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Extraplanar gas in galaxies 


One key property of star-forming galaxies is the presence of extended discs of "cold" gas. This gas can be atomic and neutral at various temperatures below 10000 K or molecular and very cold (typically temperatures <30 K). The atomic gas is mostly studied thanks to the emission at 21 cm of the neutral atomic hydrogen (HI), which constitutes the main component of the interstellar medium (ISM) of a disc galaxy at z=0. The local HI kinematics is characterised by relatively large velocity dispersions, typically 10 km/s, that tell us that the ISM is turbulent. The HI discs of spiral galaxies like the Milky Way extend to distances of tens of kiloparsecs from the galaxy centres but are vertically very thin. The typically scale height is of the order of 100 parsec (giving similar dimensions as a music compact disc (CD)!). However, this does not constitute all the HI that a galaxy contains.


Another component, discovered rather recently, is the so-called extraplanar HI or, more generally, extraplanar gas, as it is also observed ionised. This component typically comprises 10-30% of all the HI in a disc galaxy and extends, vertically, up to much larger distances from the disc plane, typically a few kiloparsecs. The existence of extraplanar gas can be unveiled only with observations with high sensitivity as its emission is much fainter that of the thin disc, being the gas in this extended layer at much lower column density. We see an example of this here below. In dynamical terms the HI in discs can be considered roughly in hydrostatic equilibrium, in the sense that its pressure (turbulent pressure mostly) balances the gravitational pull of stars, dark matter and the gas itself. Instead, the extraplanar gas is not in hydrostatic equilibrium and its ubiquitous presence testify to a continuous regeneration of these layers (typical timescales for gas clouds at a few kpc the galaxy plane are of only tens of million years compared to galaxy lifetimes of ~10 gigayears). For this reason, we think that at least a fraction of the extraplanar gas is produced by galactic fountains powered by stellar feedback.


Three generations of neutral hydrogen observations of the edge-on spiral galaxy NGC 891, obtained with the Westerbork Synthesis Radio Telescope in 1979 (left panel), 1997 (middle panel) and 2007 (right panel). The improvement in sensitivity of one order of magnitude between each observation led to the discovery of the extraplanar HI (middle panel) and next to the realization that this layer comprises 25% of the HI mass of the gas and hosts gas filaments extending up to more than 20 kpc from the disk (right panel). In the coming future, SKA will go down two more orders of magnitude in column density, we can only wonder what we will find.

The table below (from Marasco, Fraternali et al. 2019) reports the properties of the extraplanar gas layers in nearby (non-edge-on) galaxies from the HALOGAS survey.

For comparison the edge-on galaxy shown above, N891, has M_HI,EPG=1.2 billion M_Sun, h=2.2 kpc and velocity gradient 15 km/s, thus similar values to other galaxies.

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