Munch: Monday, October 30, 2006

                               


 

    usual time and place:  12:30, 6th floor conference room

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


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What is Munch?
       Munch Archive

Hubble diagram of gamma-ray bursts: Robust evidence for a Chaplygin gas expansion-driven universe with phase transition at $z \simeq 3$

Authors: Herman J. Mosquera Cuesta, Habib Dumet M., Rodrigo Turcati, Carlos A. Bonilla Quintero, Cristina Furlanetto, Jefferson Morais
Comments: 5 pages, 11 *.eps figures, revtex4 style
The Hubble diagram (HD) of Gamma-Ray Bursts (GRBs) having properly estimated redshifts is compared with the predicted one for the Chaplygin gas (CG), a dark energy candidate. The CG cosmology and that of Friedmann and $\Lambda$-CDM models are studied and confronted to the GRBs observations. The model-to-sample $\chi^2$ statistical analysis indicates the CG model as the best fit. The present GRBs HD plot exhibits a marked trend: as one goes back in time, it gets much closer to the predict HD for a Friedmann universe. This clear trend conclusively demonstrates that a transition from decelerate to accelerate expansion did take place. However, contrarily to claims based on supernovae type Ia, the transition redshift lies somewhere between $\sim 2.5 < z \simeq 3.5$ rather than at $z \sim 0.5-1$. All of these striking features of the GRBs HD constitute the most robust demonstration that the Chaplygin gas can in fact be the universe's driving dark energy field.

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Constraining Alternative Theories of Gravity using Solar System Tests

Authors: Gianluca Allemandi, Matteo Luca Ruggiero
Comments: 4 Pages, RevTeX
Solar System tests give nowadays constraints on the estimated value of the cosmological constant, which can be accurately derived from different experiments regarding gravitational redshift, light deflection, gravitational time-delay and geodesic precession. Assuming that each reasonable theory of gravitation should satisfy Solar System tests we use this limits on the estimated value of the cosmological constant to constrain alternative theories of Gravity, which are nowadays studied as possible theories for cosmological models and provide viable solutions to the cosmological constant problem and the explanation of the present acceleration of the Universe. We obtain that the estimated values, from Solar System tests, for the parameters appearing in the alternative theories of Gravity are orders of magnitude bigger than the values obtained in the framework of cosmologically relevant theories.

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Testing MOND with Local Group spiral galaxies

Authors: Edvige Corbelli, Paolo Salucci
Comments: 6 pages, 4 figures. To be published in MNRAS
The rotation curves and the relative mass distributions of the two nearby Local Group spiral galaxies, M31 and M33, show discrepancies with Modified Newtonian dynamic (MOND) predictions. In M33 the discrepancy lies in the kinematics of the outermost regions. It can be alleviated by adopting tilted ring models compatible with the 21-cm datacube but different from the one that best fits the data. In M31 MOND fails to fit the falling part of the rotation curve at intermediate radii, before the curve flattens out in the outermost regions. Newtonian dynamics in a framework of a stellar disc embedded in a dark halo can explain the complex rotation curve profiles of these two galaxies, while MOND has some difficulties. However, given the present uncertainties in the kinematics of these nearby galaxies, we cannot address the success or failure of MOND theory in a definite way. More sensitive and extended observations around the critical regions, suggested by MOND fits discussed in this paper, may lead to a definite conclusion.

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The baryon fraction of LambdaCDM haloes

Authors: Robert A. Crain (1), Vincent R. Eke (1), Carlos S. Frenk (1), Adrian Jenkins (1), Ian G. McCarthy (1), Julio F. Navarro (2), Frazer R. Pearce (3) ((1) ICC, University of Durham, (2) University of Victoria, (3) University of Nottingham)
Comments: 9 pages, 5 figures (Figs 1. and 2 reduced in quality), 1 table, submitted to MNRAS. Version with high-resolution figures can be obtained from this http URL
We investigate the baryon fraction in dark matter haloes formed in non-radiative gas-dynamical simulations of the LambdaCDM cosmogony. By combining a realisation of the Millennium Simulation (Springel et al.) with a simulation of a smaller volume focussing on dwarf haloes, our study spans five decades in halo mass, from 10^10 Msun/h to 10^15 Msun/h. We find that the baryon fraction within the halo virial radius is typically 90% of the cosmic mean, with an rms scatter of 6%, independently of redshift and of halo mass down to the smallest resolved haloes. Our results show that, contrary to the proposal of Mo et al. (2005), pre-virialisation gravitational heating is unable to prevent the collapse of gas within galactic and proto-galactic haloes, and confirm the need for non-gravitational feedback in order to reduce the efficiency of gas cooling and star formation in dwarf galaxy haloes. Simulations including a simple photoheating model (where a gas temperature floor of T_{floor} = 2x10^4 K is imposed from z=11) confirm earlier suggestions that photoheating can only prevent the collapse of baryons in systems with virial temperatures T_{200} < ~2.2 T_{floor} ~ 4.4x10^4 K (corresponding to a virial mass of M_{200} ~ 10^10 Msun/h and a circular velocity of V_{200} ~ 35 km/s). Photoheating may thus help regulate the formation of dwarf spheroidals and other galaxies at the extreme faint-end of the luminosity function, but it cannot, on its own, reconcile the abundance of sub-L* galaxies with the vast number of dwarf haloes expected in the LambdaCDM cosmogony. The lack of evolution or mass dependence seen in the baryon fraction augurs well for X-ray cluster studies that assume a universal and non-evolving baryon fraction to place constraints on cosmological parameters.

