Michael Odenkirchen

The Ghost of Sagittarius and Lumps in the Halo of the Milky Way

We identify new structures in the halo of the Milky Way from positions, colors, and magnitudes of five million stars detected in the Sloan Digital Sky Survey. Most of these stars are within 126 of the celestial equator. We present color-magnitude diagrams (CMDs) for stars in two previously discovered, tidally disrupted structures. The CMDs and turnoff colors are consistent with those of the Sagittarius dwarf galaxy, as had been predicted. In one direction, we are even able to detect a clump of red stars, similar to that of the Sagittarius dwarf, from stars spread across 110 deg2 of sky. Focusing on stars with the colors of F turnoff objects, we identify at least five additional overdensities of stars. Four of these may be pieces of the same halo structure, which would cover a region of the sky at least 40° in diameter, at a distance of 11 kpc from the Sun (18 kpc from the center of the Galaxy). The turnoff is significantly bluer than that of thick-disk stars, yet the stars lie closer to the Galactic plane than a power-law spheroid predicts. We suggest two models to explain this new structure. One possibility is that this new structure could be a new dwarf satellite of the Milky Way, hidden in the Galactic plane and in the process of being tidally disrupted. The other possibility is that it could be part of a disklike distribution of stars which is metal-poor, with a scale height of approximately 2 kpc and a scale length of approximately 10 kpc. The fifth overdensity, which is 20 kpc away, is some distance from the Sagittarius dwarf streamer orbit and is not associated with any known Galactic structure. We have tentatively identified a sixth overdensity in the halo. If this sixth structure is instead part of a smooth distribution of halo stars (the spheroid), then the spheroid must be very flattened, with axial ratio q = 0.5. It is likely that there are many smaller streams of stars in the Galactic halo.

A Low-Latitude Halo Stream around the Milky Way

We present evidence for a ring of stars in the plane of the Milky Way, extending at least from l = 180° to 227° with turnoff magnitude g ~ 19.5; the ring could encircle the Galaxy. We infer that the low Galactic latitude structure is at a fairly constant distance of R = 18 ± 2 kpc from the Galactic center above the Galactic plane and has R = 20 ± 2 kpc in the region sampled below the Galactic plane. The evidence includes 500 Sloan Digital Sky Survey spectroscopic radial velocities of stars within 30° of the plane. The velocity dispersion of the stars associated with this structure is found to be 27 km s-1 at (l, b) = (198°, - 27°), 22 km s-1 at (l, b) = (225°, 28°), 30 km s-1 at (l, b) = (188°, 24°), and 30 km s-1 at (l, b) = (182°, 27°). The structure rotates in the same prograde direction as the Galactic disk stars but with a circular velocity of 110 ± 25 km s-1. The narrow measured velocity dispersion is inconsistent with power-law spheroid or thick-disk populations. We compare the velocity dispersion in this structure with the velocity dispersion of stars in the Sagittarius dwarf galaxy tidal stream, for which we measure a velocity dispersion of 20 km s-1 at (l, b) = (165°, - 55°). We estimate a preliminary metallicity from the Ca II (K) line and color of the turnoff stars of [Fe/H] = -1.6 with a dispersion of 0.3 dex and note that the turnoff color is consistent with that of the spheroid population. We interpret our measurements as evidence for a tidally disrupted satellite of 2 × 107 to 5 × 108 M☉ that rings the Galaxy.

Theoretical isochrones in several photometric systems. II. The Sloan Digital Sky Survey ugriz system

Following Paper I, we provide extended tables of bolometric corrections, extinction coeffi cients, stellar isochrones, and integrated magnitudes and colours of single-burst stellar populations, for the Sloan Digital Sky Survey (SDSS) ugriz photometric system. They are tested on comparisons with DR1 data for a few stellar systems, namely the Palomar 5 and NGC 2419 globular clusters and the Draco dSph galaxy.

