2,994 research outputs found
Accurate masses for dispersion-supported galaxies
We derive an accurate mass estimator for dispersion-supported stellar systems
and demonstrate its validity by analyzing resolved line-of-sight velocity data
for globular clusters, dwarf galaxies, and elliptical galaxies. Specifically,
by manipulating the spherical Jeans equation we show that the dynamical mass
enclosed within the 3D deprojected half-light radius r_1/2 can be determined
with only mild assumptions about the spatial variation of the stellar velocity
dispersion anisotropy. We find M_1/2 = 3 \sigma_los^2 r_1/2 / G ~ 4
\sigma_los^2 R_eff / G, where \sigma_los^2 is the luminosity-weighted square of
the line-of-sight velocity dispersion and R_eff is the 2D projected half-light
radius. While deceptively familiar in form, this formula is not the virial
theorem, which cannot be used to determine accurate masses unless the radial
profile of the total mass is known a priori. We utilize this finding to show
that all of the Milky Way dwarf spheroidal galaxies (MW dSphs) are consistent
with having formed within a halo of mass approximately 3 x 10^9 M_sun in Lambda
CDM cosmology. The faintest MW dSphs seem to have formed in dark matter halos
that are at least as massive as those of the brightest MW dSphs, despite the
almost five orders of magnitude spread in luminosity. We expand our analysis to
the full range of observed dispersion-supported stellar systems and examine
their I-band mass-to-light ratios (M/L). The M/L vs. M_1/2 relation for
dispersion-supported galaxies follows a U-shape, with a broad minimum near M/L
~ 3 that spans dwarf elliptical galaxies to normal ellipticals, a steep rise to
M/L ~ 3,200 for ultra-faint dSphs, and a more shallow rise to M/L ~ 800 for
galaxy cluster spheroids.Comment: 20 pages, 13 figures. Accepted to MNRAS on March 27th, 201
Amateur Astronomy Research Telescope
The ERAU Amateur Astronomy Club Research Team is designing and building an amateur research telescope. The primary goal is high portability and compactness, with no compromise of optical quality. The team began with a set of mirrors pre-owned by the Amateur Astronomy Club, and will be constructing the telescope frame from medium density particle board. In the traditional style of John Dobson, the secondary mirror will be mounted on a truss system. This will reduce the weight and size of the telescope during transportation, as it can be disassembled and transported in a very small area. The primary purpose of the telescope is to observe and catalog messier objects in the night sky. After completion of the Messier Catalog, it shall be presented before physics faculty of ERAU
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
An Adaptive Optics Survey of Stellar Variability at the Galactic Center
We present a year adaptive optics (AO) study of stellar
variability and search for eclipsing binaries in the central pc
() of the Milky Way nuclear star cluster. We measure the photometry
of 563 stars using the Keck II NIRC2 imager (-band, ). We achieve a photometric uncertainty floor of (), comparable to the highest precision achieved
in other AO studies. Approximately half of our sample () shows
variability. of known early-type young stars and of
known late-type giants are variable. These variability fractions are higher
than those of other young, massive star populations or late-type giants in
globular clusters, and can be largely explained by two factors. First, our
experiment time baseline is sensitive to long-term intrinsic stellar
variability. Second, the proper motion of stars behind spatial inhomogeneities
in the foreground extinction screen can lead to variability. We recover the two
known Galactic center eclipsing binary systems: IRS 16SW and S4-258 (E60). We
constrain the Galactic center eclipsing binary fraction of known early-type
stars to be at least . We find no evidence of an eclipsing
binary among the young S-stars nor among the young stellar disk members. These
results are consistent with the local OB eclipsing binary fraction. We identify
a new periodic variable, S2-36, with a 39.43 day period. Further observations
are necessary to determine the nature of this source.Comment: 69 pages, 28 figures, 12 tables. Accepted for publication in The
Astrophysical Journa
Measuring the Orbits of the Arches and Quintuplet Clusters using HST and Gaia: Exploring Scenarios for Star Formation Near the Galactic Center
We present new absolute proper motion measurements for the Arches and
Quintuplet clusters, two young massive star clusters near the Galactic center.
