Evolution of the Velocity Ellipsoids in the Thin Disk of the Galaxy and the Radial Migration of Stars

Data from the revised Geneva--Copenhagen catalog are used to study the influence of radial migration of stars on the age dependences of parameters of the velocity ellipsoids for nearby stars in the thin disk of the Galaxy, assuming that the mean radii of the stellar orbits remain constant. It is demonstrated that precisely the radial migration of stars, together with the negative metallicity gradient in the thin disk,are responsible for the observed negative correlation between the metallicities and angular momenta of nearby stars, while the angular momenta of stars that were born at the same Galactocentric distances do not depend on either age or metallicity. (abridged)Comment: Astronomy Reports, Vol. 86 No. 9, P.1117-1126 (2009

Relationship between the Velocity Ellipsoids of Galactic-Disk Stars and their Ages and Metallicities

The dependences of the velocity ellipsoids of F-G stars of the thin disk of the Galaxy on their ages and metallicities are analyzed based on the new version of the Geneva-Copenhagen Catalog. The age dependences of the major, middle, and minor axes of the ellipsoids, and also of the dispersion of the total residual veltocity, obey power laws with indices 0.25,0.29,0.32, and 0.27 (with uncertainties \pm 0.02). Due to the presence of thick-disk objects, the analogous indices for all nearby stars are about a factor of 1.5 larger. Attempts to explain such values are usually based on modeling relaxation processes in the Galactic disk. With increasing age, the velocity ellipsoid increases in size and becomes appreciably more spherical, turns toward the direction of the Galactic center, and loses angular momentum. The shape of the velocity ellipsoid remains far from equilibrium. With increasing metallicity, the velocity ellipsoid for stars of mixed age increases in size, displays a weak tendency to become more spherical, and turns toward the direction of the Galactic center (with these changes occurring substantially more rapidly in the transition through the metallicity [Fe/H]= -0.25). Thus, the ellipsoid changes similarly to the way it does with age; however, with decreasing metallicity, the rotational velocity about the Galactic center monotonically increases, rather than decreases(!). Moreover, the power-law indices for the age dependences of the axes depend on the metallicity, and display a maximum near [Fe/H]=-0.1. The age dependences of all the velocity-ellipsoid parameters for stars with equal metallicity are roughly the same. It is proposed that the appearance of a metallicity dependence of the velocity ellipsoids for thin-disk stars is most likely due to the radial migration of stars.Comment: 15 pages, 6 figures, accepted 2009, Astronomy Reports, Vol. 53 No. 9, P.785-80

The influence of nitrogen and phosphorus ferilization on nutrient status and profitability of Bromegrass on Ida soils

This study was undertaken to determine the profitability and the feasibility of fertilizing bromegrass for grazing in the Monona-Ida-Hamburg soil association area. This area is well adapted to growing forage crops. Because of the high content of calcium and potassium, alfalfa grows well if phosphorus is applied. Bromegrass is able in some way to get nitrogen from alfalfa, and the two crops grow well together. The bloat danger in pasturing alfalfa or bromegrass-alfalfa mixtures, however, is well known to cattlemen in the area. Many believe the cost of nitrogen fertilizer to maintain productivity of bromegrass pastures is less than the cost of losses from bloat on bromegrass-alfalfa pastures. The profitability of fertilizing bromegrass stands is examined in Part I of the study. The feasibility is examined in Part II. In Part I, returns at three levels of nitrogen cost and beef price and at three conversion ratios of forage to beef are calculated on the basis of experimental yields

Merging Galaxies in the SDSS EDR

We present a new catalog of merging galaxies obtained through an automated systematic search routine. The 1479 new pairs of merging galaxies were found in approximately 462 sq deg of the Sloan Digital Sky Survey Early Data Release (SDSS EDR; Stoughton et al. 2002) photometric data, and the pair catalog is complete for galaxies in the magnitude range 16.0 <= g* <= 20. The selection algorithm, implementing a variation on the original Karachentsev (1972) criteria, proved to be very efficient and fast. Merging galaxies were selected such that the inter-galaxy separations were less than the sum of the component galaxies' radii. We discuss the characteristics of the sample in terms of completeness, pair separation, and the Holmberg effect. We also present an online atlas of images for the SDSS EDR pairs obtained using the corrected frames from the SDSS EDR database. The atlas images also include the relevant data for each pair member. This catalog will be useful for conducting studies of the general characteristics of merging galaxies, their environments, and their component galaxies. The redshifts for a subset of the interacting and merging galaxies and the distribution of angular sizes for these systems indicate the SDSS provides a much deeper sample than almost any other wide-area catalog to date.Comment: 58 pages, which includes 15 figures and 6 tables. Figures 2, 8, 9, 10, 11, 13, and 14 are provided as JPEG files. For online atlas, see http://home.fnal.gov/~sallam/MergePair/ . Accepted for publication in A

