Dana I. Dinescu

Space Velocities of Globular Clusters. III. Cluster Orbits and Halo Substructure

We have compiled a catalog of absolute proper motions of globular clusters from various sources. The sample consists of 38 clusters, from which most of the southern ones (15 clusters) were measured in our previous papers in this series. We have integrated orbits assuming two different Galactic potential models adopted from the literature and have calculated orbital parameters. The uncertainties associated with the orbital parameters were derived in a Monte Carlo approach, and we conclude that, overall, at the present level of measurement errors, orbital differences due to Galactic potential models are not significant. Three metal-poor clusters are found to have orbits similar to prototypical metal-rich disk clusters. These clusters are NGC 6254 (M10), NGC 6626 (M28), and NGC 6752. We interpret this as a potentially significant constraint on the formation of the disk. It is thus possible that part of the inner metal-poor halo is the low-metallicity tail of the thick disk. In this case, the ages of these clusters indicate that the formation of the disk partially overlapped with that of the halo. The clusters classified as young halo or red horizontal-branch by Zinn show a radially anisotropic velocity distribution, their orbits are of high total energy, with apocentric radii larger than 10 kpc and highly eccentric. In this sense they may represent an accreted component of our Galaxy. We also discuss ω Cens orbit characteristics in the view of an accreted origin. We investigate the effect of the orbital motion on the internal dynamics of clusters. Adopting the formalism from Gnedin & Ostriker and their destruction rates due to two-body relaxation, we find that, in most cases, this internal process is more important than the destruction processes due to disk and bulge shocking. Hubble Space Telescope (HST) observations argue that NGC 6397s luminosity function is depleted at the faint end, and this is blamed on its high total destruction rate. We propose a list of clusters with similar destruction rates that may also have depleted luminosity functions. We also note the bias toward deriving higher destruction rates in studies that statistically assign tangential velocities based on a kinematic model of the globular cluster system, in contrast to the rates derived from the measured tangential velocities. Clusters prone to such biases are those that have circular orbits (kinematically thick-disk clusters) and some of those with orbits of high total energy.

The Closest View of a Dwarf Galaxy: New Evidence on the Nature of the Canis Major Overdensity

We present the first deep color-magnitude diagram of the putative central region (05 × 05) of the Canis Major stellar overdensity (l, b) = (240, - 8) found recently by Martin and coworkers, which has been proposed as the remnant of a dwarf satellite accreted onto the Milky Way on a near-equatorial orbit. We find a narrow (in apparent magnitude) main sequence extending 6 mag below the turnoff to our limiting magnitude of B ~ 24.5 mag. This main sequence has very high contrast (>3) with respect to the thin/thick disk/halo background; its narrowness at brighter magnitudes clearly implies the presence of a distinct and possibly still bound stellar system. We derived the line-of-sight size (r1/2) of this system based on the B-band width of the lower main sequence, obtaining 0.94 ± 0.18 (random) ± 0.18 (systematic) kpc. That size matches a model prediction for the main body of the parent galaxy of the Monoceros tidal stream. The high-density contrast and limited spatial extent in the radial direction are very hard to reconcile with the alternative explanation put forward to explain the Canis Major stellar overdensity: a flared or warped Galactic disk viewed in projection, as found in the recent work of Momany and coworkers. We also derived a central surface brightness of μV,0 = 23.3 ± 0.1 mag arcsec-2 and an absolute magnitude of MV = -14.5 ± 0.1 mag. These values place the Canis Major object in the category of dwarf galaxy, considering the LV-size and MV - μV planes for such objects. However, like the Sagittarius dwarf, it is an outlier in the [Fe/H] - MV plane in the sense that it is too metal-rich for its estimated absolute magnitude. This suggests that the main mechanism driving its recent and current star formation history (possibly tidal stripping) is different from that of isolated dwarfs.

