131,310 research outputs found
Infrared Magnitude-Redshift Relations for Luminous Radio Galaxies
Infrared magnitude-redshift relations for the 3CR and 6C samples of radio
galaxies are presented for a wide range of plausible cosmological models,
including those with non-zero cosmological constant OmegaLambda. Variations in
the galaxy formation redshift, metallicity and star formation history are also
considered. The results of the modelling are displayed in terms of magnitude
differences between the models and no-evolution tracks, illustrating the amount
of K-band evolution necessary to account for the observational data. Given a
number of plausible assumptions, the results of these analyses suggest that:
(i) cosmologies which predict T_0xH_0>1 (where T_0 denotes the current age of
the universe) can be excluded; (ii) the star formation redshift should lie in
the redshift interval 5<z<20, values towards the lower end of the range being
preferred in cosmologies with larger values of T_0xH_0; (iii) the Einstein-de
Sitter model provides a reasonable fit to the data; (iv) models with finite
values of OmegaLambda can provide good agreement with the observations only if
appropriate adjustments of other parameters such as the galaxy metallicities
and star-formation histories are made. Without such modifications, even after
accounting for stellar evolution, the high redshift radio galaxies are more
luminous (ie. more massive) than those nearby in models with finite
OmegaLambda, including the favoured model with Omega=0.3, OmegaLambda=0.7. For
cosmological models with larger values of T_0xH_0, the conclusions are the same
regardless of whether any adjustments are made or not. The implications of
these results for cosmology and models of galaxy formation are discussed.Comment: 14 pages, LaTeX, 9 figures, accepted for publication in MNRAS.
Replacement corrects some annoying typo
Alpha, Betti and the Megaparsec Universe: on the Topology of the Cosmic Web
We study the topology of the Megaparsec Cosmic Web in terms of the
scale-dependent Betti numbers, which formalize the topological information
content of the cosmic mass distribution. While the Betti numbers do not fully
quantify topology, they extend the information beyond conventional cosmological
studies of topology in terms of genus and Euler characteristic. The richer
information content of Betti numbers goes along the availability of fast
algorithms to compute them.
For continuous density fields, we determine the scale-dependence of Betti
numbers by invoking the cosmologically familiar filtration of sublevel or
superlevel sets defined by density thresholds. For the discrete galaxy
distribution, however, the analysis is based on the alpha shapes of the
particles. These simplicial complexes constitute an ordered sequence of nested
subsets of the Delaunay tessellation, a filtration defined by the scale
parameter, . As they are homotopy equivalent to the sublevel sets of
the distance field, they are an excellent tool for assessing the topological
structure of a discrete point distribution. In order to develop an intuitive
understanding for the behavior of Betti numbers as a function of , and
their relation to the morphological patterns in the Cosmic Web, we first study
them within the context of simple heuristic Voronoi clustering models.
Subsequently, we address the topology of structures emerging in the standard
LCDM scenario and in cosmological scenarios with alternative dark energy
content. The evolution and scale-dependence of the Betti numbers is shown to
reflect the hierarchical evolution of the Cosmic Web and yields a promising
measure of cosmological parameters. We also discuss the expected Betti numbers
as a function of the density threshold for superlevel sets of a Gaussian random
field.Comment: 42 pages, 14 figure
Detecting hierarchical stellar systems with LISA
A significant fraction of stars are members of gravitationally bound
hierarchies containing three or more components. Almost all low mass stars in
binaries with periods shorter three days are part of a hierarchical system. We
therefore anticipate that a large fraction of compact galactic binaries
detected by the Laser Interferometer Space Antenna (LISA) will be members of
hierarchical triple or quadruple system. The acceleration imparted by the
hierarchical companions can be detected in the gravitational wave signal for
outer periods as large as 100 years. For systems with periods that are shorter
than, or comparable to, the mission lifetime, it will be possible to measure
the period and eccentricity of the outer orbit. LISA observations of
hierarchical stellar systems will provide insight into stellar evolution,
including the role that Kozai-Lidov oscillations play in driving systems
towards merger.Comment: 15 pages, 14 figure
Cosmic Voids: structure, dynamics and galaxies
In this review we discuss several aspects of Cosmic Voids. Voids are a major
component of the large scale distribution of matter and galaxies in the
Universe. They are of instrumental importance for understanding the emergence
of the Cosmic Web. Their relatively simple shape and structure makes them into
useful tools for extracting the value of a variety cosmic parameters, possibly
including even that of the influence of dark energy. Perhaps most promising and
challenging is the issue of the galaxies found within their realm. Not only
does the pristine environment of voids provide a promising testing ground for
