568 research outputs found
Discriminating among theories of spiral structure using Gaia DR2
We compare the distribution in position and velocity of nearby stars from the
Gaia DR2 radial velocity sample with predictions of current theories for
spirals in disc galaxies. Although the rich substructure in velocity space
contains the same information, we find it more revealing to reproject the data
into action-angle variables, and we describe why resonant scattering would be
more readily identifiable in these variables. We compute the predicted changes
to the phase space density, in multiple different projections, that would be
caused by a simplified isolated spiral pattern, finding widely differing
predictions from each theory. We conclude that the phase space structure
present in the Gaia data shares many of the qualitative features expected in
the transient spiral mode model. We argue that the popular picture of
apparently swing-amplified spirals results from the superposition of a few
underlying spiral modes.Comment: Revised version accepted to appear in MNRAS. Some significant
improvements. A full resolution version of Fig 4 is available from
http://www.physics.rutgers.edu/~sellwood/mult_res.pd
Intercomparison of four different in-situ techniques for ambient formaldehyde measurements in urban air
International audienceResults from an intercomparison of several currently used in-situ techniques for the measurement of atmospheric formaldehyde (CH2O) are presented. The measurements were carried out at Bresso, an urban site in the periphery of Milan (Italy) as part of the FORMAT-I field campaign. Eight instruments were employed by six independent research groups using four different techniques: Differential Optical Absorption Spectroscopy (DOAS), Fourier Transform Infra Red (FTIR) interferometry, the fluorimetric Hantzsch reaction technique (five instruments) and a chromatographic technique employing C18-DNPH-cartridges (2,4-dinitrophenylhydrazine). White type multi-reflection systems were employed for the optical techniques in order to avoid spatial CH2O gradients and ensure the sampling of nearly the same air mass by all instruments. Between 23 and 31 July 2002, up to 13 ppbv of CH2O were observed. The concentrations lay well above the detection limits of all instruments. The formaldehyde concentrations determined with DOAS, FTIR and the Hantzsch instruments were found to agree within ±11%, with the exception of one Hantzsch instrument, which gave systematically higher values. The two hour integrated samples by DNPH yielded up to 25% lower concentrations than the data of the continuously measuring instruments averaged over the same time period. The consistency between the DOAS and the Hantzsch method was better than during previous intercomparisons in ambient air with slopes of the regression line not significantly differing from one. The differences between the individual Hantzsch instruments could be attributed in part to the calibration standards used. Possible systematic errors of the methods are discussed
Haptic subitizing across the fingers
Numerosity judgments of small sets of items (≤ 3) are generally fast and errorfree, while response times and error rates increase rapidly for larger numbers of items. We investigated an efficient process used for judging small numbers of items (known as subitizing) in active touch. We hypothesized that this efficient process for numerosity judgment might be related to stimulus properties that allow for efficient (parallel) search. Our results showed that subitizing was not possible forraised lines among flat surfaces, whereas this type of stimulus could be detected in parallel over the fingers. However, subitizing was possible when the number of fingers touching a surface had to be judged while the other fingers were lowered in mid-air. In the latter case, the lack of tactile input is essential, since subitizing was not enabled by differences in proprioceptive information from the fingers. Our results show that subitizing using haptic information from the fingers is possible only whensome fingers receive tactile information while other fingers do not
An ever-present snail shell triggered by a dark matter wake
We utilize a novel numerical technique to model star formation in
cosmological simulations of galaxy formation - called Superstars - to simulate
a Milky Way-like galaxy with star particles to study the
formation and evolution of out-of-equilibrium stellar disc structures in a full
cosmological setting. In the plane defined by the coordinate and velocity
perpendicular to the mid-plane (vertical phase space, ), stars in
Solar-like volumes at late times exhibit clear spirals qualitatively similar in
shape and amplitude to the ``Snail shell'' phase spiral. We show that
the phase spiral forms at a look back time of Gyr during the
pericentric passage of a satellite on a polar
orbit. This satellite stimulates the formation of a resonant wake in the dark
matter halo while losing mass at a rate of - dex per orbit loop.
The peak magnitude of the wake-induced gravitational torque at the Solar radius
is times that from the satellite, and triggers the formation of a disc
warp that wraps up into a vertical phase spiral over time. As the wake decays,
the phase spiral propagates several Gigayears to present-day and can be
described as ``ever-present'' once stable disc evolution is established. These
results suggest an alternative scenario to explain the phase spiral
which does not rely on a perturbation from bar buckling or a recent direct hit
from a satellite.Comment: accepted for pub in mnras. 11 + 5 pages. For a high-cadence animated
version of figure 1, see
https://wwwmpa.mpa-garching.mpg.de/auriga/movies/multi_halo_6_sf64.mp
Network conduciveness with application to the graph-coloring and independent-set optimization transitions
We introduce the notion of a network's conduciveness, a probabilistically
interpretable measure of how the network's structure allows it to be conducive
to roaming agents, in certain conditions, from one portion of the network to
another. We exemplify its use through an application to the two problems in
combinatorial optimization that, given an undirected graph, ask that its
so-called chromatic and independence numbers be found. Though NP-hard, when
solved on sequences of expanding random graphs there appear marked transitions
at which optimal solutions can be obtained substantially more easily than right
before them. We demonstrate that these phenomena can be understood by resorting
to the network that represents the solution space of the problems for each
graph and examining its conduciveness between the non-optimal solutions and the
optimal ones. At the said transitions, this network becomes strikingly more
conducive in the direction of the optimal solutions than it was just before
them, while at the same time becoming less conducive in the opposite direction.
