1,090 research outputs found
A revised Cepheid distance to NGC 4258 and a test of the distance scale
In a previous paper (Maoz et al. 1999), we reported a Hubble Space Telescope
(HST) Cepheid distance to the galaxy NGC 4258 obtained using the calibrations
and methods then standard for the Key Project on the Extragalactic Distance
Scale. Here, we reevaluate the Cepheid distance using the revised Key Project
procedures described in Freedman et al. (2001). These revisions alter the zero
points and slopes of the Cepheid Period-Luminosity (P-L) relations derived at
the Large Magellanic Cloud (LMC), the calibration of the HST WFPC2 camera, and
the treatment of metallicity differences. We also provide herein full
information on the Cepheids described in Maoz et al. 1999. Using the refined
Key Project techniques and calibrations, we determine the distance modulus of
NGC 4258 to be 29.47 +/- 0.09 mag (unique to this determination) +/- 0.15 mag
(systematic uncertainties in Key Project distances), corresponding to a metric
distance of 7.8 +/- 0.3 +/- 0.5 Mpc and 1.2 sigma from the maser distance of
7.2 +/- 0.5 Mpc. We also test the alternative Cepheid P-L relations of Feast
(1999), which yield more discrepant results. Additionally, we place weak limits
upon the distance to the LMC and upon the effect of metallicity in Cepheid
distance determinations.Comment: 26 pages in emulateapj5 format, including 6 figures and 5 tables.
Accepted for publication in the Astrophysical Journa
Minisuperspace Quantization of "Bubbling AdS" and Free Fermion Droplets
We quantize the space of 1/2 BPS configurations of Type IIB SUGRA found by
Lin, Lunin and Maldacena (hep-th/0409174), directly in supergravity. We use the
Crnkovic-Witten-Zuckerman covariant quantization method to write down the
expression for the symplectic structure on this entire space of solutions. We
find the symplectic form explicitly around AdS_5 x S^5 and obtain a U(1)
Kac-Moody algebra, in precise agreement with the quantization of a system of N
free fermions in a harmonic oscillator potential, as expected from AdS/CFT. As
a cross check, we also perform the quantization around AdS_5 x S^5 by another
method, using the known spectrum of physical perturbations around this
background and find precise agreement with our previous calculation.Comment: 22 Pages + 2 Appendices, JHEP3; v3: explanation of factor 2 mismatch
added, references reordered, published versio
Galaxy Mergers and Gravitational Lens Statistics
We investigate the impact of hierarchical galaxy merging on the statistics of
gravitational lensing of distant sources. Since no definite theoretical
predictions for the merging history of luminous galaxies exist, we adopt a
parametrized prescription, which allows us to adjust the expected number of
pieces comprising a typical present galaxy at z=0.65. The existence of global
parameter relations for elliptical galaxies and constraints on the evolution of
the phase space density in dissipationless mergers, allow us to limit the
possible evolution of galaxy lens properties under merging.
We draw two lessons from implementing this lens evolution into statistical
lens calculations: (1) The total optical depth to multiple imaging (e.g. of
quasars) is quite insensitive to merging. (2) Merging leads to a smaller mean
separation of observed multiple images. Because merging does not reduce
drastically the expected lensing frequency it cannot make -dominated
cosmologies compatible with the existing lensing observations. A comparison
with the data from the HST Snapshot Survey shows that models with little or no
evolution of the lens population are statistically favored over strong merging
scenarios. The specific merging scenario proposed by Toomre (1977) can be
rejected (95\% level) by such a comparison. Some versions of the scenario
proposed by Broadhurst, Ellis \& Glazebrook (1992), are statistically
acceptable.Comment: uuencoded postscript file with figure
Time delay measurement of the lensed quasar HE1104-1805
We have measured the time delay between the two images of the gravitationally
lensed quasar HE1104-1805 by combining observations made with the Wise
Observatory 1m telescope and published observations of this system by Schechter
et al., spanning a total of five years, from 1997 to 2002. Based on a variety
of techniques, we find that the best fit time delay is -161 (+/-7)(+34 -11)
days. The 68% and 95% confidence intervals include the systematic errors due to
an observed component of uncorrelated variability between images. The delay is
shorter than predicted by simple models and may indicate a large external shear
or a large value of the Hubble parameter, h>0.75 (95% confidence). The residual
light curve between the two images shows a longterm trend of ~0.04 mag/yr,
presumably the result of microlensing by stars in the lens galaxy, but also
short timescale (~1 month) variability with a mean amplitude of about 0.07 mag.Comment: 7 pages, ApJ, in pres
Spectroscopic Redshifts for Seven Lens Galaxies
We report VLT observations of 11 lensed quasars, designed to measure the
redshifts of their lens galaxies. We successfully determined the redshifts for
