288 research outputs found
Malmquist Bias and the Distance to the Virgo Cluster
This paper investigates the impact of Malmquist bias on the distance to the
Virgo cluster determined by the H_0 Key Project using M100, and consequently on
the derived value of H_0. Malmquist bias is a volume-induced statistical effect
which causes the most probable distance to be different from the raw distance
measured. Consideration of the bias in the distance to the Virgo cluster raises
this distance and lowers the calculated value of H_0. Monte Carlo simulations
of the cluster have been run for several possible distributions of spirals
within the cluster and of clusters in the local universe. Simulations
consistent with known information regarding the cluster and the errors of
measurement result in a bias of about 6.5%-8.5%. This corresponds to an
unbiased distance of 17.2-17.4 Mpc and a value of H_0 in the range 80-82
km/s/Mpc.
The problem of determining the bias to Virgo illustrates several key points
regarding Malmquist bias. Essentially all conventional astronomical distance
measurements are subject to this bias. In addition, the bias accumulates when
an attempt is made to construct "distance ladders" from measurements which are
individually biased. As will be shown in the case of Virgo, the magnitude and
direction of the bias are sensitive to the spatial distribution of the parent
poputation from which the observed object is drawn - a distribution which is
often poorly known. This leads to uncertainty in the magnitude of the bias, and
adds to the importance of minimizing the number of steps in "distance ladders".Comment: 19 pages, 3 figures, Latex, To appear in Ap
A maximum-likelihood method for improving faint source flux and color estimates
Flux estimates for faint sources or transients are systematically biased high
because there are far more truly faint sources than bright. Corrections which
account for this effect are presented as a function of signal-to-noise ratio
and the (true) slope of the faint-source number-flux relation. The corrections
depend on the source being originally identified in the image in which it is
being photometered. If a source has been identified in other data, the
corrections are different; a prescription for calculating the corrections is
presented. Implications of these corrections for analyses of surveys are
discussed; the most important is that sources identified at signal-to-noise
ratios of four or less are practically useless.Comment: 9 pp., accepted for publication in PAS
The Case for an Accelerating Universe from Supernovae
The unexpected faintness of high-redshift Type Ia supernovae (SNe Ia), as
measured by two teams, has been interpreted as evidence that the expansion of
the Universe is accelerating. We review the current challenges to this
interpretation and seek to answer whether the cosmological implications are
compelling. We discuss future observations of SNe Ia which could offer
extraordinary evidence to test acceleration.Comment: To appear as an Invited Review for PASP 20 pages, 13 figure
The Correlation Between Galaxy HI Linewidths and K' Luminosities
The relationship between galaxy luminosities and rotation rates is studied
with total luminosities in the K' band. Extinction problems are essentially
eliminated at this band centered at 2.1 micron. A template luminosity-linewidth
relation is derived based on 65 galaxies drawn from two magnitude-limited
cluster samples. The zero-point is determined using 4 galaxies with accurately
known distances. The calibration is applied to give the distance to the Pisces
Cluster (60 Mpc) at a redshift in the CMB frame of 4771 km/s. The resultant
value of the Hubble Constant is 81 km/s/Mpc. The largest sources of uncertainty
arises from the small number of zero-point calibrators at this time at K' and
present application to only one cluster.Comment: 13 pages including 5 figures and 2 tables. Accepted for publication
in Astrophysical Journa
The Clustering of Extragalactic Extremely Red Objects
We have measured the angular and spatial clustering of 671 K5
Extremely Red Objects (EROs) from a 0.98 square degree sub-region of the NOAO
Deep Wide-Field Survey (NDWFS). Our study covers nearly 5 times the area and
has twice the sample size of any previous ERO clustering study. The wide field
of view and BwRIK passbands of the NDWFS allow us to place improved constraints
