of NGC 253, a nearby starburst galaxy (D~3.4 Mpc) known to possess a wind, which trace the cool molecular wind at 50 pc resolution. At this resolution the extraplanar molecular gas closely tracks the H{\alpha} filaments, and it appears connected to molecular expanding shells located in the starburst region. These observations allow us to directly measure the molecular outflow rate to be > 3 Msun/yr and likely ~9 Msun/yr. This implies a ratio of mass-outflow rate to star formation rate of at least {\eta}~1-3, establishing the importance of the starburst-driven wind in limiting the star formation activity and the final stellar content.

The Dynamics of Starburst-Driven Molecular Winds in NGC 253

The observational paper of molecular winds emanating from starburst galaxies plays a key role in understanding the impact of galactic winds on both star formation and the broader galactic environment. The paper "The Starburst-Driven Molecular Wind in NGC 253 and the Suppression of Star Formation" investigates the molecular outflows from NGC 253, a nearby starburst galaxy, utilizing data obtained from the Atacama Large Millimeter Array (ALMA). This research focuses on the detailed characterization and quantification of these outflows and their implications on the suppression of star formation activities within the galaxy.

The paper employs high-resolution CO J=1-0 transition imaging to trace the morphology and kinematics of the cold molecular component of the wind at a resolution of 50 parsecs. This spatial resolution enables a precise mapping of the extraplanar molecular gas and reveals its close association with Hα filaments. These features are spatially correlated with previously identified expanding molecular shells in the starburst region, supporting the connection between star-formation-driven winds and molecular gas displacement.

One of the significant findings of this paper is the direct measurement of the molecular outflow rate in NGC 253. The observations yield a molecular outflow rate of $$\dot{M}_{\text{mol}} \sim 9\, \text{M}_\odot \, \text{yr}^{-1}$$, with a potential upper bound reaching $$\dot{M}_{\text{mol}} \sim 30\, \text{M}_\odot \, \text{yr}^{-1}$$. This rate significantly exceeds the star formation rate (SFR) of approximately $2.8\, \text{M}_\odot \, \text{yr}^{-1}$. The mass loading factor, $\eta$, defined as the ratio of outflow rate to star formation rate, is thus estimated to be at least 1 with implications of being as high as 3, confirming the critical role of starburst-driven winds in regulating star formation processes.

Further, the paper uncovers the asymmetric nature of the ionized wind structure emerging from the central region of NGC 253. The ionized wind, constrained within a cone of 60° opening angle, emanates seemingly parallel to the plane of the sky (inclination $i\approx78^\circ$), with inclination-corrected velocities reaching several hundred km s$^{-1}$. The observed outflow velocity of the ionized component surpasses that of the molecular component, indicating differing wind-driving mechanisms and possibly the influence of advection on the molecular component.

This paper sheds light on the complex interplay between star formation and feedback processes. By elucidating the structure and magnitude of these molecular winds, the research contributes to understanding how such processes can alter galactic evolution. A noteworthy observation is the contribution of stellar winds, supernovae, and radiation pressure which act synergistically to drive these winds — mechanisms believed to be effectively active in NGC 253.

In summary, this research emphasizes the significance of molecular winds in inhibiting star formation through the removal of potential star-forming material from the central regions of galaxies. With an average mass outflow rate likely surpassing the SFR, the persistence of galactic winds could lead to substantial depletion of cold molecular gas, thus impacting future star formation and the evolution of galaxies like NGC 253. The paper highlights the capability of modern radio astronomy facilities like ALMA to enhance our comprehension of galactic feedback mechanisms, which remain a paramount topic in astrophysical research for unraveling the life cycle of baryonic matter in galaxies. Future investigations may continue to assess the escapement fraction of these winds from their host galaxies and their contribution to intergalactic enrichment processes.