assessing the role of environment on the formation and evolution of galaxies,
the dearth of dwarf galaxies may even represent a serious challenge to the
standard view of cosmic structure formation.Comment: 29 pages, 12 figures, invited review COSPA2008, Pohang, Korea. Modern
Physics Letters A, accepted. For high-res version see
http://www.astro.rug.nl/~weygaert/voids.cospa2008.weygaert.pd
Nearly 5000 Distant Early-Type Galaxies in COMBO-17: a Red Sequence and its Evolution since z~1
We present the rest-frame colors and luminosities of ~25000 m_R<24 galaxies
in the redshift range 0.2<z<1.1, drawn from 0.78 square degrees of the COMBO-17
survey. We find that the rest-frame color distribution of these galaxies is
bimodal at all redshifts out to z~1. This bimodality permits a
model-independent definition of red, early-type galaxies and blue, late-type
galaxies at any given redshift. The colors of the blue peak become redder
towards the present day, and the number density of blue luminous galaxies has
dropped strongly since z~1. Focusing on the red galaxies, we find that they
populate a color-magnitude relation. Such red sequences have been identified in
galaxy cluster environments, but our data show that such a sequence exists over
this redshift range even when averaging over all environments. The mean color
of the red galaxy sequence evolves with redshift in a way that is consistent
with the aging of an ancient stellar population. The rest-frame B-band
luminosity density in red galaxies evolves only mildly with redshift in a
Lambda-dominated cold dark matter universe. Accounting for the change in
stellar mass-to-light ratio implied by the redshift evolution in red galaxy
colors, the COMBO-17 data indicate an increase in stellar mass on the red
sequence by a factor of two since z~1. The largest source of uncertainty is
large-scale structure, implying that considerably larger surveys are necessary
to further refine this result. We explore mechanisms that may drive this
evolution in the red galaxy population, finding that both galaxy merging and
truncation of star formation in some fraction of the blue, star-forming
population are required to fully explain the properties of these galaxies.Comment: To appear in the Astrophysical Journal 20 June 2004. 16 pages, 6
embedded figures. Substantial revision of photometric redshifts and extensive
minor changes to the paper throughout: conclusions unchange
Detecting change points in the large-scale structure of evolving networks
Interactions among people or objects are often dynamic in nature and can be
represented as a sequence of networks, each providing a snapshot of the
interactions over a brief period of time. An important task in analyzing such
evolving networks is change-point detection, in which we both identify the
times at which the large-scale pattern of interactions changes fundamentally
and quantify how large and what kind of change occurred. Here, we formalize for
the first time the network change-point detection problem within an online
probabilistic learning framework and introduce a method that can reliably solve
it. This method combines a generalized hierarchical random graph model with a
Bayesian hypothesis test to quantitatively determine if, when, and precisely
how a change point has occurred. We analyze the detectability of our method
using synthetic data with known change points of different types and
magnitudes, and show that this method is more accurate than several previously
used alternatives. Applied to two high-resolution evolving social networks,
this method identifies a sequence of change points that align with known
external "shocks" to these networks
Hierarchical models of high redshift galaxies with thermally pulsing asymptotic giant branch stars: comparison with observations
In a recent paper we presented the first semi-analytic model of galaxy
formation in which the Thermally-Pulsing Asymptotic Giant Branch phase of
stellar evolution has been fully implemented. Here we address the comparison
with observations, and show how the TP-AGB recipe affects the performance of
the model in reproducing the colours and near-IR luminosities of high-redshift
galaxies. We find that the semi-analytic model with the TP-AGB better matches
the colour-magnitude and colour-colour relations at z ~ 2, both for
nearly-passive and for star-forming galaxies. The model with TP-AGB produces
star-forming galaxies with red V-K colours, thus revising the unique
interpretation of high-redshift red objects as 'red & dead'. We also show that
without the TP-AGB the semi-analytic model fails at reproducing the observed
colours, a situation that cannot be corrected by dust reddening. We also
explore the effect of nebular emission on the predicted colour-magnitude
relation of star-forming galaxies, to conclude that it does not play a
significant role in reddening their colours, at least in the range of
star-formation rates covered by the model. Finally, the rest-frame K-band
luminosity function at z ~ 2.5 is more luminous by almost 1 magnitude. This
indicates that the AGN feedback recipe that is adopted to regulate the
high-mass end of the luminosity function should be sophisticated to take the
effect of the stellar populations into account at high redshifts.Comment: 10 pages, 8 figures; effects of nebular emission included; accepted
for publication on MNRA
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