22 research outputs found
The scientific potential of space-based gravitational wave detectors
The millihertz gravitational wave band can only be accessed with a
space-based interferometer, but it is one of the richest in potential sources.
Observations in this band have amazing scientific potential. The mergers
between massive black holes with mass in the range 10 thousand to 10 million
solar masses, which are expected to occur following the mergers of their host
galaxies, produce strong millihertz gravitational radiation. Observations of
these systems will trace the hierarchical assembly of structure in the Universe
in a mass range that is very difficult to probe electromagnetically. Stellar
mass compact objects falling into such black holes in the centres of galaxies
generate detectable gravitational radiation for several years prior to the
final plunge and merger with the central black hole. Measurements of these
systems offer an unprecedented opportunity to probe the predictions of general
relativity in the strong-field and dynamical regime. Millihertz gravitational
waves are also generated by millions of ultra-compact binaries in the Milky
Way, providing a new way to probe galactic stellar populations. ESA has
recognised this great scientific potential by selecting The Gravitational
Universe as its theme for the L3 large satellite mission, scheduled for launch
in ~2034. In this article we will review the likely sources for millihertz
gravitational wave detectors and describe the wide applications that
observations of these sources could have for astrophysics, cosmology and
fundamental physics.Comment: 18 pages, 2 figures, contribution to Gravitational Wave Astrophysics,
the proceedings of the 2014 Sant Cugat Forum on Astrophysics; v2 includes one
additional referenc
Particle Probe of Horava-Lifshitz Gravity
Kehagias-Sfetsos black hole in Ho\v{r}ava-Lifshitz gravity is probed through
particle geodesics. Gravitational force of KS black hole becomes weaker than
that of Schwarzschild around horizon and interior space. Particles can be
always scattered or trapped in new closed orbits, unlike those falling forever
in Schwarzschild black. The properties of null and timelike geodesics are
classified with values of coupling constants. The precession rates of the
orbits are evaluated. The time trajectories are also classified under different
values of coupling constants for both null and timelike geodesics. Physical
phenomena that may be observable are discussed.Comment: 10 pages, 8 figure
Stability analysis of agegraphic dark energy in Brans-Dicke cosmology
Stability analysis of agegraphic dark energy in Brans-Dicke theory is
presented in this paper. We constrain the model parameters with the
observational data and thus the results become broadly consistent with those
expected from experiment. Stability analysis of the model without best fitting
shows that universe may begin from an unstable state passing a saddle point and
finally become stable in future. However, with the best fitted model, There is
no saddle intermediate state. The agegraphic dark energy in the model by itself
exhibits a phantom behavior. However, contribution of cold dark matter on the
effective energy density modifies the state of teh universe from phantom phase
to quintessence one. The statefinder diagnosis also indicates that the universe
leaves an unstable state in the past, passes the LCDM state and finally
approaches the sable state in future.Comment: 15 pages, 12 figure
The Large Scale Structure in the Universe: From Power-Laws to Acoustic Peaks
The most popular tools for analysing the large scale distribution of galaxies
are second-order spatial statistics such as the two-point correlation function
or its Fourier transform, the power spectrum. In this review, we explain how
our knowledge of cosmic structures, encapsulated by these statistical
descriptors, has evolved since their first use when applied on the early galaxy
catalogues to the present generation of wide and deep redshift surveys,
incorporating the most challenging discovery in the study of the galaxy
distribution: the detection of Baryon Acoustic Oscillations.Comment: 20 pages, 12 figures, to appear in "Data Analysis in Cosmology",
Lecture Notes in Physics, 2008, eds. V. J. Martinez, E. Saar, E.
