The long-term dynamics of Oort cloud comets are studied under the influence of both the radial and the vertical components of the Galactic tidal field. Sporadic dynamical perturbation processes are ignored, such as passing stars, since we aim to stud
y the influence of just the axisymmetric Galactic tidal field on the cometary motion and how it changes in time. We use a model of the Galaxy with a disc, bulge and dark halo, and a local disc density, and disc scale length constrained to fit the best available observational constraints. By integrating a few million of cometary orbits over 1 Gyr, we calculate the time variable flux of Oort cloud comets that enter the inner Solar System, for the cases of a constant Galactic tidal field, and a realistically varying tidal field which is a function of the Suns orbit. The applied method calculates the evolution of the comets by using first-order averaged mean elements. We find that the periodicity in the cometary flux is complicated and quasi-periodic. The amplitude of the variations in the flux are of order 30%. The radial motion of the Sun is the chief cause of this behaviour, and should be taken into account when the Galactic influence on the Oort cloud comets is studied.
We have detected stellar halo streams in the solar neighborhood using data from the 7th public data release of the Sloan Digital Sky Survey (SDSS), which includes the directed stellar program SEGUE: Sloan Extension For Galactic Understanding and Expl
oration. In order to derive distances to each star, we used the metallicity-dependent photometric parallax relation from Ivezic et al. (2008) for which we examine and quantify the accuracy. Our final sample consists of 22,321 nearby (d < 2 kpc), metal-poor ([Fe/H] < -0.5) main-sequence stars with 6D estimates of position and space velocity. We characterize the orbits of these stars through suitable kinematic proxies for their effective integrals of motion, angular momentum, eccentricity, and orbital polar angle and compare the observed distribution to expectations from a smooth distribution in four [Fe/H] bins. On this basis we identify at least five significant phase-space overdensities of stars on very similar orbits in the solar neighborhood to which we can assign unambiguously peaked [Fe/H] distributions. Three of them have been identified previously, including the halo stream discovered by Helmi et al. (1999) at a significance level of 12.0. In addition, we find at least two new genuine halo streams, judged by their kinematics and [Fe/H], at significance levels of 2.9 and 4.8, respectively. For one stream the stars even show coherence in configuration space, matching a spatial overdensity of stars found by Juric et al. (2008) at (R,z) approx (9.5,0.8) kpc. Our results demonstrate the practical power of our search method to detect substructure in the phase-space distribution of nearby stars without making a-priori assumptions about the detailed form of the gravitational potential.
We use a new method to trace backwards the star formation history of the Milky Way disk, using a sample of M dwarfs in the solar neighbourhood which is representative for the entire solar circle. M stars are used because they show H_alpha emission un
til a particular age which is a well calibrated function of their absolute magnitudes. This allows us to reconstruct the rate at which disk stars have been born over about half the disks lifetime. Our star formation rate agrees well with those obtained by using other, independent, methods and seems to rule out a constant star formation rate. The principal result of this study is to show that a relation of the Schmidt-Kennicut type (which relates the star formation rate to the interstellar gas content of galaxy disks) has pertained in the Milky Way disk during the last 5 Gyr. The star formation rate we derive from the M dwarfs and the interstellar gas content of the disk can be inferred as a function of time from a model of the chemical enrichment of the disk, which is well constrained by the observations indicating that the metallicity of the Galactic disk has remained nearly constant over the timescales involved. We demonstrate that the star formation rate and gas surface densities over the last 5 Gyrs can be accurately described by a Schmidt-Kennicutt law with an index of Gamma = 1.45 (+0.22,-0.09). This is, within statistical uncertainties, the same value found for other galaxies.
We have analyzed the phase space distribution of a sample of about 900 non--kinematically selected low metallicity stars in the solar vicinity. The stars primarily represent the thick disk and halo populations of the Milky Way. We aim to identify ove
rdensely populated regions in phase space, which we interpret as signatures of star streams passing close to the Sun. The search was conducted in a space constructed from the angular momenta and eccentricities of the stellar orbits. Besides recovering all well known star streams in the thick disk, we isolated four statistically significant phase space overdensities amongst halo stars. One of them is associated with a previously known halo star stream, but three of them are novel features, which we propose be also considered as genuine halo streams.
Deep optical/near-IR surface photometry of galaxies outside the Local Group have revealed faint and very red halos around objects as diverse as disk galaxies and starbursting dwarf galaxies. The colours of these structures are too extreme to be recon
ciled with stellar populations similar to those seen in the stellar halos of the Milky Way or M31, and alternative explanations like dust reddening, high metallicities or nebular emission are also disfavoured. A stellar population obeying an extremely bottom-heavy initial mass function (IMF), is on the other hand consistent with all available data. Because of its high mass-to-light ratio, such a population would effectively behave as baryonic dark matter and could account for some of the baryons still missing in the low-redshift Universe. Here, we give an overview of current red halo detections, alternative explanations for the origin of the red colours and ongoing searches for red halos around types of galaxies for which this phenomenon has not yet been reported. A number of potential tests of the bottom-heavy IMF hypothesis are also discussed.
