Thursday, March 31, 2011

arXiv: 29 March 2011

Extrasolar Asteroid Mining as Forensic Evidence for Extraterrestrial Intelligence

Duncan Forgan, Martin Elvis
http://arxiv.org/abs/1103.5369v1
The development of civilisations like ours into spacefaring, multi-planet entities requires significant raw materials to construct vehicles and habitats. Interplanetary debris, including asteroids and comets, may provide such a source of raw materials. In this article we present the hypothesis that extraterrestrial intelligences (ETIs) engaged in asteroid mining may be detectable from Earth. Considering the detected disc of debris around Vega as a template, we explore the observational signatures of targeted asteroid mining (TAM), such as unexplained deficits in chemical species, changes in the size distribution of debris and other thermal signatures which may be detectable in the spectral energy distribution (SED) of a debris disc. We find that individual observational signatures of asteroid mining can be explained by natural phenomena, and as such they cannot provide conclusive detections of ETIs. But, it may be the case that several signatures appearing in the same system will prove harder to model without extraterrestrial involvement. Therefore signatures of TAM are not detections of ETI in their own right, but as part of "piggy-back" studies carried out in tandem with conventional debris disc research, they could provide a means of identifying unusual candidate systems for further study using other SETI techniques.

What do we really know about Dark Energy?

Ruth Durrer
http://arxiv.org/abs/1103.5331v1
In this paper I discuss what we truly know about dark energy. I shall argue that up to date our single indication for the existence of dark energy comes from distance measurements and their relation to redshift. Supernovae, CMB anisotropies and observations of baryon acoustic oscillations, they all simply tell us that the observed distance to a given redshift is larger than the one expected from a Friedmann Lemaitre universe with matter only and the locally measured Hubble parameter.



arXiv: 28 March 2011

Radio Halos, cluster mergers and the role of future LOFAR observations

Rossella Cassano
http://arxiv.org/abs/1103.5031v1
A radio bimodality is observed in galaxy clusters: a fraction of clusters host giant radio halos while the majority of clusters do not show evidence of diffuse cluster-scale radio emission. Present data clearly suggest that the radio bi-modality has a correspondence in terms of dynamical state of the hosting clusters. I will report on these evidences in some details and discuss the role of cluster mergers in the generation of giant radio halos and their evolution. Finally I will report on expectations on the statistical properties of radio halos assuming that the emitting electrons are re-accelerated by merger-turbulence, and discuss the role of incoming LOFAR surveys.


Friday, March 25, 2011

arXiv: 25 March 2011

Cosmological Parameters from Observations of Galaxy Clusters

Authors: Steven W. Allen, August E. Evrard, Adam B. Mantz
http://arxiv.org/abs/1103.4829v1
Studies of galaxy clusters have proved crucial in helping to establish the standard model of cosmology, with a universe dominated by dark matter and dark energy. A theoretical basis for understanding clusters as massive, multi-component, quasi-equilibrium systems has been developed, and is growing in its capability to interpret multi-wavelength observations of expanding scope and sensitivity. We review current cosmological results, including contributions to fundamental physics, obtained from observations of galaxy clusters. These results are consistent with and complementary to those from other methods. We highlight several areas of opportunity for the next few years, and emphasize the need for accurate modeling of survey selection and sources of systematic error. Capitalizing on these opportunities will require a multi-wavelength approach and the application of rigorous statistical frameworks, utilizing the combined strengths of observers, simulators and theorists.

A Multiband Study of the Galaxy Populations of the First Four Sunyaev--Zeldovich Effect selected Galaxy Clusters

Authors: A. Zenteno, J. Song, S. Desai, R. Armstrong, J. J. Mohr, C.-C. Ngeow, W. A. Barkhouse, S. S. Allam, K. Andersson, G. Bazin, B. A. Benson, E. Bertin, M. Brodwin, E. J. Buckley-Geer, S. M. Hansen, F. W. High, H. Lin, Y.-T. Lin, J. Liu, A. Rest, R. C. Smith, B. Stalder, A. A. Stark, D. L. Tucker, Y. Yang
arXiv:1103.4612v1
We present first results of an examination of the optical properties of the galaxy populations in SZE selected galaxy clusters. Using clusters selected by the South Pole Telescope survey and deep multiband optical data from the Blanco Cosmology Survey, we measure the radial profile, the luminosity function, the blue fraction and the halo occupation number of the galaxy populations of these four clusters with redshifts ranging from 0.3 to 1. Our goal is to understand whether there are differences among the galaxy populations of these SZE selected clusters and previously studied clusters selected in the optical and the X-ray. The radial distributions of galaxies in the four systems are consistent with NFW profiles with a galaxy concentration of 3 to 6. We show that the characteristic luminosities in $griz$ bands are consistent with passively evolving populations emerging from a single burst at redshift $z=3$. The faint end power law slope of the luminosity function is found to be on average $\alpha \approx -1.2$ in griz. Halo occupation numbers (to $m^*+2$) for these systems appear to be consistent with those based on X-ray selected clusters. The blue fraction estimated to $0.36L^*$, for the three lower redshift systems, suggests an increase with redshift, although with the current sample the uncertainties are still large. Overall, this pilot study of the first four clusters provides no evidence that the galaxy populations in these systems differ significantly from those in previously studied cluster populations selected in the X-ray or the optical.

Analytic solutions in non-linear massive gravity

Authors: Kazuya Koyama (ICG, Portsmouth), Gustavo Niz (ICG, Portsmouth), Gianmassimo Tasinato (ICG, Portsmouth)
arXiv:1103.4708v1
We study spherically symmetric solutions in a covariant massive gravity model, which is a candidate for a ghost-free non-linear completion of the Fierz-Pauli massive gravity. We find a solution that exhibits the Vainshtein mechanism, recovering general relativity below a Vainshtein radius given by $(r_g m^2)^{1/3}$, where $m$ is the graviton mass and $r_g$ is the Schwarzschild radius of a matter source. We also found another exact solution corresponding to Schwarzschild-de Sitter spacetime, where the curvature scale of de Sitter space is proportional to the mass squared of the graviton.

