Letters of Intent received in 2016

LoI 2018-1892
Focus Meeting: Warm and Hot Baryonic Matter in the Cosmos

Topics

1. Census of cosmic baryons
2. Heating and distribution of baryons in the intergalactic medium
3. Kinematics and dynamics of galactic halos
4. Origin of soft diffuse X-ray background
5. Future prospects for dedicated spectroscopic EUV/X-ray missions

Rationale

At z~1000, the baryons produced in the Big Bang Nucleosynthesis (BBN) era are fully accounted for, based on observations of the cosmic microwave background (CMB) and detailed modeling of the acoustic oscillation peaks in the CMB power spectrum. At z~0, however, observations show that over a third of the BBN baryons are “missing”. Arguments have been put forth on the fate of these “missing” baryons – they mostly reside in shocked-heated gas in the large-scale filamentary structures or in the hot halos around galaxies. Given the very low densities and high temperatures of the gas in the cosmic web, it is challenging to “see” baryons in it even with state-of-the-art UV or X-ray instruments. The prospects for finding missing baryons improve near galaxies, but remain challenging. The problem is not merely of academic interest, as the distribution of cosmic baryons, as well as their physical properties, hinges upon the detailed physics of galaxy formation and evolution and will strongly test our current models for these processes.

On smaller scales, observations have shown that the baryon fraction is exceedingly low for dwarf galaxies, near the cosmic average for rich clusters of galaxies, and normal galaxies lie somewhere in between. Taking the Milky Way as an example, its baryon fraction is only about 10% of the cosmic average. A correlation clearly exists between the baryon fraction and the depth of the gravitational potential well of a system, raising the question what happened to the missing baryons. Are they expelled from a galaxy by a supergalactic wind during its evolution, or not enough baryons fell into the gravitational potential well of the dark matter halo during its formation? Once again, the answer depends on the detailed physics of galaxy formation and evolution.

The locally missing baryons may reside in the circumgalactic medium. Observations show that extended hot halos seem to be common among early type galaxies and are also present around a number of massive spiral galaxies. It is still an open question as to how the halo gas is heated. Supernova- or AGN-driven outflows are thought to play important roles. In many cases, however, detailed analyses show that hot halos cannot account for the missing baryons unless the density profile extends much beyond the virial radius. On the other hand, Planck observations of SZ effects seem to indicate that the halos can extend much beyond the virial radius of a galaxy, which would imply that the missing baryons are hot. A more direct way of probing these baryons is through measuring absorption and emission lines at soft X-ray energies. This has been done for the Milky Way, with results that imply the presence of an extended hot halo. Similar measurements are not yet possible for other galaxies even with state-of-the-art X-ray spectrometers.

Efforts are under way to design and develop spectroscopic missions at EUV and soft X-ray wavelengths that are dedicated to survey the distribution of baryons in circumgalactic and intergalactic media, through absorption and emission lines mainly of O VI, O VII, O VIII, and possibly also of C IV, C V, N VI, N VII, and Ne IX. The missions would be complementary to ESA’s Athena mission, with much enhanced spectroscopic capabilities in EUV and soft X-rays. For X-ray spectrometers, different technologies are available, with their respective pros and cons, so trade-offs are necessary for specific scientific priorities. The proposed Focus Meeting will bring together experts on science and technology to discuss the key science drivers and the required technologies and thus to further the mission concepts. A dedicated spectroscopic mission would also enable research on many other important topics, including the kinematics and dynamics of galactic halos, the origin of diffuse X-ray background, the properties of supernova remnants, the roles of charge exchange processes, amongst other processes.

The scientific scope of the proposed Focus Meeting goes much beyond any one Division. Although the proposed coordinating Division is Division J (Galaxies and Cosmology), the themes of the meeting are also highly relevant to Division B (Facilities, Technologies, and Data Science), Division D (High Energy Phenomena and Fundamental Physics), and Division H (Interstellar Matter and Local Universe).