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Limits on f_NL parameters from WMAP 3yr data

Authors: Paolo Creminelli, Leonardo Senatore, Matias Zaldarriaga, Max Tegmark
Comments: 10 pages, 3 figures
Report-no: IC/2006/108, HUTP-06/A0041
We analyze the 3-year WMAP data and look for a deviation from Gaussianity in the form of a 3-point function that has either of the two theoretically motivated shapes: local and equilateral. There is no evidence of departure from Gaussianity and the analysis gives the presently tightest bounds on the parameters f_ NL^local and f_NL^equil., which define the amplitude of respectively the local and the equilateral non-Gaussianity: -36 < f_NL^local < 100, -256 < f_NL^equil. < 332 at 95% C.L. In models with modified kinetic term for the inflaton, like DBI inflation, where a large equilateral non-Gaussianity is related to a reduced speed of sound c_s, our analysis gives the lower bound: c_s > 0.028 (at 95% C.L.).

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Complex structures in galaxy cluster fields: implications for gravitational lensing mass models

Authors: Lindsay King, Virginia Corless (Institute of Astronomy, Cambridge University, UK)
Comments: To appear in MNRAS letters, 5 pages, 3 figures
The distribution of mass on galaxy cluster scales is an important test of structure formation scenarios, providing constraints on the nature of dark matter itself. Several techniques have been used to probe the mass distributions of clusters, sometimes yielding results which are discrepant, or at odds with clusters formed in simulations - for example giving NFW concentration parameters much higher than expected in the standard CDM model. In addition, the velocity fields of some well studied galaxy clusters reveal the presence of several structures close to the line-of-sight, often not dynamically bound to the cluster itself. We investigate what impact such neighbouring but unbound massive structures would have on the determination of cluster profiles using weak gravitational lensing. Depending on its concentration and mass ratio to the primary halo, one secondary halo close to the line-of-sight can cause the estimated NFW concentration parameter to be significantly higher than that of the primary halo, and also cause the estimated mass to be biased high. Although it is difficult to envisage how this mechanism alone could yield concentrations as high as reported for some clusters, multiple haloes close to the line-of-sight, such as in the case of Abell 1689, can substantially increase the concentration parameter estimate. Together with the fact that clusters are triaxial, and that including baryonic physics also leads to an increase in the concentration of a dark matter halo, the tension between observations and the standard CDM model is eased. If the alignment with the secondary structure is imprecise, then the estimated concentration parameter can also be even lower than that of the primary halo, reinforcing the importance of identifying structures in cluster fields.

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A potentially pure test of cosmic geometry: galaxy clusters and the real space Alcock-Paczynski test

Authors: Young-Rae Kim (CMU) Rupert Croft (CMU)
Comments: 13 pages, 15 figures, Monthly Notices in press
We investigate the possibility of probing dark energy by measuring the isotropy of the galaxy cluster autocorrelation function (an Alcock-Paczynski test). The correlation function is distorted in redshift space because of the cluster peculiar velocities, but if these are known and can be subtracted, the correlation function measurement becomes in principle a pure test of cosmic geometry. Galaxy cluster peculiar velocities can be measured using the kinetic Sunyaev Zel'dovich (kSZ) effect. Upcoming CMB surveys, e.g., ACT, SPT, Planck, are expected to do this with varying levels of accuracy, dependent on systematic errors due to cluster temperature measurements, radio point sources, and the primary CMB anisotropy. We use the Hubble volume N-body simulation and the hydrodynamic simulation results of Nagai et. al (2003) to simulate various kSZ surveys. We find by model fitting that a measurement of the correlation function distortion can be used to recover the cosmological parameters that have been used to generate the simulation. However, the low space density of galaxy clusters requires larger surveys than are taking place at present to place tight constraints on cosmology. For example, with the SPT and ACT surveys, Omega_Lambda could be measured to within 0.1 and 0.2 respectively at one sigma, but only upper limits on the equation of state parameter w will be possible. Nevertheless, with accurate measurements of the kSZ effect, this test can eventually be used to probe the dark energy equation of state and its evolution with redshift, with different systematic errors than other methods.

