arXiv: 7 March 2011

Eternal inflation and a thermodynamic treatment of Einstein's equations

Jose Tomas Galvez Ghersi, Ghazal Geshnizjani, Federico Piazza, Sarah Shandera

http://arxiv.org/abs/1103.0783v1

In pursuing the intriguing resemblance of the Einstein equations to thermodynamic equations, most sharply seen in systems possessing horizons, we suggest that eternal inflation of the stochastic type may be a fruitful phenomenon to explore. We develop a thermodynamic first law for quasi-de Sitter space, valid on the horizon of a single observer's Hubble patch and explore consistancy with previous proposals for horizons of various types in dynamic and static situations. We use this framework to demonstrate that for the local observer fluctuations of the type necessary for stochastic eternal inflation fall within the regime where the thermodynamic approach is believed to apply. This scenario is interesting because of suggestive parallels to black hole evaporation.

arXiv: 4 March 2011

Warp or lag? The ionized and neutral hydrogen gas in the edge-on dwarf galaxy UGC 1281

P. Kamphuis, R.F. Peletier, P.C. van der Kruit, G.H. Heald

http://arxiv.org/abs/1103.0699v1

The properties of gas in the halos of galaxies constrain global models of the interstellar medium. Kinematical information is of particular interest since it is a clue to the origin of the gas. Until now mostly massive galaxies have been investigated for their halo properties. Here we report on deep HI and H{\alpha} observations of the edge-on dwarf galaxy UGC 1281 in order to determine the existence of extra-planar gas and the kinematics of this galaxy. This is the first time a dwarf galaxy is investigated for its gaseous halo characteristics. We have obtained H{\alpha} integral field spectroscopy using PPAK at Calar Alto and deep HI observations with the WSRT of this edge-on dwarf galaxy. These observations are compared to 3D models in order to determine the distribution of HI in the galaxy. We find that UGC 1281 has H{\alpha} emission up to 25"(655 pc) in projection above the plane and in general a low H{\alpha} flux. Compared to other dwarf galaxies UGC 1281 is a normal dwarf galaxy with a slowly rising rotation curve that flattens off at 60 km/s and a central depression in its HI distribution. Its HI extends 70" (1.8 kpc) in projection from the plane. This gas can be explained by either a warp partially in the line-of-sight warp or a purely edge-on warp with rotational velocities that decline with a vertical gradient of 10.6 \pm 3.7 km/s/kpc. The line-of-sight warp model is the preferred model as it is conceptually simpler. In either model the warp starts well within the optical radius.

Coupled Quintessence and the Halo Mass Function

Ewan R. M. Tarrant, Carsten van de Bruck, Edmund J. Copeland, Anne M. Green

http://arxiv.org/abs/1103.0694v1

A sufficiently light scalar field slowly evolving in a potential can account for the dark energy that presently dominates the universe. This quintessence field is expected to couple directly to matter components, unless some symmetry of a more fundamental theory protects or suppresses it. Such a coupling would leave distinctive signatures in the background expansion history of the universe and on cosmic structure formation, particularly at galaxy cluster scales. Using semi--analytic expressions for the CDM halo mass function, we make predictions for halo abundance in models where the quintessence scalar field is coupled to cold dark matter, for a variety of quintessence potentials. We evaluate the linearly extrapolated density contrast at the redshift of collapse using the spherical collapse model and we compare this result to the corresponding prediction obtained from the non--linear perturbation equations in the Newtonian limit. For all the models considered in this work, if there is a continuous flow of energy from the quintessence scalar field to the CDM component, then the predicted number of CDM haloes can only lie below that of $\Lambda$CDM, when each model shares the same cosmological parameters today. In the last stage of our analysis we perform a global MCMC fit to data to find the best fit values for the cosmological model parameters. We find that for some forms of the quintessence potential, coupled dark energy models can offer a viable alternative to $\Lambda$CDM in light of the recent detections of massive high--$z$ galaxy clusters, while other models of coupled quintessence predict a smaller number of massive clusters at high redshift compared to $\Lambda$CDM.

arXiv: 3 March 2011

Exact non-linear inhomogeneities in $\Lambda$CDM cosmology

Nikolai Meures, Marco Bruni (ICG, Portsmouth)

http://arxiv.org/abs/1103.0501v1

At a time when galaxy surveys and other observations are reaching unprecedented sky coverage and precision it seems timely to investigate the effects of general relativistic non-linear dynamics on the growth of structures and on observations. Analytic inhomogeneous cosmological models are an indispensable way of investigating and understanding these effects in a simplified context. 
In this paper we develop an exact inhomogeneous solution of general relativity with pressureless matter (dust, describing Cold Dark Matter) and cosmological constant $\Lambda$, which can be used to model an arbitrary matter distribution along one line of sight. In particular, we consider the second class Szekeres model with $\Lambda$ and split the dynamics of the model into a flat $\Lambda$CDM background and exact non-linear deviations, obtaining several new results. One single metric function $Z$ describes the deviation from the background. We show that $F$, the time dependent part of $Z$, satisfies the familiar linear differential equation for $\delta$, the first-order density perturbation of dust, with the usual growing and decaying modes. In the limit of small deviations, $\delta \approx F$ as expected, and the growth of deviations links up exactly with standard perturbation theory. In particular, we exhibit an exact conserved curvature deviation, necessary for the existence of the growing mode and corresponding to the first-order curvature perturbation. Analyzing the covariant variables associated with the space-time, we derive the associated dynamical system, which we are able to decouple and reduce to two differential equations, one for $\Omega_\Lambda$ representing the background dynamics and one for $\delta$ describing the dynamics of the inhomogeneities.

