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Cool outflows in galaxies and their implications

  • Review Article
  • Published:
The Astronomy and Astrophysics Review Aims and scope

Abstract

Neutral-atomic and molecular outflows are a common occurrence in galaxies, near and far. They operate over the full extent of their galaxy hosts, from the innermost regions of galactic nuclei to the outermost reaches of galaxy halos. They carry a substantial amount of material that would otherwise have been used to form new stars. These cool outflows may have a profound impact on the evolution of their host galaxies and environments. This article provides an overview of the basic physics of cool outflows, a comprehensive assessment of the observational techniques and diagnostic tools used to characterize them, a detailed description of the best-studied cases, and a more general discussion of the statistical properties of these outflows in the local and distant universe. The remaining outstanding issues that have not yet been resolved are summarized at the end of the review to inspire new research directions.

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Fig. 1

Images reproduced with permission from (a, b) Rupke and Veilleux (2011), (c) Veilleux et al. (2009b), copyright by AAS; and (b) Cicone et al. (2015), copyright by ESO; and (d, e) Bolatto et al. (2013a), copyright by Macmillan

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Notes

  1. Note that there is a typographical error in the numerical formulae of Rupke et al. (2005c): in their equations 13–18, the normalization factor of the column density N should be 10\(^{20}\) cm\(^{-2}\) rather than 10\(^{21}\) cm\(^{-2}\). The outflow energetics published in Rupke et al. (2005c) are based on the correct formulae and not affected by this error.

  2. Both Eqs. (28) and (29) assume \(T \sim 10^4\) K, while Eq. (29) further assumes that \(n_\mathrm{e} \lesssim 7 \times 10^5\) cm,\(^{-3}\) the critical density associated with the [O III] 5007 transition above which collisional de-excitation becomes significant.

  3. Just as for CO, high opacities can impact the critical density for e.g., HCN which may be very abundant in warm regions. In addition, the critical density can be strongly reduced (by factors 4–6) in regions of high temperature. This requires a multi-level treatment of the critical density.

  4. Sarangi et al. (2019) have recently suggested that nuclear AGN winds may also be sites of dust formation.

  5. There is also evidence that the AGN in the GC was \(\sim 10^5\) more active in the recent past (\(\sim 10^{2 -3}\) years), although still greatly sub-Eddington (\(L/L_{\mathrm{Edd}} \lesssim 10^{-5}\)), based on the detection of strong fluorescent \({\mathrm{Fe}}\,{\mathrm{K}}\alpha \) line emission off of molecular clouds near the GC (Sunyaev et al. 1993; Koyama et al. 1996; Ponti et al. 2010).

  6. For instance, Krumholz et al. (2017a) argue that the average SFR is closer to \(\sim \) 1 \(M_\odot \) \({\mathrm{year}}^{-1}\), more in line with the estimated mass supply rate to the CMZ (\(\gtrsim \) 1 \(M_\odot \) \({\mathrm{year}}^{-1}\), Sormani and Barnes 2019).

  7. As pointed out by Sparre et al. (2019), instabilities have a smaller effect in 3D than in 2D because a 3D flow has the freedom to use the z-direction to avoid disturbing dense clouds. As a result, the level of fragmentation is lower in 3D than in 2D, and the increase in covering fraction for large clouds in 3D is less than that seen in 2D.

References