The Molecular Outflow in NGC 253 at a Resolution of Two Parsecs

Abstract

We present 0.15 (∼ 2.5 pc) resolution ALMA CO(3-2) observations of the starbursting center in NGC 253. Together with archival ALMA CO(1-0) and CO(2-1) data we decompose the emission into a disk and non-disk component. We find ∼ 7 − 16% of the CO luminosity to be associated with the non-disk component (1.2 − 4.2 × 10 7 K km s −1 pc 2 ). The total molecular gas mass in the center of NGC 253 is ∼ 3.6 × 10 8 M with ∼ 0.5 × 10 8 M (∼ 15%) in the non-disk component. These measurements are consistent across independent mass estimates through three CO transitions. The high-resolution CO(3-2) observations allow us to identify the molecular outflow within the non-disk gas. Using a starburst conversion factor, we estimate the deprojected molecular mass outflow rate, kinetic energy and momentum in the starburst of NGC 253. The deprojected molecular mass outflow rate is in the range ∼ 14 − 39 M yr −1 with an uncertainty of 0.4 dex. The large spread arises due to different interpretations of the kinematics of the observed gas while the errors are due to unknown geometry. The majority of this outflow rate is contributed by distinct outflows perpendicular to the disk, with a significant contribution by diffuse molecular gas. This results in a mass loading factor η =Ṁ out /Ṁ SFR in the range η ∼ 8 − 20 for gas ejected out to ∼ 300 pc. We find the kinetic energy of the outflow to be ∼ 2.5 − 4.5 × 10 54 erg and ∼ 0.8 dex typical error which is ∼ 0.1% of the total or ∼ 8% of the kinetic energy supplied by the starburst. The outflow momentum is 4.8 − 8.7 × 10 8 M km s −1 (∼ 0.5 dex error) or ∼ 2.5 − 4% of the kinetic momentum released into the ISM by feedback. The unknown outflow geometry and launching sites are the primary source of uncertainty in this study.

Cool outflows in galaxies and their implications

“…A bi-conical wind emerges out of this region, apparent in Xrays (Strickland et al 2000(Strickland et al , 2002, Hα emission and other ionized lines (Sharp and Bland-Hawthorn 2010;Westmoquette et al 2011), neutral sodium emission (Heckman et al 2000), polycyclic aromatic hydrocarbon emission (Tacconi-Garman et al 2005), OH absorption (Turner 1985;Sturm et al 2011), and spatially resolved molecular emission (Fig. 1e; Bolatto et al 2013a;Walter et al 2017;Zschaechner et al 2018;Krieger et al 2019). The geometry of the wind is well characterized through Hα observations, emerging approximately perpendicular to the plane of the galaxy and almost in the plane of the sky (inclination of 12 • ) with an opening angle of ∼ 60 • .…”

“…The molecular outflow in NGC 253 has a CO-emitting mass of ∼ 3-4 × 10 7 M , and molecular mass loss rates estimated to be ∼ 20 M year −1 (Zschaechner et al 2018;Krieger et al 2019). By comparison, the mass of the Hα-emitting phase is 10 7 M (Westmoquette et al 2011), and the inferred mass loss rate is ∼ 4 M year −1 (Krieger et al 2019). The mass-loss rate in the hot, X-ray emitting fluid is estimated to be < 2.2 M year −1 for reasonable flow parameters (Strickland et al 2000).…”

Frequency and nature of central molecular outflows in nearby star-forming disk galaxies

“…In this section, we present our procedure to systematically identify central molecular outflows in a large sample of galaxies with similar properties. Rather than case studies tailored to individual galaxies (e.g., NGC 0253; Krieger et al 2019), we present a generic approach to identify galaxies which are likely to possess molecular outflows. This allows us to statistically compare properties of outflows (e.g., frequency, outflow masses, and outflow rates) as well as the non-detection rate of the sample in a robust, quantitative way.…”

“…Galactic outflows are known to consist of multiple gas phases: hot X-ray gas (e.g., Komossa et al 2003), warm ionized gas ([O iii], Hα; Venturi et al 2018;López-Cobá et al 2020;Hogarth et al 2021), neutral atomic gas (e.g., the Na i doublet; Sato et al 2009), warm and cold molecular gas (OH, CO; Feruglio et al 2010;Veilleux et al 2013;Zschaechner et al 2018;Lutz et al 2020), and dense molecular gas (HCN, HCO + ; Michiyama et al 2018;Krieger et al 2019). The molecular gas phase may dominate the outflow mass in certain active galaxies as revealed by multiphase observations of nearby galaxies, such as NGC 0253 (Krieger et al 2019), quasar hosts at redshift z ∼ 2.4 (Carniani et al 2015) or AGN hosts when comparing the molecular to their ionized gas phase (Fluetsch et al 2019). This motivates a detailed characterization of the molecular phase of outflows.…”

The centres of M83 and the Milky Way: opposite extremes of a common star formation cycle

“…As star formation is highly localised in both space and time, the resulting feedback will be much more efficient at driving galactic-scale nuclear outflows (e.g. the outflow currently being driven in NGC 253, Krieger et al 2019;Zschaechner et al 2018) than the same star formation integrated over the whole duty cycle.…”