texas cosmology network meeting  






Cepheid Variables in the Antennae

Joy M. Chavez, Lucas M. Macri, Adam G. Riess, and Anne Pellerin

An accurate measurement of the Hubble constant provides important constraints on the equation of state of dark energy. Currently, the most robust determination of H$ is based on Cepheid distances to nearby type Ia supernovae. The occurrence of SN 2007sr in the Antennae (NGC 4038/39) provides an important additional calibrator for this method since this galaxy pair is within a distance that Cepheid variables can be resolved with HST.

We have recently obtained a set of 12 epochs of WFPC2 images which we have analyzed using a previously obtained set of ACS data as reference. An earlier WFPC2 image was also included in our study. We present preliminary periods and magnitudes of Cepheids in this galaxy pair.




Cepheid and Long-period Variables in NGC 4258

Samantha Hoffmann, Lucas Macri (Texas A&M University)

We present results of a survey for Cepheids and long-period variables in NGC 4258. This galaxy plays a key role in the Extragalactic Distance Scale due to its very precise and accurate maser-based distance. Our observations were obtained at the Gemini North Observatory in the gri bands over 22 epochs spanning 4 years.

We have discovered long-period Cepheids which we have used to extend the P-L relation in this galaxy beyond its previous limit of P<45 days. This will enable a more accurate calibration of the P-L relation in this important galaxy. Additionally, we have identified long-period variables and present their properties.

We acknowledge support by NASA through the following grants: HST-GO-09810, -10399, & -10802




Can we detect gravitational waves from Q-ball formation?

Kohei Kamada

Q-ball formation associated with the Affleck-Dine baryo/leptogenesis provides several interesting cosmological consequences. One of them is generation of gravitational waves. When the charge of a Q-ball is large, the amplitude of the gravitational waves is also large. However, decay rate of such Q-balls is rather small and they tend to dominate the energy density of the universe. Because the gravitational wavess are diluted and their frequency is redshifted during Q-ball dominated era, their detectability is determined by the balance of above two effects. We find that there is a finite but very small parameter region where such GWs can be detected by future detectors such as DECIGO or BBO when the thermal logarithmic potential dominates the potential for the Affleck-Dine field and this case is the only possibility of detection. Moreover the suitable parameters for detection can hardly explain the present baryon asymmetry.




New Constraints on the self-interacting cold dark matter (SIDM) model from a comparison of galaxy and cluster observations with an improved theory of SIDM halos.

Jun Koda

We study the effect of Self-Interacting Dark Matter (SIDM) hypothesis on the density profiles of halos. Collisionless CDM predicts cuspy density profiles toward the center, while observations of low mass galaxies prefer cored profiles. SIDM has been proposed as a possible solution to this cuspy core problem on low-mass scales. On the other hand, observations and collisionless CDM agree on mass scales of galaxy clusters. It is also known that the SIDM hypothesis contradicts with X-ray and gravitational lensing observations of cluster of galaxies, if the cross section were too large. Our final goal is to find the range of SIDM scattering cross section models that are consistent with those astrophysical observations in two different mass scales.

There are two theoretical approaches to compute the effect of self-interacting scattering -- Gravitational N-body simulation with Monte Carlo scattering and the conducting fluid model; those two approaches, however, had not been confirmed to agree with each other. We first show that two methods are in reasonable agreement with each other for halos with realistic mass assembly history in an expanding LCDM universe; the value of cross section necessary to have a maximally relaxed low-density core in LCDM is in mutual agreement.

We then develop a semianalytic model that predicts the time evolution of SIDM halo. Our semianalytic relaxation model enables us to understand how a SIDM halo would relax to a cored profile, and obtain an ensemble of SIDM halos from simulations without scatterings with reasonable computational resources. We apply the semianalytic relaxation model to CDM halos, and compare the resulting statistical distribution of SIDM halos with astrophysical observations. Our results improve the constraints on SIDM cross section from observations of relaxed galaxy clusters.

We show that there exists a range of scattering cross section that simultaneously solve the cuspy core problem on low-mass scales and satisfy the galaxy cluster observations.