The cosmological constant: a lesson from Bose-Einstein condensates

Authors: Stefano Finazzi, Stefano Liberati, Lorenzo Sindoni
http://arxiv.org/abs/1103.4841v1
The cosmological constant is one of the most pressing problems in modern physics. In this Letter, we address the issue of its nature and computation using an analogue gravity standpoint as a toy model for an emergent gravity scenario. Even if it is well known that phonons in some condense matter systems propagate like a quantum field on a curved spacetime, only recently it has been shown that the dynamics of the analogue metric in a Bose-Einstein condensate can be described by a Poisson-like equation with a vacuum source term reminiscent of a cosmological constant. Here we directly compute this term and confront it with the other energy scales of the system. On the gravity side of the analogy, this model suggests that in emergent gravity scenarios it is natural for the cosmological constant to be much smaller than its naif value computed as the zero-point energy of the emergent effective field theory. The striking outcome of our investigation is that the value of this constant cannot be easily predicted by just looking at the ground state energy of the microscopic system from which spacetime and its dynamics should emerge. A proper computation would require the knowledge of both the full microscopic quantum theory and a detailed understanding about how Einstein equations emerge from such a fundamental theory. In this light, the cosmological constant appears even more a decisive test bench for any quantum/emergent gravity scenario.






Thursday, March 24, 2011

arXiv: 24 March 2011

Local Group Dwarf Galaxies: Nature And Nurture

Authors: Till Sawala, Cecilia Scannapieco, Simon White
http://arxiv.org/abs/1103.4562v1
We investigate the formation and evolution of dwarf galaxies in a high resolution, hydrodynamical cosmological simulation of a Milky Way sized halo and its environment. Our simulation includes gas cooling, star formation, supernova feedback, metal enrichment and UV heating. In total, 90 satellites and more than 400 isolated dwarf galaxies are formed in the simulation, allowing a systematic study of the internal and environmental processes that determine their evolution. We find that 95% of satellite galaxies are gas-free at z=0, and identify three mechanisms for gas loss: supernova feedback, tidal stripping, and photo-evaporation due to re-ionization. Gas-rich satellite galaxies are only found with total masses above ~ 5x10^9 solar masses. In contrast, for isolated dwarf galaxies, a total mass of ~ 10^9 solar masses constitutes a sharp transition; less massive galaxies are predominantly gas-free at z=0, more massive, isolated dwarf galaxies are often able to retain their gas. In general, we find that the total mass of a dwarf galaxy is the main factor which determines its star formation, metal enrichment, and its gas content, but that stripping may explain the observed difference in gas content between field dwarf galaxies and satellites with total masses close to 10^9 solar masses. We also find that a morphological transformation via tidal stripping of infalling, luminous dwarf galaxies whose dark matter is less concentrated than their stars, cannot explain the high total mass-light ratios of the faint dwarf spheroidal galaxies.


Wednesday, March 23, 2011

arXiv: 23 March 2011


The stellar and dark halo mass assembly of galaxies

Authors: V. Avila-Reese (1), C. Firmani (1,2) ((1) IA-UNAM, (2) INAF-OAB)

arXiv:1103.4329v1
The emerging empirical picture of galaxy stellar mass (Ms) assembly shows that galaxy population buildup proceeds from top to down in Ms. By connecting galaxies to LCDM halos and their histories, individual (average) Ms growth tracks can be inferred. These tracks show that massive galaxies assembled their Ms the earlier the more massive the halo, and that less massive galaxies are yet actively growing in Ms, the more active the less massive is the halo. The predicted star formation rates as a function of mass and the downsizing of the typical mass that separate active galaxies from the passive ones agree with direct observational determinations. This implies that the LCDM scenario is consistent with these observations. The challenge is now to understand the baryonic physics that drives the significant and systematical shift of the stellar mass assembly of galaxies from the mass assembly of their corresponding halos (from halo upsizing to galaxy downsizing).

Testing feasibility of scalar-tensor gravity by scale dependent mass and coupling to matter

Authors: D. F. Mota, V. Salzano, S. Capozziello
arXiv:1103.4215v1
We investigate whether there are any cosmological evidences for a scalar field with a mass and cou- pling to matter which change accordingly to the properties of the astrophysical system it "lives in", without directly focusing on the underlying mechanism that drives the scalar field scale-dependent- properties. We assume a Yukawa type of coupling between the field and matter and also that the scalar field mass grows with density, in order to overcome all gravity constraints within the solar system. We analyse three different gravitational systems assumed as "cosmological indicators": su- pernovae type Ia, low surface brightness spiral galaxies and clusters of galaxies. Results show that: a. a quite good fit to the rotation curves of low surface brightness galaxies only using visible stellar and gas mass components is obtained; b. a scalar field can fairly well reproduce the matter profile in clusters of galaxies, estimated by X-ray observations and without the need of any additional dark matter; c. there is an intrinsic difficulty in extracting information about the possibility of a scale-dependent massive scalar field (or more generally about a varying gravitational constant) from supernovae type Ia.

Limits on Dark Radiation, Early Dark Energy, and Relativistic Degrees of Freedom

Authors: Erminia Calabrese, Dragan Huterer, Eric V. Linder, Alessandro Melchiorri, Luca Pagano
arXiv:1103.4132v1
Abstract: Recent cosmological data analyses hint at the presence of an extra relativistic energy component in the early universe. This component is often parametrized as an excess of the effective neutrino number N_{eff} over the standard value of 3.046. The excess relativistic energy could be an indication for an extra (sterile) neutrino, but early dark energy and barotropic dark energy also contribute to the relativistic degrees of freedom. We examine the capabilities of current and future data to constrain and discriminate between these explanations, and to detect the early dark energy density associated with them. We found that while early dark energy does not alter the current constraints on N_{eff}, a dark radiation component, such as that provided by barotropic dark energy models, can substantially change current constraints on N_{eff}, bringing its value back to agreement with the theoretical prediction. Both dark energy models also have implications for the primordial mass fraction of Helium Y_p and the scalar perturbation index n_s. The ongoing Planck satellite mission will be able to further discriminate between sterile neutrinos and early dark energy





arXiv: 22 March 2011

Scalar radiation from Chameleon-shielded regions

Authors: Alessandra Silvestri
http://arxiv.org/abs/1103.4013v1
I study the profile of the Chameleon field around a radially pulsating mass. Focusing on the case in which the background (static) Chameleon profile exhibits a thin-shell, I add small perturbations to the source in the form of time-dependent radial pulsations. It is found that the Chameleon field inherits a time-dependence, there is a resultant scalar radiation from the region of the source and the metric outside the spherically symmetric mass is not static. This has several interesting and potentially testable consequences.