on the clustering of z=1 EROs. We find the angular clustering of EROs is
slightly weaker than in previous measurements, and w(1')=0.25+/-0.05 for
K<18.40 EROs. We find no significant correlation of ERO spatial clustering with
redshift, apparent color or absolute magnitude, although given the
uncertainties, such correlations remain plausible. We find the spatial
clustering of K5 EROs is well approximated by a power-law, with
r_0=9.7+/-1.1 Mpc/h in comoving coordinates. This is comparable to the
clustering of 4L* early-type galaxies at z<1, and is consistent with the
brightest EROs being the progenitors of the most massive ellipticals. There is
evidence of the angular clustering of EROs decreasing with increasing apparent
magnitude, when NDWFS measurements of ERO clustering are combined with those
from the literature. Unless the redshift distribution of K>20 EROs is very
broad, the spatial clustering of EROs decreases from r_0=9.7+/-1.1 Mpc/h for
K20 EROs.Comment: Accepted for publication in the ApJ. 29 pages with 10 figures. The
NOAO Deep Wide-Field Survey Bootes data release is available online at
http://www.noao.edu/noao/noaodeep
A Complete Catalog of Swift GRB Spectra and Durations: Demise of a Physical Origin for Pre-Swift High-Energy Correlations
We calculate durations and spectral paramaters for 218 Swift bursts detected
by the BAT instrument between and including GRBs 041220 and 070509, including
77 events with measured redshifts. Incorporating prior knowledge into the
spectral fits, we are able to measure the characteristic spectral
peak energy and the isotropic equivalent energy
(1-- keV) for all events. This complete and rather extensive catalog,
analyzed with a unified methodology, allows us to address the persistence and
origin of high-energy correlations suggested in pre-Swift observations. We find
that the - correlation is present in the Swift
sample; however, the best-fit powerlaw relation is inconsistent with the
best-fit pre-Swift relation at >5 sigma significance. Moreover, it has a factor
>~ 2 larger intrinsic scatter, after accounting for large errors on . A large fraction of the Swift events are hard and subluminous
relative to (and inconsistent with) the pre-Swift relation, in agreement with
indications from BATSE GRBs without redshift. Moreover, we determine an
experimental threshold for the BAT detector and show how the -- correlation arises artificially due to partial
correlation with the threshold. We show that pre-Swift correlations found by
Amati et al.(2002), Yonetoku et al. (2004), Firmani et al.(2006) (and
independently by others) are likely unrelated to the physical properties of
GRBs and are likely useless for tests of cosmology. Also, an explanation of
these correlations in terms of a detector threshold provides a natural and
quantitative explanation for why short-duration GRBs and events at low redshift
tend to be outliers to the correlations.Comment: 25 pages, 9 figures, 2 tables, Accepted to Ap
Selection Bias in Observing the Cosmological Evolution of the Mbh-sigma and Mbh-L Relationships
Programs to observe evolution in the Mbh-sigma or Mbh-L relations typically
compare black-hole masses, Mbh, in high-redshift galaxies selected by nuclear
activity to Mbh in local galaxies selected by luminosity L, or stellar velocity
dispersion sigma. Because AGN luminosity is likely to depend on Mbh, selection
effects are different for high-redshift and local samples, potentially
producing a false signal of evolution. This bias arises because cosmic scatter
in the Mbh-sigma and Mbh-L relations means that the mean log(L) or log(sigma)
among galaxies that host a black hole of given Mbh, may be substantially
different than the log(L) or log(sigma) obtained from inverting the Mbh-L or
Mbh-sigma relations for the same nominal Mbh. The bias is particularly strong
at high Mbh, where the luminosity and dispersion functions of galaxies are
falling rapidly. The most massive black holes occur more often as rare outliers
in galaxies of modest mass than in the even rarer high-mass galaxies, which
would otherwise be the sole location of such black holes in the absence of
cosmic scatter. Because of this bias, Mbh will typically appear to be too large
in the distant sample for a given L or sigma. For the largest black holes and
the largest plausible cosmic scatter, the bias can reach a factor of 3 in Mbh
for the Mbh-sigma relation and a factor of 9 for the Mbh-L relation.
