Milky Way (milky + way)

Distribution by Scientific Domains

Selected Abstracts

An HST/ACS view of the inhomogeneous outer halo of M31,

J. C. Richardson
ABSTRACT We present a high precision photometric view of the stellar populations in the outer halo of M31, using data taken with the Hubble Space Telescope/Advanced Camera for Surveys. We analyse the field populations adjacent to 11 luminous globular clusters which sample the galactocentric radial range 18 ,R, 100 kpc and reach a photometric depth of ,2.5 mag below the horizontal branch (mF814W, 27 mag). The colour,magnitude diagrams are well populated out to ,60 kpc and exhibit relatively metal-rich red giant branches, with the densest fields also showing evidence for prominent red clumps. We use the Dartmouth isochrones to construct metallicity distribution functions which confirm the presence of dominant populations with ,[Fe/H],,,0.6 to ,1.0 dex and considerable metallicity dispersions of 0.2 to 0.3 dex (assuming a 10 Gyr population and scaled-solar abundances). The average metallicity over the range 30,60 kpc is [Fe/H]=,0.80 ± 0.14 dex, with no evidence for a significant radial gradient. Metal-poor stars ([Fe/H],,1.3) typically account for ,10,20 per cent of the population in each field, irrespective of radius. Assuming our fields are unbiased probes of the dominant stellar populations in these parts, we find that the M31 outer halo remains considerably more metal rich than that of the Milky Way out to at least 60 kpc. [source]

Quantitative analysis of clumps in the tidal tails of star clusters

A. Just
ABSTRACT Tidal tails of star clusters are not homogeneous but show well-defined clumps in observations as well as in numerical simulations. Recently, an epicyclic theory for the formation of these clumps was presented. A quantitative analysis was still missing. We present a quantitative derivation of the angular momentum and energy distribution of escaping stars from a star cluster in the tidal field of the Milky Way and derive the connection to the position and width of the clumps. For the numerical realization we use star-by-star N -body simulations. We find a very good agreement of theory and models. We show that the radial offset of the tidal arms scales with the tidal radius, which is a function of cluster mass and the rotation curve at the cluster orbit. The mean radial offset is 2.77 times the tidal radius in the outer disc. Near the Galactic Centre the circumstances are more complicated, but to lowest order the theory still applies. We have also measured the Jacobi energy distribution of bound stars and showed that there is a large fraction of stars (about 35 per cent) above the critical Jacobi energy at all times, which can potentially leave the cluster. This is a hint that the mass loss is dominated by a self-regulating process of increasing Jacobi energy due to the weakening of the potential well of the star cluster, which is induced by the mass loss itself. [source]

Spatial distribution of luminous X-ray binaries in spiral galaxies

Zhao-yu Zuo
ABSTRACT We have modelled the spatial distribution of luminous X-ray binaries (XRBs) in spiral galaxies that are like the Milky Way using an evolutionary population synthesis code. In agreement with previous theoretical expectations and observations, we find that both high- and low-mass XRBs show clear concentrations towards the galactic plane and bulge. We also compare XRB distributions under the galactic potential with a dark matter halo and the modified Newtonian dynamics potential, and we suggest that the difference may serve as potential evidence to discriminate between these two types of model. [source]

Galaxy growth in the concordance ,CDM cosmology

Q. Guo
ABSTRACT We use galaxy and dark halo data from the public database for the Millennium Simulation to study the growth of galaxies in the De Lucia et al. model for galaxy formation. Previous work has shown this model to reproduce many aspects of the systematic properties and the clustering of real galaxies, both in the nearby universe and at high redshift. It assumes the stellar masses of galaxies to increase through three processes, major mergers, the accretion of smaller satellite systems and star formation. We show the relative importance of these three modes to be a strong function of stellar mass and redshift. Galaxy growth through major mergers depends strongly on stellar mass, but only weakly on redshift. Except for massive systems, minor mergers contribute more to galaxy growth than major mergers at all redshifts and stellar masses. For galaxies significantly less massive than the Milky Way, star formation dominates the growth at all epochs. For galaxies significantly more massive than the Milky Way, growth through mergers is the dominant process at all epochs. At a stellar mass of 6 × 1010 M,, about that of the Milk Way, star formation dominates at z > 1 and mergers at later times. At every stellar mass, the growth rates through star formation increase rapidly with increasing redshift. Specific star formation rates are the decreasing function of stellar mass not only at z= 0 but also at all higher redshifts. For comparison, we carry out a similar analysis of the growth of dark matter haloes. In contrast to the galaxies, growth rates depend strongly on redshift, but only weakly on mass. They agree qualitatively with analytic predictions for halo growth. [source]

