DAAD Project B

»All-Sky Survey of Metal Absorption in the Circumgalactic Medium of the Milky Way«

Prof. Dr. Philipp Richter (UP, advisor) and Prof. Dr. Lutz Wisotzki (AIP, co-advisor)


Background: Spiral galaxies like the Milky Way are surrounded by large amounts of diffuse gas. This so-called circumgalactic medium (CGM) represents both intergalactic material that is being accreted to fuel star formation in galaxies and gas that is being expelled from galaxies by way of galactic winds and outflows. For our understanding of galaxy formation and evolution it is of crucial importance to observationally pinpoint the distribution and physical nature of infalling and outflowing circumgalactic gas [1].

Aims: In this project, we will extensively study the distribution and physical conditions in the CGM of the Milky Way. Because of the multi-phase nature of the CGM, that contains ionized and neutral gas within a large range of column densities, ultraviolet (UV) absorption spectroscopy towards extragalactic background sources provides the most efficient method to investigate the physical properties in the gas and its spatial distribution. We will use more than 300 UV quasar spectra taken with the Cosmic Origins Spectrograph (COS) [2] installed on the Hubble Space Telescope (HST) to constrain the infall and outflow rate of gas in the Milky Way, the total gas mass and metallicity of the Milky Way’s CGM, and the role of minor merger processes for the distribution of gas in the Milky Way halo.

Methods: The publicly available HST/COS spectra will be downloaded and analyzed using custom-written routines within the ESO-MIDAS software package. The student will focus on the analysis of absorption lines from low and high metal ions in the Milky Way’s CGM and determine column densities and absorption characteristics for several species including Si II, Si III, Fe II, N I, and OI in the radial-velocity range that is relevant for local circumgalactic gas (|vLSR| = 50 - 500 km/s). Using supplementary 21-cm H I emission-line data from the new Effelsberg 21-cm all-sky survey (EBHIS) [3] together with photoionization models, the measured ion column densities and their ratios will be used to constrain the overall metallicity in the gas and to search for typical enrichment patterns (e.g., from the N/O ratio) that yield insights into the chemical enrichment history of the gas. The observed distribution of radial velocities of the metal absorption lines will be analyzed to pinpoint the kinematics of infalling and outflowing gas and to determine the infall and outflow rates. Finally, the observational results will be implemented into a kinematic model of the Milky Way to compare the absorption characteristics of the Milky Way’s CGM from an interior viewpoint with spectroscopic observations of the gas distribution around other low-redshift galaxies (exterior viewpoint).




  1. P. Richter, 2012: Cold Gas Accretion by High-Velocity Clouds and their Relation to QSO Absorption Line Systems. The Astrophysical Journal, 750, 165
  2. J.C. Green, C.S. Froning, S. Osterman, et al., 2012: The Cosmic Origins Spectrograph. The Astrophysical Journal, 744, 60
  3. B. Winkel, P.M.W. Kalberla, J. Kerp, L. Flöer, 2010: The Effelsberg-Bonn HI Survey: Data Reduction. The Astrophysical Journal Supplements, 188, 488