Catalog of Nearby Isolated Galaxies in the Volume z<0.01

Authors: I.D. Karachentsev, D.I. Makarov, V.E. Karachentseva, O.V. Melnyk
http://arxiv.org/abs/1103.3990v2
We present a catalog of 520 most isolated nearby galaxies with radial velocities V_LG<3500 km/s covering the entire sky. This population of "space orphans" makes up 4.8% among 10900 galaxies with measured radial velocities. We describe the isolation criterion used to select our sample, called the "Local Orphan Galaxies" (LOG), and discuss their basic optical and HI properties. A half of the LOG catalog is occupied by the Sdm, Im and Ir morphological type galaxies without a bulge. The median ratio M_gas/M_star in the LOG galaxies exceeds 1. The distribution of the catalog galaxies on the sky looks uniform with some signatures of a weak clustering on the scale of about 0.5 Mpc. The LOG galaxies are located in the regions where the mean local density of matter is approximately 50 times lower than the mean global density. We indicate a number of LOG galaxies with distorted structures, which may be the consequence of interaction of isolated galaxies with massive dark objects.



Constraint on the early Universe by relic gravitational waves: From pulsar timing observations

Authors: Wen Zhao
arXiv:1103.3927v1
Abstract:
Recent pulsar timing observations by PPTA and EPTA teams obtained the constraint on the relic gravitational waves at the frequency $f_*=1/{\rm yr}$, which provides the opportunity to constrain $H_*$, the Hubble parameter when these waves crossed the horizon during inflation. In this Letter, we investigate this constraint by considering the general scenario for the early Universe: we assume the effective (average) equation-of-state $w$ before the Big Bang nucleosynthesis stage is a free parameter. In the standard hot big-bang scenario with $w=1/3$, we find that the current PPTA result follows a bound $H_*\leq 1.15\times10^{-1}\mpl$, and EPTA result follows $H_*\leq 6.92\times10^{-2}\mpl$. We also find that, these bounds become much tighter in the non-standard scenarios with $w>1/3$. When $w=1$, the bounds become $H_*\leq5.89\times10^{-3}\mpl$ for the current PPTA and $H_*\leq3.39\times10^{-3}\mpl$ for the current EPTA.



Monday, March 21, 2011

arXiv: 21 March 2011

Likelihood reconstruction method of real-space density and velocity power spectra from a redshift galaxy survey

Authors: Jiayu Tang, Issha Kayo, Masahiro Takada (IPMU, U. Tokyo)
http://arxiv.org/abs/1103.3614v1
We develop a maximum likelihood based method of reconstructing the real-space density and velocity power spectra from the measured galaxy clustering in redshift space. Our method reconstructs band powers of the real-space power spectra, each of which depends on the redshift-space power spectrum with different powers of angular modulations mu^2n (n=0,1,2) at each wavenumber bins, including marginalization over uncertainties in the Fingers-of-God (FoG) effect. By using N-body simulations of 70 realizations and the halo catalogs, we test our method by comparing the reconstructed power spectra with the spectra directly measured from the simulations. We show that the method can well recover the power spectrum of mu^0, or equivalently the density power spectrum, up to k~0.3 h/Mpc to a few percent accuracies in amplitudes, for both dark matter and halos, if we assume an adequate functional form of the FoG effect. For the power spectrum of mu^2, which is closely related to the density-velocity power spectrum P_dv(k), the method can recover the input power spectrum for dark matter up to k~0.15 h/Mpc. However, for the halo spectrum, the reconstructed spectrum shows greater amplitudes than the spectrum P_dv(k) inferred from the simulations over a range of wavenumbers 0.05<k<0.3 h/Mpc. We argue that the disagreement is ascribed to nonlinearity effects that arise from the cross-bispectra of density and velocity perturbations. Using the perturbation theory we derive the nonlinear correction term to the redshift-space spectrum, and find that the leading-order correction term is proportional to mu^2 and increases total powers of the mu^2-power spectrum more significantly for more massive halos. We find that adding the nonlinearity correction term to the measured P_dv(k) can fairly well reproduce the reconstructed spectra for halos over the wavenumber range up to k~0.2 h/Mpc. (Abridged)

Bias in low-multipole CMB reconstructions

http://arxiv.org/abs/1103.3505v1
The large-angle, low multipole cosmic microwave background (CMB) provides a unique view of the largest angular scales in the Universe. Study of these scales is hampered by the facts that we have only one Universe to observe, only a few independent samples of the underlying statistical distribution of these modes, and an incomplete sky to observe due to the interposing Galaxy. Techniques for reconstructing a full sky from partial sky data are well known and have been applied to the large angular scales. In this work we critically study the reconstruction process and show that, in practise, the reconstruction is biased due to leakage of information from the region obscured by foregrounds to the region used for the reconstruction. We conclude that, despite being suboptimal in a technical sense, using the unobscured region without reconstructing is the most robust measure of the true CMB sky.

Galaxy Evolution in Cosmological Simulations With Outflows I: Stellar Masses and Star Formation Rates

Authors: eel Davé (Arizona), Benjamin D. Oppenheimer (Leiden), Kristian Finlator (UC Santa Barbara)
http://arxiv.org/abs/1103.3528v1
Abstract: We examine the growth of the stellar content of galaxies from z=3-0 in cosmological hydrodynamic simulations incorporating parameterised galactic outflows. Without outflows, galaxies overproduce stellar masses (M*) and star formation rates (SFRs) compared to observations. Winds introduce a three-tier form for the galaxy stellar mass and star formation rate functions, where the middle tier depends on differential (i.e. mass-dependent) recycling of ejected wind material back into galaxies. A tight M*-SFR relation is a generic outcome of all these simulations, and its evolution is well-described as being powered by cold accretion, although current observations at z>2 suggest that star formation in small early galaxies must be highly suppressed. Roughly one-third of z=0 galaxies at masses below M^* are satellites, and star formation in satellites is not much burstier than in centrals. All models fail to suppress star formation and stellar mass growth in massive galaxies at z<2, indicating the need for an external quenching mechanism such as black hole feedback. All models also fail to produce dwarfs as young and rapidly star-forming as observed. An outflow model following scalings expected for momentum-driven winds broadly matches observed galaxy evolution around M^* from z=0-3, which is a significant success since these galaxies dominate cosmic star formation, but the failures at higher and lower masses highlight the challenges still faced by this class of models. We argue that central star-forming galaxies are well-described as living in a slowly-evolving equilibrium between inflows from gravity and recycled winds, star formation, and strong and ubiquitous outflows that regulate how much inflow forms into stars. Star-forming galaxy evolution is thus primarily governed by the continual cycling of baryons between galaxies and intergalactic gas






