329 research outputs found
When Does Evidence Suffice for Conviction?
There is something puzzling about statistical evidence. One place this manifests is in the law, where courts are reluctant to base affirmative verdicts on evidence that is purely statistical, in spite of the fact that it is perfectly capable of meeting the standards of proof enshrined in legal doctrine. After surveying some proposed explanations for this, I shall outline a new approach – one that makes use of a notion of normalcy that is distinct from the idea of statistical frequency. The puzzle is not, however, merely a legal one. Our unwillingness to base beliefs on statistical evidence is by no means limited to the courtroom, and is at odds with almost every general principle that epistemologists have proposed as to how we ought to manage our beliefs
A novel high-throughput screen for identifying lipids that stabilise membrane proteins in detergent based solution
Membrane proteins have a range of crucial biological functions and are the target of about 60% of all prescribed drugs. For most studies, they need to be extracted out of the lipid-bilayer, e.g. by detergent solubilisation, leading to the loss of native lipids, which may disturb important protein-lipid/bilayer interactions and thus functional and structural integrity. Relipidation of membrane proteins has proven extremely successful for studying challenging targets, but the identification of suitable lipids can be expensive and laborious. Therefore, we developed a screen to aid the high-throughput identification of beneficial lipids. The screen covers a large lipid space and was designed to be suitable for a range of stability assessment methods. Here, we demonstrate its use as a tool for identifying stabilising lipids for three membrane proteins: a bacterial pyrophosphatase (Tm-PPase), a fungal purine transporter (UapA) and a human GPCR (A(2A)R). A(2A)R is stabilised by cholesteryl hemisuccinate, a lipid well known to stabilise GPCRs, validating the approach. Additionally, our screen also identified a range of new lipids which stabilised our test proteins, providing a starting point for further investigation and demonstrating its value as a novel tool for membrane protein research. The pre-dispensed screen will be made commercially available to the scientific community in future and has a number of potential applications in the field.Peer reviewe
Modelling the near-IR spectra of Jupiter using line-by-line methods
We have obtained long-slit, infrared spectra of Jupiter with the Anglo
Australian Telescope in the K and H bands at a resolving power of 2260. Using a
line-by-line, radiative transfer model with the latest, improved spectral line
data for methane and ammonia, we derive a model of the zonal characteristics in
the atmosphere of this giant planet. We fit our model to the spectra of the
zones and belts visible at 2.1 {\mu}m using different distributions of cloud
opacities. The modeled spectra for each region match observations remarkably
well at K band and in low pressure regions at the H band. Our results for the
upper deck cloud distribution are consistent with previous models (Banfield et
al.1998) fitted to low resolution, grism spectra. The ability to obtain and
model high resolution planetary spectra in order to search for weakly absorbing
atmospheric constituents can provide better constraints on the chemical
composition of planetary atmospheres.Comment: Accepted by MNRAS on the 7th February 201
The globular cluster NGC 2419: a crucible for theories of gravity
We present the analysis of a kinematic data set of stars in the globular
cluster NGC 2419, taken with Keck/DEIMOS. Combined with a reanalysis of deep
HST and Subaru imaging data, which provide an accurate luminosity profile of
the cluster, we investigate the validity of a large set of dynamical models of
the system, which are checked for stability via N-body simulations. We find
that isotropic models in either Newtonian or Modified Newtonian Dynamics (MOND)
are ruled out with extremely high confidence. However, a simple Michie model in
Newtonian gravity with anisotropic velocity dispersion provides an excellent
representation of the luminosity profile and kinematics. In contrast, with MOND
we find that Michie models that reproduce the luminosity profile either
over-predict the velocity dispersion on the outskirts of the cluster if the
mass to light ratio is kept at astrophysically-motivated values, or else they
under-predict the central velocity dispersion if the mass to light ratio is
taken to be very small. We find that the best Michie model in MOND is a factor
of 10000 less likely than the Newtonian model that best fits the system. A
likelihood ratio of 350 is found when we investigate more general models by
solving the Jeans equation with a Markov-Chain Monte Carlo scheme. We verified
with N-body simulations that these results are not significantly different when
the MOND external field effect is accounted for. If the assumptions that the
cluster is in dynamical equilibrium, spherical, not on a peculiar orbit, and
possesses a single dynamical tracer population of constant M/L are correct, we
conclude that the present observations provide a very severe challenge for
MOND. [abridged]Comment: 25 pages, 19 figures, accepted for publication in Ap
Tidal Signatures in the Faintest Milky Way Satellites: The Detailed Properties of Leo V, Pisces II and Canes Venatici II
We present deep wide-field photometry of three recently discovered faint
Milky Way satellites: Leo V, Pisces II, and Canes Venatici II. Our main goals
are to study the structure and star formation history of these dwarfs; we also
search for signs of tidal disturbance. The three satellites have similar
half-light radii ( pc) but a wide range of ellipticities. Both Leo