Non-Gaussian velocity distributions - The effect on virial mass estimates of galaxy groups

Andre L.B. Ribeiro, Paulo A.A. Lopes, Marina Trevisan

http://arxiv.org/abs/1103.0426v1

We present a study of 9 galaxy groups with evidence for non-Gaussianity in their velocity distributions out to 4R200. This sample is taken from 57 groups selected from the 2PIGG catalog of galaxy groups. Statistical analysis indicates that non-Gaussian groups have masses significantly higher than Gaussian groups. We also have found that all non-Gaussian systems seem to be composed of multiple velocity modes. Besides, our results indicate that multimodal groups should be considered as a set of individual units with their own properties. In particular, we have found that the mass distribution of such units are similar to that of Gaussian groups. Our results reinforce the idea of non-Gaussian systems as complex structures in the phase space, likely corresponding to secondary infall aggregations at a stage before virialization. The understanding of these objects is relevant for cosmological studies using groups and clusters through the mass function evolution.

Model Independent Tests of Cosmic Gravity

Eric V. Linder

http://arxiv.org/abs/1103.0282v1

Gravitation governs the expansion and fate of the universe, and the growth of large scale structure within it, but has not been tested in detail on these cosmic scales. The observed acceleration of the expansion may provide signs of gravitational laws beyond general relativity. Since the form of any such extension is not clear, from either theory or data, we adopt a model independent approach to parametrising deviations to the Einstein framework. We explore the phase space dynamics of two key post-GR functions and derive a classification scheme and an absolute criterion on accuracy necessary for distinguishing classes of gravity models. Future surveys will be able to constrain the post-GR functions' amplitudes and forms to the required precision, and hence reveal new aspects of gravitation.

A Method for 21cm Power Spectrum Estimation in the Presence of Foregrounds

Adrian Liu, Max Tegmark

http://arxiv.org/abs/1103.0281v1

21cm tomography promises to be a powerful tool for estimating cosmological parameters, constraining the epoch of reionization, and probing the so-called dark ages. However, realizing this promise will require the extraction of a cosmological power spectrum from beneath overwhelmingly large sources of foreground contamination. In this paper, we develop a unified matrix-based framework for foreground subtraction and power spectrum estimation, which allows us to quantify the errors and biases that arise in the power spectrum as a result of foreground subtraction. We find that existing line-of-sight foreground subtraction proposals can lead to substantial mode-mixing as well as residual noise and foreground biases, whereas our proposed inverse variance foreground subtraction eliminates noise and foreground biases, gives smaller error bars, and produces less correlated measurements of the power spectrum. We also numerically confirm the intuitive belief in the literature that 21cm foreground subtraction is best done using frequency rather than angular informatio

arXiv: 2 March 2011

Early-Type galaxies at z ~ 1.3. III. On the dependence of Formation Epochs and Star Formation Histories on Stellar Mass and Environment

Alessandro Rettura, S. Mei, S.A. Stanford, A. Raichoor, S. Moran, B. Holden, P. Rosati, R. Ellis, F. Nakata,M. Nonino, T. Treu1, J.P. Blakeslee1, R. Demarco, P. Eisenhardt, H.C. Ford, R.A.E. Fosbury, G. Illingworth, M. Huertas-Company, M.J. Jee, T. Kodama, M. Postman, M. Tanaka, R.L. White

http://arxiv.org/abs/1103.0265v1

We study the environmental dependence of stellar population properties at z ~ 1.3. We derive galaxy properties (stellar masses, ages and star formation histories) for samples of massive, red, passive early-type galaxies in two high-redshift clusters, RXJ0849+4452 and RXJ0848+4453 (with redshifts of z = 1.26 and 1.27, respectively), and compare them with those measured for the RDCS1252.9-2927 cluster at z=1.24 and with those measured for a similarly mass-selected sample of field contemporaries drawn from the GOODS-South Field. Robust estimates of the aforementioned parameters have been obtained by comparing a large grid of composite stellar population models with extensive 8-10 band photometric coverage, from the rest-frame far-ultraviolet to the infrared. We find no variations of the overall stellar population properties among the different samples of cluster early-type galaxies. However, when comparing cluster versus field stellar population properties we find that, even if the (star formation weighted) ages are similar and depend only on galaxy mass, the ones in the field do employ longer timescales to assemble their final mass. We find that, approximately 1 Gyr after the onset of star formation, the majority (75%) of cluster galaxies have already assembled most (> 80%) of their final mass, while, by the same time, fewer (35%) field ETGs have. Thus we conclude that while galaxy mass regulates the timing of galaxy formation, the environment regulates the timescale of their star formation histories.

Probing the time dependence of dark energy

E.M. Barboza Jr., J.S. Alcaniz

http://arxiv.org/abs/1103.0257v1

A new method to investigate a possible time-dependence of the dark energy equation of state $w$ is proposed. We apply this methodology to two of the most recent data sets of type Ia supernova (Union2 and SDSS) and the baryon acoustic oscillation peak at $z = 0.35$. For some combinations of these data, we show that there is a clear departure from the standard $\Lambda$CDM model at intermediary redshifts, although a non-evolving dark energy component ($dw/dz = 0$) cannot be ruled out by these data. The approach developed here may be useful to probe a possible evolving dark energy component when applied to upcoming observational data.

Too big to fail? The puzzling darkness of massive Milky Way subhalos

Michael Boylan-Kolchin, James S. Bullock, Manoj Kaplinghat

http://arxiv.org/abs/1103.0007v1

We show that dissipationless LCDM simulations predict that the majority of the most massive subhalos of the Milky Way are too dense to host any of its bright satellites (L_V > 10^5 L_sun). These dark subhalos have circular velocities at infall of 30-70 km/s and infall masses of [0.2-4] x 10^10 M_sun. Unless the Milky Way is a statistical anomaly, this implies that galaxy formation becomes effectively stochastic at these masses. This is in marked contrast to the well-established monotonic relation between galaxy luminosity and halo circular velocity (or halo mass) for more massive halos. We show that at least two (and typically four) of these massive dark subhalos are expected to produce a larger dark matter annihilation flux than Draco. It may be possible to circumvent these conclusions if baryonic feedback in dwarf satellites or different dark matter physics can reduce the central densities of massive subhalos by order unity on a scale of 0.3 - 1 kpc.

