The Warm Hot intergalactic medium and the CIRCUMgalactic medium


In the galaxy, a fruitful way to map the distribution of ionized gas is by measuring the dispersion measure to pulsars.  Dispersion measure gives the total column of electrons to the source and is related to the temporal delay between a pulse in different frequency bands.  However, we do not yet have a confirmed extragalactic source (pulsars can only be observed within the local group) that can be used to measure the dispersion of the intergalactic medium.  However, a population of bursts that appears to be extragalactic has been found.  Motivated by this development, I wrote a paper on how extragalactic dispersion measure estimates would be very interesting for constraining the distribution of matter in the low redshift IGM.  Unfortunately, none of the plots in this paper simple enough to include here, but the conclusion is that such measurements would be very interesting and constrain plausible models for the WHIM that are difficult or impossible to constrain with other known observables.


Dispersion measure as a probe of the WHIM


The intergalactic medium becomes very difficult to observe at z<1 (i.e. for the last 10 billion years).  This difficulty occurs partly because intergalactic gas is so low density by these times owing to the expansion of the Universe, that our strongest absorption line -- the Lyα line of atomic hydrogen -- is undetectable except in regions that are overdense by a factor of >10.  In addition, more massive structures are collapsing than at earlier times.  This leads to larger velocity intergalactic shocks and, hence, significant heating of intergalactic gas well above the 104 K temperatures that are otherwise expected.  Heating also occurs as galaxies expel out most of the gas that fell into them.  The final product is hot, >105K gas that is predicted to account for half of all the baryons.  This heating further reduces the fraction of observable atomic transitions owing to collisional ionization of many ions, leaving emission and absorption from highly ionized states of oxygen as the primary observable.

A related puzzle is the gas outside of galaxies but within halos, the circumgalactic medium (CGM).  It is unclear how much gas there is, whether its at the virial temperature of the halo, and how quickly it is accreting onto the central galaxy or being blown out by winds.  Understanding the CGM is likely essential for understanding which galaxies are star forming and which are not. 

I have recently began to think about these topics, and am especially interested in OVI observations (in light of recent COS data) and the thermal Sunyaev-Zel'dovich effect (which with stacking has been used to detect hot gas in 5x10^12 Msun halos with Planck)!