Unfortunately, the actual cosmic scatter is not known well enough to correct
for the bias. Measuring evolution of the Mbh and galaxy property relations
requires object selection to be precisely defined and exactly the same at all
redshifts.Comment: 28 pages, 6 figures, submitted to the Astrophysical Journa
An Unbiased Estimate of the Global Hubble Constant in the Region of Pisces-Perseus
We obtain an unbiased estimate of the global Hubble constant H0 in the volume
of cz<12000km/s in the region of Pisces-Perseus. The Tully-Fisher (TF) relation
is applied to a magnitude limited sample of 441 spirals selected from the
Arecibo 21cm catalog. The photometry data were calibrated with CCD observations
and we achieve 0.13mag for the photometric internal error. We use a maximum
likelihood method for the TF analysis. Monte-Carlo simulations demonstrate that
our method reproduces a given H0 at the 95% confidence level. By applying the
method to our sample galaxies, we obtain the unbiased global Hubble constant
H0=65+-2(+20,-14) km/s/Mpc; the first and the second terms represent the
internal random error and the external errors, respectively. We also find a
good agreement for our H0 with those recently obtained via Cepheid observation,
the TF relation and supernovae. Hubble velocities of the spirals inferred from
our H0 show no significant systematic difference from those given in the Mark
III catalog. The same analysis for H0 is carried out using r-band photometry
data of the Pisces-Perseus region given by Willick et al.(1997). We obtain a
global H0 which is consistent with that obtained from our B-band analysis. A
bulk motion in the Pisces-Perseus region is briefly discussed, based on our
calibration of H0. Our r-band TF analysis supports the notion of a coherent
streaming motion of the Pisces-Perseus ridge with -200km/s with respect to the
CMB, in agreement with most modern studies.Comment: 40 pages, 27 postscript figures, to appear in Ap.J. Figures are
included in the tex
Recommended from our members
Assessment of Maternal Posttraumatic Stress Disorder Following Childbirth: Psychometric Properties of the Swedish Version of City Birth Trauma Scale
Objective: City Birth Trauma Scale is an instrument designed to evaluate and diagnose postpartum posttraumatic stress disorder (PTSD) according to the 5th edition of Diagnostic and Statistical Manual of Mental Disorders (DSM-5). No validated Swedish instrument exists to measure postpartum PTSD according to DSM-5. Therefore, the objective of this study was to assess the psychometric properties of the Swedish version of the City BiTS (City BiTS-Swe) and to examine the latent factor structure of postpartum PTSD.
Method: A total of 619 women, who had given birth at five clinics in the past six to 16 weeks, completed an online version of City BiTS-Swe and the Edinburgh Postnatal Depression Scale (EPDS). Additionally, sociodemographic and medical data was collected. A second questionnaire was answered by 110 women to examine reliability over time.
Results: The confirmatory factor analysis using the two-factor model gave best fit to the data. We found a high internal consistency (α = 0.89-0.87) and good test-retest reliability (ICC = 0.53 - 0.90). Divergent reliability with EPDS showed significant correlations with satisfying results for the subscale birth-related symptoms (r=0.41). We also found discriminant validity concerning mode of birth, parity, gestational age, mental illness, history of traumatic childbirth and history of traumatic event as expected.
Conclusions: The City BiTS-Swe is a valid and reliable instrument to assess and diagnose PTSD following childbirth.
Clinical impact statement: PTSD following childbirth can cause considerable suffering for the affected woman, her partner and the child. Having a valid and reliable instrument to assess and diagnose childbirth-related PTSD is of great importance for the detection and treatment of the disease. The results of this study confirms that the Swedish version of City BiTS is a reliable and valid instrument that can be recommended for use in daily clinical practice and for research purposes
The Motions of Clusters of Galaxies and the Dipoles of the Peculiar Velocity Field
In preceding papers of this series, TF relations for galaxies in 24 clusters
with radial velocities between 1000 and 9200 km/s (SCI sample) were obtained, a
Tully-Fisher (TF) template relation was constructed and mean offsets of each
cluster with respect to the template obtained. Here, an estimate of the
line-of-sight peculiar velocities of the clusters and their associated errors
are given. It is found that cluster peculiar velocities in the Cosmic Microwave
Background reference frame do not exceed 600 k/ms and that their distribution
has a line-of-sight dispersion of 300 k/ms, suggesting a more quiescent cluster
peculiar velocity field than previously reported. When measured in a reference
frame in which the Local Group is at rest, the set of clusters at cz > 3000
km/s exhibits a dipole moment in agreement with that of the CMB, both in
amplitude and apex direction. It is estimated that the bulk flow of a sphere of
6000 km/s radius in the CMB reference frame is between 140 and 320 km/s. These
results are in agreement with those obtained from an independent sample of
field galaxies (Giovanelli et al. 1998; see astro-ph/9807274)Comment: 9 pages, 2 tables, 7 figures, uses AAS LaTex; to appear in A
- …