We present the first three-dimensional internal motions for individual stars in the Draco dwarf spheroidal galaxy. By combining first epoch Hubble Space Telescope observations and second epoch Gaia Data Release 2 positions we measured the proper motions of 149 sources in the direction of Draco. We determined the line-of-sight velocities of a sub-sample of 81 red giant branch stars using medium resolution spectra acquired with the DEIMOS spectrograph at the Keck II telescope. Altogether this resulted in a final sample of 45 members of Draco with high-precision and accurate 3D motions, which we publish as a table in this paper. With this high-quality dataset we determined the velocity dispersions at a projected distance of ~120pc from the centre of Draco to be sigma_R_=11.0^+2.1^_-1.5_km/s, sigma_T_=9.9^+2.3^_-3.1_km/s and sigma_LOS_=9.0^+1.1^_-1.1_ km/s in the projected radial, tangential and line-of-sight directions. This results in a velocity anisotropy beta=0.25^+0.47^_-1.38_ at r>120pc. Tighter constraints can be obtained using the spherical Jeans equations and assuming constant anisotropy and NFW mass profiles, as well as that the 3D velocity dispersion should be lower than ~1/3 of the escape velocity of the system. In this case, we constrain the maximum circular velocity V_max_ of Draco to be in the range of 10.2-17.0km/s. The corresponding mass range is in good agreement with previous estimates based on line-of-sight velocities only. Our Jeans modelling supports the case for a cuspy dark matter profile in this galaxy. Firmer conclusions may be drawn by applying more sophisticated models on this dataset and with upcoming data releases.
The kinematics and dynamics of stellar and substellar populations within young, still-forming clusters provide valuable information for constraining theories of formation mechanisms. Using Keck II NIRSPEC+AO data, we have measured radial velocities for 56 low-mass sources within 4' of the core of the Orion Nebula Cluster (ONC). We also remeasure radial velocities for 172 sources observed with SDSS/APOGEE. These data are combined with proper motions measured using HST ACS/WFPC2/WFC3IR and Keck II NIRC2, creating a sample of 135 sources with all three velocity components. The velocities measured are consistent with a normal distribution in all three components. We measure intrinsic velocity dispersions of ({sigma}_{nu}{alpha}_, {sigma}_{nu}{beta}_, {sigma}_{nu}{delta}_ ) = (1.64+/-0.12, 2.03+/-0.13, 2.56^0.17^_0.16_) km/s. Our computed intrinsic velocity dispersion profiles are consistent with the dynamical equilibrium models from Da Rio et al. (2017ApJ...845..105D) in the tangential direction but not in the line-of-sight direction, possibly indicating that the core of the ONC is not yet virialized, and may require a nonspherical potential to explain the observed velocity dispersion profiles. We also observe a slight elongation along the north-south direction following the filament, which has been well studied in previous literature, and an elongation in the line-of-sight to tangential velocity direction. These 3D kinematics will help in the development of realistic models of the formation and early evolution of massive clusters.
3D-kinematics of white dwarfs from SPY project. II.
Short Name:
J/A+A/447/173
Date:
21 Oct 2021
Publisher:
CDS
Description:
We present the kinematics of a sample of 398 DA white dwarfs from the SPY project (ESO SN Ia Progenitor surveY) and discuss kinematic criteria for distinguishing of thin-disk, thick-disk, and halo populations. This is the largest 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 were combined with our measurements of proper motions to derive 3D space motions. Galactic orbits and further kinematic parameters were computed. We calculated individual errors of kinematic parameters by means of a Monte Carlo error propagation code. Our kinematic criteria for assigning population membership were deduced from a sample of F and G stars taken from the literature, for which chemical criteria can be used to distinguish between a thin-disk, a thick-disk and a halo star. Our kinematic population classification scheme is based on the position in the U-V-velocity diagram, the position in the J_z_-eccentricity diagram, and the Galactic orbit. We combined this with age information and found seven halo and 23 thick-disk white dwarfs in this brightness limited sample. Another four rather cool white dwarfs probably also belong to the thick disk. Correspondingly 2% of the white dwarfs belong to the halo and 7% to the thick disk. The mass contribution of the thick-disk white dwarfs is found to be substantial, but is insufficient to account for the missing dark matter.
We detail an innovative new technique for measuring the two-dimensional (2D) velocity moments (rotation velocity, velocity dispersion and Gauss-Hermite coefficients h3 and h4) of the stellar populations of galaxy haloes using spectra from Keck DEIMOS (Deep Imaging Multi-Object Spectrograph) multi-object spectroscopic observations. The data are used to reconstruct 2D rotation velocity maps. Here we present data for five nearby early-type galaxies to ~three effective radii.
We report on extensive photometry of DK Lac obtained during the interval 1990-2009, which includes a 2mag low state during 2001-2003. Much of the photometry consists of exposures obtained with a typical spacing of several days, but also includes 26 sequences of continuous photometry each lasting 2-7hr. We find no evidence for periodicities in our data.