The Anisotropic Distribution of Galactic Satellites

We present a study of the spatial distribution of subhalos in galactic dark matter halos using dissipationless cosmological simulations of the concordance LCDM model. We find that subhalos are distributed anisotropically and are preferentially located along the major axes of the triaxial mass distributions of their hosts. The Kolmogorov-Smirnov probability for drawing our simulated subhalo sample from an isotropic distribution is P_KS \simeq 1.5 \times 10^{-4}. An isotropic distribution of subhalos is thus not the correct null hypothesis for testing the CDM paradigm. The nearly planar distribution of observed Milky Way (MW) satellites is marginally consistent (probability \simeq 0.02) with being drawn randomly from the subhalo distribution in our simulations. Furthermore, if we select the subhalos likely to be luminous, we find a distribution that is consistent with the observed MW satellites. In fact, we show that subsamples of the subhalo population with a centrally-concentrated radial distribution, similar to that of the MW dwarfs, typically exhibit a comparable degree of planarity. We explore the origin of the observed subhalo anisotropy and conclude that it is likely due to (1) preferential accretion of subhalos along filaments, often closely aligned with the major axis of the host halo, and (2) evolution of satellite orbits within the prolate, triaxial potentials typical of CDM halos. Agreement between predictions and observations requires the major axis of the outer dark matter halo of the Milky Way to be nearly perpendicular to the disk. We discuss possible observational tests of such disk-halo alignment with current large galaxy surveys.Comment: 14 pages (including appendix), 9 figures. Accepted for Publication in ApJ. Minor changes to reflect referee's comment

Spatial and kinematic alignments between central and satellite halos

Based on a cosmological N-body simulation we analyze spatial and kinematic alignments of satellite halos within six times the virial radius of group size host halos (Rvir). We measure three different types of spatial alignment: halo alignment between the orientation of the group central substructure (GCS) and the distribution of its satellites, radial alignment between the orientation of a satellite and the direction towards its GCS, and direct alignment between the orientation of the GCS and that of its satellites. In analogy we use the directions of satellite velocities and probe three further types of alignment: the radial velocity alignment between the satellite velocity and connecting line between satellite and GCS, the halo velocity alignment between the orientation of the GCS and satellite velocities and the auto velocity alignment between the satellites orientations and their velocities. We find that satellites are preferentially located along the major axis of the GCS within at least 6 Rvir (the range probed here). Furthermore, satellites preferentially point towards the GCS. The most pronounced signal is detected on small scales but a detectable signal extends out to 6 Rvir. The direct alignment signal is weaker, however a systematic trend is visible at distances < 2 Rvir. All velocity alignments are highly significant on small scales. Our results suggest that the halo alignment reflects the filamentary large scale structure which extends far beyond the virial radii of the groups. In contrast, the main contribution to the radial alignment arises from the adjustment of the satellite orientations in the group tidal field. The projected data reveal good agreement with recent results derived from large galaxy surveys. (abridged)Comment: accepted for publication in Ap

Characterisation of the Galactic thin disc with Corot targets

We use kinematical and chemical properties of 754 Corot stars to characterise the stellar populations of the Milky Way disc in three beams close the Galactic plane. From the atmospheric parameters derived in Gazzano et al. (2010) with the Matisse algorithm, we derived stellar distances using isochrones. Combining these data with proper motions, we provide the complete kinematical description of stars in three Corot fields. Finally, we used kinematical criteria to identify the Galactic populations in our sample and study their characteristics, particularly their chemistry. Comparing our kinematics with the Besancon Galactic model, we show that, within 3-sigma, simulated and observed kinematical distributions are in good agreement. We study the characteristics of the thin disc, finding a correlation that is significant at a value of 2-sigma between the V-velocity component and the metallicity for two different radial distance bins (8-9kpc and 9-10kpc; but not for the most inner bin 7-8kpc, probably because of the uncertainties in the abundances) which could be interpreted as radial migration evidence. We also measured a radial metallicity gradient value of -0.097+/-0.015dex/kpc with giant stars, and -0.053+/-0.015dex/kpc with dwarfs. Finally, we identified metal-rich stars with peculiar high [alpha/Fe] values in the directions pointing to the inner part of the Galaxy. Applying the same methodology to the planet-hosting stars detected by Corot shows that they mainly belong to the thin disc population with normal chemical and kinematical properties.Comment: Astronomy & Astrophysics, in pres

