Stacking; last refuge of…

 Searching for HI around MHONGOOSE Galaxies via Spectral Stacking

[astroph]

Veronese et al. 

The circum-galactic medium is under intense scrutiny recently because of the role it is suspected to play in sustaining star-formation and the metal (anything heavier than Helium) content of galaxies. 

There are a bunch of different approaches to the question of the balance within a galaxy: one can map various species in the galaxy using spectroscopy, one can look for absorption features in ultraviolet spectra (see Tumlinson+ 2017) and one can look for gas directly. The absorption route is popular in the US, the direct observation of cold gas is more of a European/South African thing. It depends on which telescope you have most likely access to. 

This paper uses a simulation — TNG50 — to estimate how to stack the 21cm HI atomic hydrogen line signal around nearby galaxies. There is an extensive argument on how to account for any motion before stacking. Here is the map of where one can expect HI (in TNG50 again):

The 21cm HI atomic hydrogen map of two TNG galaxies.

And this is the motion of that gas in the same two galaxies:

The velocity map of the two TNG galaxies. Both are rotating and given the side of the galaxy it is on, that rotation is a reasonable prior for any stacking of the small stuff on the outskirts which is probably not directly detected in observations.

So the models show how one could stack using the motion of the galaxy itself as a prior: there is a receding and approaching side and one can extrapolate how much redshift correction one should apply before stacking the spectra into a single spectrum. 

A TNG galaxy with identified areas for stacking. 

A stacked spectrum. One can classify stacked spectra as either visually visible in the cube or not even visible as a signal in the cube, only in the stacked spectrum. 

So equipped with this approach, the authors now stack where practical in the 18 MHONGOOSE galaxies already observed for this survey. This is not the full sample but it is about half, a fair number.

This is the stacked spectrum detections and their properties. Both visible and non-visible stacks work out fine. The issue is that there are not as many easily stacked images. There is not that much signal outside these disks!

The line properties of stacked areas either visually identified in the cube or stacked without a hint in the cube.

This is bad news for deep HI surveys. The hope was that the cold gas refueling nearby galaxies could be mapped with 21cm and further examined. As it stands, deeper observations will not reveal that much more 21cm line signal in ever more diffuse disks; at those column densities, the gas is ionized. 

This remains to be seen but I think it makes a good case that just hammering away blindly to get deeper 21cm obervations on single galaxies is maybe not the best approach. Spreading the observing time around a sample is a much better use of the time. It really validates the MHONGOOSE choice of depth and trade-off between depth and sample size. 

More to come of course. How much gas galaxies are receiving from their IGM remains a big unknown.