882 research outputs found
Too Big to Fail in the Local Group
We compare the dynamical masses of dwarf galaxies in the Local Group (LG) to
the predicted masses of halos in the ELVIS suite of CDM simulations, a
sample of 48 Galaxy-size hosts, 24 of which are in paired configuration similar
to the LG. We enumerate unaccounted-for dense halos ( km s) in these volumes that at some point in their histories were
massive enough to have formed stars in the presence of an ionizing background
( km s). Within 300 kpc of the Milky Way, the
number of unaccounted-for massive halos ranges from 2 - 25 over our full
sample. Moreover, this "too big to fail" count grows as we extend our
comparison to the outer regions of the Local Group: within 1.2 Mpc of either
giant we find that there are 12-40 unaccounted-for massive halos. This count
excludes volumes within 300 kpc of both the MW and M31, and thus should be
largely unaffected by any baryonically-induced environmental processes.
According to abundance matching -- specifically abundance matching that
reproduces the Local Group stellar mass function -- all of these missing
massive systems should have been quite bright, with .
Finally, we use the predicted density structure of outer LG dark matter halos
together with observed dwarf galaxy masses to derive an
relation for LG galaxies that are outside the virial
regions of either giant. We find that there is no obvious trend in the relation
over three orders of magnitude in stellar mass (a "common mass" relation), from
, in drastic conflict with the tight relation
expected for halos that are unaffected by reionization. Solutions to the too
big to fail problem that rely on ram pressure stripping, tidal effects, or
statistical flukes appear less likely in the face of these results.Comment: 16 pages, 14 figures, 2 tables, submitted to MNRA
Modeling the induction, thrust, and power of a yaw misaligned actuator disk
Collective wind farm flow control, where wind turbines are operated in an
individually suboptimal strategy to benefit the aggregate farm, has
demonstrated potential to reduce wake interactions and increase farm energy
production. However, existing wake models used for flow control often estimate
the thrust and power of yaw misaligned turbines using simplified empirical
expressions which require expensive calibration data and do not accurately
extrapolate between turbine models. The thrust, wake velocity deficit, wake
deflection, and power of a yawed wind turbine depend on its induced velocity.
Here, we extend classical one-dimensional momentum theory to model the
induction of a yaw misaligned actuator disk. Analytical expressions for the
induction, thrust, initial wake velocities, and power are developed as a
function of the yaw angle and thrust coefficient. The analytical model is
validated against large eddy simulations of a yawed actuator disk. Because the
induction depends on the yaw and thrust coefficient, the power generated by a
yawed actuator disk will always be greater than a model
suggests, where is yaw. The power lost by yaw depends on the thrust
coefficient. An analytical expression for the thrust coefficient that maximizes
power, depending on the yaw, is developed and validated. Finally, using the
developed induction model as an initial condition for a turbulent far-wake
model, we demonstrate how combining wake steering and thrust (induction)
control can increase array power, compared to either independent steering or
induction control, due to the joint dependence of the induction on the thrust
coefficient and yaw angle.Comment: 22 pages, 9 figure
Segue 2: The Least Massive Galaxy
Segue 2, discovered by Belokurov et al., is a galaxy with a luminosity of only 900 L_☉. We present Keck/DEIMOS spectroscopy of 25 members of Segue 2—a threefold increase in spectroscopic sample size. The velocity dispersion is too small to be measured with our data. The upper limit with 90% (95%) confidence is σ_v < 2.2 (2.6) km s^(–1), the most stringent limit for any galaxy. The corresponding limit on the mass within the three-dimensional half-light radius (46 pc) is M_(1/2) < 1.5 (2.1) × 10^5 M_☉. Segue 2 is the least massive galaxy known. We identify Segue 2 as a galaxy rather than a star cluster based on the wide dispersion in [Fe/H] (from –2.85 to –1.33) among the member stars. The stars' [α/Fe] ratios decline with increasing [Fe/H], indicating that Segue 2 retained Type Ia supernova ejecta despite its presently small mass and that star formation lasted for at least 100 Myr. The mean metallicity, 〈[Fe/H]〉 = -2.22 ± 0.13 (about the same as the Ursa Minor galaxy, 330 times more luminous than Segue 2), is higher than expected from the luminosity-metallicity relation defined by more luminous dwarf galaxy satellites of the Milky Way. Segue 2 may be the barest remnant of a tidally stripped, Ursa Minor-sized galaxy. If so, it is the best example of an ultra-faint dwarf galaxy that came to be ultra-faint through tidal stripping. Alternatively, Segue 2 could have been born in a very low mass dark matter subhalo (v_(max) < 10 km s^(–1)), below the atomic hydrogen cooling limit
Stellar Kinematics in the Complicated Inner Spheroid of M31: Discovery of Substructure Along the Southeastern Minor Axis and its Relationship to the Giant Southern Stream
We present the discovery of a kinematically-cold stellar population along the
SE minor axis of the Andromeda galaxy (M31) that is likely the forward
continuation of M31's giant southern stream. This discovery was made in the
course of an on-going spectroscopic survey of red giant branch (RGB) stars in
M31 using the DEIMOS instrument on the Keck II 10-m telescope. Stellar
kinematics are investigated in eight fields located 9-30 kpc from M31's center
(in projection). A likelihood method based on photometric and spectroscopic
diagnostics is used to isolate confirmed M31 RGB stars from foreground Milky
Way dwarf stars: for the first time, this is done without using radial velocity
as a selection criterion, allowing an unbiased study of M31's stellar
kinematics. The radial velocity distribution of the 1013 M31 RGB stars shows
evidence for the presence of two components. The broad (hot) component has a
velocity dispersion of 129 km/s and presumably represents M31's virialized
spheroid. A significant fraction (19%) of the population is in a narrow (cold)
component centered near M31's systemic velocity with a velocity dispersion that
decreases with increasing radial distance, from 55.5 km/s at R_proj=12 kpc to
10.6 km/s at R_proj=18 kpc. The spatial and velocity distribution of the cold
component matches that of the "Southeast shelf" predicted by the Fardal et al.
