“Some Like It Hot!” A Journey from the Hot IGrM to the Multiphase CGM

22 April - 17 May 2024

Paola Popesso, Esra Bulbul, Marcella Brusa, Ian McCarthy, Nabila Aghanim

The exchange of mass, energy, and metals between galaxies, their surrounding circum-galactic medium (CGM), and the gas permeating halos at larger scale represents a fundamental part of the modern astrophysics. Both components still elude our knowledge. Indeed, we are very far from having a census of the CGM properties in the bulk of the galaxy population. Furthermore, the halo gas is well known at large scale only in the most massive halos of our Universe, the galaxy clusters (intra-cluster medium, ICM), but it is still very elusive in the bulk of the virtualized halo population at the group mass scale  (Intra-group medium IGrM).

For this reason, so far, the study of such baryon cycle has been linked almost exclusively to galaxy evolution in a so called "streetlight bias”. Albeit more difficult to detect, the IGrM and the CGM are the other two key players on the stage, and their evolution across cosmic time is linked to the baryon exchange as much as the galactic component. Due to the substantial lack of information on the nature of the CGM and IGrM at large scales, many key questions remain unanswered. For instance, how is the IGrM affected at large scales by the feedback imparted energy? In other words, how does the baryon cycle affect the large-scale structure of our Universe? How much energy, momentum and entropy are released to the still elusive CGM hot component around galaxies? What is the main mechanism of interplay between CGM and central galaxy?

With the advent of instrument such as eROSITA, XRISM and JWST, and the enormous variety of results collected by Alma, MUSE, KCWI and LOFAR, it is time to review the state of the art of this research field. The final goal of this workshop is to fill the knowledge gap and review in an unbiased way all the key aspects of the baryon cycle: from the hot IGrM enclosed in the bulk of the virtualized halo population, to the CGM at hundreds of kpc from the central galaxy, and down to the main mechanisms in act at the galaxy center able to trigger the baryonic exchange.

Tentative Programme:


Week 1 (22/4 to 26/4):  Down to the pc scale. At the core of the triggering mechanism.

  • Can observations firmly constrain gentler feedback as a function of the halo mass, or will the upcoming datasets provide a different picture?
  • How can observations constrain or verify the efficiencies of the feedback processes?
  • Can observations provide a statistical picture of feedback duty cycle as a function of different environments or halo mass scale?
  • Will the combination of upcoming eROSITA, XRISM and LOFAR data confirm the predicted scenario in the local Universe?
  • What do we learn by combining multiwavelength observations capturing different gas phases of AGN outflows?


Week 3 (29/4 to 3/5):  At the kpc scale. The CGM-galaxy-AGN interplay: mechanisms of the baryonic exchange.

  • Can we create a coherent picture of the nature of the CGM and its conditions: what is the incidence of a single phase hot CGM and a multiphase CGM?
  • How precisely can we constrain observationally the relation between the thermodynamic conditions of the CGM and the activity of the central galaxy?
  • Will the upcoming datasets open up the possibility to extend the diagnostic plots of the precipitation and CCA framework to the bulk of the low mass halos?
  • Do we have a sufficient theoretical understanding of the feedback-CGM interplay to design specific diagnostics able to discriminate among different feedback implementation or flavors?


Week 3  (6/5 to 10/5): Within the inner 200 kpc. From hot to multiphase CGM

  • Will we be able to finally observe the hot component of the CGM around the bulk of the galaxy population, from star forming to quiescent galaxies?
  • Are predictions in agreement with observations? More in general, will we be able to constrain the gas mass of the different phases to eventually account for all the baryon mass as a function of the halo mass?
  • Will these observations finally discriminate among different models, and will these results be in agreement with what obtained at larger scales (see previous point)?
  • What is the incidence of UV traced gas observed in galaxy groups and how often is the CGM shared among bounded galaxies?
  • Can we consider a multiphase IGrM?
  • How does this change our picture of the CGM metal enrichment and is this consistent with the predictions of hydrodynamical simulations modeling the interacting CGM in low mass halos?


Week 4  (13/5 to 17/5): At the Mpc scale. The mysterious Universe looked up in low mass halos

  • What is the slope and the scatter of the main X-ray scaling relations down to the galaxy group regime? 
  • Is the scatter related to the current and past AGN activity of the central galaxy at the group mass scale? 
  • In other words, is the X-ray appearance of low mass halos dictated by the interplay of the feedback at pc scale and the IGM at Mpc scale?
  • Can low mass halos be deprived of their gas by the feedback?
  • Which is the feedback implementation that best reproduces the upcoming observations of gas and baryon mass fraction of low mass halos down to the Milky Way sized groups?
  • Is the IGrM multiphase? How much of the warm gas from cosmic filaments is retained in groups?
  • Does the halo assembly bias affect also the gas distribution in halos?