Analysis of Peculiarities of the Stellar Velocity Field in the Solar Neighborhood

Based on a new version of the Hipparcos catalogue and an updated Geneva-Copenhagen survey of F and G dwarfs, we analyze the space velocity field of about 17000 single stars in the solar neighborhood. The main known clumps, streams, and branches (Pleiades, Hyades, Sirius, Coma Berenices, Hercules, Wolf 630-alpha Ceti, and Arcturus) have been identified using various approaches. The evolution of the space velocity field for F and G dwarfs has been traced as a function of the stellar age. We have managed to confirm the existence of the recently discovered KFR08 stream. We have found 19 Hipparcos stars, candidates for membership in the KFR08 stream, and obtained an isochrone age estimate for the stream, 13 Gyr. The mean stellar ages of the Wolf 630-alpha Ceti and Hercules streams are shown to be comparable, 4--6 Gyr. No significant differences in the metallicities of stars belonging to these streams have been found. This is an argument for the hypothesis that these streams owe their origin to a common mechanism.Comment: 23 pages, 9 figure

A peculiar class of debris disks from Herschel/DUNES - A steep fall off in the far infrared

Aims. We present photometric data of debris disks around HIP 103389 (HD 199260), HIP 107350 (HN Peg, HD206860), and HIP 114948 (HD 219482), obtained in the context of our Herschel Open Time Key Program DUNES (DUst around NEarby Stars). Methods. We used Herschel/PACS to detect the thermal emission of the three debris disks with a 3 sigma sensitivity of a few mJy at 100 um and 160 um. In addition, we obtained Herschel/PACS photometric data at 70 um for HIP 103389. Two different approaches are applied to reduce the Herschel data to investigate the impact of data reduction on the photometry. We fit analytical models to the available spectral energy distribution (SED) data. Results. The SEDs of the three disks potentially exhibit an unusually steep decrease at wavelengths > 70 um. We investigate the significance of the peculiar shape of these SEDs and the impact on models of the disks provided it is real. Our modeling reveals that such a steep decrease of the SEDs in the long wavelength regime is inconsistent with a power-law exponent of the grain size distribution -3.5 expected from a standard equilibrium collisional cascade. In contrast, a very distinct range of grain sizes is implied to dominate the thermal emission of such disks. However, we demonstrate that the understanding of the data of faint sources obtained with Herschel is still incomplete and that the significance of our results depends on the version of the data reduction pipeline used. Conclusions. A new mechanism to produce the dust in the presented debris disks, deviations from the conditions required for a standard equilibrium collisional cascade (grain size exponent of -3.5), and/or significantly different dust properties would be necessary to explain the potentially steep SED shape of the three debris disks presented. (abridged)Comment: 14 pages, 4 figures, accepted by A&

Gravitational stability and dynamical overheating of stellar disks of galaxies

We use the marginal stability condition for galactic disks and the stellar velocity dispersion data published by different authors to place upper limits on the disk local surface density at two radial scalelengths $R=2h$. Extrapolating these estimates, we constrain the total mass of the disks and compare these estimates to those based on the photometry and color of stellar populations. The comparison reveals that the stellar disks of most of spiral galaxies in our sample cannot be substantially overheated and are therefore unlikely to have experienced a significant merging event in their history. The same conclusion applies to some, but not all of the S0 galaxies we consider. However, a substantial part of the early type galaxies do show the stellar velocity dispersion well in excess of the gravitational stability threshold suggesting a major merger event in the past. We find dynamically overheated disks among both seemingly isolated galaxies and those forming pairs. The ratio of the marginal stability disk mass estimate to the total galaxy mass within four radial scalelengths remains within a range of 0.4---0.8. We see no evidence for a noticeable running of this ratio with either the morphological type or color index.Comment: 25 pages, 5 figures, accepted to Astronomy Letter