The Baryonic Assembly of Dark Matter Halos

C.-A. Faucher-Giguere, D. Keres, C.-P. Ma (UC Berkeley)

http://arxiv.org/abs/1103.0001v1

We use a suite of cosmological hydrodynamic simulations to quantify the accretion rates of baryons into dark matter halos and the resulting baryon mass fractions, as a function of halo mass, redshift, and baryon type (including cold and hot gas). We find that the net baryonic accretion rates through the virial radius are sensitive to galactic outflows and explore a range of outflow parameters to illustrate the effects. We show that the cold gas accretion rate is in general not a simple universal factor of the dark matter accretion rate, and that galactic winds can cause star formation rates to deviate significantly from the external gas accretion rates, both via gas ejection and re-accretion. Furthermore, galactic winds can inject enough energy and momentum in the surrounding medium to slow down accretion altogether, especially in low-mass halos and at low redshift. By resolving the accretion rates versus radius from the halo centers, we show how cold streams penetrate the hot atmospheres of massive halos at z>2, but gradually disappear at lower redshift. The total baryon mass fraction is also strongly suppressed by outflows in low-mass halos, but is nearly universal in the absence of feedback in halos above the UV background suppression scale. The transition halo mass, at which the gas mass in halos is equal for the cold and hot components, is roughly constant at ~10^11.5 Msun and does not depend sensitively on the wind prescription. We provide simple fitting formulae for the cold gas accretion rate into halos in the no-wind case. Finally, we show that cold accretion is broadly consistent with driving the bulk of the highly star-forming galaxies observed at z~2, but that the more intense star formers likely sample the high end of the accretion rate distribution, and may be additionally fueled by a combination of gas recycling, gas re-accretion, hot mode cooling, and mergers.

arXiv: 1 March 2011

The Total Mass of Dark Matter Haloes

Donnino Anderhalden, Juerg Diemand

http://arxiv.org/abs/1102.5736v1

The simple, conventional dark matter halo mass definitions commonly used in cosmological simulations ("virial" mass, FoF mass, $M_{50,100,200,...}$) only capture part of the collapsed material and are therefore inconsistent with the halo mass concept used in analytical treatments of structure formation. Simulations have demonstrated that typical dark matter particle orbits extend out to about 90 per cent of their turnaround radius, which results in apocenter passages outside of the current "virial" radius on the first and also on the second orbit. Here we describe how the formation history of haloes can be used to identify those particles which took part in the halo collapse, but are missed by conventional group-finders because of their remote present location. These particles are added to the part of the halo already identified by FoF. The corrected masses of dark haloes are significantly higher (the median mass increase is 25 per cent) and there is a considerable shift of the halo mass function towards the Press & Schechter form. We conclude that meaningful quantitative comparisons between (semi-)analytic predictions of halo properties (e.g. mass functions, mass accretion rates, merger rates, spatial clustering, etc.) and simulation results will require using the same halo definition in both approaches.

arXiv: 28 February 2011

Modified Gravity Makes Galaxies Brighter

Authors: Anne-Christine Davis, Eugene A. Lim, Jeremy Sakstein, Douglas Shaw
http://arxiv.org/abs/1102.5278v1
We show that modified gravity with screening mechanisms, such as the popular chameleon or symmetron models, alter the equations governing the structure of main sequence stars, resulting in an increase in their luminosity of up to three times that of their unscreened counterparts. We predict that the luminosity of unscreened dwarf galaxies due to this enhancement of their constituent stars can be up to 40%. We discuss how it may then be possible to exploit this effect observationally to increase the current bounds on such theories by an order of magnitude or more. In particular, this increase in luminosity dramatically alters stellar evolution, leading to a much shorter main sequence lifetime. These effects may lead to a plethora of unusual phenomena in unscreened galaxies, all of which are potential probes of new physics.

arXiv: 25 February 2011

Subaru and Gemini High Spatial Resolution Infrared 18 Micron Imaging Observations of Nearby Luminous Infrared Galaxies

Authors: Masatoshi Imanishi (1), Keisuke Imase (2), Nagisa Oi (2), Kohei Ichikawa (3) ((1) Subaru/NAOJ, (2) GUAS/NAOJ/Subaru, (3) Kyoto Univ.)
http://arxiv.org/abs/1102.4854v1
We present the results of a ground-based, high spatial resolution infrared 18 micron imaging study of nearby luminous infrared galaxies (LIRGs), using the Subaru 8.2-m and Gemini South 8.1-m telescopes. The diffraction-limited images routinely achieved with these telescopes in the Q-band (17-23 micron) allow us to investigate the detailed spatial distribution of infrared emission in these LIRGs. We then investigate whether the emission surface brightnesses are modest, as observed in starbursts, or are so high that luminous active galactic nuclei (AGNs; high emission surface brightness energy sources) are indicated. The sample consists of 18 luminous buried AGN candidates and starburst-classified LIRGs identified in earlier infrared spectroscopy. We find that the infrared 18 micron emission from the buried AGN candidates is generally compact, and the estimated emission surface brightnesses are high, sometimes exceeding the maximum value observed in and theoretically predicted for a starburst phenomenon. The starburst-classified LIRGs usually display spatially extended 18 micron emission and the estimated emission surface brightnesses are modest, within the range sustained by a starburst phenomenon. The general agreement between infrared spectroscopic and imaging energy diagnostic methods suggests that both are useful tools for understanding the hidden energy sources of the dusty LIRG population.