Friday, March 18, 2011

arXiv: 18 March 2011

Dynamical masses of early-type galaxies: a comparison to lensing results and implications for the stellar IMF and the distribution of dark matter

Authors: J. Thomas, R. P. Saglia, R. Bender, D. Thomas, K. Gebhardt, J. Magorrian, E. M. Corsini, G. Wegner, S. Seitz
http://arxiv.org/abs/1103.3414v1
This work aims to study the distribution of luminous and dark matter in Coma early-type galaxies. Dynamical masses obtained under the assumption that mass follows light do not match with the masses of strong gravitational lens systems of similar velocity dispersions. Instead, dynamical fits with dark matter halos are in good agreement with lensing results. We derive mass-to-light ratios of the stellar populations from Lick absorption line indices, reproducing well the observed galaxy colours. Even in dynamical models with dark matter halos the amount of mass that follows the light increases more rapidly with galaxy velocity dispersion than expected for a constant stellar initial mass function (IMF). While galaxies around sigma ~ 200 km/s are consistent with a Kroupa IMF, the same IMF underpredicts luminous dynamical masses of galaxies with sigma ~ 300 km/s by a factor of two and more. A systematic variation of the stellar IMF with galaxy velocity dispersion could explain this trend with a Salpeter IMF for the most massive galaxies. If the IMF is instead constant, then some of the dark matter in high velocity dispersion galaxies must follow a spatial distribution very similar to that of the light. A combination of both, a varying IMF and a component of dark matter that follows the light is possible as well. For a subsample of galaxies with old stellar populations we show that the tilt in the fundamental plane can be explained by systematic variations of the total (stellar + dark) mass inside the effective radius. We tested commonly used mass estimator formulae, finding them accurate at the 20-30% level.







Thursday, March 17, 2011

arXiv: 17 March 2011

The dark matter halo density profile, spiral arm morphology and black hole mass of M33

In this paper, we investigate the dark matter halo density profile of M33. We find that the HI rotation curve of M33 is best described by a NFW dark matter halo density profile model, with a halo concentration of cvir = 4.0\pm1.0 and a virial mass of Mvir = (2.2\pm0.1)\times10^11 Msun. We go on to use the NFW concentration (cvir)of M33, along with the values derived for other galaxies (as found in the literature), to show that cvir correlates with both spiral arm pitch angle and supermassive black hole mass.


Wednesday, March 16, 2011

arXiv: 16 March 2011

A 3% Solution: Determination of the Hubble Constant with the Hubble Space Telescope and Wide Field Camera 3
Adam G. Riess (JHU, STScI), Lucas Macri (Texas A&M), Stefano Casertano (STScI), Hubert Lampeitl (U of Portsmouth), Henry C. Ferguson (STScI), Alexei V. Filippenko (UCB), Saurabh W. Jha(Rutgers), Weidong Li (UCB), Ryan Chornock (Harvard CfA)
We use the Wide Field Camera 3 (WFC3) on the Hubble Space Telescope to determine the Hubble constant (H0) from optical and infrared observations of over 600 Cepheid variables in the host galaxies of 8 recent Type Ia supernovae (SNe Ia), providing the calibration for a mag-z relation of 253 SNe Ia. Increased precision over past measurements comes from: (1) more than doubling the number of infrared observations of Cepheids in nearby SN hosts; (2) increasing the sample of ideal SN Ia calibrators from six to eight; (3) increasing by 20% the number of Cepheids with infrared observations in the megamaser host NGC 4258; (4) reducing the difference in the mean metallicity of the Cepheid comparison samples from \Delta log [O/H] = 0.08 to 0.05; and (5) calibrating all optical Cepheid colors with one camera, WFC3, to remove cross-instrument zero-point errors. Uncertainty in H0 from beyond the 1st rung of the distance ladder is reduced from 3.5% to 2.3%. The measurement of H0 via the geometric distance to NGC 4258 is 74.8 \pm 3.1 km s- 1 Mpc-1, a 4.1% measurement including systematics. Better precision independent of NGC 4258 comes from two alternative Cepheid absolute calibrations: (1) 13 Milky Way Cepheids with parallaxes and (2) 92 Cepheids in the Large Magellanic Cloud with multiple eclipsing binary distances, yielding 74.4 \pm 2.5 km s- 1 Mpc-1, a 3.4% uncertainty with systematics. Our best estimate uses all three calibrations but a larger uncertainty afforded from any two: H0 = 73.8 \pm 2.4 km s- 1 Mpc-1 including systematics, a 3.3% uncertainty. The improvement in H0, combined with WMAP7yr data, results in a constraint on the EOS parameter of dark energy of w = -1.08 \pm 0.10 and Neff = 4.2 \pm 0.7 for the number of relativistic species in the early universe. It also rules out the best-fitting gigaparsec-scale void models, posited as an alternative to dark energy. (abridged)

Large-scale Structure in f(T) Gravity

In this work we study the cosmology of the general f(T) gravity theory. We express the modified Einstein equations using covariant quantities, and derive the gauge-invariant perturbation equations in covariant form. We consider a specific choice of f(T), designed to explain the observed late-time accelerating cosmic expansion without including an exotic dark energy component. Our numerical solution shows that the extra degree of freedom of such f(T) gravity models generally decays as one goes to smaller scales, and consequently its effects on scales such as galaxies and galaxies clusters are small. But on large scales, this degree of freedom can produce large deviations from the standard LCDM scenario, leading to severe constraints on the f(T) gravity models as an explanation to the cosmic acceleration.









