1,131 research outputs found
The formation of ultra-compact dwarf galaxies and nucleated dwarf galaxies
Ultra compact dwarf galaxies (UCDs) have similar properties as massive
globular clusters or the nuclei of nucleated galaxies. Recent observations
suggesting a high dark matter content and a steep spatial distribution within
groups and clusters provide new clues as to their origins. We perform
high-resolution N-body / smoothed particle hydrodynamics simulations designed
to elucidate two possible formation mechanisms for these systems: the merging
of globular clusters in the centre of a dark matter halo, or the massively
stripped remnant of a nucleated galaxy. Both models produce density profiles as
well as the half light radii that can fit the observational constraints.
However, we show that the first scenario results to UCDs that are underluminous
and contain no dark matter. This is because the sinking process ejects most of
the dark matter particles from the halo centre. Stripped nuclei give a more
promising explanation, especially if the nuclei form via the sinking of gas,
funneled down inner galactic bars, since this process enhances the central dark
matter content. Even when the entire disk is tidally stripped away, the nucleus
stays intact and can remain dark matter dominated even after severe stripping.
Total galaxy disruption beyond the nuclei only occurs on certain orbits and
depends on the amount of dissipation during nuclei formation. By comparing the
total disruption of CDM subhaloes in a cluster potential we demonstrate that
this model also leads to the observed spatial distribution of UCDs which can be
tested in more detail with larger data sets.Comment: 8 pages, 8 figures, final version accepted for publication in MNRA
Modeling the disruption of the globular cluster Pal5 by Galactic tides
In order to understand the extended massive tidal tails of the globular
cluster Pal5, its very low mass and velocity dispersion, and its size, which is
much larger than the theoretical tidal radius, we performed more than 1000
N-body simulations. Tidal shocks at disk crossings near perigalacticon dominate
the evolution of extended low-concentration clusters, resulting in massive
tidal tails and often in a quick destruction of the cluster. The overlarge size
of Pal5 can be explained as the result of an expansion following the heating
induced by the last strong disk shock ~150Myr ago. The models can reproduce the
low observed velocity dispersion and the relative fractions of stars in the
tails and between the inner and outer parts of the tails. The tidal tails of
Pal5 show substantial structure not seen in our simulations. We argue that this
structure is probably caused by Galactic substructure, such as giant molecular
clouds, spiral arms, and dark-matter clumps.
Clusters initially larger than their theoretical tidal limit remain so,
because, after being shocked, they settle into a new equilibrium near
apogalacticon, where they are unaffected by the perigalactic tidal field. This
implies that, contrary to previous wisdom, globular clusters on eccentric
orbits may well remain super-tidally limited and hence vulnerable to strong
disk shocks, which dominate their evolution until destruction. Our simulations
unambiguously predict the destruction of Pal5 at its next disk crossing in
\~110Myr. This corresponds to only 1% of the cluster lifetime, suggesting that
many more similar systems could once have populated the inner parts of the
Milky Way, but have been transformed into debris streams by the Galactic tidal
field. (Abridged)Comment: accepted for publication in The Astronomical Journa
Is the Fast Evolution Scenario for Virialized Compact Groups Really Compelling? The Role of a Dark Massive Group Halo
We report on results of N-body simulations aimed at testing the hypothesis
that galaxies in X-ray emitting (i.e., virialized) Compact Groups are not
tidally stripped when they are embedded in a common, massive, quiescent dark
matter halo. To disentangle the effects of interactions from spurious effects
due to an incorrect choice of the initial galaxy model configurations, these
have been chosen to be tidally-limited King spheres, representing systems in
quasi-equilibrium within the tidal field of the halo. The potential of the halo
has been assumed to be frozen and the braking due to dynamical friction
neglected. Our results confirm the hypothesis of low rates of tidal stripping
and suggest a scenario for virialized Compact Group evolution in their
quiescent phases with only very moderate tidally induced galaxy evolution can
be generally expected. This implies the group stability, provided that the
dynamical friction timescales in these systems are not much shorter than the
Hubble time. We discuss briefly this possibility, in particular taking account
of the similarity between the velocity dispersions of a typical virialized
Compact Groups and the internal velocity dispersion of typical member galaxies.