Characterizing the atmospheres of transiting rocky planets around late type dwarfs

Authors: E. Pallé (IAC), M. R. Zapatero Osorio (CAB, CSIC-INTA), A. García Muñoz (IAC)
http://arxiv.org/abs/1102.4989v1
Visible and near-infrared spectra of transiting hot Jupiter planets have recently been observed, revealing some of the atmospheric constituents of their atmospheres. In the near future, it is probable that primary and secondary eclipse observations of Earth-like rocky planets will also be achieved. The characterization of the Earth's transmission spectrum has shown that both major and trace atmospheric constituents may present strong absorption features, including important bio-markers such as water, oxygen and methane. Our simulations using a recently published empirical Earth's transmission spectrum, and the stellar spectra for a variety of stellar types, indicate that the new generation of extremely large telescopes, such as the proposed 42-meter European Extremely Large Telescope(E-ELT), could be capable of retrieving the transmission spectrum of an Earth-like planet around very cool stars and brown dwarfs (Teff < 3100 K). For a twin of Earth around a star with Teff around 3100 K (M4), for example, the spectral features of water vapor, methane, carbon dioxide, and oxygen in the wavelength range between 0.9 and 2.4 micron can simultaneously be detected within a hundred hours of observing time, or even less for a late-M star. Such detection would constitute a proof for the existence of life in that planet. The detection time can be reduced to a few hours for a super-Earth type of planet with twice the Earth's radius

arXiv: 24 February 2011

Spherical Collapse in f(R) Gravity

Authors: Alex Borisov, Bhuvnesh Jain, Pengjie Zhang
http://arxiv.org/abs/1102.4839v1
We use 1-dimensional numerical simulations to study spherical collapse in the f(R) gravity models. We include the nonlinear coupling of the gravitational potential to the scalar field in the theory and use a relaxation scheme to follow the collapse. We find an unusual enhancement in density near the virial radius which may provide observable tests of gravity. We also use the estimated collapse time to calculate the critical overdensity $\delta_c$ used in calculating the mass function and bias of halos. We find that analytical approximations previously used in the literature do not capture the complexity of nonlinear spherical collapse.

Nonlinear Perturbation Theory Integrated with Nonlocal Bias, Redshift-space Distortions, and Primordial Non-Gaussianity

Authors: Takahiko Matsubara
http://arxiv.org/abs/1102.4619v1
The standard nonlinear perturbation theory of the gravitational instability is extended to incorporate the nonlocal bias, redshift-space distortions, and primordial non-Gaussianity. We show that local biasing schemes in Eulerian and Lagrangian spaces are not compatible to each other in nonlinear regime. It is only when biases are allowed to be nonlocal that the Eulerian and Lagrangian biases are perturbatively related order by order in general. The relation between Eulerian and Lagrangian kernels of density perturbations with biasing are derived. The effects of primordial non-Gaussianity and redshift-space distortions are also incorporated in our formalism, and diagrammatic methods are introduced. Vertex resummations of higher-order perturbations in the presence of bias are considered.

arXiv: 23 February 2011

A model independent measure of the large scale curvature of the Universe

Authors: Edvard Mortsell, Jakob Jonsson

http://arxiv.org/abs/1102.4485v1

Cosmological distances as a function of redshift depend on the effective curvature density via the effect on the geometrical path of photons from large scale spatial curvature and its effect on the expansion history, H(z). Cosmological time, however, depends on the expansion history only. Therefore, by combining distance and lookback time observations (or other estimates of the expansion history), it is possible to isolate the geometrical curvature contribution and measure the curvature in a model independent way, i.e., free from assumptions about the energy content of the universe.
We investigate two different approaches to accomplish this task; the differential and the integral approach. The differential approach requires, in addition to distances, derivatives of distance with respect to redshift as well as knowledge of the expansion history. The integral approach is based on measuring the integral of the inverse of the expansion history via measurements of cosmic time as derived, e.g., from galaxy ages.
In this paper, we attempt to constrain the large scale curvature of the Universe using distances obtained from observations of Type Ia supernovae together with inferred ages of passively evolving galaxies and Hubble parameter estimates from the large scale clustering of galaxies. Current data are consistent with zero spatial curvature, although the uncertainty on the curvature density is of order unity. Future data sets with on the order of thousands of Type Ia supernovae distances and galaxy ages will allow us to constrain the curvature density with an uncertainty of less than 0.1 at the 95% confidence level.

arXiv: 22 February 2011

A Novel Test of the Modified Newtonian Dynamics with Gas Rich Galaxies

Authors: Stacy S. McGaugh
http://arxiv.org/abs/1102.3913v1
The current cosmological paradigm, LCDM, requires that the mass-energy of the universe be dominated by invisible components: dark matter and dark energy. An alternative to these dark components is that the law of gravity be modified on the relevant scales. A test of these ideas is provided by the Baryonic Tully-Fisher Relation (BTFR), an empirical relation between the observed mass of a galaxy and its rotation velocity. Here I report a test using gas rich galaxies for which both axes of the BTFR can be measured independently of the theories being tested and without the systematic uncertainty in stellar mass that affects the same test with star dominated spirals. The data fall precisely where predicted a priori by the modified Newtonian dynamics (MOND). The scatter in the BTFR is attributable entirely to observational uncertainty. This is consistent with the action of a single effective force law but poses a serious fine-tuning problem for LCDM.

arXiv: 21 February 2011

Nonlinear Structure Formation with the Environmentally Dependent Dilaton

Authors: Phil Brax, Carsten van de Bruck, Anne-C. Davis, Baojiu Li, Douglas J. Shaw
http://arxiv.org/abs/1102.3692v1
We have studied the nonlinear structure formation of the environmentally dependent dilaton model using $N$-body simulations. We find that the mechanism of suppressing the scalar fifth force in high-density regions works very well. Within the parameter space allowed by the solar system tests, the dilaton model predicts small deviations of the matter power spectrum and the mass function from their $\Lambda$CDM counterparts. The importance of taking full account of the nonlinearity of the model is also emphasized.