Using multi-epoch HST observations, we construct proper motion catalogs for the
Arches (35,000 stars) and Quintuplet (40,000 stars) fields in ICRF
coordinates established using stars in common with the Gaia EDR3 catalog. The
bulk proper motions of the clusters are measured to be (,
) = (-0.80 0.032, -1.89 0.021) mas/yr for the Arches
and (, ) = (-0.96 0.032, -2.29 0.023)
mas/yr for the Quintuplet, achieving 5x higher precision than past
measurements. We place the first constraints on the properties of the cluster
orbits that incorporate the uncertainty in their current line-of-sight
distances. The clusters will not approach closer than 25 pc to SgrA*,
making it unlikely that they will inspiral into the Nuclear Star Cluster within
their lifetime. Further, the cluster orbits are not consistent with being
circular; the average value of r / r is 1.9 (equivalent
to eccentricity of 0.31) for both clusters. Lastly, we find that the
clusters do not share a common orbit, challenging one proposed formation
scenario in which the clusters formed from molecular clouds on the open stream
orbit derived by Kruijssen et al. (2015). Meanwhile, our constraints on the
birth location and velocity of the clusters offer mild support for a scenario
in which the clusters formed via collisions between gas clouds on the x1 and x2
bar orbit families.Comment: Accepted for publication in ApJ. 38 pages, 25 figures. Proper motion
catalogs included in ancillary material
The Orbital Eccentricities of Directly Imaged Companions Using Observable-Based Priors: Implications for Population-level Distributions
The eccentricity of a sub-stellar companion is an important tracer of its
formation history. Directly imaged companions often present poorly constrained
eccentricities. A recently developed prior framework for orbit fitting called
''observable-based priors'' has the advantage of improving biases in derived
orbit parameters for objects with minimal phase coverage, which is the case for
the majority of directly imaged companions. We use observable-based priors to
fit the orbits of 21 exoplanets and brown dwarfs in an effort to obtain the
eccentricity distributions with minimized biases. We present the objects'
individual posteriors compared to their previously derived distributions,
showing in many cases a shift toward lower eccentricities. We analyze the
companions' eccentricity distribution at a population level, and compare this
to the distributions obtained with the traditional uniform priors. We fit a
Beta distribution to our posteriors using observable-based priors, obtaining
shape parameters and . This represents an approximately flat distribution of
eccentricities. The derived and parameters are consistent with
the values obtained using uniform priors, though uniform priors lead to a tail
at high eccentricities. We find that separating the population into high and
low mass companions yields different distributions depending on the
classification of intermediate mass objects. We also determine via simulation
that the minimal orbit coverage needed to give meaningful posteriors under the
assumptions made for directly imaged planets is 15% of the inferred
period of the orbit.Comment: Accepted for publication in AJ, 20 pages, 13 figure
Combined collider constraints on neutralinos and charginos
Searches for supersymmetric electroweakinos have entered a crucial phase, as
the integrated luminosity of the Large Hadron Collider is now high enough to
compensate for their weak production cross-sections. Working in a framework
where the neutralinos and charginos are the only light sparticles in the
Minimal Supersymmetric Standard Model, we use gambit to perform a detailed
likelihood analysis of the electroweakino sector. We focus on the impacts of
recent ATLAS and CMS searches with 36 fb of 13 TeV proton-proton
collision data. We also include constraints from LEP and invisible decays of
the and Higgs bosons. Under the background-only hypothesis, we show that
current LHC searches do not robustly exclude any range of neutralino or
chargino masses. However, a pattern of excesses in several LHC analyses points
towards a possible signal, with neutralino masses of = (8-155,
103-260, 130-473, 219-502) GeV and chargino masses of
= (104-259, 224-507) GeV
at the 95% confidence level. The lightest neutralino is mostly bino, with a
possible modest Higgsino or wino component. We find that this excess has a
combined local significance of , subject to a number of cautions. If
one includes LHC searches for charginos and neutralinos conducted with 8 TeV
proton-proton collision data, the local significance is lowered to 2.9.
