X-Ray Detection of the Galaxy's Missing Baryons in the Circum-Galactic Medium of L$^*$ Galaxies

Abstract

The amount of baryons hosted in the disks of galaxies is lower than expected based on the mass of their dark-matter halos and the fraction of baryon-to-total matter in the universe, giving rise to the so called galaxy missing-baryon problem. The presence of cool circum-galactic matter gravitationally bound to its galaxy's halo up to distances of at least ten times the size of the galaxy's disk, mitigates the problem but is far from being sufficient for its solution. It has instead been suggested, that the galaxy missing baryons may hide in a much hotter gaseous phase of the circum-galactic medium, possibly near the halo virial temperature and co-existing with the cool phase. Here we exploit the best available X-ray spectra of known cool circum-galactic absorbers of L * galaxies to report the first direct high-statistical-significance (5.3−6.8σ) detection of associated O vii and N vi absorption in the stacked XMM-Newton and Chandra spectra of three quasars. We show that these absorbers trace hot medium in the X-ray halo of these systems, at logT(in k) 5.88 − 6.1 K (comprising the halo virial temperature T vir = 10 6 K). We estimate a mass of the X-ray halo M hot−CGM (1.4−1.6)×10 11 M , corresponding, for these systems, to a galaxy missing baryon fraction ξ = M hot−CGM /M missing 0.99 − 1.13 and thus closing the galaxy baryon census in typical L * galaxies. Our measurements contribute significantly to the solution of the long-standing galaxy missing baryon problem and to the understanding of the continuous cycle of baryons in-and-out of galaxies throughout the life of the universe.

Detection of Thermal Sunyaev–Zel’dovich Effect in the Circumgalactic Medium of Low-mass Galaxies—A Surprising Pattern in Self-similarity and Baryon Sufficiency

“…Also, X-ray absorption entirely depends on metal ions, which makes results dependent on the assumed abundance ratios of metals and the metal-to-hydrogen ratio. In addition to that, the lack of sufficient X-ray-bright QSO sightlines passing through the CGM of external galaxies limits the scope to probe the hot CGM of external galaxies in absorption (e.g., Mathur et al 2021;Nicastro et al 2023).…”

Constraining Circumgalactic Turbulence with QSO Absorption Line Measurements

“…The primary source of this information comes from absorption line spectroscopy, with surveys such as COS-Halos (Tumlinson et al 2013;Werk et al 2013Werk et al , 2016, the Keck Baryonic Structure Survey (Rudie et al 2012(Rudie et al , 2019Erb et al 2023), COS-Dwarfs (Bordoloi et al 2014), COS-GASS (Borthakur et al 2015(Borthakur et al , 2016, and the Cosmic Ultraviolet Baryon Survey (Chen et al 2020;Zahedy et al 2021;Qu et al 2022), using bright background quasars as light sources. Most of this work has been at rest-frame UV wavelengths, although some work has been done inX-ray wavelengths (e.g., Comparat et al 2022;Nicastro et al 2023). Observations such as these have shown that the CGM is highly ionized, extended, and multiphase, with warm ions such as C IV and Si IV being found at similar velocities as colder ions, such as Mg II, N II, and Si II (Tripp et al 2011;Meiring et al 2013;Burchett et al 2015Burchett et al , 2019.…”