37 research outputs found
Galaxy Disks
The formation and evolution of galactic disks is particularly important for
understanding how galaxies form and evolve, and the cause of the variety in
which they appear to us. Ongoing large surveys, made possible by new
instrumentation at wavelengths from the ultraviolet (GALEX), via optical (HST
and large groundbased telescopes) and infrared (Spitzer) to the radio are
providing much new information about disk galaxies over a wide range of
redshift. Although progress has been made, the dynamics and structure of
stellar disks, including their truncations, are still not well understood. We
do now have plausible estimates of disk mass-to-light ratios, and estimates of
Toomre's parameter show that they are just locally stable. Disks are mostly
very flat and sometimes very thin, and have a range in surface brightness from
canonical disks with a central surface brightness of about 21.5 -mag
arcsec down to very low surface brightnesses. It appears that galaxy
disks are not maximal, except possibly in the largest systems. Their HI layers
display warps whenever HI can be detected beyond the stellar disk, with
low-level star formation going on out to large radii. Stellar disks display
abundance gradients which flatten at larger radii and sometimes even reverse.
The existence of a well-defined baryonic Tully-Fisher relation hints at an
approximately uniform baryonic to dark matter ratio. Thick disks are common in
disk galaxies and their existence appears unrelated to the presence of a bulge
component; they are old, but their formation is not yet understood. Disk
formation was already advanced at redshifts of , but at that epoch
disks were not yet quiescent and in full rotational equilibrium. Downsizing is
now well-established. The formation and history of star formation in S0s is
still not fully understood.Comment: This review has been submitted for Annual Reviews of Astronomy &
Astrophysics, vol. 49 (2011); the final printed version will have fewer
figures and a somewhat shortened text. A pdf-version of this preprint with
high-resolution figures is available from
http://www.astro.rug.nl/~vdkruit/jea3/homepage/disks-ph.pdf. (table of
contents added; 71 pages, 24 figures, 529 references
Formation of Galaxy Clusters
In this review, we describe our current understanding of cluster formation:
from the general picture of collapse from initial density fluctuations in an
expanding Universe to detailed simulations of cluster formation including the
effects of galaxy formation. We outline both the areas in which highly accurate
predictions of theoretical models can be obtained and areas where predictions
are uncertain due to uncertain physics of galaxy formation and feedback. The
former includes the description of the structural properties of the dark matter
halos hosting cluster, their mass function and clustering properties. Their
study provides a foundation for cosmological applications of clusters and for
testing the fundamental assumptions of the standard model of structure
formation. The latter includes the description of the total gas and stellar
fractions, the thermodynamical and non-thermal processes in the intracluster
plasma. Their study serves as a testing ground for galaxy formation models and
plasma physics. In this context, we identify a suitable radial range where the
observed thermal properties of the intra-cluster plasma exhibit the most
regular behavior and thus can be used to define robust observational proxies
for the total cluster mass. We put particular emphasis on examining assumptions
and limitations of the widely used self-similar model of clusters. Finally, we
discuss the formation of clusters in non-standard cosmological models, such as
non-Gaussian models for the initial density field and models with modified
gravity, along with prospects for testing these alternative scenarios with
large cluster surveys in the near future.Comment: 66 pages, 17 figures, review to be published in 2012 Annual Reviews
of Astronomy & Astrophysic
The Hubble Constant
I review the current state of determinations of the Hubble constant, which
gives the length scale of the Universe by relating the expansion velocity of
objects to their distance. There are two broad categories of measurements. The
first uses individual astrophysical objects which have some property that
allows their intrinsic luminosity or size to be determined, or allows the
determination of their distance by geometric means. The second category
comprises the use of all-sky cosmic microwave background, or correlations
between large samples of galaxies, to determine information about the geometry
of the Universe and hence the Hubble constant, typically in a combination with
other cosmological parameters. Many, but not all, object-based measurements
give values of around 72-74km/s/Mpc , with typical errors of 2-3km/s/Mpc.