V and CVn II show hints of stream-like overdensities at large radii. An
analysis of the satellite color-magnitude diagrams shows that all three objects
are old ( 10 Gyr) and metal-poor ([Fe/H] ), though neither the
models nor the data have sufficient precision to assess when the satellites
formed with respect to cosmic reionization. The lack of an observed younger
stellar population (\la 10 Gyr) possibly sets them apart from the other
satellites at Galactocentric distances \ga 150 kpc. We present a new
compilation of structural data for all Milky Way satellite galaxies and use it
to compare the properties of classical dwarfs to the ultra-faints. The
ellipticity distribution of the two groups is consistent at the
2- level. However, the faintest satellites tend to be more
aligned toward the Galactic center, and those satellites with the highest
ellipticity (\ga 0.4) have orientations () in the range
. This latter
observation is in rough agreement with predictions from simulations of dwarf
galaxies that have lost a significant fraction of their dark matter halos and
are being tidally stripped.Comment: 25 pages, 7 figures, ApJ accepted; version updated to match ApJ
accepte
From Galaxy Clusters to Ultra-Faint Dwarf Spheroidals: A Fundamental Curve Connecting Dispersion-supported Galaxies to Their Dark Matter Halos
We examine scaling relations of dispersion-supported galaxies over more than
eight orders of magnitude in luminosity by transforming standard fundamental
plane parameters into a space of mass (M1/2), radius (r1/2), and luminosity
(L1/2). We find that from ultra-faint dwarf spheroidals to giant cluster
spheroids, dispersion-supported galaxies scatter about a one-dimensional
"fundamental curve" through this MRL space. The weakness of the M1/2-L1/2 slope
on the faint end may imply that potential well depth limits galaxy formation in
small galaxies, while the stronger dependence on L1/2 on the bright end
suggests that baryonic physics limits galaxy formation in massive galaxies. The
mass-radius projection of this curve can be compared to median dark matter halo
mass profiles of LCDM halos in order to construct a virial mass-luminosity
relationship (Mvir-L) for galaxies that spans seven orders of magnitude in
Mvir. Independent of any global abundance or clustering information, we find
that (spheroidal) galaxy formation needs to be most efficient in halos of Mvir
~ 10^12 Msun and to become inefficient above and below this scale. Moreover,
this profile matching technique is most accurate at the high and low luminosity
extremes (where dark matter fractions are highest) and is therefore quite
complementary to statistical approaches that rely on having a well-sampled
luminosity function. We also consider the significance and utility of the
scatter about this relation, and find that in the dSph regime observational
errors are almost at the point where we can explore the intrinsic scatter in
the luminosity-virial mass relation. Finally, we note that purely stellar
systems like Globular Clusters and Ultra Compact Dwarfs do not follow the
fundamental curve relation. This allows them to be easily distinguished from
dark-matter dominated dSph galaxies in MRL space. (abridged)Comment: 27 pages, 18 figures, ApJ accepted. High-res movies of 3D figures are
available at http://www.physics.uci.edu/~bullock/fcurve/movies.htm
Stealth Galaxies in the Halo of the Milky Way
We predict that there is a population of low-luminosity dwarf galaxies
orbiting within the halo of the Milky Way that have surface brightnesses low
enough to have escaped detection in star-count surveys. The overall count of
stealth galaxies is sensitive to the presence (or lack) of a low-mass threshold
in galaxy formation. These systems have luminosities and stellar velocity
dispersions that are similar to those of known ultrafaint dwarf galaxies but
they have more extended stellar distributions (half light radii greater than
about 100 pc) because they inhabit dark subhalos that are slightly less massive
than their higher surface brightness counterparts. As a result, the typical
peak surface brightness is fainter than 30 mag per square arcsec. One
implication is that the inferred common mass scale for Milky Way dwarfs may be
an artifact of selection bias. If there is no sharp threshold in galaxy
formation at low halo mass, then ultrafaint galaxies like Segue 1 represent the
high-mass, early forming tail of a much larger population of objects that could
number in the hundreds and have typical peak circular velocities of about 8
km/s and masses within 300 pc of about 5 million solar masses. Alternatively,
if we impose a low-mass threshold in galaxy formation in order to explain the
unexpectedly high densities of the ultrafaint dwarfs, then we expect only a
handful of stealth galaxies in the halo of the Milky Way. A complete census of
these objects will require deeper sky surveys, 30m-class follow-up telescopes,
and more refined methods to identify extended, self-bound groupings of stars in
the halo.Comment: 12 pages, 7 figures, accepted by ApJ. Several crucial references
added and the discussion has been expanded. Conclusions are unchanged
The population of variable stars in M54 (NGC6715)
We present new B, V and I CCD time-series photometry for 177 variable stars
in a 13'X 13' field centered on the globular cluster M54 (lying at the center
of the Sagittarius dwarf spheroidal galaxy), 94 of which are newly identified
variables. The total sample is composed of 2 anomalous Cepheids, 144 RR Lyrae
stars (108 RR0 and 36 RR1), 3 SX Phoenicis, 7 eclipsing binaries (5 W UMA and 2
Algol binaries), 3 variables of uncertain classification and 18 long-period
variables. The large majority of the RR Lyrae variables likely belong to M54.
