K. S. de Boer

Local-Group tests of dark-matter concordance cosmology - Towards a new paradigm for structure formation

Predictions of the concordance cosmological model (CCM) of the structures in the environment of large spiral galaxies are compared with observed properties of Local Group galaxies. Five new, most probably irreconcilable problems are uncovered: 1) A wide variety of published CCM models consistently predict some form of relation between dark-matter-mass and luminosity for the Milky Way (MW) satellite galaxies, but none is observed. 2) The mass function of luminous sub-haloes predicted by the CCM contains too few satellites with dark matter (DM) mass ≈10 7 Mwithin their innermost 300 pc than in the case of the MW satellites. 3) The Local Group galaxies and data from extragalactic surveys indicate there is a correlation between bulge-mass and the number of luminous satellites that is not predicted by the CCM. 4) The 13 new ultra-faint MW satellites define a disc-of-satellites (DoS) that is virtually identical to the DoS previously found for the 11 classical MW satellites, implying that most of the 24 MW satellites are correlated in phase-space. 5) The occurrence of two MW-type DM halo masses hosting MW-like galaxies is unlikely in the CCM. However, the properties of the Local Group galaxies provide information leading to a solution of the above problems. The DoS and bulge-satellite correlation suggest that dissipational events forming bulges are related to the processes forming phase-space correlated satellite populations. These events are well known to occur since in galaxy encounters energy and angular momentum are expelled in the form of tidal tails, which can fragment to form populations of tidal-dwarf galaxies (TDGs) and associated star clusters. If Local Group satellite galaxies are to be interpreted as TDGs then the substructure predictions of the CCM are internally in conflict. All findings thus suggest that the CCM does not account for the Local Group observations and that therefore existing as well as new viable alternatives have to be further explored. These are discussed and natural solutions for the above problems emerge.

Ultraviolet absorption by interstellar gas at large distances from the galactic plane

Eighteen high dispersion International Ultraviolet Exploration spectra of 6 stars in the large magellanic cloud (LMC) 3 stars in the small magellanic cloud (SMC) and 2 foreground stars were studied. Fourteen spectra cover the wavelengths lambda 1150-2000 A and 4 cover lambda 1900-3200 A. All the Magellanic Cloud star spectra exhibit exceedingly strong interstellar absorption lines due to a wide range of ionization stages at galactic velocities and at velocities associated with the LMC or SMC. The analysis is restricted to the Milky Way absorption features. Toward the LMC stars, the strong interstellar lines have a positive velocity extension, which exceeds the extension recorded toward the SMC stars. The most straightforward interpretation of these velocity extensions is obtained by assuming that gas at large distances away from the plane of the galaxy participates in the rotation of the galaxy as found in the galactic disk.

Making counter-orbiting tidal debris - The origin of the Milky Way disc of satellites?

Using stellar-dynamical calculations it is shown for the fir st time that counter-orbiting material emerges naturally i n tidal interactions of disc galaxies. Model particles on both pro- and retrograde orbits can be formed as tidal debris in single encounters with disc galaxies of 1-to-1 and 4-to-1 mass ratios. A total of 74 model calculations are performed for a range of different initial parameters. Interactions include fly-by and merger cases. The fraction o f counter-orbiting material produced varies over a wide range (from a few up to 50 percent). All fly-by models show a similar two-pha se behaviour, with retrograde material forming first. Prope rties of the prograde and retrograde populations are extracted to make an observational discrimination possible. During such encounters the tidal debris occupies a certain region in phase space. In this material, tidal-dwarf galaxie s may form. The modelling therefore can explain why galaxies may have dwarf galaxies orbiting counter to the bulk of their dwarf gala xies. An example is the Sculptor dwarf of the Milky Way, which orbits counter to the bulk of the disc of satellites. The modelling th us supports the scenario of the MW satellites being ancient tidal-dwarf galaxies formed from gaseous material stripped from another galaxy during an encounter with the young MW. A possible candidate for this galaxy is identified as the Mage llanic Cloud progenitor galaxy. Its angular motion fits the a ngular motion of the MW disc of satellites objects. This scenario is in agre ement with Lynden-Bell’s original suggestion for the origi n of the dSph satellites and the near-unbound orbit of the LMC.

Discovery of molecular hydrogen in a high-velocity cloud of the Galactic halo

The Milky Ways halo contains clouds of neutral hydrogen with high radial velocities which do not follow the general rotational motion of the Galaxy. Few distances to these high-velocity clouds are known, so even gross properties such as total mass are hard to determine. As a consequence, there is no generally accepted theory regarding their origin. One idea is that they result from gas that has cooled after being ejected from the Galaxy through fountain-like flows powered by supernovae; another is that they are composed of gas, poor in heavy elements, which is falling onto the disk of the Milky Way from intergalactic space. The presence of molecular hydrogen, whose formation generally requires the presence of dust (and therefore gas, enriched in heavy elements), could help to distinguish between these possibilities. Here we report the discovery of molecular hydrogen absorption in a high-velocity cloud along the line of sight to the Large Magellanic Cloud. We also derive for the same cloud an iron abundance which is half of the solar value. From these data, we conclude that gas in this cloud originated in the disk of the Milky Way.

Observational evidence for a hot gaseous Galactic corona

Galactic absorption features observed in high-dispersion far-UV spectra of two Large Magellanic Cloud (LMC) stars obtained with the IUE satellite are discussed. The stars observed were HD 38268 (OB + WN5) and HD 38282 (WN6), which lie at a distance of about 55 kpc and in the galactic direction corresponding to a latitude of about -32 deg and a longitude of about 280 deg. The line of sight to these stars passes through the disk and halo of the Milky Way and the halo and disk of the LMC. Evidence is presented for the existence of a hot (somewhat below 1 million K), low-density (about 0.001 H atom per cu cm) Galactic corona.