Tracing intermediate-mass black holes in the Galactic Centre

U. Löckmann
ABSTRACT We have developed a new method for post-Newtonian, high-precision integration of stellar systems containing a super-massive black hole (SMBH), splitting the forces on a particle between a dominant central force and perturbations. We used this method to perform fully collisional N -body simulations of inspiralling intermediate-mass black holes (IMBHs) in the centre of the Milky Way. We considered stellar cusps of different power-law indices and analysed the effects of IMBHs of different masses, all starting from circular orbits at an initial distance of 0.1 pc. Our simulations show how IMBHs deplete the central cusp of stars, leaving behind a flatter cusp with slope consistent with what has recently been observed. If an additional IMBH spirals into such a flat cusp, it can take 50 Myr or longer to merge with the central SMBH, thus allowing for direct observation in the near future. The final merger of the two black holes involves gravitational wave radiation which may be observable with planned gravitational wave detectors. Furthermore, our simulations reveal detailed properties of the hypervelocity stars (HVSs) created, and how generations of HVSs can be used to trace IMBHs in the Galactic Centre. We find that significant rotation of HVSs (which would be evidence for an IMBH) can only be expected among very fast stars (v > 1000 km s,1). Also, the probability of creating a hypervelocity binary star is found to be very small. [source]

Kinematics of hypervelocity stars in the triaxial halo of the Milky Way

Qingjuan Yu
ABSTRACT Hypervelocity stars (HVSs) ejected by the massive black hole at the Galactic Centre have unique kinematic properties compared to other halo stars. Their trajectories will deviate from being exactly radial because of the asymmetry of the Milky Way potential produced by the flattened disc and the triaxial dark matter halo, causing a change of angular momentum that can be much larger than the initial small value at injection. We study the kinematics of HVSs and propose an estimator of dark halo triaxiality that is determined only by instantaneous position and velocity vectors of HVSs at large Galactocentric distances (r, 50 kpc). We show that, in the case of a substantially triaxial halo, the distribution of deflection angles (the angle between the stellar position and velocity vector) for HVSs on bound orbits is spread uniformly over the range 10°,180°. Future astrometric and deep wide-field surveys should measure the positions and velocities of a significant number of HVSs, and provide useful constraints on the shape of the Galactic dark matter halo. [source]

The UKIRT Infrared Deep Sky Survey (UKIDSS)

A. Lawrence
ABSTRACT We describe the goals, design, implementation, and initial progress of the UKIRT Infrared Deep Sky Survey (UKIDSS), a seven-year sky survey which began in 2005 May. UKIDSS is being carried out using the UKIRT Wide Field Camera (WFCAM), which has the largest étendue of any infrared astronomical instrument to date. It is a portfolio of five survey components covering various combinations of the filter set ZYJHK and H2. The Large Area Survey, the Galactic Clusters Survey, and the Galactic Plane Survey cover approximately 7000 deg2 to a depth of K, 18; the Deep Extragalactic Survey covers 35 deg2 to K, 21, and the Ultra Deep Survey covers 0.77 deg2 to K, 23. Summed together UKIDSS is 12 times larger in effective volume than the 2MASS survey. The prime aim of UKIDSS is to provide a long-term astronomical legacy data base; the design is, however, driven by a series of specific goals , for example, to find the nearest and faintest substellar objects, to discover Population II brown dwarfs, if they exist, to determine the substellar mass function, to break the z= 7 quasar barrier; to determine the epoch of re-ionization, to measure the growth of structure from z= 3 to the present day, to determine the epoch of spheroid formation, and to map the Milky Way through the dust, to several kpc. The survey data are being uniformly processed. Images and catalogues are being made available through a fully queryable user interface , the WFCAM Science Archive ( The data are being released in stages. The data are immediately public to astronomers in all ESO member states, and available to the world after 18 months. Before the formal survey began, UKIRT and the UKIDSS consortia collaborated in obtaining and analysing a series of small science verification (SV) projects to complete the commissioning of the camera. We show some results from these SV projects in order to demonstrate the likely power of the eventual complete survey. Finally, using the data from the First Data Release, we assess how well UKIDSS is meeting its design targets so far. [source]

Mass modelling of dwarf spheroidal galaxies: the effect of unbound stars from tidal tails and the Milky Way