(2007) orbital model of the progenitor of the giant southern stream. The
metallicity distribution of the cold component matches that of the giant
southern stream, but is about 0.2 dex more metal rich on average than that of
the hot spheroidal component. We discuss the implications of our discovery on
the interpretation of the intermediate-age spheroid population found in this
region in recent ultra-deep HST imaging studies.Comment: 23 pages, 16 figures, 2 tables, accepted for publication in the
Astrophysical Journal. Changes from previous version: expanded discussion in
sections 4.2 and 7.2, removal of section 7.1.4 and associated figure
(discussion moved to section 7.1.2
Ionic Tuning of Cobaltites at the Nanoscale
Control of materials through custom design of ionic distributions represents
a powerful new approach to develop future technologies ranging from spintronic
logic and memory devices to energy storage. Perovskites have shown particular
promise for ionic devices due to their high ion mobility and sensitivity to
chemical stoichiometry. In this work, we demonstrate a solid-state approach to
control of ionic distributions in (La,Sr)CoO thin films. Depositing a Gd
capping layer on the perovskite film, oxygen is controllably extracted from the
structure, up-to 0.5 O/u.c. throughout the entire 36 nm thickness. Commensurate
with the oxygen extraction, the Co valence state and saturation magnetization
show a smooth continuous variation. In contrast, magnetoresistance measurements
show no-change in the magnetic anisotropy and a rapid increase in the
resistivity over the same range of oxygen stoichiometry. These results suggest
significant phase separation, with metallic ferromagnetic regions and
oxygen-deficient, insulating, non-ferromagnetic regions, forming percolated
networks. Indeed, X-ray diffraction identifies oxygen-vacancy ordering,
including transformation to a brownmillerite crystal structure. The unexpected
transformation to the brownmillerite phase at ambient temperature is further
confirmed by high-resolution scanning transmission electron microscopy which
shows significant structural - and correspondingly chemical - phase separation.
This work demonstrates room-temperature ionic control of magnetism, electrical
resistivity, and crystalline structure in a 36 nm thick film, presenting new
opportunities for ionic devices that leverage multiple material
functionalities
Moons Are Planets: Scientific Usefulness Versus Cultural Teleology in the Taxonomy of Planetary Science
We argue that taxonomical concept development is vital for planetary science
as in all branches of science, but its importance has been obscured by unique
historical developments. The literature shows that the concept of planet
developed by scientists during the Copernican Revolution was theory-laden and
pragmatic for science. It included both primaries and satellites as planets due
to their common intrinsic, geological characteristics. About two centuries
later the non-scientific public had just adopted heliocentrism and was
motivated to preserve elements of geocentrism including teleology and the
assumptions of astrology. This motivated development of a folk concept of
planet that contradicted the scientific view. The folk taxonomy was based on
what an object orbits, making satellites out to be non-planets and ignoring
most asteroids. Astronomers continued to keep primaries and moons classed
together as planets and continued teaching that taxonomy until the 1920s. The
astronomical community lost interest in planets ca. 1910 to 1955 and during
that period complacently accepted the folk concept. Enough time has now elapsed
so that modern astronomers forgot this history and rewrote it to claim that the
folk taxonomy is the one that was created by the Copernican scientists.
Starting ca. 1960 when spacecraft missions were developed to send back detailed
new data, there was an explosion of publishing about planets including the
satellites, leading to revival of the Copernican planet concept. We present
evidence that taxonomical alignment with geological complexity is the most
useful scientific taxonomy for planets. It is this complexity of both primary
and secondary planets that is a key part of the chain of origins for life in
the cosmos.Comment: 68 pages, 16 figures. For supplemental data files, see
https://www.philipmetzger.com/moons_are_planets
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