The Impact of Peculiar Velocity on 21cm Cosmology from the Epoch of Reionization

Yi Mao, Paul R. Shapiro, Ilian T. Iliev, Garrelt Mellema, Jun Koda, and Ue-Li Pen

Neutral hydrogen atoms in the intergalactic medium at high redshift contribute a diffuse background of redshifted 21cm radiation which encodes information about the physical conditions in the early universe at z>6 during and before the epoch of reionization (EOR). Tomography of this 21cm background has emerged as a promising cosmological probe. The assumption that cosmological information in the 21cm signal can be separated from astrophysical information (i.e. that fluctuations in the total matter density can be measured separately from the dependence on patchy reionization and spin temperature) is based on linear perturbation theory and the anisotropy introduced by peculiar velocity. While it is true that fluctuations in the matter density at such high redshift are likely to be of linear amplitude on the large scales which correspond to the beam- and bandwidths of upcoming experiments, the nonlinearity of smaller scale structure in both density and velocity can leave its imprint on the signal, which might then spoil the linear separation scheme. We shall address this issue here by using large-scale reionization simulations which combine a large-box, high-resolution N-body simulation of the LCDM universe (with 29 billion particles in a comoving box 114/h Mpc on a side in present units) with a radiative transfer simulation of reionization, to predict the 21cm brightness temperature fluctuations. We will compare these results with the expectations from linear theory, to test the validity of the latter.




Adiabaticity Bounds on Non-Gaussianity in Multiple Field Inflation

Joel Meyers

A class of models has been shown to generate primordial non-gaussianities due to superhorizon evolution of the curvature perturbation in the presence of non-adiabatic fluctuations during multiple field inflation. We argue on general grounds that the observed adiabaticity of the power spectrum places limits on the non-adiabatic contribution to the observable non-gaussianities in these models.




A minimal set of invariants as a systematic approach to higher order gravity models: physical and cosmological constraints

Jacob Moldenhauer, Mustapha Ishak-Boushaki, Damien Easson, John Thompson

We present some new results about a systematic approach to higher-order gravity (HOG) models. These models have recently attracted a lot of attention with over 300 papers in the last 3 years focusing mostly on the so-called f(R) models. The models are derived from curvature invariants that are more general than the Einstein-Hilbert action. Some of the models exhibit late-time self-acceleration without the need for dark energy and fit some current observations. The open question is that there are an infinite number of invariants that one could select, and many published papers have stressed the need to find a systematic approach that will allow one to study methodically the various possibilities. We present here some new results on a novel systematic approach to these models. We explore a new connection that we made between theorems from the theory of invariants in general relativity and these cosmological models. In summary, the theorems demonstrate that curvature invariants are not all independent from each other and that for a given Ricci Segre type and Petrov type (symmetry classification) of the space-time, there exists a complete minimal set of independent invariants (a basis) in terms of which all the other invariants can be expressed. As an immediate consequence of the proposed approach, the number of invariants to consider is dramatically reduced from infinity to four invariants in the worst case and to only two invariants in the cases of interest, including all Friedmann-Lemaitre-Robertson-Walker (FLRW) metrics. We derive models that pass stability and physical acceptability conditions. We present dynamical equations and phase portrait analyses that show the promise of the systematic approach. We consider observational constraints from magnitude-redshift Supernovae Type Ia data, distance to the last scattering surface of the Cosmic Microwave Background (CMB) radiation, and Baryon Acoustic Observations (BAO). We put observational constraints on general (HOG) models AND also different forms of the Gauss-Bonnet, f(G), modified gravity models. We find models that pass physical and observational constraints and give fits to the data that are as good as those of the standard Lambda-Cold-Dark-Matter model. Those models have also been shown in previous work to pass solar system tests. Finding accelerating higher order gravity models with late time acceleration that pass physical acceptability conditions, solar system tests, and cosmological constraints constitute serious contenders to explain cosmic acceleration. Our next step is to compare these models to growth of large scale structure and full CMB analysis that require the development of perturbations for these models.




Fitting Observations to Inhomogeneous Cosmological Models and the Question of Apparent Acceleration

Anthony E. Nwankwo

The Szekeres inhomogeneous models are used in order to fit supernova combined data sets. We show that with a choice of the spatial curvature function that is guided by current observations, the models fit supernova data almost as well as the LCDM model without requiring a dark energy component. As an exact solution to Einstein's equations, the Szekeres models are regarded as good candidates to model the true lumpy universe that we observe. The null geodesics in these models are not radial and require analytical and numerical integrations. The best fit model found is also consistent with the requirement of spatial flatness at CMB scales. This raises the question of apparent acceleration due to living in one of the under-dense regions of the universe. This is also linked to the open problem of averaging in cosmology.

In a more general context, how do inhomogeneities and non-linearities affect precision cosmology? The first results presented seem to encourage further investigations using inhomogeneous models versus constraints from full CMB analysis and large scale structure.




Keeping the Universe ionised: photoheating and the high-redshift clumping factor

Andreas Pawlik

The critical star formation rate density required to keep the intergalactic hydrogen ionised depends crucially on the average rate of recombinations in the intergalactic medium (IGM). This rate is proportional to the clumping factor C = / avg(rho_b)^2, where rho_b and avg(rho_b) are the local and cosmic mean baryon density, respectively, and the brackets < > indicate spatial averaging over the recombining gas in the IGM.