Jaros, aw Klimentowski
ABSTRACT We study the origin and properties of the population of unbound stars in the kinematic samples of dwarf spheroidal (dSph) galaxies. For this purpose we have run a high-resolution N -body simulation of a two-component dwarf galaxy orbiting in a Milky Way potential. In agreement with the tidal stirring scenario of Mayer et al., the dwarf is placed on a highly eccentric orbit, its initial stellar component is in the form of an exponential disc and it has a NFW-like dark matter (DM) halo. After 10 Gyr of evolution the dwarf produces a spheroidal stellar component and is strongly tidally stripped so that mass follows light and the stars are on almost isotropic orbits. From this final state, we create mock kinematic data sets for 200 stars by observing the dwarf in different directions. We find that when the dwarf is observed along the tidal tails the kinematic samples are strongly contaminated by unbound stars from the tails. We also study another source of possible contamination by adding stars from the Milky Way. We demonstrate that most of the unbound stars can be removed by the method of interloper rejection proposed by den Hartog & Katgert and recently tested on simulated DM haloes. We model the cleaned-up kinematic samples using solutions of the Jeans equation with constant mass-to-light ratio (M/L) and velocity anisotropy parameter. We show that even for such a strongly stripped dwarf the Jeans analysis, when applied to cleaned samples, allows us to reproduce the mass and M/L of the dwarf with accuracy typically better than 25 per cent and almost exactly in the case when the line of sight is perpendicular to the tidal tails. The analysis was applied to the new data for the Fornax dSph galaxy. We show that after careful removal of interlopers the velocity dispersion profile of Fornax can be reproduced by a model in which mass traces light with a M/L of 11 solar units and isotropic orbits. We demonstrate that most of the contamination in the kinematic sample of Fornax probably originates from the Milky Way. [source]

Structural parameters of Mayall II = G1 in M31

J. Ma
ABSTRACT Mayall II = G1 is one of the most luminous globular clusters (GCs) known in M31. New deep, high-resolution observations with the Advanced Camera for Surveys on the Hubble Space Telescope are used to provide accurate photometric data to the smallest radii yet. In particular, we present the precise variation of ellipticity and position angle, and of surface brightness for the core of the object. Based on these accurate photometric data, we redetermine the structural parameters of G1 by fitting a single-mass isotropic King model. We derive a core radius, rc= 0.21 ± 0.01 arcsec (= 0.78 ± 0.04 pc), a tidal radius, rt= 21.8 ± 1.1 arcsec (= 80.7 ± 3.9 pc), and a concentration index c= log (rt/rc) = 2.01 ± 0.02. The central surface brightness is 13.510 mag arcsec,2. We also calculate the half-light radius, at rh= 1.73 ± 0.07 arcsec (= 6.5 ± 0.3 pc). The results show that, within 10 core radii, a King model fits the surface brightness distribution well. We find that this object falls in the same region of the MV versus log Rh diagram as , Centauri, M54 and NGC 2419 in the Milky Way. All three of these objects have been claimed to be the stripped cores of now defunct dwarf galaxies. We discuss in detail whether GCs, stripped cores of dwarf spheroidals and normal dwarf galaxies form a continuous distribution in the MV versus log Rh plane, or if GCs and dwarf spheroidals constitute distinct classes of objects; we present arguments in favour of this latter view. [source]

Cold dark matter microhalo survival in the Milky Way

G. W. Angus
ABSTRACT A special purpose N -body simulation has been built to understand the tidal heating of the smallest dark matter substructures (10,6 M, and 0.01 pc) from the grainy potential of the Milky Way due to individual stars in the disc and the bulge. To test the method, we first run simulations of single encounters of microhaloes with an isolated star, and compare with analytical predictions of the dark particle bound fraction as a function of impact parameter. We then follow the orbits of a set of microhaloes in a realistic flattened Milky Way potential. We concentrate on (detectable) microhaloes passing near the Sun with a range of pericentre and apocentre. Stellar perturbers near the orbital path of a microhalo would exert stochastic impulses, which we apply in a Monte Carlo fashion according to the Besançon model for the distribution of stars of different masses and ages in our Galaxy. Also incorporated are the usual pericentre tidal heating and disc shocking. We give a detailed diagnosis of typical microhaloes and find microhaloes with internal tangential anisotropy are slightly more robust than the ones with radial anisotropy. In addition, the dark particles generally go through of a random walk in velocity space and diffuse out of the microhaloes. We show that the typical destruction time-scales are strongly correlated with the stellar density averaged along a microhalo's orbit over the age of the stellar disc. We also present the morphology of a microhalo at several epochs which may hold the key to dark matter detections. We checked our results against different choices of microhalo mass, virial radius and anisotropy. [source]