Sagittarius Tidal Debris 90 Kiloparsecs from the Galactic Center

A new overdensity of A-colored stars in distant parts of the Milky Ways stellar halo, at a dereddened Sloan Digital Sky Survey magnitude of g0 = 20.3, is presented. Identification of associated variable RR Lyrae candidates supports the claim that these are blue horizontal branch stars. The inferred distance of these stars from the Galactic center is 90 kpc, assuming that the absolute magnitude of these stars is M = 0.7 and that the Sun is 8.5 kpc from the Galactic center. The new tidal debris is within 10 kpc of the same plane as other confirmed tidal debris from the disruption of the Sagittarius dwarf galaxy and could be associated with the trailing tidal arm. Distances to the Sagittarius stream estimated from M stars are about 13% smaller than our inferred distances. The tidal debris has a width of at least 10° and is traced for more than 20° across the sky. The globular cluster NGC 2419 is located within the detected tidal debris and may also have once been associated with the Sagittarius dwarf galaxy.

Modeling the disruption of the globular cluster Palomar 5 by Galactic tides

The globular cluster Palomar 5 is remarkable not only because of its extended massive tidal tails, but also for its very low mass and velocity dispersion, and its size, which is much larger than its theoretical tidal radius. In order to understand these extreme properties, we performed more than 1000 N-body simulations of clusters traversing the Milky Way on the orbit of Pal 5. Tidal shocks at disk crossings near perigalacticon dominate the evolution of extended low-concentration clusters, resulting in massive tidal tails and often in a quick destruction of the cluster. The very large size of Pal 5 can be explained as the result of an expansion following the heating induced by the last strong disk shock ~150 Myr ago. Some of the models can reproduce the low observed velocity dispersion and the relative fractions of stars in the tails and between the inner and outer parts of the tails. Our simulations illustrate to what extent the observable tidal tails trace out the orbit of the parent object. The tidal tails of Pal 5 show substantial structure not seen in our simulations. We argue that this structure is probably caused by Galactic substructure, such as giant molecular clouds, spiral arms, and dark matter clumps, which was ignored in our modeling. Clusters initially larger than their theoretical tidal limit remain so, because after being shocked, they settle into a new equilibrium near apogalacticon where they are unaffected by the perigalactic tidal field. This implies that, contrary to previous wisdom, globular clusters on eccentric orbits may well remain supertidally limited and hence vulnerable to strong disk shocks, which dominate their evolution until destruction. Our simulations unambiguously predict the destruction of Pal 5 at its next disk crossing in ~110 Myr. This corresponds to only 1% of the cluster lifetime, suggesting that many more similar systems could once have populated the inner parts of the Milky Way but have been transformed into debris streams by the Galactic tidal field.

A Matched-Filter Analysis of the Tidal Tails of the Globular Cluster Palomar 5

The tidal tails of the globular cluster Palomar 5 are analyzed over a 41 deg2 area of the Sloan Digital Sky Survey photometric catalogs. The matched filter algorithm provides the maximum possible signal-to-noise detection of the cluster stars over the measured background, and the expected and actual effectiveness of the technique in the context of this data set is discussed. The stellar background is examined in some detail for systematic variation as a function of Galactic position in order to assess its effect on the detection efficiency. Of the total number of Pal 5 stars detected, 45% are out in the tails. The tails are found as the only additional 3 σ overdensity of cluster stars over the entire 41 deg2 area studied. The annular-averaged density of stars along the tails is fitted to a power law in radius with best-fit index -1.58 ± 0.07, significantly steeper than that predicted from a constant orbit-averaged mass-loss rate.