We perform a suite of cosmological smoothed particle hydrodynamics simulations that include radiative cooling to calculate the volume-weighted clumping factor of the IGM at redshifts z >= 6. We focus on the effect of photo-ionisation heating by a uniform ultra-violet background and find that photo-heating strongly reduces the clumping factor because the increased pressure support smoothes out small-scale density fluctuations. Because the reduction of the clumping factor makes it easier to keep the IGM ionised, photo-heating provides a positive feedback on reionisation.

We demonstrate that this positive feedback is in fact very strong: even our most conservative estimate for the clumping factor (C ≈ 6) is five times smaller than the clumping factor that is usually employed to determine the capacity of star-forming galaxies to keep the z = 6 IGM ionised. Our results imply that the observed population of star-forming galaxies at z ≈ 6 may be sufficient to keep the IGM ionised, provided that the IGM was reheated at z & 9 and that the fraction of ionising photons that escape the star-forming regions to ionise the IGM is larger than ≈ 0.2.




The Search for Cepheids and other Variable Stars in M33

Anne Pellerin

We are conducting a long-term photometric survey of the nearby galaxy M33 to discover Cepheids, eclipsing binaries, and long-period variables. The main goal of the project is to provide an absolute calibration of the Cepheid Period-Luminosity relation and study its metallicity dependence.

This work combines previously-obtained data from the DIRECT project with new observations acquired at the WIYN 3.5-m telescope. The entire data set spans over 9 years with excellent synoptic coverage which will enable the discovery and characterization of stars displaying variability over a wide range of timescales (days, weeks, months, years).

We present representative light curves of different variables, color-magnitude diagrams, and optical Cepheid Period-Luminosity relations for M33.




Vortices (and the Angular Momentum Problem) in Bose-Einstein-Condensed Cold Dark Matter Halos

Tanja Rindler-Daller and Paul R. Shapiro

Several suggestions have appeared in the literature that cold dark matter (CDM) may be in the form of a Bose-Einstein condensate (BEC), including axionic and other forms of CDM. This has important implications for the physics of structure formation, notably at small scales where one expects significant deviations from the more standard CDM, due to the superfluidity exhibited by BECs. However, even on the scales of individual galactic halos, the issue of acquiring angular momentum during galaxy formation is affected. Laboratory BECs are known to develop vortices when rotated with sufficient angular velocity. In cosmology, simulations of structure formation in the CDM model show that halos acquire angular momentum as they form, consistent with that expected from gravitational tidal torquing by the surrounding large-scale structure. Vortices could, in principle, then result if the CDM is a BEC. We address this point by calculating the critical angular velocity for vortex creation in some simple models of BEC/CDM galactic halos and comparing the results with the angular velocity expected from cosmological N-body simulations of CDM. We start from the Gross-Pitaevskii equation of motion for the BEC wave function, coupled self-consistently to the Poisson equation, to describe self-gravitating BEC halos of ellipsoidal shape with varying degrees of rotational support. The implications of these results for cosmological models of CDM involving BECs will be discussed.




Can IceCube Observations of Dwarf Galaxies Bound the Dark Matter Annihilation Cross Section?

Pearl Sandick

We investigate whether the IceCube neutrino telescope will be competitive with Atmospheric Cerenkov Telescopes (ACTs), such as HESS and VERITAS, in bounding the dark matter annihilation cross section. We examine the potential of IceCube to detect neutrinos from dark matter annihilations in the nearby Dwarf Spheroidal Galaxy Willman I and compare with current limits from ACTs. We find that IceCube will have competitive or superior sensitivity with an exposure time of 10 years for heavy ($m_\chi \gtrsim$ few GeV) leptophilic WIMPs. If annihilations proceed directly to neutrinos with some substantial branching fraction, IceCube may have better sensitivity to the annihilation cross section than ACTs for all WIMP masses.




A Robust Cosmic Shear Measurement Method

Jun Zhang

We propose to measure the weak cosmic shear using the spatial derivatives of the galaxy surface brightness field. The measurement is carried out in Fourier space, in which the point spread function (PSF) can be transformed to a desired form with multiplications, and the spatial derivatives can be easily measured. This method is mathematically well defined regardless of the galaxy morphology and the form of the PSF, and involves simple procedures of image processing. Furthermore, with high resolution galaxy images, this approach allows one to probe the shape distortions of galaxy substructures, which can potentially provide much more independent shear measurements than the ellipticities of the whole galaxy. The photon noise and the pixelation effect can also be reliably treated in this method. We demonstrate the efficiency of this method using computer-generated mock galaxy images.

26 October 2009
Astronomy Program · The University of Texas at Austin · Austin, Texas 78712
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