A number of puzzling observational data on Compact Groups can be easily
explained in this framework. Other observations would be better understood as
the result of enhanced merging activity in the proto-group environment, leading
to virialized Compact Group formation through mergers of lower mass halos, as
predicted by hierarchical scenarios of structure formation.Comment: 18 pages, 1 postscript file, 2 tables, to be published in ApJLet
Anisotropic static solutions in modelling highly compact bodies
Einstein field equations for anisotropic spheres are solved and exact
interior solutions obtained. This paper extends earlier treatments to include
anisotropic models which accommodate a wider variety of physically viable
energy densities. Two classes of solutions are possible. The first class
contains the limiting case for the energy density which
arises in many astrophysical applications. In the second class the singularity
at the center of the star is not present in the energy density. The models
presented in this paper allow for increasing and decreasing profiles in the
behavior of the energy density.Comment: 9 pages, to appear in Pramana - J. Phy
Binding Energy and the Fundamental Plane of Globular Clusters
A physical description of the fundamental plane of Galactic globular clusters
is developed which explains all empirical trends and correlations in a large
number of cluster observables and provides a small but complete set of truly
independent constraints on theories of cluster formation and evolution in the
Milky Way. Within the theoretical framework of single-mass, isotropic King
models, it is shown that (1) 39 regular (non--core-collapsed) globulars with
measured core velocity dispersions share a common V-band mass-to-light ratio of
1.45 +/- 0.10, and (2) a complete sample of 109 regular globulars reveals a
very strong correlation between cluster binding energy and total luminosity,
regulated by Galactocentric position: E_b \propto (L^{2.05} r_{\rm gc}^{-0.4}).
The observational scatter about either of these two constraints can be
attributed fully to random measurement errors, making them the defining
equations of a fundamental plane for globular clusters. A third, weaker
correlation, between total luminosity and the King-model concentration
parameter, c, is then related to the (non-random) distribution of globulars on
the plane. The equations of the FP are used to derive expressions for any
cluster observable in terms of only L, r_{\rm gc}, and c. Results are obtained
for generic King models and applied specifically to the globular cluster system
of the Milky Way.Comment: 60 pages with 19 figures, submitted to Ap
Identifying effective behavioural components of Intervention and Comparison group support provided in SMOKing cEssation (IC-SMOKE) interventions: a systematic review protocol
BACKGROUND: Systematic reviews of behaviour change interventions for smoking cessation vary in scope, quality, and applicability. The current review aims to generate more accurate and useful findings by (1) a detailed analysis of intervention elements that change behaviour (i.e. behaviour change techniques (BCTs)) and potential moderators of behaviour change (i.e. other intervention and sample characteristics) and (2) assessing and controlling for variability in support provided to comparison groups in smoking cessation trials. METHODS: A systematic review will be conducted of randomized controlled trials of behaviour change interventions for smoking cessation in adults (with or without pharmacological support), with a minimum follow-up of 6 months, published after 1995. Eligible articles will be identified through the Cochrane Tobacco Addiction Group Specialized Register. Study authors will be asked for detailed descriptions of smoking cessation support provided to intervention and comparison groups. All data will be independently coded by two researchers. The BCT taxonomy v1 (tailored to smoking cessation interventions) and template for intervention description and replication criteria will be used to code intervention characteristics. Data collection will further include sample and trial characteristics and outcome data (smoking cessation rates). Multilevel mixed-effects meta-regression models will be used to examine which BCTs and/or BCT clusters delivered to intervention and comparison groups explain smoking cessation rates in treatment arms (and effect sizes) and what key moderators of behaviour change are. Predicted effect sizes of each intervention will be computed assuming all interventions are compared against comparison groups receiving the same levels of behavioural support (i.e. low, medium, and high levels). Multi-disciplinary advisory board members (policymakers, health care providers, and (ex-)smokers) will provide strategic input throughout the project to ensure the review's applicability to policy and practice. DISCUSSION: By capturing BCTs in intervention and comparison groups, this systematic review will provide more accurate estimates of the effectiveness of smoking cessation interventions, the most promising BCTs and/or BCT clusters associated with smoking cessation rates in intervention and comparison arms, and important moderators of behaviour change. The results could set new standards for conducting meta-analyses of behaviour change interventions and improve research, service delivery, and training in the area of smoking cessation
HST/STIS Imaging of the Host Galaxy of GRB980425/SN1998bw
We present HST/STIS observations of ESO 184-G82, the host galaxy of the
gamma-ray burst GRB 980425 associated with the peculiar Type Ic supernova
SN1998bw. ESO 184-G82 is found to be an actively star forming SBc sub-luminous
galaxy. We detect an object consistent with being a point source within the
astrometric uncertainty of 0.018 arcseconds of the position of the supernova.
