JOY CHAVEZ (LUCAS MACRI GRAD STUDENT)
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.
SAMANTHA HOFFMANN (LUCAS MACRI GRAD STUDENT)
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
KOHEI KAMADA (TCC INVITED BY KOMATSU)
Can we detect gravitational waves from Q-ball formation?
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.
JUN KODA (UNIVERSITY OF TEXAS AT AUSTIN)
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.
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
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.
YI MAO (UNIVERSITY OF TEXAS AT AUSTIN)
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.
JOEL MEYERS (UNIVERSITY OF TEXAS AT AUSTIN)
Adiabaticity Bounds on Non-Gaussianity in Multiple Field Inflation
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.
JACOB MOLDENHAUER (MUSTAPHA ISHAK GRAD STUDENT)
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.
ANTHONY E. NWANKWO (MUSTAPHA ISHAK GRAD STUDENT)
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.
ANDREAS PAWLIK (UNIVERSITY OF TEXAS AT AUSTIN)
Keeping the Universe ionised: photoheating and the high-redshift
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.
ANNE PELLERIN (LUCAS MACRI POSTDOC)
The Search for Cepheids and other Variable Stars in M33
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.
TANJA RINDLER-DALLER (PAUL SHAPIRO VISITOR)
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.
PEARL SANDICK (UNIVERSITY OF TEXAS AT AUSTIN)
Can IceCube Observations of Dwarf Galaxies Bound the
Dark Matter Annihilation Cross Section?
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.
JUN ZHANG (UNIVERSITY OF TEXAS AT AUSTIN)
A Robust Cosmic Shear Measurement Method
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.