Star formation efficiency as a function of metallicity: from star clusters to galaxies

Authors: Sami Dib (1), Laurent Piau (2), Subhanjoy Mohanty (1), Jonathan Braine (3) ((1) Imperial College London (2) Latmos (3) Bordeaux)
http://arxiv.org/abs/1102.3839v1
We explore how the star formation efficiency in a protocluster clump is regulated by metallicity dependent stellar winds from the newly formed OB stars (Mstar > 5 Msol). The model describes the co-evolution of the mass function of gravitationally bound cores and of the IMF in a protocluster clump. Dense cores are generated uniformly in time at different locations in the clump, and contract over lifetimes that are a few times their free fall times. The cores collapse to form stars that power strong stellar winds whose cumulative kinetic energy evacuates the gas from the clump and quenches further core and star formation. This sets the final star formation efficiency, SFEf. Models are run with various metallicities in the range Z/Zsol=[0.1,2]. We find that the SFEf decreases strongly with increasing metallicity. The SFEf-metallicity relation is well described by a decaying exponential whose exact parameters depend weakly on the value of the core formation efficiency. We find that there is almost no dependence of the SFEf-metallicity relation on the clump mass. This is due to the fact that an increase (decrease) in the clump mass leads to an increase (decrease) in the feedback from OB stars which is opposed by an increase (decrease) in the gravitational potential of the clump.The clump mass-cluster relation relations we find for all of the different metallicity cases imply a negligible difference between the exponent of the mass function of the protocluster clumps and that of the young clusters mass function. By normalizing the SFEf to their value for the solar metallicity case, we compare our results to SFE-metallicity relations derived on galactic scales and find a good agreement. As a by-product of this study, we also provide ready-to-use prescriptions for the power of stellar winds of main sequence OB stars in the mass range [5,80] Msol in the metallicity range we have considered.

arXiv: 18 February 2011

Precise cosmological parameter estimation using CosmoRec

Authors: J. R. Shaw, J. Chluba
http://arxiv.org/abs/1102.3683v1
We use the new cosmological recombination code, CosmoRec, for parameter estimation in the context of (future) precise measurements of the CMB temperature and polarization anisotropies. We address the question of how previously neglected physical processes in the recombination model of Recfast affect the determination of key cosmological parameters, for the first time performing a model-by-model computation of the recombination problem. In particular we ask how the biases depend on different combinations of parameters, e.g. when varying the helium abundance or the effective number of neutrino species in addition to the standard six parameters. We also forecast how important the recombination corrections are for a combined Planck, ACTPol and SPTpol data analysis. Furthermore, we ask which recombination corrections are really crucial for CMB parameter estimation, and whether an approach based on a redshift-dependent correction function to Recfast is sufficient in this context.

Dark Subhaloes and Disturbances in Extended HI Discs

Authors: Philip Chang, Sukanya Chakrabarti
http://arxiv.org/abs/1102.3436v1
We develop a perturbative approach to study the excitation of disturbances in the extended atomic hydrogen (HI) discs of galaxies produced by passing dark matter subhaloes. The shallow gravitational potential of the dark matter subhaloes (compared to the primary halo) allows us to use the epicyclic approximation, the equations of which we solve by modal analysis, i.e., assuming a disc is composed of N radial rings with M modes. We show that properties of dark matter subhaloes can be inferred from the profile and amplitude of the modal energy of the disc. Namely, we find that the overall amplitude of the response gives the mass of the dark sub-halo. Motivated by this modal analysis, we then show that the density response shows similar features. Finally, we show that our results agree with those from full hydrodynamic simulation. We find a simple scaling relation between the satellite mass and Fourier amplitudes of the resultant surface density of the gas disc where the effective Fourier amplitude (essentially a sum over the low order modes) scales as $m_{s}^{1/2}$, where $m_{s}$ is the satellite mass. The utility of this relation is that it can be readily applied to an observed HI map to deduce the satellite mass without recourse to full numerical simulation. This will greatly aid us in analyzing large samples of spiral galaxies to constrain the population of dwarf satellites in the Local Volume.

Time-Resolved Properties of the White Light Continuum During Stellar Flares/ Seminar Series (4)

Speaker:Adam Kowalski
Affiliation:
Astronomy Department, University of Washington
Title: Time-Resolved Properties of the White Light Continuum During Stellar Flares
Date: 17 February  2011
Place: ING+NOT Astro-seminar- Santa Cruz-La Palma- Spain

Abstract:
The primary mode of radiative energy release in stellar flares is in the optical and near-ultraviolet (NUV) continuum. This white light radiation carries a large fraction of the total radiated flare energy. However, radiative hydrodynamic models of stellar flares using a solar flare paradigm and the sparse observations of solar and stellar flare continua are all seemingly in disagreement over the type(s) of emission that contribute to the optical/NUV continuum during flares. We have begun a long-term spectroscopic flare monitoring campaign to fully characterize the  optical/NUV white light continuum emission on short timescales for large and  small flares. To date, our most significant results come from spectroscopic (3350A - 9260A) and photometric (U band) observations during  1.3 hours of the decay phase of a megaflare on the dM4.5e star YZ CMi, where  we have detected multiple continuum components that contribute to the white  light. I will present the continuum and emission line properties of this  flare and initial phenomenological modeling of the flaring atmosphere. I will  also compare the continuum properties to ultra-high speed observations of 
smaller flares

arXiv: 17 February 2011

Improved constraints on cosmological parameters from SNIa data

M.C. March (Imperial), R. Trotta (Imperial), P. Berkes (Volen Center for Complex Systems), G.D. Starkman(CWRU), P.M. Vaudrevange (CWRU/DESY)