Tuesday, March 15, 2011

arXiv: 15 March 2011

     Observational evidence for a metal rich atmosphere on the super-Earth GJ1214b
We report observations of two consecutive transits of the warm super-Earth exoplanet GJ1214b at 3.6 and 4.5 microns with the Infrared Array Camera instrument on-board the Spitzer Space Telescope. The two transit light curves allow for the determination of the transit parameters for this system. We find these paremeters to be consistent with the previously determined values and no evidence for transit timing variations. The main investigation consists of measuring the transit depths in each bandpass to constrain the planet's transmission spectrum. Fixing the system scale and impact parameters, we measure R_p/R_star=0.1176 (+0.0008/-0.0009) and 0.1163 (+0.0010/-0.0008) at 3.6 and 4.5 microns, respectively. Combining these data with the previously reported MEarth Observatory measurements in the red optical yields constraints on the GJ1214b's transmission spectrum and allows us to rule-out a cloud-free, solar composition (i.e., hydrogen-dominated) atmosphere at 4.5 sigma confidence. This independently confirms a recent finding that was based on a measurement of the planet's transmission spectrum using the VLT. The Spitzer, MEarth, and VLT observations together yield a remarkably flat transmission spectrum over the large wavelength domain spanned by the data. Consequently, cloud-free atmospheric models require more than 30% metals (assumed to be in the form of H2O by volume to be consistent with all the observations.

WMAP7 and future CMB constraints on annihilating dark matter: implications on GeV-scale WIMPs

(Context) We calculate constraints from current and future cosmic microwave background (CMB) measurements on annihilating dark matter (DM) with masses below the electroweak scale: m_{DM}=5-100 GeV. In particular, we focus our attention on the lower end of this mass range, as DM particles with masses m_{DM} ~ 10 GeV have been recently claimed to be consistent with the CoGeNT and DAMA/LIBRA results, while also providing viable DM candidates to explain the measurements of Fermi and WMAP haze. (Aims) We study the model (in)dependence of CMB spectrum on particle physics DM models, large scale structure formation and cosmological uncertainties. We attempt to find a simple and practical recipe for estimating current and future CMB bounds on a broad class of DM annihilation models. (Results) We show that in the studied DM mass range the CMB signal of DM annihilations is independent of the details of large scale structure formation, distribution and profile of DM halos and other cosmological uncertainties. All particle physics models of DM annihilation can be described with only one parameter, the fraction of energy carried away by neutrinos in DM annihilation. As the main result we provide a simple and rather generic fitting formula for calculating CMB constraints on the annihilation cross section of light WIMPs. We show that thermal relic DM in the CoGeNT, DAMA favoured mass range is in a serious conflict with present CMB data for the annihilation channels with few neutrinos, and will definitely be tested by the Planck mission for all possible DM annihilation channels. Also, our findings strongly disfavor the claim that thermal relic DM annihilations with m_{DM} ~ 10 GeV and $<sigma_av> ~ 9x10^{-25} cm^{3}s^{-1} could be a cause for Fermi and WMAP haze.

Disentangling star formation and merger growth in the evolution of Luminous Red Galaxies

Rita Tojeiro (ICG, Portsmouth), Will J. Percival (ICG, Portsmouth)
(Submitted on 14 Mar 2011)
We use the empirically determined stellar evolution models of Tojeiro et al. (2011) to predict the past evolution of the Sloan Digital Sky Survey (SDSS-II) Luminous Red Galaxy (LRG) sample without an a-priori assumption that the galaxies evolve passively. By carefully contrasting the evolution of the predicted and observed number and luminosity densities we test the passive evolution scenario for galaxies of different luminosity, and determine minimum merger rates. We find that the LRG population is not purely coeval, with some of galaxies targeted at z<0.23 and at z>0.34 showing different dynamical growth than galaxies targeted throughout the sample. Our results show that the LRG population is dynamically growing, and that this growth must be dominated by the faint end. For the most luminous galaxies, we find lower minimum merger rates than required by previous studies that assume passive stellar evolution, suggesting that some of the dynamical evolution measured previously was actually due to galaxies with non-passive stellar evolution being incorrectly modelled. Our methodology can be used to identify and match coeval populations of galaxies across cosmic times, over one or more surveys.


Modification of the halo mass function by kurtosis associated with primordial non-Gaussianity

We study the halo mass function in the presence of the kurtosis type of primordial non-Gaussianity. The kurtosis corresponds to the trispectrum as defined in Fourier space. The primordial trispectrum is commonly characterized by two parameters, $\tau_{\rm NL}$ and $g_{\rm NL}$. As applications of the derived non-Gaussian mass function, we consider the effect on the abundance of void structure, the effect on early star formation and on formation of the most massive object at high redshift. We show that by comparing the effects of primordial non-Gaussianity on cluster abundance with that on void abundance, we can distinguish between the skewness and the kurtosis types of primordial non-Gaussianity. As for early star formation, we show that the kurtosis type of primordial non-Gaussianity seems not to affect the reionization history of the Universe on average. However, at high redshifts (up to $z\simeq 20$) such non-Gaussianity does somewhat affect the early stages of reionization.

On new variational principles as alternatives to the Palatini method

A variational principle was recently suggested by Goenner, where an independent metric generates the spacetime connection. It is pointed out here that the resulting theory is equivalent to the usual Palatini theory. However, a bimetric reformulation of the variational principle leads to theories which are physically distinct from both the metric and the metric-affine ones, even for the Einstein-Hilbert action. They are obtained at a decoupling limit of C-theories, which contain also other viable generalizations of the Palatini theories








arXiv: 14 March 2011

Inhomogeneity effects in Cosmology

This article looks at how inhomogeneous spacetime models may be significant for cosmology. First it looks at how the averaging process may affect large scale dynamics, with backreaction effects leading to effective contributions to the averaged energy-momentum tensor. Secondly it considers how local inhomogeneities may affect cosmological observations in cosmology, possibly significantly affecting the concordance model parameters. Thirdly it presents the possibility that the universe is spatially inhomogeneous on Hubble scales, with a violation of the Copernican principle leading to an apparent acceleration of the universe. This could perhaps even remove the need for the postulate of dark energy.

