A huge reservoir of ionized gas around the Milky Way: Accounting for the Missing Mass? (1205.5037v4)

Abstract: Most of the baryons from galaxies have been "missing" and several studies have attempted to map the circumgalactic medium (CGM) of galaxies in their quest. Recent studies with the Hubble Space Telescope have shown that many galaxies contain a large reservoir of ionized gas with temperatures of about 10^5 K. Here we report on X-ray observations made with the Chandra X-ray Observatory probing an even hotter phase of the CGM of our Milky Way at about 10^6 K. We show that this phase of the CGM is massive, extending over a large region around the Milky Way, with a radius of over 100 kpc. The mass content of this phase is over ten billion solar masses, many times more than that in cooler gas phases and comparable to the total baryonic mass in the disk of the Galaxy. The missing mass of the Galaxy appears to be in this warm-hot gas phase.

• The paper finds that O VII absorption in 21 of 29 AGNs yields a 72% covering fraction for the ionized circumgalactic medium.
• The paper estimates the CGM extends beyond 100 kpc and contains over ten billion solar masses, potentially resolving the missing baryon problem.
• The paper combines absorption and emission measures to constrain gas temperature and density, supporting models of extensive warm-hot halos in galaxy evolution.

A Huge Reservoir of Ionized Gas Around the Milky Way: Accounting for the Missing Mass?

This paper investigates the elusive issue of missing baryons in the Milky Way by examining the warm-hot circumgalactic medium (CGM) using X-ray observations from the Chandra X-ray Observatory. The researchers focus on detecting absorption lines of highly ionized oxygen, specifically $\text{O\,VII}$ and $\text{O\,VIII}$, along extragalactic sight lines. Their findings suggest that a significant portion of the Milky Way's missing baryonic mass resides in this ionized gas phase.

Methodology and Observations

The authors utilize both the High and Low Energy Transmission Gratings onboard the Chandra telescope to discern the $\text{O\,VII}$ and $\text{O\,VIII}$ absorption features in the spectra of bright active galactic nuclei (AGNs). A robust sample was constructed by selecting AGN targets with publicly available Chandra observations, ensuring sufficient exposure time for high signal-to-noise ratios. Importantly, the paper combines these absorption measurements with existing emission measure data to determine the density and extent of the CGM.

The results show $\text{O\,VII}$ K$\alpha$ absorption lines are present in 21 out of the 29 sources studied, indicating a covering fraction of approximately 72%. The $\text{O\,VII}$ K$\beta$ absorption lines are observed in about 30% of the targets. By employing the technique of comparing multiple absorption lines from the same ion, the authors derive constraints on the column densities and Doppler parameters, finding the $\text{O\,VII}$ column densities to span from $10^{15.82}$ to $10^{16.50}$ cm$^{-2}$.

Results and Implications

The authors find that the temperature of the warm-hot CGM ranges from $\log T = 6.1-6.4~K$, consistent with collisional ionization equilibrium conditions. They report that this ionized gas phase is surprisingly extensive, stretching over a radius exceeding 100 kpc, and possessing a mass exceeding ten billion solar masses. This mass is comparable to the total baryonic mass within the Milky Way's disk and far exceeds the cooler gas phases previously studied.

Through the combination of absorption and emission measures, the paper estimates the pathlength and electron density of the ionized CGM. Assuming a metallicity of $Z = 0.3 Z_{\odot}$, plausible given typical estimates for circumgalactic environments, the pathlength could be as extensive as $239 \pm 100$ kpc. The calculated baryonic mass is significant, potentially accounting for the Milky Way's missing baryonic fraction and suggesting that the warm-hot CGM may play a crucial role in galactic and large-scale structure dynamics.

Theoretical Considerations and Future Directions

The findings presented align well with recent cosmological simulations that predict the presence of extensive ionized gas halos around galaxies. These high mass estimates challenge some models but agree with others that propose substantial warm-hot baryonic reservoirs in the Local Group or Milky Way's CGM.

Conclusively, this paper significantly advances our understanding of baryonic matter distribution in galaxy halos and the role of CGMs in galactic dynamics and evolution. Future theoretical modeling and simulations incorporating these empirical findings are essential. Furthermore, upcoming observational campaigns, possibly with next-generation X-ray observatories, could offer higher spectral resolution data to refine these estimates and provide greater insights into the baryonic matter's configuration and movements within galactic environments.