http://arxiv.org/abs/1102.3237v1

We present a new method based on a Bayesian hierarchical model to extract constraints on cosmological parameters from SNIa data obtained with the SALT-II lightcurve fitter. We demonstrate with simulated data sets that our method delivers considerably tighter statistical constraints on the cosmological parameters and that it outperforms the usual chi-square approach 2/3 of the times. As a further benefit, a full posterior probability distribution for the dispersion of the intrinsic magnitude of SNe is obtained. We apply this method to recent SNIa data and find that it improves statistical constraints on cosmological parameters from SNIa data alone by about 40% w.r.t. the standard approach. From the combination of SNIa, CMB and BAO data we obtain Omega_m=0.29 +/- 0.01, Omega_Lambda=0.72 +/- 0.01 (assuming w=-1) and Omega_m =0.28 +/- 0.01, w=-0.90 +/- 0.04 (assuming flatness; statistical uncertainties only). We constrain the intrinsic dispersion of the B-band magnitude of the SNIa population, obtaining sigma_int = 0.13 +/- 0.01 [mag]. Applications to systematic uncertainties will be discussed in a forthcoming paper.

N-body simulations with generic non-Gaussian initial conditions II: Halo bias

Christian Wagner, Licia Verde

http://arxiv.org/abs/1102.3229v1

We present N-body simulations for generic non-Gaussian initial conditions with the aim of exploring and modelling the scale-dependent halo bias. This effect is evident at very large scales requiring large simulation boxes. In addition, the previously available prescription to implement generic non-Gaussian initial conditions has been improved to keep under control higher-order terms which were spoiling the power spectrum on large scales. We pay particular attention to the differences between physical, inflation-motivated primordial bispectra and their factorizable templates, and to the operational definition of the non-Gaussian halo bias (which has both a scale-dependent and an approximately scale-independent contributions). We find that analytic predictions for both the non-Gaussian halo mass function and halo bias work well once a calibration factor (which was introduced before) is calibrated on simulations. The halo bias remains therefore an extremely promising tool to probe primordial non-Gaussianity and thus to give insights into the physical mechanism that generated the primordial perturbations. The simulation outputs and tables of the analytic predictions will be made publicly available via the non-Gaussian comparison project web site this http URL

The other side of Bulge Formation in a Lambda-CDM cosmology: Bulgeless Galaxies in the Local Universe

Fabio Fontanot (1), Gabriella De Lucia (1), David Wilman (2), Pierluigi Monaco (3,1) ((1) INAF-Osservatorio Astronomico di Trieste, (2) Max-Planck-Institute fuer Extraterrestrische Physik, Garching bei Muenchen (3) Dipartimento di Fisica, sez. Astronomia, Universita' di Trieste)

http://arxiv.org/abs/1102.3188v1

We study the physical properties, formation histories, and environment of galaxies without a significant ``classical'' spheroidal component, as predicted by semi-analytical models of galaxy formation and evolution. This work is complementary to the analysis presented in De Lucia et al., (2011), where we focus on the relative contribution of various physical mechanisms responsible for bulge assembly in a Lambda-CDM cosmology. We find that the fraction of bulgeless galaxies is a strong decreasing function of stellar mass: they represent a negligible fraction of the galaxy population with M* > 10^12 Msun, but dominate at M* < 10^10 Msun. We find a clear dichotomy in this galaxy population, between central galaxies of low-mass dark matter haloes, and satellite galaxies in massive groups/clusters. We show that bulgeless galaxies are relatively young systems, that assemble most of their mass at low-redshift, but they can also host very old stellar populations. Since galaxy-galaxy mergers are assumed to lead to the formation of a spheroidal component, in our models these galaxies form preferentially in low-mass haloes that host a small number of satellites galaxies. We show that the adopted modelling for galaxy mergers represents a key ingredient in determining the actual number of bulgeless galaxies. Our results show that these galaxies are not a rare population in theoretical models: at z~0, galaxies with no classical bulge (but often including galaxies with the equivalent of pseudo-bulges) account for up to 14% of the galaxies with 10^11 < M*/Msun < 10^12.

arXiv: 16 February 2011

Observational constraints on scalar field models of dark energy with barotropic equation of state

Olga Sergijenko, Ruth Durrer, Bohdan Novosyadlyj

http://arxiv.org/abs/1102.3168v2

We constrain the parameters of dynamical dark energy in the form of a classical or tachyonic scalar field with barotropic equation of state jointly with other cosmological ones using the combined datasets which include the CMB power spectra from WMAP7, the baryon acoustic oscillations in the space distribution of galaxies from SDSS DR7, the power spectrum of luminous red galaxies from SDSS DR7 and the light curves of SN Ia from 2 different compilations: Union2 (SALT2 light curve fitting) and SDSS (SALT2 and MLCS2k2 light curve fittings). It has been found that the initial value of dark energy equation of state parameter is constrained very weakly by most of the data while the rest of main cosmological parameters are well constrained: their likelihoods and posteriors are similar, have the forms close to Gaussian (or half-Gaussian) and their confidential ranges are narrow. The most reliable determinations of the best fitting value and $1\sigma$ confidence range for the initial value of dark energy equation of state parameter were obtained from the combined datasets including SN Ia data from the full SDSS compilation with MLCS2k2 fitting of light curves. In all such cases the best fitting value of this parameter is lower than the value of corresponding parameter for current epoch. Such dark energy loses its repulsive properties and in future the expansion of the Universe will change into contraction. We also perform an error forecast for the Planck mock data and show that they narrow essentially the confidential ranges of cosmological parameters values, moreover, their combination with SN SDSS compilation with MLCS2k2 light curve fitting may exclude the fields with initial equation of state parameter $>-0.1$ at 2$\sigma$ confidential level.