Introduction to Early Universe Cosmology
Robert H. Brandenberger (McGill University)
Observational cosmology is in its "golden age" with a vast amount of recent data on the distribution of matter and light in the universe. This data can be used to probe theories of the very early universe. It is small amplitude cosmological fluctuations which encode the information about the very early universe and relate it to current data. Hence, a central topic in these lectures is the "theory of cosmological perturbations", the theory which describes the generation of inhomogeneities in the very early universe and their evolution until the current time. I will apply this theory to three classes of models of the very early universe. The first is "Inflationary Cosmology", the current paradigm for understanding the early evolution of the universe. I will review the successes of inflationary cosmology, but will also focus on some conceptual challenges which inflationary cosmology is facing, challenges which motivate the search for possible alternatives. I will introduce two alternative scenarios, the "Matter Bounce" model and "String Gas Cosmology", and I will discuss how cosmological fluctuations which can explain the current data are generated in those models.

The Cauchy problem for f(R)-gravity: an overview

We review the Cauchy problem for f(R) theories of gravity, in metric and metric-affine for- mulations, pointing out analogies and differences with respect to General Relativity. The role of conformal transformations, effective scalar fields and sources in the field equations is discussed in view of the well-posedness of the problem. Finally, criteria of viability of the f(R)-models are considered according to the various matter fields acting as sources


Friday, March 11, 2011

arXiv: 11 March 2011

The Atacama Cosmology Telescope: Detection of the Power Spectrum of Gravitational Lensing

We report the first detection of the gravitational lensing of the cosmic microwave background through a measurement of the four-point correlation function in the temperature maps made by the Atacama Cosmology Telescope. We verify our detection by calculating the levels of potential contaminants and performing a number of null tests. The resulting convergence power spectrum at 2-degree angular scales measures the amplitude of matter density fluctuations on comoving length scales of around 100 Mpc at redshifts around 1 to 3. The measured amplitude of the signal agrees with Lambda Cold Dark Matter cosmology predictions. Since the amplitude of the convergence power spectrum scales as the square of the amplitude of the density fluctuations, the 4-sigma detection of the lensing signal measures the amplitude of density fluctuations to 12%.

A theoretical framework for combining techniques that probe the link between galaxies and dark matter

We develop a theoretical framework that combines measurements of galaxy-galaxy lensing, galaxy clustering, and the galaxy stellar mass function in a self-consistent manner. While considerable effort has been invested in exploring each of these probes individually, attempts to combine them are still in their infancy despite the potential of such combinations to elucidate the galaxy-dark matter connection, to constrain cosmological parameters, and to test the nature of gravity. In this paper, we focus on a theoretical model that describes the galaxy-dark matter connection based on standard halo occupation distribution techniques. Several key modifications enable us to extract additional parameters that determine the stellar-to-halo mass relation and to simultaneously fit data from multiple probes while allowing for independent binning schemes for each probe. In a companion paper, we demonstrate that the model presented here provides an excellent fit to galaxy-galaxy lensing, galaxy clustering, and stellar mass functions measured in the COSMOS survey from z=0.2 to z=1.0. We construct mock catalogs from numerical simulations to investigate the effects of sample variance and covariance on each of the three probes. Finally, we analyze and discuss how trends in each of the three observables impact the derived parameters of the model. In particular, we investigate the various features of the observed galaxy stellar mass function (low-mass slope, plateau, knee, and high-mass cut-off) and show how each feature is related to the underlying relationship between stellar and halo mass. We demonstrate that the observed plateau feature in the stellar mass function at Mstellar~2x10^10 Msun is due to the transition that occurs in the stellar-to-halo mass relation at Mhalo ~ 10^12 Msun from a low-mass power-law regime to a sub-exponential function at higher stellar mass.

Thursday, March 10, 2011

arXiv: 10 March 2011

Properties of Dark Matter Haloes and their Correlations: the Lesson from Principal Component Analysis

Authors: Ramin A. Skibba, Andrea V. Maccio'
http://arxiv.org/abs/1103.1641v1

We study the correlations between the structural parameters of dark matter haloes using Principal Component Analysis (PCA). We consider a set of eight parameters, six of which are commonly used to characterize dark matter halo properties: mass, concentration, spin, shape, overdensity, and the angle ($\Phi_L$) between the major axis and the angular momentum vector. Two additional parameters ($\x_{off}$ and $\rho_{rms}$) are used to describe the degree of `relaxedness' of the halo. We find that we can account for much of the variance of these properties with halo mass and concentration, on the one hand, and halo relaxedness on the other. Nonetheless, three principle components are usually required to account for most of the variance. We argue that halo mass is not as dominant as expected, which is a challenge for halo occupation models and semi-analytic models that assume that mass determines other halo (and galaxy) properties. In addition, we find that the angle $\Phi_L$ is not significantly correlated with other halo parameters, which may present a difficulty for models in which galaxy disks are oriented in haloes in a particular way. Finally, at fixed mass, we find that a halo's environment (quantified by the large-scale overdensity) is relatively unimportant.

Probing the dark-matter halos of cluster galaxies with weak lensing

Authors: E. Pastor Mira, S. Hilbert, J. Hartlap, P. Schneider
http://arxiv.org/abs/1103.1635v1
Context: Understanding the evolution of the dark matter halos of galaxies after they become part of a cluster is essential for understanding the evolution of these satellite galaxies. Aims: We investigate the potential of galaxy-galaxy lensing to map the halo density profiles of galaxies in clusters. Methods: We propose a method that separates the weak-lensing signal of the dark-matter halos of galaxies in clusters from the weak-lensing signal of the cluster's main halo. Using toy cluster models as well as ray-tracing through N-body simulations of structure formation along with semi-analytic galaxy formation models, we test the method and assess its performance. Results: We show that with the proposed method, one can recover the density profiles of the cluster galaxy halos in the range 30 - 300 kpc. Using the method, we find that weak-lensing signal of cluster member galaxies in the Millennium Simulation is well described by an Navarro-Frenk-White (NFW) profile. In contrast, non-singular isothermal mass distribution (like PIEMD) model provide a poor fit. Furthermore, we do not find evidence for a sharp truncation of the galaxy halos in the range probed by our method. Instead, there is an observed overall decrease of the halo mass profile of cluster member galaxies with increasing time spent in the cluster. This trend, as well as the presence or absence of a truncation radius, should be detectable in future weak-lensing surveys like the Dark Energy Survey (DES) or the Large Synoptic Survey Telescope (LSST) survey. Such surveys should also allow one to infer the mass-luminosity relation of cluster galaxies with our method over two decades in mass. Conclusions: It is possible to recover in a non-parametric way the mass profile of satellite galaxies and their dark matter halos in future surveys, using our proposed weak lensing method.



