The circum-galactic medium of the Milky Way

Our Milky Way galaxy, like other nearby galaxies, is missing most of its baryons (i.e. normal matter). Galaxies have also lost most of the metals that they produced. Cosmological simulations of galaxy formation suggest that the missing baryonic mass should reside in the circum-galactic medium (CGM), in a warm-hot gas phase at temperatures between one million and 10 million K. Although theoretical models predict the existence of the warm-hot gas in the CGM, detecting and characterizing the diffuse CGM has been difficult. At the expected temperatures the baryons are in the form of highly ionized plasma, observable in soft X-rays. A combination of absorption and emission studies at soft X-ray energies is required to fully characterize this warm-hot CGM. Recently, combining the Chandra observations of OVII and OVIII absorption lines and XMM-Newton and Suzaku measurements of the Galactic halo emission measure, we found that there is a huge reservoir of ionized gas around the Milky Way, with the mass of over 2 billion solar masses and the radius of over 100 kpc. Thus there appears to be more baryonic mass in the warm-hot CGM than in the entire disk of the Galaxy and as much mass in metals as in all the stars in the disk! While this has been an exciting new discovery, there are several avenues to make further progress by alleviating initial caveats and assumptions. In the initial work we compared values of absorption and emission averaged over the whole sky. However, studies of soft X-ray diffuse background show that the emission measure of the Galactic halo varies by an order of magnitude in different directions. Therefore it is crucial to determine the emission measure of warmhot gas near absorption sight-lines to understand the differences in physical properties of the CGM across the sky; this is one of the main goals of this proposal. Our proposed program has two parts: (1) new observations with Suzaku (now archived), and (2) archival XMM-Newton observations. We have been awarded 339.7ks of Suzaku time to determine the Galactic halo emission measure close to four sight-lines for which we have accurate absorption measurements. We request support for the analysis of these observations. With our novel XMM-Newton program we will determine emission and absorption parameters from the same observations. XMM-Newton has the ideal combination of large field of view and CCD spectrographs for emission studies and high-resolution grating spectrographs for absorption studies. We will (1) measure the absorption and emission parameters of the CGM along a large number of sight-lines; (2) determine the physical properties of the CGM such as temperature, density and path-length; (3) characterize the halo anisotropy by measuring variations from sight-line to sight-line; (4) model different radial density and temperature profiles and provide constraints from data; and finally (5) measure the mass of the CGM and its contribution to the missing baryons and missing metals problem. Mapping the warm-hot gaseous halo of the Milky Way is not just important to measure its mass content; accurate characterization of the CGM is also critical for comparison with theoretical models that predict the distribution, extent and physical properties of the CGM. With the proposed study we will present the best and the most comprehensive phenomenological picture of the circumgalactic medium which we will compare with theoretical models of galaxy formation. The proposed study is relevant to the NASA Strategic Goal to discover how the Universe works, explore how the Universe began and evolved into its present form. NASA has also partnered with ESA for the next generation X-ray mission Athena; missing baryons and the warm-hot intergalactic and circumgalactic medium is one of the key science goals of Athena for which the proposed study will provide the necessary ground work. PI Mathur has been appointed as a member of the missing baryons science working group of Athena.