We believe that, besides becoming useful also in other areas in which network
theory has a role to play, network conduciveness may become instrumental in
helping clarify further issues related to NP-hardness that remain poorly
understood
Range dependent processing of visual numerosity: similarities across vision and haptics
‘Subitizing’ refers to fast and accurate judgement of small numerosities, whereas for larger numerosities either counting or estimation are used. Counting is slow and precise, whereas estimation is fast but imprecise. In this study consisting of five experiments we investigated if and how the numerosity judgement process is affected by the relative spacing between the presented numerosities. To this end we let subjects judge the number of dots presented on a screen and recorded their response times. Our results show that subjects switch from counting to estimation if the relative differences between subsequent numerosities are large (a factor of 2), but that numerosity judgement in the subitizing range was still faster. We also show this fast performance for small numerosities only occurred when numerosity information is present. This indicates this is typical for number processing and not magnitude estimation in general. Furthermore, comparison with a previous haptic study suggests similar processing in numerosity judgement through haptics and vision
The globular cluster system of the Auriga simulations
We investigate whether the galaxy and star formation model used for the
Auriga simulations can produce a realistic globular cluster (GC) population. We
compare statistics of GC candidate star particles in the Auriga haloes with
catalogues of the Milky Way (MW) and Andromeda (M31) GC populations. We find
that the Auriga simulations do produce sufficient stellar mass for GC
candidates at radii and metallicities that are typical for the MW GC system
(GCS). We also find varying mass-ratios of the simulated GC candidates relative
to the observed mass in the MW and M31 GC systems for different bins of
galactocentric radius-metallicity (r -[Fe/H]). Overall, the
Auriga simulations produce GC candidates with higher metallicities than the MW
and M31 GCS and they are found at larger radii than observed. The Auriga
simulations would require bound cluster formation efficiencies higher than ten
percent for the metal-poor GC candidates, and those within the Solar radius
should experience negligible destruction rates to be consistent with
observations. GC candidates in the outer halo, on the other hand, should either
have low formation efficiencies, or experience high mass loss for the Auriga
simulations to produce a GCS that is consistent with that of the MW or M31.
Finally, the scatter in the metallicity as well as in the radial distribution
between different Auriga runs is considerably smaller than the differences
between that of the MW and M31 GCSs. The Auriga model is unlikely to give rise
to a GCS that can be consistent with both galaxies.Comment: Accepted by MNRAS 2020 May 15. Received 2020 May 14; in original form
2019 September
The effect of feature saliency on haptic subitizing
‘Subitizing’ refers to fast and error-free numerosity judgment for small (<4) sets of items. For larger sets, the slower process of ‘counting’ is used. Counting has a serial character, whereas subitizing is believed to have a parallel character. While subitizing was initially found in vision, it has been shown to exist in touch as well. In vision, it has been demonstrated that adding distractor items to a set of target items influences numerosity judgment of the target items. Subitizing was in this case only possible if the distractor item is highly salient among the targets. In the present study, we investigated the effect of adding a distractor item on haptic judgement of a set of target items. To this end, we asked subjects to judge the number of spheres grasped in their hand. Either a cube or an ellipsoid could be added to the set. A cube among spheres has been shown to be highly salient, while an ellipsoid among spheres is not. Our results show that adding a distractor item led to an increase in the response time slopes regardless of the distractor shape. Subitizing was, however, only possible in the case of a salient distractor. This is in agreement with results from vision
Focused Local Search for Random 3-Satisfiability
A local search algorithm solving an NP-complete optimisation problem can be
viewed as a stochastic process moving in an 'energy landscape' towards
eventually finding an optimal solution. For the random 3-satisfiability
problem, the heuristic of focusing the local moves on the presently
unsatisfiedclauses is known to be very effective: the time to solution has been
observed to grow only linearly in the number of variables, for a given
clauses-to-variables ratio sufficiently far below the critical
satisfiability threshold . We present numerical results
on the behaviour of three focused local search algorithms for this problem,
considering in particular the characteristics of a focused variant of the
simple Metropolis dynamics. We estimate the optimal value for the
``temperature'' parameter for this algorithm, such that its linear-time
regime extends as close to as possible. Similar parameter
optimisation is performed also for the well-known WalkSAT algorithm and for the
less studied, but very well performing Focused Record-to-Record Travel method.
We observe that with an appropriate choice of parameters, the linear time
regime for each of these algorithms seems to extend well into ratios -- much further than has so far been generally assumed. We discuss the
statistics of solution times for the algorithms, relate their performance to
the process of ``whitening'', and present some conjectures on the shape of
their computational phase diagrams.Comment: 20 pages, lots of figure
- …