Satellite systems around galaxies in hydrodynamic simulations

Noam I. Libeskind
ABSTRACT We investigate the properties of satellite galaxies formed in N -body/SPH simulations of galaxy formation in the ,CDM cosmology. The simulations include the main physical effects thought to be important in galaxy formation and, in several cases, produce realistic spiral discs. In total, a sample of nine galaxies of luminosity comparable to the Milky Way was obtained. At magnitudes brighter than the resolution limit, MV=,12, the luminosity function of the satellite galaxies in the simulations is in excellent agreement with data for the Local Group. The radial number density profile of the model satellites, as well as their gas fractions also match observations very well. In agreement with previous N -body studies, we find that the satellites tend to be distributed in highly flattened configurations whose major axis is aligned with the major axis of the (generally triaxial) dark halo. In two out of three systems with sufficiently large satellite populations, the satellite system is nearly perpendicular to the plane of the galactic disc, a configuration analogous to that observed in the Milk Way. The discs themselves are perpendicular to the minor axis of their host haloes in the inner parts, and the correlation between the orientation of the galaxy and the shape of the halo persists even out to the virial radius. However, in one case the disc's minor axis ends up, at the virial radius, perpendicular to the minor axis of the halo. The angular momenta of the galaxies and their host halo tend to be well aligned. [source]

Discovery and analysis of three faint dwarf galaxies and a globular cluster in the outer halo of the Andromeda galaxy,

N. F. Martin
ABSTRACT We present the discovery of three faint dwarf galaxies and a globular cluster in the halo of the Andromeda galaxy (M31), found in our MegaCam survey that spans the southern quadrant of M31, from a projected distance of ,50 to ,150 kpc. Though the survey covers 57 deg2, the four satellites lie within 2° of one another. From the tip of the red giant branch (RGB), we estimate that the globular cluster lies at a distance of 631 ± 58 kpc from the Milky Way and along with a ,100 kpc projected distance from M31 we derive a total distance of 175 ± 55 kpc from its host, making it the farthest M31 globular cluster known. It also shows the typical characteristics of a bright globular cluster, with a half-light radius of 2.3 ± 0.2 pc and an absolute magnitude in the V band of MV,0=,8.5 ± 0.3. Isochrone fitting reveals that it is dominated by a very old population with a metallicity of [Fe/H],,1.3. The three dwarf galaxies are revealed as overdensities of stars that are aligned along the RGB tracks in their colour,magnitude diagrams. These satellites are all very faint, with absolute magnitudes in the range ,7.3 ,MV,0,,6.4, and show strikingly similar characteristics with metallicities of [Fe/H],,1.4 and half-light radii of ,120 ± 45 pc, making these dwarf galaxies two to three times smaller than the smallest previously known satellites of M31. Given their faintness, their distance is difficult to constrain, but we estimate them to be between 740 and 955 kpc which places them well within the virial radius of the host galaxy. The panoramic view of the MegaCam survey can provide an unbiased view of the satellite distribution of the Andromeda galaxy and, extrapolating from its coverage of the halo, we estimate that up to 45 ± 20 satellites brighter than MV,,6.5 should be orbiting M31. Hence faint dwarf galaxies cannot alone account for the missing satellites that are predicted by , cold dark matter models, unless they reside in dark matter minihaloes that are more massive than the typical masses of 107 M, currently inferred from their central radial velocity dispersion. [source]

Effects of dynamical evolution on the distribution of substructures

Jorge Peñarrubia
ABSTRACT We develop a semi-analytical model that determines the evolution of the mass and position of dark matter substructures orbiting in dark matter haloes. We apply this model to the case of the Milky Way. We focus in particular on the effects of mass loss, dynamical friction and substructure,substructure interactions, the last of which has previously been ignored in analytic models of substructure evolution. Our semi-analytical treatment reproduces both the spatial distribution of substructures and their mass function as obtained from the most recent N -body cosmological calculations of Gao et al. We find that, if mass loss is taken into account, the present distribution of substructures is practically insensitive to dynamical friction and scatterings from other substructures. Implementing these phenomena leads to a slight increase (,5 per cent) in the number of substructures at r < 0.25rvir, whereas their effects on the mass function are negligible. We find that mass-loss processes lead to the disruption of substructures before dynamical friction and gravitational scattering can significantly alter their orbits. Our results suggest that the present substructure distribution at r > 0.25rvir reflects the orbital properties at infall and is therefore purely determined by the dark matter environment around the host halo and has not been significantly altered by dynamical evolution. [source]

MSX mid-infrared imaging of massive star birth environments , II.

Giant H ii regions
ABSTRACT We conduct a Galactic census of giant H ii (GH ii) regions, based on the all-sky 6-cm data set of Kuchar & Clark, plus the kinematic distances obtained by Russeil. From an inspection of mid-infrared (MIR) Mid-course Space Experiment (MSX) and far-IR IRAS Sky Survey Atlas images, we identify a total of 56 GH ii regions in the Milky Way, of which 15 per cent (65 per cent) can be seen at optical (near-IR) wavelengths. The mid to far-IR fluxes from each GH ii region are measured, and sample the thermal emission from the ubiquitous dust present within the exciting clusters of OB stars, arising from the integrated luminosity of the hot stars heating the cluster dust, for which we obtain log L(IR) = 5.5,7.3 L,. The MIR 21-,m spatial morphology is presented for each GH ii region, and often indicates multiple emission sources, suggesting complicated cluster formation. IR colour,colour diagrams are presented, providing information concerning the temperature distribution and the optical depth of the dust. For the clusters of our study, the dust is not optically thick to all stellar radiation, thus the measured infrared luminosity is lower than Lbol. As the dust environment of a cluster begins to dissipate, the thermal emission and its optical depth ought to decrease even before the stars evolve appreciably. We see evidence of this in our empirical relationship between the integrated IR and Lyman continuum luminosities. [source]

Is the dark halo of our Galaxy spherical?