Martinez-Gonzalez, and M.J. Pons-Borderia, Springer-Verla
The chemical enrichment of the ICM from hydrodynamical simulations
The study of the metal enrichment of the intra-cluster and inter-galactic
media (ICM and IGM) represents a direct means to reconstruct the past history
of star formation, the role of feedback processes and the gas-dynamical
processes which determine the evolution of the cosmic baryons. In this paper we
review the approaches that have been followed so far to model the enrichment of
the ICM in a cosmological context. While our presentation will be focused on
the role played by hydrodynamical simulations, we will also discuss other
approaches based on semi-analytical models of galaxy formation, also critically
discussing pros and cons of the different methods. We will first review the
concept of the model of chemical evolution to be implemented in any
chemo-dynamical description. We will emphasise how the predictions of this
model critically depend on the choice of the stellar initial mass function, on
the stellar life-times and on the stellar yields. We will then overview the
comparisons presented so far between X-ray observations of the ICM enrichment
and model predictions. We will show how the most recent chemo-dynamical models
are able to capture the basic features of the observed metal content of the ICM
and its evolution. We will conclude by highlighting the open questions in this
study and the direction of improvements for cosmological chemo-dynamical models
of the next generation.Comment: 25 pages, 11 figures, accepted for publication in Space Science
Reviews, special issue "Clusters of galaxies: beyond the thermal view",
Editor J.S. Kaastra, Chapter 18; work done by an international team at the
International Space Science Institute (ISSI), Bern, organised by J.S.
Kaastra, A.M. Bykov, S. Schindler & J.A.M. Bleeke
Effect of Local Environment and Stellar Mass on Galaxy Quenching and Morphology at 0.5 < z < 2.0
Large scale structure and cosmolog
Observations of the High Redshift Universe
(Abridged) In these lectures aimed for non-specialists, I review progress in
understanding how galaxies form and evolve. Both the star formation history and
assembly of stellar mass can be empirically traced from redshifts z~6 to the
present, but how the various distant populations inter-relate and how stellar
assembly is regulated by feedback and environmental processes remains unclear.
I also discuss how these studies are being extended to locate and characterize
the earlier sources beyond z~6. Did early star-forming galaxies contribute
significantly to the reionization process and over what period did this occur?
Neither theory nor observations are well-developed in this frontier topic but
the first results presented here provide important guidance on how we will use
more powerful future facilities.Comment: To appear in `First Light in Universe', Saas-Fee Advanced Course 36,
Swiss Soc. Astrophys. Astron. in press. 115 pages, 64 figures (see
http://www.astro.caltech.edu/~rse/saas-fee.pdf for hi-res figs.) For lecture
ppt files see
http://obswww.unige.ch/saas-fee/preannouncement/course_pres/overview_f.htm
Statistical analysis of galaxy surveys - I. Robust error estimation for two-point clustering statistics
20 pages, 13 figures, 1 table.-- Pre-print archive.We present a test of different error estimators for two-point clustering statistics, appropriate for present and future large galaxy redshift surveys. Using an ensemble of very large dark matter ΛCDM N-body simulations, we compare internal error estimators (jackknife and bootstrap) to external ones (Monte Carlo realizations). For three-dimensional clustering statistics, we find that none of the internal error methods investigated is able to reproduce either accurately or robustly the errors of external estimators on 1 to 25 h−1 Mpc scales. The standard bootstrap overestimates the variance of ξ(s) by ∼40 per cent on all scales probed, but recovers, in a robust fashion, the principal eigenvectors of the underlying covariance matrix. The jackknife returns the correct variance on large scales, but significantly overestimates it on smaller scales. This scale dependence in the jackknife affects the recovered eigenvectors, which tend to disagree on small scales with the external estimates. Our results have important implications for fitting models to galaxy clustering measurements. For example, in a two-parameter fit to the projected correlation function, we find that the standard bootstrap systematically overestimates the 95 per cent confidence interval, while the jackknife method remains biased, but to a lesser extent. Ignoring the systematic bias, the scatter between realizations, for Gaussian statistics, implies that a 2σ confidence interval, as inferred from an internal estimator, corresponds in practice to anything from 1σ to 3σ. By oversampling the subvolumes, we find that it is possible, at least for the cases we consider, to obtain robust bootstrap variances and confidence intervals that agree with external error estimates. Our results are applicable to two-point statistics, like ξ(s) and wp(rp) , measured in large redshift surveys, and show that the interpretation of clustering measurements with internally estimated errors should be treated with caution.PN is supported