Space Velocities of Southern Globular Clusters. II. New Results for 10 Clusters

Absolute proper motions have been measured for 10 clusters, in addition to the sample of five clusters presented in an earlier paper. These newly measured clusters are NGC 1904 (M79), NGC 2298, NGC 4590 (M68), NGC 5139 (ω Cen), NGC 5897, NGC 6093 (M80), NGC 6121 (M4), NGC 6144, NGC 6809 (M55), and NGC 7099 (M30). The correction to absolute proper motion is determined based on galaxies, except for one field with high extinction, for which we used the Hipparcos system. This latter correction proves to be very useful for fields at low Galactic latitude, provided magnitude-dependent systematics are well controlled. The errors in absolute proper motion per cluster range between 0.4 and 1.0 mas yr-1. Space velocities are also determined, and the kinematics are briefly discussed; a companion paper will present a detailed analysis of orbits in conjunction with other physical parameters of clusters.

Absolute Proper Motion of the Fornax Dwarf Spheroidal Galaxy from Photographic and Hubble Space Telescope WFPC2 Data

We have measured the absolute proper motion of the Fornax dwarf spheroidal galaxy from a combination of photographic plate material and Hubble Space Telescope Wide Field Planetary Camera 2 data that provide a time baseline of up to 50 yr. The extragalactic reference frame consists of eight QSO images and 48 galaxies. The absolute proper motion is μα cos δ = 0.59 ± 0.16 mas yr-1 and μδ = -0.15 ± 0.16 mas yr-1. The corresponding orbit of Fornax is polar, with an eccentricity of 0.27 and a radial period of 4.5 Gyr. Fornaxs current location is near pericenter. The direction of the motion of Fornax supports the notion that Fornax belongs to the Fornax–Leo I–Leo II–Sculptor–Sextans stream as hypothesized by both Lynden-Bell and Majewski. According to our orbit determination, Fornax crossed the Magellanic plane ~190 Myr ago, a time that coincides with the termination of the star formation process in Fornax. We propose that ram pressure stripping due to the passage of Fornax through a gaseous medium denser than the typical intragalactic medium left behind from the LMC may have caused the end of star formation in Fornax. The excess, anomalous clouds within the south Galactic pole region of the Magellanic Stream, whose origin has long been debated in the literature as constituents of either the Magellanic Stream or of the extragalactic Sculptor group, are found to lie along the orbit of Fornax. We speculate that these clouds are stripped material from Fornax as the dwarf crossed the orbit of the Magellanic Clouds.

The Absolute Proper Motion of Palomar 12: A Case for Tidal Capture from the Sagittarius Dwarf Spheroidal Galaxy

We have measured the absolute proper motion of the young globular cluster Pal 12 with respect to background galaxies, using plate material spanning a 40 yr time baseline, and measuring stars down to a magnitude V ~ 22. The measured absolute proper motion has an uncertainty of 0.3 mas yr-1 in each coordinate. Pal 12s young age for a globular cluster led to the hypothesis that the cluster originated in the Large Magellanic Cloud (LMC) and was later captured by the Milky Way (Lin & Richer). Here we investigate this hypothesis using the complete kinematical data. We present the orbital characteristics of Pal 12 and compare them with those of the LMC and Sagittarius dwarf galaxy (Sgr). The present kinematical data suggest that, from the two parent candidates for Pal 12, Sgr presents a more plausible case for the host galaxy than the LMC. We explore this scenario in the context of the uncertainties in the orbits and using two different analyses: the direct comparison of the orbits of Pal 12 and Sgr as a function of time, and the analytical model of Sgrs tidal disruption developed by Johnston. We find that, within the present uncertainties of the observables, this scenario is viable in both methods. Moreover, both methods place this event at the same point in time. Our best estimate of the time of Pal 12s tidal capture from Sgr is ~1.7 Gyr ago.

Local Surface Density of the Galactic Disk from a Three-Dimensional Stellar Velocity Sample

We have reestimated the surface density of the Galactic disk in the solar neighborhood within ±0.4 kpc of the Sun using the parallaxes and proper motions of a kinematically and spatially unbiased sample of 1476 old bright red giant stars from the Hipparcos catalog with measured radial velocities from Barbier-Brossat & Figon. We determine the vertical distribution of the red giants as well as the vertical velocity dispersion of the sample (14.4 ± 0.3 km s-1) and combine these to derive the surface density of the gravitating matter in the Galactic disk as a function of the Galactic coordinate z. The surface density of the disk increases from 10.5 ± 0.5 M⊙ pc-2 within ±50 pc to 42 ± 6 M⊙ pc-2 within ±350 pc. The estimated volume density of the Galactic disk within ±50 pc is about 0.1 M⊙ pc-3 which is close to the volume density estimates of the observed baryonic matter in the solar neighborhood.