Amina Helmi
ABSTRACT It has been recently claimed that the confined structure of the debris from the Sagittarius dwarf implies that the dark matter halo of our Galaxy should be nearly spherical, in strong contrast with predictions from cold dark matter simulations, where dark haloes are found to have typical density axis ratios of 0.6,0.8. In this paper, numerical simulations are used to show that the Sagittarius streams discovered thus far are too young dynamically to be sensitive to the shape of the dark halo of the Milky Way. The data presently available are entirely consistent with a Galactic dark matter halo that could be either oblate or prolate, with minor-to-major density axis ratios as low as 0.6 within the region probed by the orbit of the Sagittarius dwarf. [source]

Spatial distribution of galaxies in the Puppis region

Pierre Chamaraux
ABSTRACT We determine the spatial distribution of the galaxies located behind the part of the zone of avoidance of the Milky Way defined by 220° < l < 260°, |b| < 20°, ,, 0°, up to a distance of 8000 km s,1. We use a sample of 369 galaxies with measured redshifts, of which 97 have been detected with the Nançay radio telescope. We show that our sample can be considered to be complete in apparent diameter down to 1.9 arcmin, a property that allows us to correct the density of galaxies for the loss of objects with distance. We then search for groups of galaxies using a companionship method and find 12 groups with at least five members, of which five are new. The members of one group are H i deficient by a factor of 1.6 on average. The method is then used to search for large structures and allows us to characterize the Puppis wall at 1400 < V0 < 2600 km s,1; it is 30 Mpc long, with the main axis being parallel to the sky plane, and it connects the Antlia cluster to the Fornax cluster through the zone of avoidance. The density of galaxies in the wall is approximately 20 times the general density of galaxies, i.e. half that observed in the densest part of the Pisces,Perseus supercluster. No internal motions are found along the line of sight, indicating that the Puppis wall has not yet collapsed. [source]

Galaxies as fluctuations in the ionizing background radiation at low redshift

Suzanne M. Linder
ABSTRACT Some Lyman continuum photons are likely to escape from most galaxies, and these can play an important role in ionizing gas around and between galaxies, including gas that gives rise to Lyman-alpha absorption. Thus the gas surrounding galaxies and in the intergalactic medium will be exposed to varying amounts of ionizing radiation depending upon the distances, orientations and luminosities of any nearby galaxies. The ionizing background can be recalculated at any point within a simulation by adding the flux from the galaxies to a uniform quasar contribution. Normal galaxies are found to almost always make some contribution to the ionizing background radiation at a redshift of zero, as seen by absorbers and at random points in space. Assuming that ,2 per cent of ionizing photons escape from a galaxy such as the Milky Way, we find that normal galaxies make a contribution of at least 30,40 per cent of the assumed quasar background. Lyman-alpha absorbers with a wide range of neutral column densities are found to be exposed to a wide range of ionization rates, although the distribution of photoionization rates for absorbers is found to be strongly peaked. On average, fewer highly ionized absorbers are found to arise further from luminous galaxies, while local fluctuations in the ionization rate are seen around galaxies having a wide range of properties. [source]

Setting new constraints on the age of the Universe

Ignacio Ferreras
There are three independent techniques for determining the age of the Universe: via cosmochronology of long-lived radioactive nuclei, via stellar modelling and population synthesis of the oldest stellar populations, and, most recently, via the precision cosmology that has become feasible with the mapping of the acoustic peaks in the cosmic microwave background. We demonstrate that all three methods give completely consistent results, and enable us to set rigorous bounds on the maximum and minimum ages that are allowed for the Universe. We present new constraints on the age of the Universe by performing a multiband colour analysis of bright cluster ellipticals over a large redshift range , which allows us to infer the ages of their stellar populations over a wide range of possible formation redshifts and metallicities. Applying a prior to Hubble's constant of we find the age of the Universe to be (1,), in agreement with the estimates from Type Ia supernovae, as well as with the latest uranium decay estimates, which yield an age for the Milky Way of . If we combine the results from cluster ellipticals with the analysis of the angular power spectrum of the cosmic microwave background and with the observations of Type Ia supernovae at high redshift, we find a similar age: . Without the assumption of any priors, universes older than 18 Gyr are ruled out by the data at the 90 per cent confidence level. [source]