Wednesday, March 9, 2011

arXiv: 9 March 2011

HI signatures of galaxy evolution

HI in and around galaxies provides unique information about the various processes shaping galaxies: merging, cold gas accretion, feedback. Observations of galaxies in the nearby universe are beginning to reveal the HI signatures of these processes by pushing the sensitivity limits of existing radio synthesis telescopes to their limits. This paper gives a brief inventory of these signatures. The capabilities of new instruments such as the SKA pathfinders and precursors is briefly addressed, though ultimately SKA will provide the adequate sensitivity to find these HI signatures beyond the local universe.


An Unbiased Method of Modeling the Local Peculiar Velocity Field with Type Ia Supernovae


We apply statistically rigorous methods of nonparametric risk estimation to the problem of inferring the local peculiar velocity field from nearby supernovae (SNIa). We use two nonparametric methods - Weighted Least Squares (WLS) and Coefficient Unbiased (CU) - both of which employ spherical harmonics to model the field and use the estimated risk to determine at which multipole to truncate the series. We show that if the data are not drawn from a uniform distribution or if there is power beyond the maximum multipole in the regression, a bias is introduced on the coefficients using WLS. CU estimates the coefficients without this bias by including the sampling density making the coefficients more accurate but not necessarily modeling the velocity field more accurately. After applying nonparametric risk estimation to SNIa data, we find that there are not enough data at this time to measure power beyond the dipole. The WLS Local Group bulk flow is moving at 538 +- 86 km/s towards (l,b) = (258 +- 10 deg, 36 +- 11 deg) and the CU bulk flow is moving at 446 +- 101 km/s towards (l,b) = (273 +- 11 deg, 46 +- 8 deg). We find that the magnitude and direction of these measurements are in agreement with each other and previous results in the literature.


Tuesday, March 8, 2011

arXiv: 8 March 2011

Can the James Webb Space Telescope detect isolated population III stars?

Isolated population III stars are postulated to exist at approximately z=10-30 and may attain masses up to a few hundred solar masses. The James Webb Space telescope (JWST) is the next large space based infrared telescope and is scheduled for launch in 2014. Using a 6.5 meter primary mirror, it will probably be able to detect some of the first galaxies forming in the early Universe. A natural question is whether it will also be able to see any isolated population III stars. Here, we calculate the apparent broadband AB-magnitudes for 300 solar masses population III stars in JWST filters at z=10-20. Our calculations are based on realistic stellar atmospheres and take into account the potential flux contribution from the surrounding HII region. The gravitational magnification boost achieved when pointing JWST through a foreground galaxy cluster is also considered. Using this machinery, we derive the conditions required for JWST to be able to detect population III stars in isolation. We find that a detection of individual population III stars with JWST is unlikely at these redshifts. However, the main problem is not necessarily that these stars are too faint, once gravitational lensing is taken into account, but that their surface number densities are too low.

Figures of merit and constraints from testing General Relativity using the latest cosmological data sets including refined COSMOS 3D weak lensing

Jason DossettJacob MoldenhauerMustapha Ishak (The University of Texas at Dallas)
We use cosmological constraints from current data sets and a figure of merit (FoM) approach to probe any deviations from general relativity (GR) at cosmological scales. The FoM approach is used to study the constraining power of various combinations of data sets on modified gravity (MG) parameters. We use recently refined HST-COSMOS weak lensing tomography data, ISW-galaxy cross correlations from 2MASS and SDSS LRG surveys, matter-power spectrum from SDSS-DR7, WMAP7 temperature and polarization spectra, BAO from 2DF and SDSS-DR7, and Union2 compilation of supernovae, in addition to other bounds from H_0 measurements and BBN. We use 3 parameterizations of MG parameters that enter the perturbed field equations. In order to allow for variations with redshift and scale, the first 2 parameterizations use recently suggested functional forms while the third is based on binning methods. Using the first parameterization, we find that CMB+ISW+WL provides the strongest constraints on MG parameters followed by CMB+WL or CMB+MPK+ISW. Using the second parameterization or binning methods, CMB+MPK+ISW consistently provides some of the strongest constraints. This shows that the constraints are parameterization dependent. We find that adding up current data sets does not improve consistently the uncertainties on MG parameters due to tensions between best fit MG parameters preferred by different data sets. Furthermore, some functional forms imposed by the parameterizations can lead to an exacerbation of these tensions. Next, unlike some studies that used the CFHTLS lensing data, we do not find any deviation from GR using the refined HST-COSMOS data, confirming previous claims in those studies that their result may have been due to some systematic effect. Finally, we find in all cases that the values corresponding to GR are within the 95% confidence contours for all data set combinations. (abridged)

Evolution of the Milky Way halo by accretion of dwarf satellite galaxies

Within the Cold Dark Matter scenario the hierarchical merging paradigm is the natural result to form massive galactic halos by the minor mergers of sub-halos and, by this, inherently their stellar halo. Although this must be also invoked for the Milky Way, the context of chemical and kinematic coherence of halo stars and dwarf spheroidal galaxies is yet unsolved a focus of present-day research. To examine this issue we model the chemo-dynamical evolution of the system of satellites selected from the cosmological Via Lactea II simulations to be similar for the Milky Way environment but at an early epoch.

Primordial non-Gaussianities in general modified gravitational models of inflation

We compute the three-point correlation function of primordial scalar density perturbations in a general single-field inflationary scenario, where a scalar field phi has a direct coupling with the Ricci scalar R and the Gauss-Bonnet term GB. Our analysis also covers the models in which the Lagrangian has a non-linear kinetic energy X=-(nabla phi)^2/2 and a Galileon-type field self-interaction G(phi, X)*(Box phi), where G is a function of phi and X. We provide a general analytic formula for the equilateral non-Gaussianity parameter f_{NL}^{equil} associated with the bispectrum of curvature perturbations. A quasi de Sitter approximation in terms of slow-roll parameters allows us to derive a simplified form of f_{NL}^{equil} convenient to constrain various inflation models observationally. If the propagation speed of the scalar perturbations is much smaller than the speed of light, the Gauss-Bonnet term as well as the Galileon-type field self-interaction can give rise to large non-Gaussianities testable in future observations. We also show that, in Brans-Dicke theory with a field potential (including f(R) gravity), f_{NL}^{equil} is of the order of slow-roll parameters as in standard inflation driven by a minimally coupled scalar field.