arXiv: 15 February 2011

Breaking down the link between luminous and dark matter in massive galaxies

Sebastien Foucaud, Christopher J. Conselice

http://arxiv.org/abs/1102.2543v2

We present a study on the clustering of a stellar mass selected sample of galaxies with stellar masses M*>10^10Msol at redshifts 0.4<z<2.0, taken from the Palomar Observatory Wide-field Infrared Survey. We examine the clustering properties of these stellar mass selected samples as a function of redshift and stellar mass, and find that galaxies with high stellar masses have a progressively higher clustering strength than galaxies with lower stellar masses. We also find that galaxies within a fixed stellar mass range have a higher clustering strength at higher redshifts. We further estimate the average total masses of the dark matter haloes hosting these stellar-mass selected galaxies. For all galaxies in our sample the stellar-mass-to-total-mass ratio is always lower than the universal baryonic mass fraction and the stellar-mass-to-total-mass ratio is strongly correlated with the halo masses for central galaxies, such that more massive haloes contain a lower fraction of their mass in the form of stars. The remaining baryonic mass is included partially in stars within satellite galaxies in these haloes, and as diff?use hot and warm gas. We also find that, at a fixed stellar mass, the stellar-to-total-mass ratio increases at lower redshifts. This suggests that galaxies at a fixed stellar mass form later in lower mass dark matter haloes, and earlier in massive haloes. We interpret this as a `halo downsizing' effect.

arXiv: 14 February 2011

Stress-Energy Connection and Cosmological Constant Problem

Durmus A. Demir

http://arxiv.org/abs/1102.2276v1

We study gravitational effects of vacuum energy in a geometry based upon the stress-energy tensor of matter and radiation. By proposing that the stress-energy tensor can be incorporated into the matter-free gravitational field equations also by modifying the connection, we end up with varied geometro-dynamical equations which properly comprise the usual gravitational field equations with a vital novelty that the vacuum energy does act not as the cosmological constant but as the source for the gravitational constant. In addition, the field equations involve non-local, Planck-suppressed, higher-dimension terms in excess of ones in the usual gravitational field equations. The formalism thus deafens the cosmological constant problem by channeling vacuum energy to gravitational constant. Nonetheless, quantum gravitational effects, if any, restore the problem, and mechanism proposed here falls short of taming such contributions.

On detection of the stochastic gravitational-wave background using the Parkes pulsar timing array

D. R. B. Yardley, W. A. Coles, G. B. Hobbs, J. P. W. Verbiest, R. N. Manchester, W. van Straten, F. A. Jenet, M. Bailes, N. D. R. Bhat, S. Burke-Spolaor, D. J. Champion, A. W. Hotan, S. Oslowski, J. E. Reynolds, J. M. Sarkissian

http://arxiv.org/abs/1102.2230v1

We search for the signature of an isotropic stochastic gravitational-wave background in pulsar timing observations using a frequency-domain correlation technique. These observations, which span roughly 12 yr, were obtained with the 64-m Parkes radio telescope augmented by public domain observations from the Arecibo Observatory. A wide range of signal processing issues unique to pulsar timing and not previously presented in the literature are discussed. These include the effects of quadratic removal, irregular sampling, and variable errors which exacerbate the spectral leakage inherent in estimating the steep red spectrum of the gravitational-wave background. These observations are found to be consistent with the null hypothesis, that no gravitational-wave background is present, with 76 percent confidence. We show that the detection statistic is dominated by the contributions of only a few pulsars because of the inhomogeneity of this data set. The issues of detecting the signature of a gravitational-wave background with future observations are discussed.

arXiv: 11 February 2011

On infrared and ultraviolet divergences of cosmological perturbations

Giovanni Marozzi, Massimiliano Rinaldi, Ruth Durrer

http://arxiv.org/abs/1102.2206v1

We study a consistent infrared and ultraviolet regularization scheme for the cosmological perturbations. The infrared divergences are cured by assuming that the Universe undergoes a transition between a non-singular pre-inflationary, radiation-dominated phase and a slow-roll inflationary evolution. The ultraviolet divergences are eliminated via adiabatic subtraction. A consistent regularization of the field fluctuations through this transition is obtained by performing a mode matching for both the gauge invariant Mukhanov variable and its adiabatic expansion. We show that these quantities do not generate ultraviolet divergences other than the standard ones, when evolving through the matching time. We also show how the de Witt-Schwinger expansion, which can be used to construct the counter-terms regularizing the ultraviolet divergences, ceases to be valid well before horizon exit of the scales of interest. Thus, such counter-terms should not be used beyond the time of the horizon exit so it is unlikely that the observed power spectrum is modified by adiabatic subtraction as claimed in the literature. However, the infrared regularization might have an impact on the observed spectrum, and we briefly discuss this possibility.

The Redshift Evolution of LCDM Halo Parameters

J.C. Muñoz-Cuartas (AIP), Andrea V. Macciò (MPIA), Stefan Gottlöber (AIP), Aaron Dutton (UVic)

http://arxiv.org/abs/1102.2186v2

We study the mass and redshift dependence of the concentration parameter in Nbody simulations spanning masses from $10^{10} \hMsun$ to $10^{15} \hMsun$ and redshifts from 0 to 2. We present a series of fitting formulas that accurately describe the time evolution of the concentration-mass relation since z=2. Using arguments based on the spherical collapse model we study the behaviour of the scale length of the density profile during the assembly history of haloes, obtaining physical insights on the origin of the observed time evolution of the concentration mass relation. We present preliminary results of the implementation of this model in the prediction of the values of the concentration parameter for different masses and redshifts.