Can filamentary accretion explain the orbital poles of the Milky Way satellites

Several scenarios have been suggested to explain the phase-space distribution of the Milky Way (MW) satellite galaxies in a disc of satellites (DoS). To quantitatively compare these different possibilities, a new method analysing angular momentum directions in modelled data is presented. It determines how likely it is to find sets of angular momenta as concentrated and as close to a polar orientation as is observed for the MW satellite orbital poles. The method can be easily applied to orbital pole data from different models. The observed distribution of satellite orbital poles is compared to published angular momentum directions of subhalos derived from six cosmological state-of-the-art simulations in the Aquarius project. This tests the possibility that filamentary accretion might be able to naturally explain the satellite orbits within the DoS. For the most likely alignment of main halo and MW disc spin, the probability to reproduce the MW satellite orbital pole properties turns out to be less than 0.5 per cent in Aquarius models. Even an isotropic distribution of angular momenta has a higher likelihood to produce the observed distribution. The two Via Lactea cosmological simulations give results similar to the Aquarius simulations. Comparing instead with numerical models of galaxy-interactions gives a probability of up to 90 per cent for some models to draw the observed distribution of orbital poles from the angular momenta of tidal debris. This indicates that the formation as tidal dwarf galaxies in a single encounter is a viable, if not the only, process to explain the phase-space distribution of the MW satellite galaxies.

Evidence for hot gaseous coronae around the Magellanic Clouds

High-dispersion IUE far-UV spectra were obtained for five stars in the LMC and two stars in the SMC. At Magellanic Cloud velocities, all the spectra exhibit very strong interstellar lines of Al III, Si IV, and C IV, in addition to the usual lower ionization stage lines associated with H I region absorption. In two stars interstellar N V is also detected at the 50 mA level. From a consideration of the strength, width, and radial velocity of these high ionization stage lines, it is argued that regions of hot halo gas have probably been detected in front of the Magellanic Clouds. The regions detected exhibit absorption characteristics very similar to that of the Milky Way corona described previously by Savage and de Boer (1979). A cloud detected in the line of sight to HD 5980 in the SMC at a radial velocity of 300 km/s is of unknown origin.

Studying the populations of our Galaxy using the kinematics of sdB stars

We have analysed the kinematics of a sample of 114 hot subdwarf stars. For 2/3 of the stars, new proper motions, spectroscopic and photometric data are presented. The vast majority of the stars show a kinematic behaviour that is similar to that of Thick Disk stars. Some stars have velocities rather fitting to solar, i.e. Thin Disk, kinematics. About ∼15 objects have orbital velocities which differ considerably from those of Disk stars. These are members of the Galactic Halo. We investigated the velocity dispersions and calculated the orbits. Most stars feature orbits with disk character (eccentricity of less than 0.5), a few reach far above the Galactic plane and have very eccentric orbits (eccentricity of more than 0.7). The intermediate eccentricity range is poorly populated. This seems to indicate that the (Thick) Disk and the Halo are kinematically disjunct. Plotting a histogram of the orbit data points along z leads to the z-distance probability distribution of the star; doing this for the whole sample leads to the z-distance probability distribution of the sample. The logarithmic histogram shows two slopes, each representing the scale height of a population. The disk component has a scale height of 0.9 (±0.1) kpc, which is consistent with earlier results and is similar to that of the Thick Disk. The other slope represents a component with a scale height ∼7 kpc, a much flatter gradient than for the disk component. This shows that the vast majority of the sdBs are disk stars, but a Halo minority is present, too. The kinematic history and population membership of the sdB stars on the whole is different from that of the cooler HBA stars, which are predominantly or even exclusively Halo objects. This leads to the question, whether the Halo sdB stars are of similar origin as the HBA stars, or whether their kinematical behaviour possibly represents another origin, such as infalling stellar aggregates or inner disk events.

Kinematics of RHB stars to trace the structure of the galaxy

Red horizontal-branch (RHB) stars have been selected from the Hipparcos catalogue to investigate their kinematics and spatial distribution. Hipparcos parallaxes, literature radial velocities and Hipparcos proper motions, together with models for the gravitational potential of the Milky Way allow a calculation of the actual velocity vectors and the orbits of the RHB stars. The velocity characteristics are used to define a halo population sample (HPS) in the collection of RHBs. The orbits lead statistically to an overall z-distance probability distri- bution, showing that the RHBs exhibit two populations, a disk one having a scale height of hdisk ≃ 0.6 kpc and a halo one of ≃ 4 kpc. We have investigated the influence on our results of parallax accuracy and of a demarcation line in the HRD between the RHB and the red-giant (RG) star region. Neither of them show marked effects. We have performed the orbit analysis using the potential model of Allen & Santillan as well as of Dehnen & Binney. The results differ only slightly for the disk population, showing that these potential models are not a critical part of such orbit investigations. RHB scale height values are smaller than those found earlier for sdB stars, most likely because the samples of stars used had different spatial distributions a priori. The data do not allow us to specify a trend in the kinematic behaviour of star types along the horizontal branch.

ORFEUS echelle spectra: Molecular hydrogen in disk, IVC, and HVC gas in front of the LMC

In front of the LMC molecular hydrogen is found in absorption near 0 km s -1 , being local disk gas, near +60 km s -1 in an intermediate velocity cloud, and near +120 km s -1 , being a high velocity halo cloud. The nature of the gas is discussed based on four Orfeus far UV spectra of LMC stars and including data from the ground and from the IUE satellite. The local gas is cool and, given a span of sight lines of only