The Morphological Origin of Dwarf Galaxies

Dwarf galaxies (DGs) serve as extremely challenging objects in extragalactic astrophysics. Their origin is expected to be set as the first units in CDM cosmology. Nevertheless they are the galaxy type most sensitive to environmental in uences and their division into multiple types with various properties have invoked the picture of their variant morphological transformations. Detailed observations reveal characteristics which allow to deduce the evolutionary paths and to witness how the environment has a?ected the evolution. Here we refer to general morphological DG types and review some general processes, most of which deplete gas-rich irregular DGs. Moreover, the variety of pecularities is brie y refered, but cannot be comprehensively analyzed because of limited paper space.

Effects of Chameleon Scalar Field on Rotation Curves of the Galaxies

We investigate the effects of chameleon scalar field to the effective density and pressure of a dark matter halo. The pressure is generated from the chameleonic fifth force on the matter. We demonstrate that the thick-shell non-singular boundary condition which forbids singular point leads to extremely stringent constraint on the matter-chameleon coupling when applied to galaxy. We argue that chameleon profile with central singularity is more likely to develop in general physical situation. The chameleonic fifth force from the chameleon profile with central singularity experienced by the dark matter could significantly modify the rotation curve of galaxies. The chameleonic fifth force could generate steeper cusp to the rotation curves in any dark matter profiles starting from the Navarro-Frenk-White (NFW) to the pseudo-isothermal (ISO) profile. Upper limits on the coupling constant between the chameleon and the dark matter are estimated from observational data of the late-type Low-Surface-Brightness galaxies (LSB). It is in the order of $\beta < 10^{-3}$

HI Content and Optical Properties of Field Galaxies from the ALFALFA Survey. II. Multivariate Analysis of a Galaxy Sample in Low Density Environments

This is the second paper of two reporting results from a study of the HI content and stellar properties of nearby galaxies detected by the Arecibo Legacy Fast ALFA blind 21-cm line survey and the Sloan Digital Sky Survey in a 2160 deg^2 region covered by both surveys. We apply strategies of multivariate data analysis to a complete HI flux-limited subset of 1624 objects extracted from the control sample of HI emitters assembled by Toribio et al. (2011a) in order to: i) investigate the correlation structure of the space defined by an extensive set of observables describing gas-rich systems; ii) identify the intrinsic parameters that best define their HI content; and iii) explore the scaling relations arising from the joint distributions of the quantities most strongly correlated with the HI mass. The principal component analysis performed over a set of five galaxy properties reveals that they are strongly interrelated, supporting previous claims that nearby HI emitters show a high degree of correlation. The best predictors for the expected value of MHI are the diameter of the stellar disk, D25r, followed by the total luminosity (both in the r-band), and the maximum rotation speed, while morphological proxies such as color show only a moderately strong correlation with the gaseous content attenuated by observational error. The simplest and most accurate prescription is log(MHI/Msun)= 8.72 + 1.25*log(D25r/kpc). We find a slope of $-8.2 \pm 0.5$ for the relation between optical magnitude and log rotation speed, in good agreement with Tully-Fisher studies, and a log slope of $1.55 \pm 0.06$ for the HI mass-optical galaxy size relation. Given the homogeneity of the measurements and the completeness of our dataset, the latter outcome suggests that the constancy of the average (hybrid) HI surface density advocated by some authors for the spiral population is a crude approximation.

Constraint propagation equations of the 3+1 decomposition of f(R) gravity

Theories of gravity other than general relativity (GR) can explain the observed cosmic acceleration without a cosmological constant. One such class of theories of gravity is f(R). Metric f(R) theories have been proven to be equivalent to Brans-Dicke (BD) scalar-tensor gravity without a kinetic term. Using this equivalence and a 3+1 decomposition of the theory it has been shown that metric f(R) gravity admits a well-posed initial value problem. However, it has not been proven that the 3+1 evolution equations of metric f(R) gravity preserve the (hamiltonian and momentum) constraints. In this paper we show that this is indeed the case. In addition, we show that the mathematical form of the constraint propagation equations in BD-equilavent f(R) gravity and in f(R) gravity in both the Jordan and Einstein frames, is exactly the same as in the standard ADM 3+1 decomposition of GR. Finally, we point out that current numerical relativity codes can incorporate the 3+1 evolution equations of metric f(R) gravity by modifying the stress-energy tensor and adding an additional scalar field evolution equation. We hope that this work will serve as a starting point for relativists to develop fully dynamical codes for valid f(R) models.

Effective Field Theory Methods in Gravitational Physics and Tests of Gravity

In this PhD thesis I make use of the effective field theory approach to General Relativity to investigate theories of gravity and to take a different point of view on the physical information that can be extracted from experiments. In the first work I present, I study a scalar-tensor theory of gravity and I address the renormalization of the energy-momentum tensor for point-like and string-like sources. The second and third study I report are set in the context of testing gravity. So far experiments have tested dynamical regimes only up to order (v/c)^5 in the post-Newtonian expansion, i.e. the very first term of the radiative sector in General Relativity. In contrast, by means of gravitational-wave astronomy, one aims at testing General Relativity up to (v/c)^(12)! It is then relevant to envisage testing frameworks which are appropriate to this strong-field/radiative regime. In the last two chapters of this thesis a new such framework is presented. Using the effective field theory approach, General Relativity non-linearities are described by Feynman diagrams in which classical gravitons interact with matter sources and among themselves. Tagging the self-interaction vertices of gravitons with parameters it is possible, for example, to translate the measure of the period decay of Hulse-Taylor pulsar in a constraint on the three-graviton vertex at the 0.1% level; for comparison, LEP constraints on the triple-gauge-boson couplings of weak interactions are accurate at 3%. With future observations of gravitational waves, higher order graviton vertices can in principle be constrained through a Fisher matrix analysis.