Low-column density intermediate- and high-velocity gas in the halo of the Milky Way
More than 40 years ago the occurrence of several narrow absorption lines in QSO (quasi-stellar object) spectra was recognized for the first time. Soon after their detection it became clear that the absorption lines are related to intervening gaseous structures in the intergalactic medium (IGM). In recent years, substantial instrumental progress has been made to measure circumgalactic gas structures around the Milky Way and other galaxies. Now it is obvious that the gas structures play a fundamental role in the formation and evolution of the Milky Way and other spiral galaxies.
The most promising local counterparts of intervening circumgalactic metal-absorbers seen in QSO absorption line data are the so-called low-, intermediate-, and high-velocity clouds in the halo of the Milky Way. They represent clouds of neutral atomic hydrogen seen in 21 cm emission at radial velocities inconsistent with a simple model of Galactic disk rotation.
While there are a large number of recent absorption studies on the nature of LVCs, IVCs, and HVCs and their role for the evolution of the Milky Way, relatively little effort has been made to investigate their connection to the distribution and nature of intervening metal-absorption systems seen in QSO spectra around other galaxies. In fact, almost all recent absorption studies of IVCs and HVCs were carried out in the FUV to study in detail metal abundances and ionisation conditions of halo clouds using the many available transitions of low and high ions in the ultraviolet regime. These studies were designed as follow-up absorption observations of known IVCs and HVCs, thus providing an 21 cm emission-selected data set. However, to statistically compare the absorption characteristics of the extraplanar Galactic halo structures with the properties of intervening metal-absorption systems towards QSOs we use an absorption-selected data set of IVCs and HVCs. Since in the UV band there is currently only a very limited number (< 50) of high-quality spectra available such a statistical comparison can be done best in the optical regime, where a large number of high-quality spectra of low- and high-redshift QSOs are available.
Together with Prof. Philipp Richter and Michael Murphy we analyse low-column density extraplanar gaseous structures detected in optical CaII and NaI absorption towards quasars using data from the UVES/VLT data archive. In total 400 lines of sight were observed, providing one of the largest samples today for analyses of IVC and HVC gas which is most likely located in the inner and outer halo of the Milky Way. The study allows us to directly compare the observed absorption column-density distribution of gas in the Milky Way halo with the overall column-density distribution of intervening absorbers towards QSOs. Moreover, the analysis enables us to identify the neutral and ionised absorption structures at low gas column densities and small angular extent that remain unseen in the large 21 cm IVC and HVC all-sky surveys, but that possibly have a considerable absorption cross section. We supplement our absorption-line data with new HI 21 cm observations (using the EBHIS and GASS survey) to investigate the relation between intermediate- and high-velocity CaII absorption features and halo 21cm emission.
The large sample of detected clouds enables for the first time a systematic statistical analysis of the properties of the low-column density cloud population in the Galactic halo. With these observations we demonstrate that the Milky Way halo contains a large number of low-column density neutral gas structures that give rise to intermediate- and high-velocity CaII and NaI absorption. In some cases, the CaII and NaI absorption lines are associated with known intermediate- and high-velocity clouds, but in other cases the observed absorption has no 21 cm counterpart. The observed CaII column density distribution is similar to the distribution found for intervening MgII systems that trace the gaseous environment of other galaxies at low and high redshift. The follow-up observations with radio synthesis telescopes uncovers several cold and compact (sub-pc scale) clumps.
With our work, we want to answer the question whether these low-column density small-scale structures are common in the extraplanar environment of the Milky Way and how they influence the evolution of our Galaxy. The goal is to place the gaseous environment of the Milky Way into a cosmological context.
Interstellar and Intergalactic magnetic fields
'Cosmic Magnetism' is one of six Key Science Projects of LOFAR. LOFAR stands for Low Frequency Array and is a new-generation radio telescope array. The stations are distributed in the Netherlands, Germany and other European countries. Its high sensitivity and angular resolution will open a new window to low-frequency observations below 250 MHz.
Using this new instrument we want to probe interstellar and intergalactic magnetic fields. Although, there are magnetic field everywhere in the universe their origin, structure and, evolution are still under scientific debate. Baryonic matter is coupled to magnetic fields, magnetic fields contribute to the nonlinear interplay of turbulent motions in the intracluster/interstellar medium and magnetic energy affects the evolution of galaxies (star formation, spiral structure). The expected data from the LOFAR array will allow us to investigate the distribution and origin of magnetic fields in galaxies and galaxy clusters. For more details see the homepage of Prof. Uli Klein
Kinematics and structures of dwarf galaxies and their interstellar and intergalactic medium
In this project we study the distribution of Dark and baryonic matter in dwarf galaxies. Our focus is on dwarf galaxies with giant HI disks because they are ideal candidates for a rotation curve analysis out to large distances. For this purpose we use Single-Dish radio telescopes like the Effelsberg 100-m telescope and the GMRT and radio interferometers like WSRT, VLA and ATCA.
Velocity field of the dwarf galaxy UGCa105 created by Philip Schmidt in the framework of his diploma thesis.
Mass decomposition
We perform mass decompositions (stellar, gaseous and Dark matter) and compare with different Dark Matter (DM) halo profiles. Cosmological simulations of structure formation have shown that the density distribution of DM halos follows the Navarro, Frenk and White (NFW) profile. Here, the DM distribution is "cusp-dominated", which means that the distribution increases steeply towards the center. The NFW model seems not to be appropriate for all observed galaxies. An alternative are "core-dominated" models, where the DM distribution is increasing towards smaller radii but with a constant density in the center. For a detailed desciption see the homepage of Prof. Uli Klein.
Ongoing accretion
In the hierarchical structure formation scenario, galaxies undergo serveral merging and accretion processes during their history. Observations and simulations show that the accretion of gas plays a crucial role in the formation and evolution of galaxies. The dwarf galaxies in our samples are perfect candidates to observe the ongoing gas accretion since we probe large galactocentric radii where the gas is not yet settled into quasi-circular orbits.
Warps
Observing dwarf galaxies with large HI disks enables us to study deviations from flat disks, such as warps or lopsidedness Warps are expected to be the result of interaction with other galaies. Interestingly, warps also occur in isolated galaxies, so that in these cases they cannot be tidally disrupted by other galaxies.
Other projects
I am interested in the new generation radio telescope arrays ASKAP and MeerKAT. I participate in several science proposals like WALLABY and GASKAP. ASKAP and MeerKat are pathfinder instruments for the Square Kilometre Array (SKA). To systematic study the properties and distribution of galactic halo gas I am involved in several projects: EBHIS, HALOGAS, LOFAR, APERTIF and WNSHS