Peter M. W. Kalberla

The cored distribution of dark matter in spiral galaxies

We present the Hi data for 5 spiral galaxies that, along with their Hα rotation curves, are used to derive the distribution of dark matter within these objects. A new method for extracting rotation curves from Hi data cubes is presented; this takes into account the existence of a warp and minimises projection effects. The rotation curves obtained are tested by taking them as input to construct model data cubes that are compared to the observed ones: the agreement is excellent. On the contrary, the model data cubes built using rotation curves obtained with standard methods, such as the first-moment analysis, fail the test. The Hi rotation curves agree well with the Hα data, where they coexist. Moreover, the combined Hα + Hi rotation curves are smooth, symmetric and extended to large radii. The rotation curves are decomposed into stellar, gaseous and dark matter contributions and the inferred density distribution is compared to various mass distributions: dark haloes with a central density core, � Cold Dark Matter (�CDM) haloes (NFW, Moore profiles), Hi scaling and MOND. The observations point to haloes with con

Accretion of low-metallicity gas by the Milky Way

Models of the chemical evolution of the Milky Way suggest that the observed abundances of elements heavier than helium (‘metals’) require a continuous infall of gas with metallicity (metal abundance) about 0.1 times the solar value. An infall rate integrated over the entire disk of the Milky Way of ∼1 solar mass per year can solve the ‘G-dwarf problem’—the observational fact that the metallicities of most long-lived stars near the Sun lie in a relatively narrow range. This infall dilutes the enrichment arising from the production of heavy elements in stars, and thereby prevents the metallicity of the interstellar medium from increasing steadily with time. However, in other spiral galaxies, the low-metallicity gas needed to provide this infall has been observed only in associated dwarf galaxies and in the extreme outer disk of the Milky Way. In the distant Universe, low-metallicity hydrogen clouds (known as ‘damped Lyα absorbers’) are sometimes seen near galaxies. Here we report a metallicity of 0.09 times solar for a massive cloud that is falling into the disk of the Milky Way. The mass flow associated with this cloud represents an infall per unit area of about the theoretically expected rate, and ∼0.1–0.2 times the amount required for the whole Galaxy.

The Diversity of High- and Intermediate-Velocity Clouds: Complex C versus IV Arch

We present Far Ultraviolet Spectroscopic Explorer (FUSE) and Space Telescope Imaging Spectrograph (STIS) observations of interstellar ultraviolet absorption lines in the Galactic high-velocity cloud Complex C and the Intermediate-Velocity Arch (IV Arch) in the direction of the quasar PG 1259+593 (l = 1206, b = +581). Absorption lines from C II, N I, N II, O I, Al II, Si II, P II, S II, Ar I, Fe II, and Fe III are used to study the atomic abundances in these two halo clouds at VLSR ~ -130 km s-1 (Complex C) and -55 km s-1 (IV Arch). The O I/H I ratio provides the best measure of the overall metallicity in the diffuse interstellar medium because ionization effects do not alter the ratio, and oxygen is at most only lightly depleted from the gas into dust grains. For Complex C, we find an oxygen abundance of 0.093 times solar, consistent with the idea that Complex C represents the infall of low-metallicity gas onto the Milky Way. In contrast, the oxygen abundance in the IV Arch is 0.98 times solar, which indicates a Galactic origin. We report the detection of an intermediate-velocity absorption component at +60 km s-1 that is not seen in H I 21 cm emission. The clouds along the PG 1259+593 sight line have a variety of properties, proving that multiple processes are responsible for the creation and circulation of intermediate and high-velocity gas in the Milky Way halo.

The FUSE Spectrum of PG 0804+761: A Study of Atomic and Molecular Gas in the Lower Galactic Halo and Beyond

We present an analysis of interstellar and intergalactic absorption lines in the Far-Ultraviolet Spectroscopic Explorer (FUSE) spectrum of the low-redshift quasar PG 0804]761 at intermediate (z em \ 0.100) resolution (FWHM D 25 km s~1) in the direction With a good signal-to-noise ratio l \ 138i.3, b \ 31i.0. (S/N) and the presence of several interesting Galactic and extragalactic absorption components along the sight line, this spectrum provides a good opportunity to demonstrate the ability of FUSE to do both interstellar and extragalactic science. Although the spectrum of PG 0804]761 is dominated by strong absorption from local Galactic gas at 0 km s~1, we concentrate our study on absorption by molecular hydrogen and neutral and ionized metals related to an intermediate-velocity cloud (IVC) in the lower Galactic halo at [55 km s~1, and on absorption from O VI extended to negative velocities. In the IVC, weak molecular hydrogen absorption is found in six lines for rotational levels 0 and 1, leading to a total column density of log N \ 14.71 ^ 0.30. We derive an O I gas-phase abundance for the IVC of H 2 solar. Lower abundances of other elements (Fe, Si) imply depletion onto dust grains or the 1.03 ~0.420.71 presence of higher, undetected ionization states. The presence of N II and Fe III absorption at [55 km s~1 indicates that a fraction of the hydrogen is ionized. From the relative abundances of O I and P II we estimate a degree of ionization H‘/(H0]H‘ )o fD19%. Absorption by O VI is found at velocities as negative as [110 km s~1, but no absorption from any species is found at velocities of D[180 km s~1, where absorption from the nearby high-velocity cloud complex A would be expected. We suggest that the extended O VI absorption traces hot gas situated above the Perseus spiral arm. Finally, we —nd intergalactic absorption by an intervening H I Lyb absorber at

