479 research outputs found
Constraining star formation rates in cool-core brightest cluster galaxies
We used broad-band imaging data for 10 cool-core brightest cluster galaxies
(BCGs) and conducted a Bayesian analysis using stellar population synthesis to
determine the likely properties of the constituent stellar populations.
Determination of ongoing star formation rates (SFRs), in particular, has a
direct impact on our understanding of the cooling of the intracluster medium
(ICM), star formation and AGN-regulated feedback. Our model consists of an old
stellar population and a series of young stellar components. We calculated
marginalized posterior probability distributions for various model parameters
and obtained 68% plausible intervals from them. The 68% plausible interval on
the SFRs is broad, owing to a wide range of models that are capable of fitting
the data, which also explains the wide dispersion in the star formation rates
available in the literature. The ranges of possible SFRs are robust and
highlight the strength in such a Bayesian analysis. The SFRs are correlated
with the X-ray mass deposition rates (the former are factors of 4 to 50 lower
than the latter), implying a picture where the cooling of the ICM is a
contributing factor to star formation in cool-core BCGs. We find that 9 out of
10 BCGs have been experiencing starbursts since 6 Gyr ago. While four out of 9
BCGs seem to require continuous SFRs, 5 out of 9 seem to require periodic star
formation on intervals ranging from 20 Myr to 200 Myr. This time scale is
similar to the cooling-time of the ICM in the central (< 5 kpc) regions.Comment: 33 pages, 14 Figures, 14 Tables. Accepted for publication in MNRA
Stochastic Gravitational Wave Measurements with Bar Detectors: Dependence of Response on Detector Orientation
The response of a cross-correlation measurement to an isotropic stochastic
gravitational-wave background depends on the observing geometry via the overlap
reduction function. If one of the detectors being correlated is a resonant bar
whose orientation can be changed, the response to stochastic gravitational
waves can be modulated. I derive the general form of this modulation as a
function of azimuth, both in the zero-frequency limit and at arbitrary
frequencies. Comparisons are made between pairs of nearby detectors, such as
LIGO Livingston-ALLEGRO, Virgo-AURIGA, Virgo-NAUTILUS, and EXPLORER-AURIGA,
with which stochastic cross-correlation measurements are currently being
performed, planned, or considered.Comment: 17 pages, REVTeX (uses rcs, amsmath, hyperref, and graphicx style
files), 4 figures (8 eps image files
Model-Based Cross-Correlation Search for Gravitational Waves from Scorpius X-1
We consider the cross-correlation search for periodic GWs and its potential
application to the LMXB Sco X-1. This method coherently combines data from
different detectors at the same time, as well as different times from the same
or different detectors. By adjusting the maximum time offset between a pair of
data segments to be coherently combined, one can tune the method to trade off
sensitivity and computing costs. In particular, the detectable signal amplitude
scales as the inverse fourth root of this coherence time. The improvement in
amplitude sensitivity for a search with a coherence time of 1hr, compared with
a directed stochastic background search with 0.25Hz wide bins is about a factor
of 5.4. We show that a search of 1yr of data from Advanced LIGO and Advanced
Virgo with a coherence time of 1hr would be able to detect GWs from Sco X-1 at
the level predicted by torque balance over a range of signal frequencies from
30-300Hz; if the coherence time could be increased to 10hr, the range would be
20-500Hz. In addition, we consider several technical aspects of the
cross-correlation method: We quantify the effects of spectral leakage and show
that nearly rectangular windows still lead to the most sensitive search. We
produce an explicit parameter-space metric for the cross-correlation search in
general and as applied to a neutron star in a circular binary system. We
consider the effects of using a signal template averaged over unknown amplitude
parameters: the search is sensitive to a combination of the intrinsic signal
amplitude and the inclination of the neutron star rotation axis, and the peak
of the expected detection statistic is systematically offset from the true
signal parameters. Finally, we describe the potential loss of SNR due to
unmodelled effects such as signal phase acceleration within the Fourier
transform timescale and gradual evolution of the spin frequency.Comment: 27 pages, 12 figures, 4 tables, pdflatex; synchronized to final
version published in Phys Rev
The challenging task of determining star formation rates: the case of a massive stellar burst in the brightest cluster galaxy of Phoenix galaxy cluster
Star formation in galaxies at the center of cooling-flow galaxy clusters is
an important phenomenon in the context of formation and evolution of massive
galaxies in the Universe. Yet, star formation rates (SFRs) in such systems
continue to be elusive. We use our Bayesian-motivated spectral energy
distribution (SED)-fitting code, BAYESCOOL, to estimate the plausible SFR
values in the brightest cluster galaxy of a massive, X-ray luminous galaxy
cluster, Phoenix. Previous studies of Phoenix have resulted in the highest
measurement of SFR for any galaxy, with the estimates reaching up to 1000 solar
masses/yr. However, a very small number of models have been considered in those
studies. BAYESCOOL allows us to probe a large parameter space. We consider two
models for star formation history, instantaneous bursts and continuous star
formation, a wide range of ages for the old and the young stellar population,
along with other discrete parameters, such as the initial mass function,
metallicities, internal extinction and extinction law. We find that in the
absence of any prior except that the maximum cooling rate < 3000 solar
masses/yr, the SFR lies in the range (2230-2890) solar masses/yr. If we impose
an observational prior on the internal extinction, E(B-V) < 0.6, the best-fit
SFR lies in (454-494) solar masses/yr, and we consider this as the most
probable range of SFR values for Phoenix. The SFR dependence on the extinction
is a reflection of the standard age-extinction degeneracy, which can be
overcome by using a prior on one of the two quantities in question.Comment: 12 pages, 4 figures, 1 Table, accepted for publication in MNRA
New Coordinates for the Amplitude Parameter Space of Continuous Gravitational Waves
The parameter space for continuous gravitational waves (GWs) can be divided
into amplitude parameters (signal amplitude, inclination and polarization
angles describing the orientation of the source, and an initial phase) and
phase-evolution parameters. The division is useful in part because the
Jaranowski-Krolak-Schutz (JKS) coordinates on the four-dimensional amplitude
parameter space allow the GW signal to be written as a linear combination of
four template waveforms with the JKS coordinates as coefficients. We define a
new set of coordinates on the amplitude parameter space, with the same
properties, which is more closely connected to the physical amplitude
parameters. These naturally divide into two pairs of Cartesian-like coordinates
on two-dimensional subspaces, one corresponding to left- and the other to
right-circular polarization. We thus refer to these as CPF (circular
polarization factored) coordinates. The corresponding two sets of polar
coordinates (known as CPF-polar) can be related in a simple way to the physical
parameters. We illustrate some simplifying applications for these various
coordinate systems, such as: a calculation of Jacobians between various
coordinate systems; an illustration of the signal coordinate singularities
associated with left- and right-circular polarization, which correspond to the
origins of the two two-dimensional subspaces; and an elucidation of the form of
the log-likelihood ratio between hypotheses of Gaussian noise with and without
a continuous GW signal. These are used to illustrate some of the prospects for
approximate evaluation of a Bayesian detection statistic defined by
marginalization over the physical parameter space. Additionally, in the
presence of simplifying assumptions about the observing geometry, we are able
to explicitly evaluate the integral for the Bayesian detection statistic, and
compare it to the approximate results.Comment: REVTeX, 18 pages, 8 image files included in 7 figure
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