by a PPARC/STFC PDRA Fellowship. EG acknowledges support
from the Spanish Ministerio deCiencia y Tecnologia (MEC), project
AYA2006-06341 and research project 2005SGR00728 from Generalitat
de Catalunya.CMBis supported by a Royal Society University Research Fellowship. DJC acknowledges the financial support from
NSF grant AST00-71048. This work was supported by the European
Commission’s ALFA-II programme via its funding of the Latin
American European Network for Astrophysics and Cosmology.Peer reviewe
Statistical analysis of galaxy surveys - II. The three-point galaxy correlation function measured from the 2dFGRS
We present new results for the 3-point correlation function, \zeta, measured
as a function of scale, luminosity and colour from the final version of the
two-degree field galaxy redshift survey (2dFGRS). The reduced three point
correlation function, Q_3 is estimated for different triangle shapes and sizes,
employing a full covariance analysis. The form of Q_3 is consistent with the
expectations for the \Lambda-cold dark matter model, confirming that the
primary influence shaping the distribution of galaxies is gravitational
instability acting on Gaussian primordial fluctuations. However, we find a
clear offset in amplitude between Q_3 for galaxies and the predictions for the
dark matter. We are able to rule out the scenario in which galaxies are
unbiased tracers of the mass at the 9-sigma level. On weakly non-linear scales,
we can interpret our results in terms of galaxy bias parameters. We find a
linear bias term that is consistent with unity, b_1 = 0.93^{+0.10}_{-0.08} and
a quadratic bias c_2 = b_2 /b_1 = -0.34^{+0.11}_{-0.08}. This is the first
significant detection of a non-zero quadratic bias, indicating a small but
important non-gravitational contribution to the three point function. Our
estimate of the linear bias from the three point function is independent of the
normalisation of underlying density fluctuations, so we can combine this with
the measurement of the power spectrum of 2dFGRS galaxies to constrain the
amplitude of matter fluctuations. We find that the rms linear theory variance
in spheres of radius 8Mpc/h is \sigma_8 = 0.88^{+0.12}_{-0.10}, providing an
independent confirmation of values derived from other techniques. On non-linear
scales, where \xi>1, we find that Q_3 has a strong dependence on scale, colour
and luminosity.Comment: 16 pages, 10 figures, minor changes, extended comparison to previous
results, accepted for publication in MNRA
Statistical analysis of galaxy surveys - III. The non-linear clustering of red and blue galaxies in the 2dFGRS
We present measurements of the higher-order clustering of red and blue
galaxies as a function of scale and luminosity made from the two-degree field
galaxy redshift survey (2dFGRS). We use a counts-in-cells analysis to estimate
the volume averaged correlation functions, xi_p, as a function of scale up to
order p=5, and also the reduced void probability function. Hierarchical
amplitudes are constructed using the estimates of the correlation functions:
S_p=(xi_p/xi_2)^(p-1). We find that: 1) Red galaxies display stronger
clustering than blue galaxies at all orders measured. 2) Red galaxies show
values of S_p that are strongly dependent on luminosity, whereas blue galaxies
show no segregation in S_p within the errors; this is remarkable given the
segregation in the variance. 3) The linear relative bias shows the opposite
trend to the hierarchical amplitudes, with little segregation for the red
sequence and some segregation for the blue. 4) Faint red galaxies deviate
significantly from the "universal" negative binomial reduced void probabilities
followed by all other galaxy populations. Our results show that the
characteristic colour of a galaxy population reveals a unique signature in its
spatial distribution. Such signatures will hopefully further elucidate the
physics responsible for shaping the cosmological evolution of galaxies.Comment: 10 pages, 3 figures, accepted by MNRA