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Sahar S. Allam, Douglas L. Tucker, Huan Lin, H. Thomas Diehl, James Annis, Joshua A. Frieman, E. Buckley-Geer 
TYPE: Scientific Paper in Journal based on Public SDSS Data
CATEGORY: SDSS-I
STATUS: To be submitteds to ApJ Letters in one week
PDF
Abstract:We report on the serendipitous discovery of the brightest Lyman Break 
Galaxy (LBG) currently known, a galaxy at z=2.73 that is being 
strongly lensed by the z=0.38 Luminous Red Galaxy (LRG) SDSS 
J002240.91+143110.4.  The arc of this gravitational lens system, 
which we have dubbed the ``8 o'clock arc'' due to its time of 
discovery, was initially identified in the imaging data of the 
Sloan Digital Sky Survey Data Release 4 (SDSS DR4); followup 
observations on the Astrophysical Research Consortium (ARC) 3.5m 
telescope at Apache Point Observatory confirmed the lensing nature 
of this system and led to the identification of the arc's spectrum as 
that of an LBG.  The arc has a spectrum and a redshift remarkably 
similar to those of the previous record-holder for brightest LBG 
(MS 1512-cB58, a.k.a ``cB58''), but, with an estimated total 
magnitude of (g,r,i) = (20.0,19.2,19.0) and surface brightness of 
(mu_g,mu_r, mu_i) = (23.3, 22.5, 22.3) mag/arcsec^2, 
the 8 o'clock arc is thrice as bright.  The 8 o'clock arc, which 
consists of three lensed images of the LBG, is 162deg (9.6 arcsec) 
long and has a length-to-width ratio of 6:1.  A fourth image of the 
LBG --- a counter-image --- can also be identified in the ARC 3.5m 
g-band images.  A simple lens model for the system assuming 
a singular isothermal ellipsoid potential yields an Einstein radius 
of theta_Ein=2.91 arcsec +/- 0.14 arcsec, a total mass for the 
lensing LRG (within the 10.6+/-0.5/h kpc enclosed by the lensed 
images) of 1.04*10^{12}/h M_sun, and a magnification factor for 
the LBG of 12.3(+15/-3.6).  The LBG itself is intrinsically quite 
luminous (approx. 6xL*) and shows indications of massive recent 
star formation, perhaps as high as 160/h M_sun/yr.

Collisionally Regenerated Dark Matter Structures in Galactic Nuclei

Authors: David Merritt, Stefan Harfst, Gianfranco Bertone
Comments: 11 pages, uses revtex4
We show that the presence of a rho~1/r^{3/2} dark matter overdensity can be robustly predicted at the center of any galaxy old enough to have grown a power-law density cusp in the stars via the Bahcall-Wolf mechanism. Using both Fokker-Planck and direct N-body integrations, we demonstrate collisional generation of these dark matter "crests" (Collisionally REgenerated STtructures) even in the extreme case that the density of both stars and dark matter were previously lowered by slingshot ejection from a binary supermassive black hole. The time scale for collisional growth of the crest is approximately the two-body relaxation time as defined by the stars, which is < 10 Gyr at the centers of stellar spheroids with luminosities comparable to that of the Milky Way bulge or less. The presence of crests can robustly be predicted in such galaxies, unlike the steeper enhancements, called "spikes," produced by the adiabatic growth of black holes. We discuss special cases where the prospects for detecting dark matter annihilations from the centers of galaxy haloes are significantly affected by the formation of crests.

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DUNE: The Dark Universe Explorer

Authors: A. Refregier, O. Boulade, Y. Mellier, B. Milliard, R. Pain, J. Michaud, F. Safa, A. Amara, P. Astier, E. Barrelet, E. Bertin, S. Boulade, C. Cara, A. Claret, L. Georges, R. Grange, J. Guy, C. Koeck, L. Kroely, C. Magneville, N. Palanque-Delabrouille, N. Regnault, G. Smadja, C. Schimd, Z. Sun
Comments: 12 latex pages, including 7 figures and 2 tables. Procs. of SPIE symposium "Astronomical Telescopes and Instrumentation", Orlando, may 2006
Understanding the nature of Dark Matter and Dark Energy is one of the most pressing issues in cosmology and fundamental physics. The purpose of the DUNE (Dark UNiverse Explorer) mission is to study these two cosmological components with high precision, using a space-based weak lensing survey as its primary science driver. Weak lensing provides a measure of the distribution of dark matter in the universe and of the impact of dark energy on the growth of structures. DUNE will also include a complementary supernovae survey to measure the expansion history of the universe, thus giving independent additional constraints on dark energy. The baseline concept consists of a 1.2m telescope with a 0.5 square degree optical CCD camera. It is designed to be fast with reduced risks and costs, and to take advantage of the synergy between ground-based and space observations. Stringent requirements for weak lensing systematics were shown to be achievable with the baseline concept. This will allow DUNE to place strong constraints on cosmological parameters, including the equation of state parameter of the dark energy and its evolution from redshift 0 to 1. DUNE is the subject of an ongoing study led by the French Space Agency (CNES), and is being proposed for ESA's Cosmic Vision programme.

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