This is in mild discrepancy with CMB-based measurements, in particular those
from the Planck satellite, which give values of 67-68km/s/Mpc and typical
errors of 1-2km/s/Mpc. The size of the remaining systematics indicate that
accuracy rather than precision is the remaining problem in a good determination
of the Hubble constant. Whether a discrepancy exists, and whether new physics
is needed to resolve it, depends on details of the systematics of the
object-based methods, and also on the assumptions about other cosmological
parameters and which datasets are combined in the case of the all-sky methods.Comment: Extensively revised and updated since the 2007 version: accepted by
Living Reviews in Relativity as a major (2014) update of LRR 10, 4, 200
Large Scale Structure of the Universe
Galaxies are not uniformly distributed in space. On large scales the Universe
displays coherent structure, with galaxies residing in groups and clusters on
scales of ~1-3 Mpc/h, which lie at the intersections of long filaments of
galaxies that are >10 Mpc/h in length. Vast regions of relatively empty space,
known as voids, contain very few galaxies and span the volume in between these
structures. This observed large scale structure depends both on cosmological
parameters and on the formation and evolution of galaxies. Using the two-point
correlation function, one can trace the dependence of large scale structure on
galaxy properties such as luminosity, color, stellar mass, and track its
evolution with redshift. Comparison of the observed galaxy clustering
signatures with dark matter simulations allows one to model and understand the
clustering of galaxies and their formation and evolution within their parent
dark matter halos. Clustering measurements can determine the parent dark matter
halo mass of a given galaxy population, connect observed galaxy populations at
different epochs, and constrain cosmological parameters and galaxy evolution
models. This chapter describes the methods used to measure the two-point
correlation function in both redshift and real space, presents the current
results of how the clustering amplitude depends on various galaxy properties,
and discusses quantitative measurements of the structures of voids and
filaments. The interpretation of these results with current theoretical models
is also presented.Comment: Invited contribution to be published in Vol. 8 of book "Planets,
Stars, and Stellar Systems", Springer, series editor T. D. Oswalt, volume
editor W. C. Keel, v2 includes additional references, updated to match
published versio
Constructing Biological Pathways by a Two-Step Counting Approach
Networks are widely used in biology to represent the relationships between genes
and gene functions. In Boolean biological models, it is mainly assumed that
there are two states to represent a gene: on-state and off-state. It is
typically assumed that the relationship between two genes can be characterized
by two kinds of pairwise relationships: similarity and prerequisite. Many
approaches have been proposed in the literature to reconstruct biological
relationships. In this article, we propose a two-step method to reconstruct the
biological pathway when the binary array data have measurement error. For a pair
of genes in a sample, the first step of this approach is to assign counting
numbers for every relationship and select the relationship with counting number
greater than a threshold. The second step is to calculate the asymptotic
p-values for hypotheses of possible relationships and select relationships with
a large p-value. This new method has the advantages of easy calculation for the
counting numbers and simple closed forms for the p-value. The simulation study
and real data example show that the two-step counting method can accurately
reconstruct the biological pathway and outperform the existing methods. Compared
with the other existing methods, this two-step method can provide a more
accurate and efficient alternative approach for reconstructing the biological
network
A review of elliptical and disc galaxy structure, and modern scaling laws
A century ago, in 1911 and 1913, Plummer and then Reynolds introduced their
models to describe the radial distribution of stars in `nebulae'. This article
reviews the progress since then, providing both an historical perspective and a
contemporary review of the stellar structure of bulges, discs and elliptical
galaxies. The quantification of galaxy nuclei, such as central mass deficits
and excess nuclear light, plus the structure of dark matter halos and cD galaxy
envelopes, are discussed. Issues pertaining to spiral galaxies including dust,
bulge-to-disc ratios, bulgeless galaxies, bars and the identification of
pseudobulges are also reviewed. An array of modern scaling relations involving
sizes, luminosities, surface brightnesses and stellar concentrations are
presented, many of which are shown to be curved. These 'redshift zero'
relations not only quantify the behavior and nature of galaxies in the Universe
today, but are the modern benchmark for evolutionary studies of galaxies,
whether based on observations, N-body-simulations or semi-analytical modelling.