The object is located inside a star-forming region and is at least one
magnitude brighter than expected for the supernova based on a simple
radioactive decay model. This implies either a significant flattening of the
light curve or a contribution from an underlying star cluster.Comment: 12 pages, 2 figures, AASTeX v5.02 accepted for publication in ApJ
Letter
Epidural Auditory Event-Related Potentials in the Rat to Frequency and duration Deviants: Evidence of Mismatch Negativity?
The capacity of the human brain to detect deviance in the acoustic environment pre-attentively is reflected in a brain event-related potential (ERP), mismatch negativity (MMN). MMN is observed in response to the presentation of rare oddball sounds that deviate from an otherwise regular pattern of frequent background standard sounds. While the primate and cat auditory cortex (AC) exhibit MMN-like activity, it is unclear whether the rodent AC produces a deviant response that reflects deviance detection in a background of regularities evident in recent auditory stimulus history or differential adaptation of neuronal responses due to rarity of the deviant sound. We examined whether MMN-like activity occurs in epidural AC potentials in awake and anesthetized rats to high and low frequency and long and short duration deviant sounds. ERPs to deviants were compared with ERPs to common standards and also with ERPs to deviants when interspersed with many different standards to control for background regularity effects. High frequency (HF) and long duration deviant ERPs in the awake rat showed evidence of deviance detection, consisting of negative displacements of the deviant ERP relative to ERPs to both common standards and deviants with many standards. The HF deviant MMN-like response was also sensitive to the extent of regularity in recent acoustic stimulation. Anesthesia in contrast resulted in positive displacements of deviant ERPs. Our results suggest that epidural MMN-like potentials to HF sounds in awake rats encode deviance in an analogous manner to the human MMN, laying the foundation for animal models of disorders characterized by disrupted MMN generation, such as schizophrenia
Laser transmitter for cubesat-class applications
Laser communications onboard CubeSats is an emerging technology for enabling high-speed space-based communication links. In this paper we present the development of a 25 cm 3 and second iteration 0.3 U CubeSat-class laser transmitter operating at data rates of up to 500 Mbps using OOK modulation and an output power of up to 300 mW over the entire C-band. We present results of the development and characterization of the transmitter. From this testing the design will be demonstrated up to TRL 4/5 with the view for future qualification work and electronics integration
On the Stability of Quasi-Equilibrium Self-Gravitating Configurations in a Tidal Field
The possibility that quasi-equilibrium self-gravitating galaxy-like
configurations exist in a tidal field is analyzed in this paper. More
specifically, we address the question of how to predict initial configurations
modeling galaxies that are able to survive environmental effects in a dense
environment for a Hubble time or so, provided thay dynamical friction is
neglected. The initial quasi-equilibrium configurations have been built up
taking into account the external tidal field produced by the halo. It modifies
the escape velocity field of the configuration, compared with isolated
configurations. The survival of the configurations as they orbit inside the
halos has been studied through N-body simulations. As a general result, it has
been found out that the bulk of the models is conserved along 12.5 Gyears of
evolution, and that the low rates of mass losses they experience are consistent
with those expected when the adiabatic protection hypothesis is at work. So,
solutions for galaxy configurations in tidal quasi-equilibrium have been found,
showing that tidal stripping in quiescent phases does not seem to be very
important, unless that the density of the galaxy environment at its formation
had been much lower than that of the galaxy environment at the point of its
orbit where the tidal perturbation is maximum. (Abridged)Comment: 36 pages, 7 postscript figures, to be published in Ap
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