Leonidas Dedes

Dark matter in the Milky Way II. The HI gas distribution as a tracer of the gravitational potential

Context. Gas within a galaxy is forced to establish pressure balance against gravitational forces. The shape of an unperturbed gaseous disk can be used to constrain dark matter models. Aims. We derive the 3D H i volume density distribution for the Milky Way out to a galactocentric radius of 40 kpc and a height of 20 kpc to constrain the Galactic mass distribution. Methods. We used the Leiden/Argentine/Bonn all sky 21-cm line survey. The transformation from brightness temperatures to densities depends on the rotation curve. We explored several models, reflecting different dark matter distributions. Each of these models was set up to solve the combined Poisson-Boltzmann equation in a self-consistent way and optimized to reproduce the observed flaring. Results. Besides a massive extended halo of M ∼ 1.8 × 10 12 M� , we find a self-gravitating dark matter disk with M = 2t o 3 × 10 11 M� , including a dark matter ring at 13 < R < 18.5 kpc with M = 2. 2t o 2.8 × 10 10 M� . The existence of the ring was previously postulated from EGRET data and coincides with a giant stellar structure that surrounds the Galaxy. The resulting Milky Way rotation curve is flat up to R ∼ 27 kpc and slowly decreases outwards. The H i gas layer is strongly flaring. The HWHM scale height is 60 pc at R = 4 kpc and increases to ∼2700 pc at R = 40 kpc. Spiral arms cause a noticeable imprint on the gravitational field, at least out to R = 30 kpc. Conclusions. Our mass model supports previous proposals that the giant stellar ring structure is due to a merging dwarf galaxy. The fact that the majority of the dark matter in the Milky Way for R < 40 kpc can be successfully modeled by a self-gravitating isothermal disk raises the question of whether this massive disk may have been caused by similar merger events in the past. The substructure in the Galactic dark matter disk suggests a dissipative nature for the dark matter disk.

Global properties of the H I distribution in the outer Milky Way - Planar and extra-planar gas

Aims: We derive the 3-D HI volume density distribution for the Galactic disk out to R = 60 kpc. Methods: Our analysis is based on parameters for the warp and rotation curve derived previously. The data are taken from the Leiden/Argentine/Bonn all sky 21-cm line survey. Results: The Milky Way HI disk is significantly warped but shows a coherent structure out to R = 35 kpc. The radial surface density distribution, the densities in the middle of the warped plane, and the HI scale heights all follow exponential relations. The radial scale length for the surface density distribution of the HI disk is 3.75 kpc. Gas at the outskirts for 40 < R < 60 kpc is described best by a distribution with an exponential radial scale length of 7.5 kpc and a velocity dispersion of 74 km/s. Such a highly turbulent medium fits also well with the average shape of the high velocity profile wings observed at high latitudes. The turbulent pressure gradient of such extra-planar gas is on average in balance with the gravitational forces. About 10% of the Milky Way HI gas is in this state. The large scale HI distribution is lopsided; for R < 15 kpc there is more gas in the south. The HI flaring indicates that this asymmetry is caused by a dark matter wake, located at R = 25 kpc in direction of the Magellanic System. Conclusions: The HI disk is made up of two major components. Most prominent is the normal HI disk which can be traced to R = 35 kpc. This is surrounded by a patchy distribution of highly turbulent gas reaching large scale heights but also large radial distances. At the position of the Sun the exponential scale height in the z direction is 3.9 kpc. This component resembles the anomalous gas discovered previously in some galaxies.

EVIDENCE FOR CHIMNEY BREAKOUT IN THE GALACTIC SUPERSHELL GSH 242 03+37

We present new high-resolution neutral hydrogen (H I) images of the Galactic supershell GSH 242-03+37. These data were obtained with the Parkes Radiotelescope as part of the Galactic All-Sky Survey (GASS). GSH 242-03+37 is one of the largest and most energetic H I supershells in the Galaxy, with a radius of 565 ± 65 pc and an expansion energy of 3 × 1053 ergs. Our images reveal a complicated shell with multiple chimney structures on both sides of the Galactic plane. These chimneys appear capped by narrow filaments about 1.6 kpc above and below the Galactic midplane, confirming structures predicted in simulations of expanding supershells. The structure of GSH 242-03+37 is extremely similar to the only other Galactic supershell known to have blown out of both sides of the plane, GSH 277+00+36. We compare the GASS H I data with X-ray and Hα images, finding no strong correlations.

HI4PI: a full-sky H i survey based on EBHIS and GASS

Deutsche Forschungsgemeinschaft (DFG) [KA1265/5-1, KA1265/5-2, KE757/71, KE757/7-2, KE757/7-3, KE757/11-1.]; International Max Planck Research School for Astronomy and Astrophysics at the Universities of Bonn and Cologne (IMPRS Bonn/Cologne); Estonian Research Council [IUT26-2]; European Regional Development Fund [TK133]; Australian Research Council Future Fellowship [FT150100024]; NSF CAREER grant [AST-1149491]

Does the stellar distribution flare? A comparison of stellar scale heights with LAB H I data

The question of whether the stellar populations in the Milky Way take part in the flaring of scale heights as observed for the H I gas is a matter of debate. Standard mass models for the Milky Way assume a constant scale height for each of the different stellar distributions. However, there is mounting evidence that at least some of the stellar distributions reach, at large galactocentric distances, high altitudes, which are incompatible with a constant scale height. We discuss recent observational evidence for stellar flaring and compare it with H I data from the Leiden/Argentine/Bonn survey. Within the systemic and statistical uncertainties we find a good agreement between both.

The Outskirts of Milky Way

We investigate the disk-halo connection in the Milky Way and analyze the thick disk layer of HI gas which surrounds the Milky Way. To extract this part of the HI gas we determine first the global properties of the neutral gas disk. We use the Leiden/Argentine/Bonn (LAB) HI survey (Kalberla et al. 2005) to determine the shape of the disk. We first convert our dataset to Galactocentric coordinates. Then the first and second moments are used to infer the Warp and the flaring of the disk, respectively. Finally we subtracted from the data modes W0,W1,W2 and we fitted the residuals for possible modes 3 < n < 20 to search for scalloping in the disk. The disk flares with scale height 600pc at R ∼ 20kpc. We find three well defined modes for the warp, out to R = 40kpc. Higher modes have relative power 13% and the mode frequency is not constant through out the rings. So we are unable to detect scalloping in the Milky way disk, concluding that if present, it is a local phenomenon. The determination of the properties of the Milky Way disk, together with a Milky Way mass model (Kalberla 2003), enable us to disentangle disk from the halo. In the LAB database we mask those parts which, according to the model, are expected to originate from the disk. After filtering, we derive maps representing the expected emission from the disk-halo interface. We observed at λ = 21cm an number of interesting regions using the 100-m Effelsberg Radio Telescope. A number of unresolved clumps were detected. They are at R ∼ 12.5kpc and z ∼ 2.5kpc, having TK ∼ 200K and showing a core-envelope structure. Follow up observations with WSRT resolved the clump into a number of arc-min cores column densities several times 1019cm−2.