The SCUBA Local Universe Galaxy Survey , II.

m data: evidence for cold dust in bright IRAS galaxies
This is the second in a series of papers presenting results from the SCUBA Local Universe Galaxy Survey. In our first paper we provided 850-,m flux densities for 104 galaxies selected from the IRAS Bright Galaxy Sample and we found that the 60-, 100-,m (IRAS) and 850-,m (SCUBA) fluxes could be adequately fitted by emission from dust at a single temperature. In this paper we present 450-,m data for the galaxies. With the new data, the spectral energy distributions of the galaxies can no longer be fitted with an isothermal dust model , two temperature components are now required. Using our 450-,m data and fluxes from the literature, we find that the 450/850-,m flux ratio for the galaxies is remarkably constant, and this holds from objects in which the star formation rate is similar to our own Galaxy, to ultraluminous infrared galaxies (ULIRGs) such as Arp 220. The only possible explanation for this is if the dust emissivity index for all of the galaxies is ,2 and the cold dust component has a similar temperature in all galaxies . The 60-,m luminosities of the galaxies were found to depend on both the dust mass and the relative amount of energy in the warm component, with a tendency for the temperature effects to dominate at the highest L60. The dust masses estimated using the new temperatures are higher by a factor of ,2 than those determined previously using a single temperature. This brings the gas-to-dust ratios of the IRAS galaxies into agreement with those of the Milky Way and other spiral galaxies which have been intensively studied in the submm. [source]

Two measures of the shape of the dark halo of the Milky Way

Rob P. Olling
In order to test the reliability of determinations of the shapes of dark-matter haloes of the galaxies, we have made such measurements for the Milky Way by two independent methods. First, we have combined the measurements of the overall mass distribution of the Milky Way derived from its rotation curve and the measurements of the amount of dark matter in the solar neighbourhood obtained from stellar kinematics to determine the flattening of the dark halo. Secondly, we have used the established technique based on the variation in thickness of the Milky Way's H i layer with radius: by assuming that the H i gas is in hydrostatic equilibrium in the gravitational potential of a galaxy, one can use the observed flaring of the gas layer to determine the shape of the dark halo. These techniques are found to produce a consistent estimate for the flattening of the dark-matter halo, with a shortest-to-longest axis ratio of q,0.8, but only if one adopts somewhat non-standard values for the distance to the Galactic centre, R0, and the local Galactic rotation speed, ,0. For consistency, one requires values of R0,7.6 kpc and ,0,190 km s,1. The results depend on the Galactic constants because the adopted values affect both distance measurements within the Milky Way and the shape of the rotation curve, which, in turn, alter the inferred halo shape. Although differing significantly from the current IAU-sanctioned values, these upper limits are consistent with all existing observational constraints. If future measurements confirm these lower values for the Galactic constants, then the validity of the gas-layer-flaring method will be confirmed. Further, dark-matter candidates such as cold molecular gas and massive decaying neutrinos, which predict very flat dark haloes with q,0.2, will be ruled out. Conversely, if the Galactic constants were found to be close to the more conventional values, then there would have to be some systematic error in the methods for measuring dark halo shapes, so the existing modelling techniques would have to be viewed with some scepticism. [source]

X-ray evidence for multiphase hot gas with nearly solar Fe abundances in the brightest groups of galaxies

David A. Buote
We analyse the ASCA spectra accumulated within ,100 kpc radii of 12 of the brightest groups of galaxies. Upon fitting isothermal models (1T) jointly to the ASCA SIS and GIS spectra we obtain fits for most groups that are of poor or at best marginal quality and give very subsolar metallicities similar to previous studies, ,Z,=0.29±0.12 Z,. Two-temperature models (2T) provide significantly better fits for 11 out of the 12 groups, and in every case have metallicities that are substantially larger than obtained for the 1T models, ,Z,=0.75±0.24 Z,. Though not very well constrained, for most of the groups absorption in excess of the Galactic value is indicated for the cooler temperature component of the 2T models. A simple multiphase cooling flow model gives results analogous to the 2T models including large metallicities, ,Z,=0.65±0.17 Z,. The nearly solar Fe abundances and also solar ,/Fe ratios indicated by the 2T and cooling flow models are consistent with models of the chemical enrichment of ellipticals, groups, and clusters which assume ratios of Type Ia to Type II supernovae and an initial mass function (IMF) similar to those of the Milky Way. Thus we have shown that the very subsolar Fe abundances and Si/Fe enhancements obtained from most previous studies within r,100 kpc of galaxy groups are an artefact of fitting isothermal models to the X-ray spectra, which also has been recently demonstrated for the brightest elliptical galaxies. Owing to the importance of these results for interpreting X-ray spectra, in an appendix we use simulated ASCA observations to examine in detail the ,Fe bias' and ,Si bias' associated with the spectral fitting of ellipticals, groups and clusters of galaxies. [source]