seven systems, five of which were previously unknown. The securely measured
redshifts for the lensing galaxies are: HE0047-1756 z=0.408; PMNJ0134-0931
z=0.766; HE0230-2130 z=0.522; HE0435-1223 z=0.455; SDSS0924+021 z=0.393;
LBQS1009-025 z=0.871; and WFIJ2033-472 z=0.658. For four additional systems
(BRI0952-0115, Q1017-207, Q1355-2257 and PMNJ1632-003) we estimate tentative
redshifts based on some features in their spectra.Comment: 8 pages, ApJ, submitte
Optical Versus Mid-Infrared Spectroscopic Classification of Ultraluminous Infrared Galaxies
The origin of huge infrared luminosities of ultraluminous infrared galaxies
(ULIGs) is still in question. Recently, Genzel et al. made mid-infrared (MIR)
spectroscopy of a large number of ULIGs and found that the major energy source
in them is massive stars formed in the recent starburst activity; i.e.,
70% -- 80% of the sample are predominantly powered by the starburst. However,
it is known that previous optical spectroscopic observations showed that the
majority of ULIGs are classified as Seyferts or LINERs (low-ionization nuclear
emission-line regions). In order to reconcile this difference, we compare types
of emission-line activity for a sample of ULIGs which have been observed in
both optical and MIR. We confirm the results of previous studies that the
majority of ULIGs classified as LINERs based on the optical emission-line
diagnostics turn to be starburst-dominated galaxies based on the MIR ones.
Since the MIR spectroscopy can probe more heavily-reddened, inner parts of the
ULIGs, it is quite unlikely that the inner parts are powered by the starburst
while the outer parts are powered by non-stellar ionization sources. The most
probable resolution of this dilemma is that the optical emission-line nebulae
with the LINER properties are powered predominantly by shock heating driven by
the superwind activity; i.e., a blast wave driven by a collective effect of a
large number of supernovae in the central region of galaxy mergers.Comment: 15 pages, 2 tables, and 3 eps figures. The Astrophysical Journal
(Part 1), in pres
Central Masses and Broad-Line Region Sizes of Active Galactic Nuclei. II. A Homogeneous Analysis of a Large Reverberation-Mapping Database
We present improved black hole masses for 35 active galactic nuclei (AGNs)
based on a complete and consistent reanalysis of broad emission-line
reverberation-mapping data. From objects with multiple line measurements, we
find that the highest precision measure of the virial product is obtained by
using the cross-correlation function centroid (as opposed to the
cross-correlation function peak) for the time delay and the line dispersion (as
opposed to full width half maximum) for the line width and by measuring the
line width in the variable part of the spectrum. Accurate line-width
measurement depends critically on avoiding contaminating features, in
particular the narrow components of the emission lines. We find that the
precision (or random component of the error) of reverberation-based black hole
mass measurements is typically around 30%, comparable to the precision attained
in measurement of black hole masses in quiescent galaxies by gas or stellar
dynamical methods. Based on results presented in a companion paper by Onken et
al., we provide a zero-point calibration for the reverberation-based black hole
mass scale by using the relationship between black hole mass and host-galaxy
bulge velocity dispersion. The scatter around this relationship implies that
the typical systematic uncertainties in reverberation-based black hole masses
are smaller than a factor of three. We present a preliminary version of a
mass-luminosity relationship that is much better defined than any previous
attempt. Scatter about the mass-luminosity relationship for these AGNs appears
to be real and could be correlated with either Eddington ratio or object
inclination.Comment: 61 pages, including 8 Tables and 16 Figures. Accepted for publication
in The Astrophysical Journa
Hydrodynamical Simulations of the Lyman Alpha Forest: Model Comparisons
We investigate the properties of the Lyman alpha forest as predicted by
numerical simulations for a range of currently viable cosmological models. This
is done in order to understand the dependencies of the forest on cosmological
parameters. Focusing on the redshift range from two to four, we show that: (1)
most of the evolution in the distributions of optical depth, flux and column
density can be understood by simple scaling relations, (2) the shape of optical
depth distribution is a sensitive probe of the amplitude of density
fluctuations on scales of a few hundred kpc, (3) the mean of the b distribution
(a measure of the width of the absorption lines) is also very sensitive to
fluctuations on these scales, and decreases as they increase. We perform a
preliminary comparison to observations, where available. A number of other
properties are also examined, including the evolution in the number of lines,
the two-point flux distribution and the HeII opacity.Comment: 37 pages, 21 figures, submitted to Ap
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