28 research outputs found
Correcting Velocity Dispersions of Dwarf Spheroidal Galaxies for Binary Orbital Motion
We show that the measured velocity dispersions of dwarf spheroidal galaxies from about 4 to 10 km s^(–1) are unlikely to be inflated by more than 30% due to the orbital motion of binary stars and demonstrate that the intrinsic velocity dispersions can be determined to within a few percent accuracy using two-epoch observations with 1-2 yr as the optimal time interval. The crucial observable is the threshold fraction—the fraction of stars that show velocity changes larger than a given threshold between measurements. The threshold fraction is tightly correlated with the dispersion introduced by binaries, independent of the underlying binary fraction and distribution of orbital parameters. We outline a simple procedure to correct the velocity dispersion to within a few percent accuracy by using the threshold fraction and provide fitting functions for this method. We also develop a methodology for constraining properties of binary populations from both single- and two-epoch velocity measurements by including the binary velocity distribution in a Bayesian analysis
A Complete Spectroscopic Survey of the Milky Way Satellite Segue 1: The Darkest Galaxy
Spectroscopic study of the Segue 1 dwarf galaxy. --author-supplied descriptio
Robust velocity dispersion and binary population modeling of the ultra-faint dwarf galaxy Reticulum II
We apply a Bayesian method to model multi-epoch radial velocity measurements
in the ultra-faint dwarf galaxy Reticulum II, fully accounting for the effects
of binary orbital motion and systematic offsets between different spectroscopic
datasets. We find that the binary fraction of Ret II is higher than 0.5 at the
90% confidence level, if the mean orbital period is assumed to be 30 years or
longer. Despite this high binary fraction, we infer a best-fit intrinsic
dispersion of 2.8 km/s, which is smaller than previous
estimates, but still indicates Ret II is a dark-matter dominated galaxy. We
likewise infer a 1% probability that Ret II's dispersion is due to
binaries rather than dark matter, corresponding to the regime
2. Our inference of a high close binary fraction
in Ret II echoes previous results for the Segue 1 ultra-faint dwarf and is
consistent with studies of Milky Way halo stars that indicate a high close
binary fraction tends to exist in metal-poor environments.Comment: 9 pages, 5 figures, to be submitted to MNRA
A Complete Spectroscopic Survey of the Milky Way satellite Segue 1: Dark matter content, stellar membership and binary properties from a Bayesian analysis
We introduce a comprehensive analysis of multi-epoch stellar line-of-sight
velocities to determine the intrinsic velocity dispersion of the ultrafaint
satellites of the Milky Way. Our method includes a simultaneous Bayesian
analysis of both membership probabilities and the contribution of binary
orbital motion to the observed velocity dispersion within a 14-parameter
likelihood. We apply our method to the Segue 1 dwarf galaxy and conclude that
Segue 1 is a dark-matter-dominated galaxy at high probability with an intrinsic
velocity dispersion of 3.7^{+1.4}_{-1.1} km/sec. The dark matter halo required
to produce this dispersion must have an average density of 2.5^{+4.1}_{-1.9}
solar mass/pc^3 within a sphere that encloses half the galaxy's stellar
luminosity. This is the highest measured density of dark matter in the Local
Group. Our results show that a significant fraction of the stars in Segue 1 may
be binaries with the most probable mean period close to 10 years, but also
consistent with the 180 year mean period seen in the solar vicinity at about 1
sigma. Despite this binary population, the possibility that Segue 1 is a bound
star cluster with the observed velocity dispersion arising from the orbital
motion of binary stars is disfavored by the multi-epoch stellar velocity data
at greater than 99% C.L. Finally, our treatment yields a projected
(two-dimensional) half-light radius for the stellar profile of Segue 1 of
28^{+5}_{-4} pc, in excellent agreement with photometric measurements.Comment: 15 pages, 19 figure
Effects of galaxy-halo alignment and adiabatic contraction on gravitational lens statistics
We study the strong gravitational lens statistics of triaxial cold dark
matter (CDM) halos occupied by central early-type galaxies. We calculate the
image separation distribution for double, cusp and quad configurations. The
ratios of image multiplicities at large separations are consistent with the
triaxial NFW model, and at small separations are consistent with the singular
isothermal ellipsoid (SIE) model. At all separations, the total lensing
probability is enhanced by adiabatic contraction. If no adiabatic contraction
is assumed, naked cusp configurations become dominant at approximately 2.5'',
which is inconsistent with the data. We also show that at small-to-moderate
separations, the image multiplicities depend sensitively on the alignment of
the shapes of the luminous and dark matter projected density profiles. In
constrast to other properties that affect these ratios, the degree of alignment
does not have a significant effect on the total lensing probability. These
correlations may therefore be constrained by comparing the theoretical image
separation distribution to a sufficiently large lens sample from future wide
and deep sky surveys such as Pan-Starrs, LSST and JDEM. Understanding the
correlations in the shapes of galaxies and their dark matter halo is important
for future weak lensing surveys.Comment: 10 pages, 7 figure