We briefly consider the implications for dark matter, finding that the correct
relic density can be obtained through the Higgs-funnel and -funnel
mechanisms, even assuming that all other sparticles are decoupled. All samples,
gambit input files and best-fit models from this study are available on Zenodo.Comment: 38 pages, 16 figures, v3 is the version accepted by EPJ
Sources of Nonnative Centrarchids in the Upper Colorado River Revealed by Stable Isotope and Microchemical Analyses of Otoliths
Nonnative fishes represent a significant impediment to the recovery of imperiled fishes, including those endemic to the Colorado River in the southwestern United States. Efforts to control nonindigenous fish abundance in the upper Colorado River basin have been unsuccessful owing in part to lack of knowledge regarding nonnative fish recruitment sources. We determined the source habitat (floodplain pond versus riverine habitats) for nonnative centrarchid fishes (largemouth bass Micropterus salmoides, green sunfish Lepomis cyanellus, bluegill L. macrochirus, and black crappie Pomoxis nigromaculatus) in the upper Colorado River using stable hydrogen isotopic composition (δD) and strontium:calcium (Sr:Ca) ratios in fish otoliths as natural markers of environmental history. Stable hydrogen isotope analysis revealed that 59% of centrarchids exhibited the otolith core signatures expected for riverine-origin fish, while 22% had emigrated from floodplain ponds and 19% were of uncertain origin. Strontium:calcium ratio data were consistent with the δD assays and indicated that relatively few fish immigrated to the river from high-salinity habitats. Black crappie was the only species that originated primarily from floodplain ponds. Efforts to control the abundance of most of the fishes included in this study should be concentrated in riverine habitats given the hydrologic conditions (below-average river discharge) present during our study. However, the proportion of pond-origin fish increased with fish age, which, coupled with historical river discharge data, suggested that floodplain pond contributions to riverine nonnative fish populations fluctuate with the interannual variations in flow regime and river–pond connectivity. Our results are the first to demonstrate the utility of δD as a natural marker of fish environmental history that will probably provide valuable insights into the management of fish in other environments
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Patterns of sediment-associated fecal indicator bacteria in an urban estuary: benthic-pelagic coupling and implications for shoreline water quality.
Estuarine and coastal waterways are commonly monitored for fecal and sewage contamination to protect recreator health and ecosystem functions. Such monitoring programs commonly rely on cultivation-based counts of fecal indicator bacteria (FIB) in water column samples. Recent studies demonstrate that sediments and beach sands can be heavily colonized by FIB, and that settling and resuspension of colonized particles may significantly influence the distribution of FIB in the water column. However, measurements of sediment FIB are rarely incor- porated into monitoring programs, and geographic surveys of sediment FIB are uncommon. In this study, the dis- tribution of FIB and the extent of benthic-pelagic FIB coupling were examined in the urbanized, lower Hudson River Estuary. Using cultivation-based enumeration, two commonly-measured FIB, enterococci and Escherichia coli, were widely distributed in both sediment and water, and were positively correlated with each other. The tax- onomic identity of FIB isolates from water and sediment was confirmed by DNA sequencing. The geometric mean of FIB concentration in sediment was correlated with both the geometric mean of FIB in water samples from the same locations and with sediment organic carbon. These two positive associations likely reflect water as the FIB source for underlying sediments, and longer FIB persistence in the sediments compared to the water, respec- tively. The relative representation of other fecal associated bacterial genera in sediment, determined by 16S rRNA gene sequencing, increased with the sequence representation of the two FIB, supporting the value of these FIB for assessing sediment contamination. Experimental resuspension of sediment increased shoreline water column FIB concentrations, which may explain why shoreline water samples had higher average FIB con- centrations than samples collected nearby but further from shore. In combination, these results demonstrate extensive benthic-pelagic coupling of FIB in an urbanized estuary and highlight the importance of sediment FIB distribution and ecology when interpreting water quality monitoring data
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