The open cluster Berkeley 53,

G. Maciejewski
Abstract We present a photometric study of the neglected open cluster Berkeley 53. We derived its fundamental parameters, such as the age, the interstellar reddening, and the distance from the Sun, based on BV photometry combined with near-infrared JHKS data. The structure and the mass function of the cluster were also studied and the total number of members and the total mass were estimated. The cluster was found to be a rich and massive stellar system, located in the Perseus Arm of the Milky Way, 3.1 ± 0.1 kpc from the Sun. Its age exceeds 1 Gy but it seems tobe very young in the context of its dynamical evolution. The analysis of the two-color diagrams and color-magnitude diagrams indicates that the cluster is significantly reddened. However, both methods resulted in different values of E (B , V), i.e. 1.21 ± 0.04 and 1.52 ± 0.01, respectively. This discrepancy suggests the presence of an abnormal interstellar extinction law toward the cluster (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Galactic Sun's motion in the cold dark matter, MOdified Newtonian Dynamics and modified gravity scenarios

L. Iorio
Abstract We numerically integrate the equations of motion of the Sun in Galactocentric Cartesian rectangular coordinates for ,4.5 Gyr , t , 0 in Newtonian mechanics with two different models for the Cold Dark Matter (CDM) halo, in MOdified Newtonian Dynamics (MOND) and in MOdified Gravity (MOG) without resorting to CDM. The initial conditions used come from the latest kinematical determination of the 3D Sun's motion in the Milky Way (MW) by assuming for the rotation speed of the Local Standard of Rest (LSR) the recent value ,0 = 268 km s,1 and the IAU recommended value ,0 = 220 km s,1; the Sun is assumed located at 8.5 kpc from the Galactic Center (GC). For ,0 = 268 km s,1 the birth of the Sun, 4.5 Gyr ago, would have occurred at large Galactocentric distances (12,27 kpc depending on the model used), while for ,0 = 220 km s,1 it would have occurred at about 8.8,9.3 kpc for almost all the models used. The integrated trajectories are far from being circular, especially for ,0 = 268 km s,1, and differ each other with the CDM models yielding the widest spatial extensions for the Sun's orbital path (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Foreground removal from Planck Sky Model temperature maps using a MLP neural network

H.U. Nørgaard-Nielsen
Abstract Unfortunately, the Cosmic Microwave Background (CMB) radiation is contaminated by emission originating in the Milky Way (synchrotron, free-free and dust emission). Since the cosmological information is statistically in nature, it is essential to remove this foreground emission and leave the CMB with no systematic errors. To demonstrate the feasibility of a simple multilayer perceptron (MLP) neural network for extracting the CMB temperature signal, we have analyzed a specific data set, namely the Planck Sky Model maps, developed for evaluation of different component separation methods before including them in the Planck data analysis pipeline. It is found that a MLP neural network can provide a CMB map of about 80 % of the sky to a very high degree uncorrelated with the foreground components. Also the derived power spectrum shows little evidence for systematic errors (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Multiverses of the past

V. Trimble
Abstract More than 2000 years ago, Epicurus taught that there are an infinite number of other worlds, both like and unlike ours, and Aristotle taught that there are none. Neither hypothesis can currently be falsified, and some versions of current multiverses perhaps never can be, which has contributed to occasional claims that "this isn't science!" (a common complaint about cosmology for centuries). Define "cosmos", or "world", or "universe" to mean the largest structure of which you and the majority of knowledgeable contemporaries will admit to being a part. This begins with the small, earth-centered worlds of ancient Egyptian paintings, Greek mythology, and Genesis, which a god could circumnavigate in a day and humans in a generation. These tend to expand and become helio-rather than geo-centric (not quite monotonically in time) and are succeeded by various assemblages of sun-like stars with planets of their own. Finite vs. infinite assemblages are debated and then the issue of whether the Milky Way is unique (so that "island universes" made sense, even if you were against the idea) for a couple of centuries. Today one thinks as a rule of the entire 4-dimensional space-time we might in principle communicate with and all its contents. Beyond are the modern multi-verses, sequential (cyclic or oscillating), hierarchical, or non-communicating entities in more than four dimensions. Each of these has older analogues, and, in every milieu where the ideas have been discussed, there have been firm supporters and firm opponents, some of whose ideas are explored here. Because astronomical observations have firmly settled some earlier disputes in favor of very many galaxies and very many stars with planets, "other worlds" can now refer only to other planets like Earth or to other universes. The focus is on the latter (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Star cluster kinematics in the GAIA era

