Research Byte

Published in the RSAA Lunations
Vol1 Issue8 1–30 September 2020

 Molecular Gas in the Galactic Centre Wind

Nuclear winds from galaxies are complicated beasts and the Milky Way’s nuclear wind is no exception.  In a paper recently published in Nature we showed that the Milky Way wind needs some more explanation.  For many years we have known that the Galactic Centre harbours a wind, which not only powers the Fermi Bubbles (Su et al 2010), but also drives multi-phase gas, including hundreds of atomic hydrogen clouds, outwards at velocities up to 330 km/s (McClure-Griffiths et al 2013, Di Teodoro et al 2018).  The jury has been somewhat divided as to whether the Galactic wind is powered by star formation within the Galaxy’s Central Molecular Zone (e.g. Crocker et al 2015) or from the activity associated with the supermassive black hole, Sagittarius A* (e.g. Bland-Hawthorn et al. 2019).  Last year we used ESO time on the APEX sub-millimetre telescope to search for molecular gas associated with the outflowing hydrogen clouds, in hopes of better constraining the amount of mass driven by the Milky Way’s wind and shedding some additional light on the wind origin.  As an initial search we targeted two clouds for 12CO (2-1) emission and found strong detections in both!  

The two clouds detected in CO have masses of about 300 solar masses and lie 0.8 and 1.8 kpc, respectively, from the Galactic Centre. Based on the detection of these two clouds we extrapolated to estimate that the total molecular mass expelled from the Galactic Centre is on the order of 0.1 solar mass/yr.  Given that the star formation rate is also approximately 0.1 solar mass /yr, the high mass loss rate implies a surprisingly high mass loading factor, similar to what is observed in starburst galaxies.  That mass loss rate is surprising if it is driven by star formation.  Alternatively, the wind could be driven by AGN activity, but we don’t yet know whether a relatively small black hole like Sgr A* could expel large gas masses like this.  In either scenario it is difficult to see how molecular gas can be accelerated to hundreds of km/s over ~2 kpc and not be disrupted.  In conclusion, the large amount of outflowing molecular gas from the Milky Way is challenging our understanding of galactic winds. 

We have a new large program approved on APEX to extend the study and place better constraints on where and how much molecular gas is associated with the Milky Ways.

Enrico Di Teodoro (formerly at RSAA, now at JHU/STScI), Naomi McClure-Griffiths (ANU-RSAA), F. Jay Lockman (GBO) & Lucia Armillotta (formerly at RSAA, now at Princeton), Nature on 20 August 2020, vol 584, 364 

(see also: https://arxiv.org/abs/2008.09121 )

Schematic figure from Mark Morris News & Views, Nature Astronomy showing the outflowing clouds in the interior of the Fermi Bubbles.

Professor Naomi McClure-Griffiths