For example, it is shown that some of the recently discovered compact
elliptical galaxies at 1.5 < z < 2.5 may be the bulges of modern disc galaxies.Comment: Condensed version (due to Contract) of an invited review article to
appear in "Planets, Stars and Stellar
Systems"(www.springer.com/astronomy/book/978-90-481-8818-5). 500+ references
incl. many somewhat forgotten, pioneer papers. Original submission to
Springer: 07-June-201
Measuring our universe from galaxy redshift surveys
Galaxy redshift surveys have achieved significant progress over the last
couple of decades. Those surveys tell us in the most straightforward way what
our local universe looks like. While the galaxy distribution traces the bright
side of the universe, detailed quantitative analyses of the data have even
revealed the dark side of the universe dominated by non-baryonic dark matter as
well as more mysterious dark energy (or Einstein's cosmological constant). We
describe several methodologies of using galaxy redshift surveys as cosmological
probes, and then summarize the recent results from the existing surveys.
Finally we present our views on the future of redshift surveys in the era of
Precision Cosmology.Comment: 82 pages, 31 figures, invited review article published in Living
Reviews in Relativity, http://www.livingreviews.org/lrr-2004-
Fluids in cosmology
We review the role of fluids in cosmology by first introducing them in
General Relativity and then by applying them to a FRW Universe's model. We
describe how relativistic and non-relativistic components evolve in the
background dynamics. We also introduce scalar fields to show that they are able
to yield an inflationary dynamics at very early times (inflation) and late
times (quintessence). Then, we proceed to study the thermodynamical properties
of the fluids and, lastly, its perturbed kinematics. We make emphasis in the
constrictions of parameters by recent cosmological probes.Comment: 34 pages, 4 figures, version accepted as invited review to the book
"Computational and Experimental Fluid Mechanics with Applications to Physics,
Engineering and the Environment". Version 2: typos corrected and references
expande
Cluster Lenses
Clusters of galaxies are the most recently assembled, massive, bound
structures in the Universe. As predicted by General Relativity, given their
masses, clusters strongly deform space-time in their vicinity. Clusters act as
some of the most powerful gravitational lenses in the Universe. Light rays
traversing through clusters from distant sources are hence deflected, and the
resulting images of these distant objects therefore appear distorted and
magnified. Lensing by clusters occurs in two regimes, each with unique
observational signatures. The strong lensing regime is characterized by effects
readily seen by eye, namely, the production of giant arcs, multiple-images, and
arclets. The weak lensing regime is characterized by small deformations in the
shapes of background galaxies only detectable statistically. Cluster lenses
have been exploited successfully to address several important current questions
in cosmology: (i) the study of the lens(es) - understanding cluster mass
distributions and issues pertaining to cluster formation and evolution, as well
as constraining the nature of dark matter; (ii) the study of the lensed objects
- probing the properties of the background lensed galaxy population - which is
statistically at higher redshifts and of lower intrinsic luminosity thus
enabling the probing of galaxy formation at the earliest times right up to the
Dark Ages; and (iii) the study of the geometry of the Universe - as the
strength of lensing depends on the ratios of angular diameter distances between
the lens, source and observer, lens deflections are sensitive to the value of
cosmological parameters and offer a powerful geometric tool to probe Dark
Energy. In this review, we present the basics of cluster lensing and provide a
current status report of the field.Comment: About 120 pages - Published in Open Access at:
http://www.springerlink.com/content/j183018170485723/ . arXiv admin note:
text overlap with arXiv:astro-ph/0504478 and arXiv:1003.3674 by other author