1,045 research outputs found
The Local Group: The Ultimate Deep Field
Near-field cosmology -- using detailed observations of the Local Group and
its environs to study wide-ranging questions in galaxy formation and dark
matter physics -- has become a mature and rich field over the past decade.
There are lingering concerns, however, that the relatively small size of the
present-day Local Group ( Mpc diameter) imposes insurmountable
sample-variance uncertainties, limiting its broader utility. We consider the
region spanned by the Local Group's progenitors at earlier times and show that
it reaches co-moving Mpc in linear size (a volume of ) at . This size at early cosmic epochs is large enough
to be representative in terms of the matter density and counts of dark matter
halos with . The Local
Group's stellar fossil record traces the cosmic evolution of galaxies with
(reaching
at ) over a region that is comparable to or larger than
the Hubble Ultra-Deep Field (HUDF) for the entire history of the Universe. It
is highly complementary to the HUDF, as it probes much fainter galaxies but
does not contain the intrinsically rarer, brighter sources that are detectable
in the HUDF. Archaeological studies in the Local Group also provide the ability
to trace the evolution of individual galaxies across time as opposed to
evaluating statistical connections between temporally distinct populations. In
the JWST era, resolved stellar populations will probe regions larger than the
HUDF and any deep JWST fields, further enhancing the value of near-field
cosmology.Comment: 6 pages, 5 figures; MNRAS Letters, in pres
ELVIS: Exploring the Local Volume in Simulations
We introduce a set of high-resolution dissipationless simulations that model
the Local Group (LG) in a cosmological context: Exploring the Local Volume in
Simulations (ELVIS). The suite contains 48 Galaxy-size halos, each within
high-resolution volumes that span 2-5 Mpc in size, and each resolving thousands
of systems with masses below the atomic cooling limit. Half of the ELVIS galaxy
halos are in paired configurations similar to the Milky Way (MW) and M31; the
other half are isolated, mass-matched analogs. We find no difference in the
abundance or kinematics of substructure within the virial radii of isolated
versus paired hosts. On Mpc scales, however, LG-like pairs average almost twice
as many companions and the velocity field is kinematically hotter and more
complex. We present a refined abundance matching relation between stellar mass
and halo mass that reproduces the observed satellite stellar mass functions of
the MW and M31 down to the regime where incompleteness is an issue, . Within a larger region spanning approximately 3
Mpc, the same relation predicts that there should be 1000 galaxies with
awaiting discovery. We show that up to 50% of halos
within 1 Mpc of the MW or M31 could be systems that have previously been within
the virial radius of either giant. By associating never-accreted halos with
gas-rich dwarfs, we show that there are plausibly 50 undiscovered dwarf
galaxies with HI masses within the Local Volume. The radial
velocity distribution of these predicted gas-rich dwarfs can be used to inform
follow-up searches based on ultra-compact high-velocity clouds found in the
ALFALFA survey.Comment: 22 pages, 19 figures, 3 tables; v2 -- accepted to MNRAS. Movies,
images, and data are available at http://localgroup.ps.uci.edu/elvi
Learning and Production of Movement Sequences: Behavioral, Neurophysiological, and Modeling Perspectives
A growing wave of behavioral studies, using a wide variety of paradigms that were introduced or greatly refined in recent years, has generated a new wealth of parametric observations about serial order behavior. What was a mere trickle of neurophysiological studies has grown to a more steady stream of probes of neural sites and mechanisms underlying sequential behavior. Moreover, simulation models of serial behavior generation have begun to open a channel to link cellular dynamics with cognitive and behavioral dynamics. Here we summarize the major results from prominent sequence learning and performance tasks, namely immediate serial recall, typing, 2XN, discrete sequence production, and serial reaction time. These populate a continuum from higher to lower degrees of internal control of sequential organization. The main movement classes covered are speech and keypressing, both involving small amplitude movements that are very amenable to parametric study. A brief synopsis of classes of serial order models, vis-Γ -vis the detailing of major effects found in the behavioral data, leads to a focus on competitive queuing (CQ) models. Recently, the many behavioral predictive successes of CQ models have been joined by successful prediction of distinctively patterend electrophysiological recordings in prefrontal cortex, wherein parallel activation dynamics of multiple neural ensembles strikingly matches the parallel dynamics predicted by CQ theory. An extended CQ simulation model-the N-STREAMS neural network model-is then examined to highlight issues in ongoing attemptes to accomodate a broader range of behavioral and neurophysiological data within a CQ-consistent theory. Important contemporary issues such as the nature of working memory representations for sequential behavior, and the development and role of chunks in hierarchial control are prominent throughout.Defense Advanced Research Projects Agency/Office of Naval Research (N00014-95-1-0409); National Institute of Mental Health (R01 DC02852
Proxy Support for HTTP Adaptive Streaming
Not long ago streaming video over the Internet included only short clips of low quality video. Now the possibilities seem endless as professional productions are made available in high definition. This explosion of growth is the result of several factors, such as increasing network performance, advancements in video encoding technology, improvements to video streaming techniques, and a growing number of devices capable of handling video. However, despite the improvements to Internet video streaming this paradigm is still evolving.
