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Recent observations of microlensing events in the Large Magellanic Cloud suggest that a sizeable fraction of the galactic halo is in form of MACHOs with mass less than abou 0.1 M_{odot}. Here we argue that molecular clouds (mainly H_2) located in the galactic halo can contribute substantially to its total mass. We outline a scenario in which dark clusters of MACHOs and molecular clouds naturally form in the halo at large galactocentric distances. Possible ways of detecting MACHOs via infrared emission and molecular clouds via the induced gamma-ray flux are discussed. Molecular clouds located in the M31 dark halo could be discovered through cosmic background radiation (CBR) anisotropies or emission lines in the microwave band.
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Oscillations in the baryon-photon fluid prior to recombination imprint different signatures on the power spectrum and correlation function of matter fluctuations. The measurement of these features using galaxy surveys has been proposed as means to determine the equation of state of the dark energy. The accuracy required to achieve competitive constraints demands an extremely good understanding of systematic effects which change the baryonic acoustic oscillation (BAO) imprint. We use 50 very large volume N-body simulations to investigate the BAO signature in the two-point correlation function. The location of the BAO bump does not correspond to the sound horizon scale at the level of accuracy required by future measurements, even before any dynamical or statistical effects are considered. Careful modelling of the correlation function is therefore required to extract the cosmological information encoded on large scales. We find that the correlation function is less affected by scale dependent effects than the power spectrum. We show that a model for the correlation function proposed by Crocce & Scoccimarro (2008), based on renormalised perturbation theory, gives an essentially unbiased measurement of the dark energy equation of state. This means that information from the large scale shape of the correlation function, in addition to the form of the BAO peak, can be used to provide robust constraints on cosmological parameters. The correlation function therefore provides a better constraint on the distance scale (~50% smaller errors with no systematic bias) than the more conservative approach required when using the power spectrum (i.e. which requires amplitude and long wavelength shape information to be discarded).
We investigated a hydrostatic equilibrium model of the Milky Way following Parker (1966), to constrain the large scale properties of the interstellar medium. In our approach we found an excellent agreement between our simple hydrostatic equilibrium model of the Milky Way and the recent all-sky survey data rangeing from the gamma-ray to the radio regime. On large scales the galactic disk-halo system is found to be stable against Parker-instabilities. Pressure support from the Galactic disk is essential to stabilise the halo. In particular the diffuse ionised gas layer acts as a disk-halo interface. Assuming that the distribution of the soft X-ray emitting plasma traces the gravitational potential, we derived the dark matter content of the Milky Way to be about M ~ 2.8 10^11 M_o. Our findings are consistent with the rotation curve of the Galaxy.
We have used RR Lyrae and Blue HB stars as tracers of the old Galactic halo, in order to study the halo structure and the galactic rotation as a function of height above the plane. Our sample includes 40 RR Lyrae and 80 BHB stars that are about 2 to 15 kpc above the plane, in a roughly 250 sq. deg. area around the North Galactic Pole (NGP). We use proper motions (derived from the GSC-II database) and radial velocities to determine the rotation of the halo. From the whole sample the motion appears to be significantly more retrograde than the samples in the solar neighborhood, confirming Majewski (1992) results and our own preliminary results based on 1/3 the present sample (Kinman et al. 2003; Spagna et al. 2003). However, the better statistics has now revealed the likely existence of two components, whose characteristics need an accurate analysis of systematic errors on the proper motions in order to be assessed in detail.
We investigate the convective stability of two popular types of model of the gas distribution in the hot Galactic halo. We first consider models in which the halo density and temperature decrease exponentially with height above the disk. These halo models were created to account for the fact that, on some sight lines, the halos X-ray emission lines and absorption lines yield different temperatures, implying that the halo is non-isothermal. We show that the hot gas in these exponential models is convectively unstable if $gamma<3/2$, where $gamma$ is the ratio of the temperature and density scale heights. Using published measurements of $gamma$ and its uncertainty, we use Bayes Theorem to infer posterior probability distributions for $gamma$, and hence the probability that the halo is convectively unstable for different sight lines. We find that, if these exponential models are good descriptions of the hot halo gas, at least in the first few kiloparsecs from the plane, the hot halo is reasonably likely to be convectively unstable on two of the three sight lines for which scale height information is available. We also consider more extended models of the halo. While isothermal halo models are convectively stable if the density decreases with distance from the Galaxy, a model of an extended adiabatic halo in hydrostatic equilibrium with the Galaxys dark matter is on the boundary between stability and instability. However, we find that radiative cooling may perturb this model in the direction of convective instability. If the Galactic halo is indeed convectively unstable, this would argue in favor of supernova activity in the Galactic disk contributing to the heating the hot halo gas.
We explore differences in Galactic halo kinematic properties derived from two commonly employed Galactic potentials: the St$ddot{a}$ckel potential and the default Milky Way-like potential used in the Galpy package (MWPotential2014), making use of stars with available metallicities, radial velocities, and proper motions from Sloan Digital Sky Survey Data Release 12. Adopting the St$ddot{a}$ckel potential, we find that the shape of the metallicity distribution function (MDF) and the distribution of orbital rotation abruptly change at $Z_{rm max}$ = 15 kpc and $r_{rm max}$ = 30 kpc (where $Z_{rm max}$ and $r_{rm max}$ are the maximum distances reached by a stellar orbit from the Galactic plane and from the Galactic center, respectively), indicating that the transition from dominance by the inner-halo stellar population to the outer-halo population occurs at those distances. Stars with $Z_{rm max}$ $>$ 15 kpc show an average retrograde motion of $V_{rm phi}$ = $-$60 km s$^{-1}$, while stars with $r_{rm max}$ $>$ 30 kpc exhibit an even larger retrograde value, $V_{rm phi}$ = $-$150 km s$^{-1}$. This retrograde signal is also confirmed using the sample of stars with radial velocities obtained by $Gaia$ Data Release 2, assuming the St$ddot{a}$ckel potential. In comparison, when using the shallower Galpy potential, a noticeable change in the MDF occurs only at $Z_{rm max}$ = 25 kpc, and a much less extreme retrograde motion is derived. This difference arises because stars with highly retrograde motions in the St$ddot{a}$ckel potential are unbound in the shallower Galpy potential, and stars with lower rotation velocities reach larger $Z_{rm max}$ and $r_{rm max}$. The different kinematic characteristics derived from the two potentials suggest that the nature of the adopted Galactic potential can strongly influence interpretation of the properties of the Galactic halo.
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