SPACE VELOCITIES OF SOUTHERN GLOBULAR CLUSTERS. IV. FIRST RESULTS FOR INNER GALAXY CLUSTERS

We have measured the absolute proper motions of four low-latitude, inner Galaxy globular clusters. These clusters are NGC 6266 (M62), NGC 6304, NGC 6316, and NGC 6723. The proper motions are on the Hipparcos system, since no background extragalactic objects are found in these high-extinction regions. The proper-motion uncertainties range between 0.3 and 0.6 mas yr � 1 . We discuss the kinematics of these clusters and of three additional bulge clusters—NGC 6522, NGC 6528, and NGC 6553—whose proper motions with respect to bulge stars had been determined previously. We find that all of the clusters have velocities that confine them to the bulge region. Of the three metal-poor clusters ([Fe/H] < � 1.0), NGC 6522, and NGC 6723 have kinematics consistent with halo membership. The third cluster, NGC 6266 however, appears to belong to a rotationally supported system. Of the four metal-rich clusters ([Fe/H] �� 1.0), NGC 6304 and NGC 6553 also have kinematics consistent with membership to a rotationally supported system. NGC 6528 has kinematics, metallicity, and mass that argue in favor of a genuine Milky Way bar cluster. NGC 6316’s kinematics indicate membership to a hotter system than the bar.

The Southern Proper Motion Program. III. A Near-Complete Catalog to V = 17.5

We present the third installment of the Yale/San Juan Southern Proper Motion Catalog, SPM3. Absolute proper motions, positions, and photographic BV photometry are given for roughly 10.7 million objects, primarily stars, down to a magnitude of V = 17.5. The catalog covers an irregular area of 3700 deg2, between the declinations of -20? and -45?, excluding the Galactic plane. The proper-motion precision for well-measured stars is estimated to be 4.0 mas yr-1. Unlike previous releases of the SPM Catalog, the proper motions are on the International Celestial Reference System by way of Hipparcos catalog stars and have an estimated systematic uncertainty of 0.4 mas yr-1. The SPM3 Catalog is available via electronic transfer. As an example of the potential of the SPM3 proper motions, we examine the Galactocentric velocities of a group of metal-poor, main-sequence A stars. The majority of these exhibit thick-disk kinematics, lending support to their interpretation as thick-disk blue stragglers, as opposed to being an accreted component.

Absolute Proper Motion of the Sagittarius Dwarf Galaxy and of the Outer Regions of the Milky Way Bulge

We have determined the absolute proper motion of the Sagittarius dwarf spheroidal galaxy (Sgr) from the Southern Proper Motion Catalog 3, by selecting red giant candidates from the Two Micron All Sky Survey. We obtain μl cos b = -2.35 ± 0.20 and μb = 2.07 ± 0.20 mas yr-1. Using the same procedure, we also determine the absolute proper motion of the bulge in the region of the sky that overlaps with Sgr. For the bulge, we obtain μl cos b = -5.86 ± 0.14 and μb = -0.59 ± 0.14 mas yr-1, at l = 76 and b = -212. The absolute proper motions are on the International Celestial Reference System via Hipparcos Catalog stars.

A Capture Scenario for the Globular Cluster ω Centauri

We explore an accretion origin for ω Cen by N-body modeling of the orbital decay and disruption of a Milky Way dwarf satellite. This work is focused on studying a particular satellite model that aims to reproduce the present orbit of ω Cen, as recently determined from absolute proper motions. The model satellite is launched from 58 kpc from the Galactic center, on a radial low-inclination orbit. We find that a capture scenario can produce an ω Cen-like object with the current low-energy orbit of the cluster. Our best model is a nucleated dwarf galaxy with a Hernquist density profile that has a mass of 8 × 109 M☉ and a half-mass radius of 1.4 kpc.