HTTP adaptive streaming involves encoding a video at multiple quality levels then dividing those quality levels into small chunks. The player can then determine which quality level to retrieve the next chunk from in order to optimize video playback when considering the underlying network conditions. This thesis first presents an experimental framework that allows for adaptive streaming players to be analyzed and evaluated. Evaluation is beneficial because there are several concerns with the adaptive video streaming ecosystem such as achieving a high video playback quality while also ensuring stable playback quality.
The primary contribution of this thesis is the evaluation of prefetching by a proxy server as a means to improve streaming performance. This work considers an implementation of a proxy server that is functional with the extremely popular Netflix streaming service, and it is evaluated using two Netflix players. The results show its potential to improve video streaming performance in several scenarios. It effectively increases the buffer capacity of the player as chunks can be prefetched in advance of the player's request then stored on the proxy to be quickly delivered once requested. This allows for degradation in network conditions to be hidden from the player while the proxy serves prefetched data, preventing a reduction to the video quality as a result of an overreaction by the player. Further, the proxy can reduce the impact of the bottleneck in the network, achieving higher throughput by utilizing parallel connections to the server
Self-organising agent communities for autonomic resource management
The autonomic computing paradigm addresses the operational challenges presented by increasingly complex software systems by proposing that they be composed of many autonomous components, each responsible for the run-time reconfiguration of its own dedicated hardware and software components. Consequently, regulation of the whole software system becomes an emergent property of local adaptation and learning carried out by these autonomous system elements. Designing appropriate local adaptation policies for the components of such systems remains a major challenge. This is particularly true where the systemβs scale and dynamism compromise the efficiency of a central executive and/or prevent components from pooling information to achieve a shared, accurate evidence base for their negotiations and decisions.In this paper, we investigate how a self-regulatory system response may arise spontaneously from local interactions between autonomic system elements tasked with adaptively consuming/providing computational resources or services when the demand for such resources is continually changing. We demonstrate that system performance is not maximised when all system components are able to freely share information with one another. Rather, maximum efficiency is achieved when individual components have only limited knowledge of their peers. Under these conditions, the system self-organises into appropriate community structures. By maintaining information flow at the level of communities, the system is able to remain stable enough to efficiently satisfy service demand in resource-limited environments, and thus minimise any unnecessary reconfiguration whilst remaining sufficiently adaptive to be able to reconfigure when service demand changes
Organized Chaos: Scatter in the relation between stellar mass and halo mass in small galaxies
We use Local Group galaxy counts together with the ELVIS N-body simulations
to explore the relationship between the scatter and slope in the stellar mass
vs. halo mass relation at low masses, .
Assuming models with log-normal scatter about a median relation of the form
, the preferred log-slope steepens from
in the limit of zero scatter to in the
case of dex of scatter in at fixed halo mass. We provide fitting
functions for the best-fit relations as a function of scatter, including cases
where the relation becomes increasingly stochastic with decreasing mass. We
show that if the scatter at fixed halo mass is large enough ( dex)
and if the median relation is steep enough (), then the
"too-big-to-fail" problem seen in the Local Group can be self-consistently
eliminated in about of realizations. This scenario requires that
the most massive subhalos host unobservable ultra-faint dwarfs fairly often; we
discuss potentially observable signatures of these systems. Finally, we compare
our derived constraints to recent high-resolution simulations of dwarf galaxy
formation in the literature. Though simulation-to-simulation scatter in
at fixed is large among separate authors (
dex), individual codes produce relations with much less scatter and usually
give relations that would over-produce local galaxy counts.Comment: 15 pages, 6 figures, 1 table. Accepted for publication into MNRA
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