H. Baumgardt
Abstract The GAIA astrometric satellite will measure positions, proper motions and parallaxes of millions of stars with microarcsecond accuracy. This will greatly increase our understanding of the stellar populations of the Milky Way and their dynamics. In particular, it will be possible to determine internal and space motions of a large number of open or globular clusters with an accuracy of a few km/s or better, which will bring new insights into the way star clusters evolve and how they and the Milky Way as a whole have formed. It will also be possible to look for clusters which have a common space motion and follow tidal streams from dissolving globular clusters over many orbits by kinematically selecting their members, which will constrain the form of the galactic potential. I illustrate how GAIA will improve our knowledge on the kinematics and dynamics of star clusters and what can be learned by comparing the GAIA data with realistic N -body simulations of star clusters, possible with e.g. future GPU or GRAPE computers. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Planar distribution of the galaxies in the Local Group

S. Pasetto
Abstract Adopting known data on positions and distances, we make use of analytical geometry and look for the plane that minimizes the distances of all galaxies to it. A planar distribution is found. We apply Hamilton's principle of minimum action to investigate the dynamics of the two major systems of the Local Group, the Milky Way and Andromeda, under the action of forces exerted by nearby galaxies or groups external to the Local Group. We find that the planar distribution is fully compatible with the minimum action and that the external force field is likely parallel to the plane. It pulls the galaxies of the Local Group without altering their planar distribution. Special care is paid to evaluate the robustness of this result. We present an explanation in the linear regime of the numerical results based on the compression effect described with the tidal tensor. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Dynamical evolution of the mass function and radial profile of the Globular Cluster systems of the Milky Way and M87

J. Shin
Abstract Evolution of the mass function (MF) and radial distribution (RD) of the globular cluster (GC) systems of the Milky Way and M87 are calculated using an advanced and realistic Fokker-Planck (FP) model that considers dynamical friction, disk/bulge shocks, and eccentric cluster orbits. We perform hundreds of FP calculations with different initial cluster conditions, and then search a wide parameter space for the best-fit initial GC MF and RD that evolves into the observed present-day GC MF and RD. By allowing both MF and RD of the initial GC system to vary, we find that in case of the Milky Way, our best-fit models have a higher peak mass for a log-normal initial MF and a higher cutoff mass for a powerlaw initial MF than previous estimates. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Ir A.H. de Voogt: life and career of a radio pioneer

R.G. Strom
Abstract There are probably few radio astronomers who would be able to recall A.H. de Voogt, which is unfortunate, but at the same time unsurprising: for he published no original astronomical research, never carried out pioneering observations, nor is his name linked to either theoretical or instrumental breakthroughs. Yet he was described by the man who first observed the 21 cm hydrogen line from the Netherlands as a radio astronomy pioneer, at the very birth of the Dutch effort. He was, moreover, a trail blazer at the cutting edge of radio, not once but twice in his career. Without him it is unlikely that the 21 cm line would have been observed in the Netherlands in 1951, and arguably the H I mapping of the Milky Way under Jan Oort's leadership would have taken place much later, if at all. Radio astronomy observing itself might well have been compromised by interference had it not been for De Voogt's foresight. Anthonet Hugo de Voogt (1892,1969) built, while still a teenager, one of the very first amateur radio stations (call letters VO: · · · ,/, , ,) in Holland, earned the radio-telegrapher's diploma during his student days, and was intimately involved in the foundation of the Dutch Society for Radio-Telegraphy in 1916. Until the 1920s, he was very active in amateur radio and astronomy circles. Trained in electrical engineering at Delft, he joined the PTT (Post Office) as a telegraph engineer in 1919, worked his way through the ranks to become head of the telephone district of Breda in 1939, and was promoted to head the PTT Radio Service just days after the end of the war. As his department was responsible for overseas radio communication, he initiated a research effort to study radio propagation in the ionosphere and the effects of solar activity. To this end, he rescued a number of Würzburg-Riese 7.5-m radar antennas abandoned at the end of the war, made one available for Jan Oort's H I work, and launched a series of radio astronomical initiatives. His group also built a number of antennas, monitored solar emission, and participated in the International Geophysical Year (1957,1958). (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Toru, 6-cm polarimetry of the MilkyWay

S. Ry
Abstract We present first results of a radio continuum and polarization survey of the Milky Way at 4.7 GHz carried out with the 32-m radio telescope in Toru,. The survey is much less affected by depolarization effects than previous measurements at lower frequencies. This enables the study of magnetic field structure and its interrelations with particular features in the interstellar medium to much larger distances from the Sun. Our preliminary total power map shows a good association of radio features with galactic star-forming regions while the polarization data reveal an increasing coherence of magnetic fields away from the Galaxy's mid-plane. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]