The Circumgalactic Medium with the Sunyaev-Zeldovich Effect

Galaxies are physically complex systems. They are believed to contain multiple components, with only a few being visible to the eye. The visible components of the galaxy include the stars, dense gas clouds and filaments, and the overall structure of the galaxy one usually thinks of as a “galaxy” (for example, the beautiful spiral structures pictured by the Hubble Space Telescope and others). However, there are also less dense components that can only be observed by telescopes operating at different wavelengths, such as X-ray and radio waves. These telescopes allow astronomers to see extended structures beyond the central part of the galaxy, into the region known as the circumgalactic medium (CGM).

The CGM consists of a large reservoir of multiphase (in density, temperature, and kinematics) gas surrounding galaxies. There are no set definitions for the starting and stopping distances of the CGM, but it is loosely defined to extend out away from the galaxy several hundreds of kiloparsecs (and perhaps beyond to ~megaparsec scales). The CGM is believed to act as a medium through which the flow of material in and out of galaxies cycles. This cycle includes gas falling onto the disks of galaxies from the even larger scales of the intergalactic medium (IGM; the space between galaxies) and then returning back to the CGM and IGM through various feedback channels. All of these processes together help to shape the galaxy and processes occurring within it, dictating how the galaxy will change over time. This is a topic astronomers call galaxy evolution, and is an unsolved problem of modern extragalactic astronomy. The CGM is tied to the processes affecting galaxy evolution and is thus a very important component of the galaxy to be explored further.

While much more diffuse than the central parts of the galaxy, it is possible to observe the CGM in both emission and absorption. Additionally, it is possible to observe the CGM with the Sunyaev-Zeldovich (SZ) effect, which is further separated into the thermal and kinetic SZ effects (tSZ, kSZ). The tSZ and kSZ effects can give us information about the physical properties of the CGM. In more detail, the tSZ effect describes the increase in cosmic microwave background (CMB) photon energies due to scattering off electrons in galaxies and galaxy clusters, and is proportional to the pressure of the scattering system. The kSZ effect is the Doppler shift of CMB photons scattering off electrons in galaxies and clusters with non-zero velocity (in addition to the Hubble flow), and is proportional to the density of the scattering system. Combining the tSZ and kSZ results in complete thermodynamic information of the CGM that we can use to provide constraints on the physical processes governing galaxy evolution.

Along with observations such as the SZ effect, there has been significant progress in learning about the CGM using hydrodynamical simulations. This is where the recently introduced CAMELS suite comes into light. In this new study, we create simulated density and pressure radial profiles from the 1P set within CAMELS, which is a set of simulations only varying one of the feedback parameters at a time. In this way, we can see the differences among the radial profiles and properly attribute them to changing one parameter over another. We project these simulated profiles into SZ profiles (i.e., project the density profiles into kSZ profiles and project the pressure profiles into tSZ profiles) as they would be observed by a near-future CMB experiment, the Simons Observatory. Once we have all of the “observed” SZ profiles, we can see how they differ depending on the parameters being varied by the CAMELS 1P set and forecast how well we would be able to constrain these parameters if our simulated profiles were real.

We find that given the modeled galaxy sample and forecasted errors of the Simons Observatory in this work, we are able to constrain all four of the feedback parameters being varied in the CAMELS 1P simulations. We are even able to constrain some parameters within the 10% level. This result indicates that as our instruments continue to improve, we will be able to further restrict the parameter space of these astrophysical models and better understand the kinds of thermodynamic processes occurring within the CGM.

In summary, the CGM is a large and diffuse component of galactic structure. It is believed to govern the flow of material in and out of galaxies that affects how the galaxy changes over time, and is thus very important to understand. The CGM can be observed in several ways, including the SZ effects and hydrodynamical simulations. In this new study, we use the CAMELS simulations to create simulated CGM thermodynamic radial profiles of gas density and pressure and project them into observed SZ profiles. We quantify how the profiles differ as the simulations vary a few feedback models, and estimate how well we would be able to constrain the thermodynamics of the CGM through this method given real profiles of the same quality.

Further reading:


Post author:

Emily Moser

Graduate Student, Cornell University

Ithaca, NY, 14850, USA