Ephemerides are provided for all the observed short-period variables. The
pulsational properties of the M54 RR Lyrae variables are close to those of
Oosterhoff I clusters, but a significant number of long-period ab type RR Lyrae
are present. We use the observed properties of the RR Lyrae to estimate the
reddening and the distance modulus of M54, E(B-V)=0.16 +/- 0.02 and
(m-M)_0=17.13 +/- 0.11, respectively, in excellent agreement with the most
recent estimates. The metallicity has been estimated for a subset of 47 RR
Lyrae stars, with especially good quality light curves, from the Fourier
parameters of the V light curve. The derived metallicity distribution has a
symmetric bell shape, with a mean of =-1.65 and a standard deviation
sigma=0.16 dex. Seven stars have been identified as likely belonging to the
Sagittarius galaxy, based on their too high or too low metallicity. This
evidence, if confirmed, might suggest that old stars in this galaxy span a wide
range of metallicities.Comment: 15 pages, 11 figures, accepted for publication by MNRA
SMASH 1 : A VERY FAINT GLOBULAR CLUSTER DISRUPTING in the OUTER REACHES of the LMC?
We present the discovery of a very faint stellar system, SMASH 1, that is potentially a satellite of the Large Magellanic Cloud. Found within the Survey of the Magellanic Stellar History (SMASH), SMASH 1 is a compact (rh=9.1-3.4+5.9pc) and very low luminosity (Mv=-1.0±0.9,Lv=102.3±0.4L⊙ ) stellar system that is revealed by its sparsely populated main sequence and a handful of red giant branch candidate member stars. The photometric properties of these stars are compatible with a metal-poor ([Fe/H]=-2.2) and old (13 Gyr) isochrone located at a distance modulus of ∼18.8, i.e., a distance of . Situated at 11.°3 from the LMC in projection, its three-dimensional distance from the Cloud is 13 kpc, consistent with a connection to the LMC, whose tidal radius is at least . Although the nature of SMASH 1 remains uncertain, its compactness favors it being a stellar cluster and hence dark-matter free. If this is the case, its dynamical tidal radius is only ≲19 pc at this distance from the LMC, and smaller than the system's extent on the sky. Its low luminosity and apparent high ellipticity (ϵ=0.62-0.21+0.17) with its major axis pointing toward the LMC may well be the tell-tale sign of its imminent tidal demise.Peer reviewe
A comparison between the stellar and dynamical masses of six globular clusters
We present the results of a comprehensive analysis of the structure and
kinematics of six Galactic globular clusters. By comparing the results of the
most extensive photometric and kinematical surveys available to date with
suited dynamical models, we determine the stellar and dynamical masses of these
stellar systems taking into account for the effect of mass segregation,
anisotropy and unresolved binaries. We show that the stellar masses of these
clusters are on average smaller than those predicted by canonical integrated
stellar evolution models because of the shallower slope of their mass
functions. The derived stellar masses are found to be also systematically
smaller than the dynamical masses by ~40%, although the presence of systematics
affecting our estimates cannot be excluded. If confirmed, this evidence can be
linked to an increased fraction of retained dark remnants or to the presence of
a modest amount of dark matter.Comment: 13 pages, 8 figures, accepted for publication by Ap
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