Gravitational energy as dark energy: Cosmic structure and apparent acceleration

David L. Wiltshire

http://arxiv.org/abs/1102.2045v1

Below scales of about 100/h Mpc our universe displays a complex inhomogeneous structure dominated by voids, with clusters of galaxies in sheets and filaments. The coincidence that cosmic expansion appears to start accelerating at the epoch when such structures form has prompted a number of researchers to question whether dark energy is a signature of a failure of the standard cosmology to properly account, on average, for the distribution of matter we observe. Here I discuss the timescape scenario, in which cosmic acceleration is understood as an apparent effect, due to gravitational energy gradients that grow when spatial curvature gradients become significant with the nonlinear growth of cosmic structure. I discuss conceptual issues related to the averaging problem, and their impact on the calibration of local geometry to the solutions of the volume-average evolution equations corrected by backreaction, and the question of nonbaryonic dark matter in the timescape framework. I further discuss recent work on defining observational tests for average geometric quantities which can distinguish the timescape model from a cosmological constant or other models of dark energy.

Correlated Supernova Systematics and Ground Based Surveys

Alex G. Kim, Eric V. Linder

http://arxiv.org/abs/1102.1992v1

Supernova distances provide a direct probe of cosmic acceleration, constraining dark energy. This leverage increases with survey redshift depth at a rate bounded by the systematic uncertainties. We investigate the impact of a wavelength-dependent, global correlation model of systematics in comparison to the standard local-redshift correlation model. This can arise from subclass uncertainties as features in the supernova spectrum redshift out of the observer photometric filters or spectral range. We explore the impact of such a systematic on ground-based supernova surveys such as Dark Energy Survey and LSST, finding distinctive implications. Extending the wavelength sensitivity to 1.05 microns through "extreme red" CCDs can improve the dark energy figure of merit by up to a factor 2.

arXiv: 10 February 2011

Cosmological Large-scale Structures beyond Linear Theory in Modified Gravity

Francis Bernardeau, Philippe Brax (IPhT Saclay)

http://arxiv.org/abs/1102.1907v1

We consider the effect of modified gravity on the growth of large-scale structures at second order in perturbation theory. We show that modified gravity models changing the linear growth rate of fluctuations are also bound to change, although mildly, the mode coupling amplitude in the density and reduced velocity fields. We present explicit formulae which describe this effect. We then focus on models of modified gravity involving a scalar field coupled to matter, in particular chameleons and dilatons, where it is shown that there exists a transition scale around which the existence of an extra scalar degree of freedom induces significant changes in the coupling properties of the cosmic fields. We obtain the amplitude of this effect for realistic dilaton models at the tree-order level for the bispectrum, finding them to be comparable in amplitude to those obtained in the DGP model.

Gravitational Cerenkov losses in MOND theories

Mordehai Milgrom (DPPA, Weizmann Institute of Science)

http://arxiv.org/abs/1102.1818v1

Survival of high-energy cosmic rays (HECRs) against gravitational Cerenkov losses has been used to constrain theories with subluminal gravitational waves. I maintain that the loss rates in MOND theories can be many orders of magnitude smaller than those derived in the literature for theories with no extra scale, such as Einstein-Aether theories. The gravitational acceleration produced by a HECR in its vicinity is much higher than the MOND acceleration a0. So, modification to general relativity (GR), which underlies the Cerenkov losses, enters only far from the hadron. The Cerenkov spectrum diverges at high wavenumbers, and the total emitted flux depends quadratically on the cutoff wavenumber. I argue that in GR-compatible MOND theories: those that coincide with GR in the high-acceleration limit, the correct cutoff is the inverse of the MOND radius of the particle, within which GR holds sway: r_M=(Gp/ca0)^{1/2}. The rates appearing in the literature, which take the inverse de Broglie wavenumber as cutoff, should be reduced by a factor ~10^{39}(pc/3x10^{11} Gev)^3 to be used for such MOND theories. With the MOND radius as cutoff, the length over which a particle can travel without major losses is qc^2/a0, where q is a dimensionless factor. Since c^2/a0 is somewhat larger than the Hubble distance, survival of EHCRs does not strongly constrain GR-compatible, MOND theories. (Abridged)

Penrose At Work

Renyue Cen (Princeton University Observatory)

http://arxiv.org/abs/1102.1937v1

If a supermassive black hole has some material orbiting around it at close to its innermost stable circular orbit (ISCO), then, when it plunges into a second supermassive black hole, the orbiting material has a velocity dispersion of order of speed of light about the orbital velocity of its host black hole. It becomes plausible that some of the orbiting material will be "catapulted" to the negative-energy ergosphere orbits of the second black hole at the plunge. This may provide an astrophysically plausible way to extract energy from the black hole, originally suggested by Penrose.

Non-Abelian Gauge Field Inflation

A. Maleknejad, M.M. Sheikh-Jabbari

http://arxiv.org/abs/1102.1932v1

In [arXiv:1102.1513] we introduced an inflationary scenario, Non-Abelian Gauge Field Inflation or gauge-flation for short, in which slow-roll inflation is driven by non-Abelian gauge field minimally coupled to gravity. We present a more detailed analysis, both numerical and analytical, of the gauge-flation. By studying the phase diagrams of the theory, we show that getting enough number of e-folds during a slow-roll inflation is fairly robust to the choice of initial gauge field values. In addition, we present a detailed analysis of the cosmic perturbation theory in gauge-flation which has many special and interesting features compared the standard scalar-driven inflationary models. The specific gauge-flation model we study in this paper has two parameters, a cutoff scale Lambda and the gauge coupling g. Fitting our results with the current cosmological data fixes \Lambda\sim 10 H \sim 10^{15} GeV (H is the Hubble parameter) and g\sim 10^{-4}, which are in the natural range of parameters in generic particle physics beyond standard models. Our model also predicts a tensor-to-scalar ratio r>0.05, in the range detectable by the Planck satellite.