3D-Kinematics of White Dwarfs from the SPY-Project

We present kinematics of a sample of 107 DA white dwarfs from the SPY project (ESO SN Ia Progenitor surveY) and discuss kinematic criteria for a distinction of thin disk, thick disk, and halo populations. This is the first homogeneous sample of white dwarfs for which 3D space motions have been determined. Since the percentage of old stars among white dwarfs is higher than among main-sequence stars, they are presumably valuable tools in studies of old populations such as the halo and the thick disk. Studies of white dwarf kinematics can help to determine the fraction of the total mass of our Galaxy contained in the form of thick disk and halo white dwarfs, an issue which is still under discussion. Radial velocities and spectroscopic distances obtained by the SPY project are combined with our measurements of proper mo- tions to derive 3D space motions. Galactic orbits and further kinematic parameters are computed. We calculate individual errors of kinematic parameters by means of a Monte Carlo error propagation code. Our kinematic criteria for assigning population membership are deduced from a sample of F and G stars taken from the literature for which chemical criteria can be used to distinguish between thin disk, thick disk and halo. Candidates for thick disk and halo members are selected in a first step from the classical U-V-velocity diagram. Our final assignment of population membership is based on orbits and position in the Jz-eccentricity diagram. We find four halo and twelve thick disk white dwarfs. We also present a systematic study of the eects of ignoring the radial velocity in kinematic investigations.

Mass segregation in the globular cluster Palomar 5 and its tidal tails

We present the stellar main-sequence luminosity function (LF) of the disrupted, low-mass, low-concentration globular cluster Palomar 5 and its well-defined tidal tails, which emanate from the cluster as a result of its tidal interaction with the Milky Way. The results of our deep (B ~ 24.5) wide-field photometry unequivocally indicate that preferentially fainter stars were removed from the cluster so that the LF of the clusters main body exhibits a significant degree of flattening compared with other globular clusters. There is clear evidence of mass segregation, which is reflected in a radial variation of the LFs. The LF of the tidal tails is distinctly enhanced with faint, low-mass stars. Pal 5 exhibits a binary main sequence, and we estimate a photometric binary frequency of roughly 10%. The binaries also show evidence of mass segregation, with more massive binary systems being more strongly concentrated toward the cluster center.

Detection of Massive Tidal Tails around the Globular Cluster Palomar 5 with Sloan Digital Sky Survey Commissioning Data

Abstract : Globular clusters are self-gravitating stellar systems that experience a time-varying tidal potential as they orbit through their parent galaxy. Their dynamical evolution is driven by internal effects such as stellar evolution, two-body relaxation and binary heating, and by external effects induced by the galactic force field, i.e., heating by tidal shocks during disk and bulge passages and tidal stripping. Both internal and external effects should lead to a permanent loss of cluster members and to the eventual dissolution of the cluster. The Galactic globular clusters observed today are believed to be survivors from an initially much more numerous population. They appear to be in various stages of evolution and dissolution, depending on their initial conditions and their galactic orbits (e.g., Cherno & Weinberg 1990, Djorgovsky & Meylan 1994). Numerical simulations predict that possibly as many as half of the present-day Galactic globulars will not survive for another Hubble time (Gnedin & Ostriker 1997). Observational conformation of the gradual dissolution of globular clusters and determination of their mass loss rates is important in itself, but can also shed light on the formation history and structure of the Galactic halo and provide constraints on the Galactic potential.

KINEMATICS OF THE TIDAL DEBRIS OF THE GLOBULAR CLUSTER PALOMAR 5

This paper presents the first observational study of the kinematics of debris from a disrupting globular cluster. Seventy-four bright member candidates of the tidal stream of the cluster Palomar 5, spread over an arc of 85 on the sky, have been observed using high-resolution spectroscopy on the VLT and 17 of them were identified as members of the stream. Their velocities are very close to the radial velocity of the cluster and show a linear trend with position along the stream. We measure the local gradient of the line-of-sight velocity along the stream to be 1.0 ± 0.4 km s–1 deg–1. The stars in the stream show an overall velocity dispersion of 4.7 km s–1, which reduces to only 2.2 km s–1 when removing two stars. This demonstrates that the tidal debris from Pal 5 forms a kinematically cold structure in the Galactic halo, and hence explains the narrowness of the tails. The velocity gradient along the stream provides an important constraint on the orbit of the cluster and thus also on the potential in the Galactic halo. This leads to the conclusion that, in contrast to what was assumed in earlier studies, the clusters orbit on the sky is not exactly aligned with its tidal tails. A revised model for the orbit of Pal 5 is thus proposed.