Dark Matter in the Milky Way. I. The Isothermal Disk Approximation

We solve the combined Poisson-Boltzmann equation for the gravitational potential of the Milky Way for an isothermal distribution of stars and gas perpendicular to the Galactic disk. In contrast to the standard assumption that the dark matter is distributed in a spheroidal way, we focus our analysis on the question of whether a fraction of the Galactic dark matter might be distributed in a disklike way. Using gaseous tracers for the gravitational potential and taking all relevant observational constraints into account, we find that a disklike dark matter distribution with a local scale height of 4 kpc provides a self-consistent and stable solution of the Poisson-Boltzmann equation. This implies that both the mass distribution and the dynamics of the Milky Way for R 35 kpc are dominated by a massive dark matter disk. For larger galactocentric radii the gravitational potential approaches spherical symmetry, implying an approximately spherical dark matter distribution on such scales.

GASS: The Parkes Galactic All-Sky Survey - Update: improved correction for instrumental effects and new data release

The Galactic All-Sky Survey is a survey of Galactic atomic hydrogen emission in the southern sky observed with the Parkes 64-m Radio Telescope. The first data release (GASS I) concerned survey goals and observing techniques, the second release (GASS II) focused on stray radiation and instrumental corrections. We seek to remove the remaining instrumental effects and present a third data release. We use the HEALPix tessellation concept to grid the data on the sphere. Individual telescope records are compared with averages on the nearest grid position for significant deviations. All averages are also decomposed into Gaussian components with the aim of segregating unacceptable solutions. Improved priors are used for an iterative baseline fitting and cleaning. In the last step we generate 3-D FITS data cubes and examine them for remaining problems. We have removed weak, but systematic baseline offsets with an improved baseline fitting algorithm. We have unraveled correlator failures that cause time dependent oscillations; errors cause stripes in the scanning direction. The remaining problems from radio frequency interference (RFI) are spotted. Classifying the severeness of instrumental errors for each individual telescope record (dump) allows us to exclude bad data from averages. We derive parameters that allow us to discard dumps without compromising the noise of the resulting data products too much. All steps are reiterated several times: in each case, we check the Gaussian parameters for remaining problems and inspect 3-D FITS data cubes visually. We find that in total ~1.5% of the telescope dumps need to be discarded in addition to ~0.5% of the spectral channels that were excluded in GASS II.The new data release facilitates data products with improved quality. A new web interface, compatible with the previous version, is available for download of GASS III FITS cubes and spectra.

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.

H I Clouds in the Lower Halo. I. The Galactic All-Sky Survey Pilot Region

We have detected over 400 H I clouds in the lower halo of the Galaxy within the pilot region of the Galactic All-Sky Survey (GASS), a region of the fourth quadrant that spans -->18° in longitude, -->40° in latitude, and is centered on the Galactic equator. These clouds have a median peak brightness temperature of 0.6 K, a median velocity width of 12.8 km s−1, and angular sizes 1°. The motion of these clouds is dominated by Galactic rotation with a random cloud-to-cloud velocity dispersion of 18 km s−1. A sample of clouds likely to be near tangent points was analyzed in detail. These clouds have radii on the order of 30 pc and a median H I mass of 630 -->M☉. The population has a vertical scale height of 400 pc and is concentrated in Galactocentric radius, peaking at -->R = 3.8 kpc. This confined structure suggests that the clouds are linked to spiral features, while morphological evidence that many clouds are aligned with loops and filaments is suggestive of a relationship with star formation. The clouds might result from supernovae and stellar winds in the form of fragmenting shells and gas that has been pushed into the halo rather than from a galactic fountain.

AN INTERACTION OF A MAGELLANIC LEADING ARM HIGH-VELOCITY CLOUD WITH THE MILKY WAY DISK

The Leading Arm of the Magellanic system is a tidally formed H I feature extending ~60° from the Magellanic Clouds ahead of their direction of motion. Using atomic hydrogen (H I) data from the Galactic All-Sky Survey (GASS), supplemented with data from the Australia Telescope Compact Array, we have found evidence for an interaction between a cloud in the Leading Arm and the Galactic disk where the Leading Arm crosses the Galactic plane. The interaction occurs at velocities permitted by Galactic rotation, which allows us to derive a kinematic distance to the cloud of 21 kpc, suggesting that the Leading Arm crosses the Galactic plane at a Galactic radius of -->R ≈ 17 kpc.

Modelling and simulation of large-scale polarized dust emission over the southern Galactic cap using the GASS Hi data

The Planck survey has quantified polarized Galactic foregrounds and established that they are a main limiting factor in the quest for the cosmic microwave background (CMB) B-mode signal induced by primordial gravitational waves during cosmic inflation. The necessity of achieving an accurate separation of the Galactic foregrounds therefore binds the search for the signal from cosmic inflation to our understanding of the magnetized interstellar medium (ISM). The two most relevant observational results coming out of Planck data analysis are the line of sight depolarization due to the fluctuations of the Galactic magnetic field orientation and the alignment of the dust filamentary structures with the magnetic field at high Galactic latitude. Furthermore, Planck and HI emission data in combination indicate that most of the dust filamentary structures are present in the cold neutral medium. The goal of this paper is to test whether together these salient observational results can account fully for the statistical properties of the dust polarization over a selected low column density portion within the southern Galactic cap (