91 research outputs found
The precision of line position measurements of unresolved quasar absorption lines and its influence on the search for variations of fundamental constants
Optical quasar spectra can be used to trace variations of the fine-structure
constant alpha. Controversial results that have been published in last years
suggest that in addition to to wavelength calibration problems systematic
errors might arise because of insufficient spectral resolution. The aim of this
work is to estimate the impact of incorrect line decompositions in fitting
procedures due to asymmetric line profiles. Methods are developed to
distinguish between different sources of line position shifts and thus to
minimize error sources in future work. To simulate asymmetric line profiles,
two different methods were used. At first the profile was created as an
unresolved blend of narrow lines and then, the profile was created using a
macroscopic velocity field of the absorbing medium. The simulated spectra were
analysed with standard methods to search for apparent shifts of line positions
that would mimic a variation of fundamental constants. Differences between
position shifts due to an incorrect line decomposition and a real variation of
constants were probed using methods that have been newly developed or adapted
for this kind of analysis. The results were then applied to real data. Apparent
relative velocity shifts of several hundred meters per second are found in the
analysis of simulated spectra with asymmetric line profiles. It was found that
each system has to be analysed in detail to distinguish between different
sources of line position shifts. A set of 16 FeII systems in seven quasar
spectra was analysed. With the methods developed, the mean alpha variation that
appeared in these systems was reduced from the original
Dalpha/alpha=(2.1+/-2.0)x10^-5 to Dalpha/alpha=(0.1+/-0.8)x10^-5. We thus
conclude that incorrect line decompositions can be partly responsible for the
conflicting results published so far
Probing the variability of the fine-structure constant with the VLT/UVES
We assess the cosmological variability of the fine-structure constant from
the analysis of an ensemble of Fe II absorption lines at the redshift z=1.15
toward the QSO HE 0515-4414 by means of the standard many-multiplet (MM)
technique and its revision based on linear regression (RMM). This is the first
time the MM technique is applied to exceptional high-resolution and high
signal-to-noise QSO spectra recorded with the UV-Visual Echelle Spectrograph
(UVES) at the ESO Very Large Telescope (VLT). Our analysis results in the most
stringent bounds hitherto infered from QSO absorption lines. Our results
support the null hypothesis of a non-varying fine-structure constant at a
significance level of 91 percent, whereas the support for the results presented
in former MM studies indicating a variation in the fine-structure constant is
12 percent.Comment: 7 pages, 5 figures, accepted for publication in Astronomy &
Astrophysics Letter
Asset price misalignments and the role of money and credit
This paper contributes to the literature on the properties of money and credit indicators for detecting asset price misalignments. After a review of the evidence in the literature on this issue, the paper discusses the approaches that can be considered to detect asset price busts. Considering a sample of 17 OECD industrialised countries and the euro area over the period 1969 Q1 – 2008 Q3, we construct an asset price composite indicator which incorporates developments in both the stock price and house price markets and propose a criterion to identify the periods characterised by asset price busts, which has been applied in the currency crisis literature. The empirical analysis is based on a pooled probit-type approach with several macroeconomic monetary, financial and real variables. According to statistical tests, credit aggregates (either in terms of annual changes or growth gap), changes in nominal long-term interest rates and investment-to-GDP ratio combined with either house prices or stock prices dynamics turn out to be the best indicators which help to forecast asset price busts up to 8 quarters ahead. JEL Classification: E37, E44, E51asset price busts, asset prices, credit aggregates, financial crisis, House prices, monetary aggregates, probit models, stock prices
The Fluctuating Intergalactic Radiation Field at Redshifts z = 2.3-2.9 from He II and H I Absorption towards HE 2347-4342
We provide an in-depth analysis of the He II and H I absorption in the
intergalactic medium (IGM) at redshifts z = 2.3-2.9 toward HE 2347-4342, using
spectra from the Far Ultraviolet Spectroscopic Explorer (FUSE) and the
Ultraviolet-Visual Echelle Spectrograph (UVES) on the VLT telescope. Following
up on our earlier study (Kriss et al. 2001, Science, 293, 1112), we focus here
on two major topics: (1) small-scale variability (Delta z = 10^-3) in the ratio
eta = N(He II)/N(H I); and (2) an observed correlation of high-eta absorbers
(soft radiation fields) with voids in the (H I) Ly-alpha distribution. These
effects may reflect fluctuations in the ionizing sources on scales of 1 Mpc,
together with radiative transfer through a filamentary IGM whose opacity
variations control the penetration of 1-5 ryd radiation over 30-40 Mpc
distances. Owing to photon statistics and backgrounds, we can measure optical
depths over the ranges 0.1 < tau(HeII) < 2.3 and 0.02 < tau(HI) < 3.9, and
reliably determine values of eta = 4 tau(HeII)/tau(HI) over the range 0.1 to
460. Values of eta = 20-200 are consistent with models of photoionization by
quasars with observed spectral indices alpha_s = 0-3. Values of eta > 200 may
require additional contributions from starburst galaxies, heavily filtered
quasar radiation, or density variations. Regions with eta < 30 may indicate the
presence of local hard sources. We find that eta is higher in "void" regions,
where H I is weak or undetected and 80% of the path length has eta > 100. These
voids may be ionized by soft sources (dwarf starbursts) or by QSO radiation
softened by escape from the AGN cores or transfer through the "cosmic web". The
apparent differences in ionizing spectra may help to explain the 1.45 Gyr lag
between the reionization epochs, z(HI) = 6.2 +/-0.2 and z(HeII) = 2.8 +/-0.2.Comment: 27 pages, 7 figures, to appear in Ap
Variability of the proton-to-electron mass ratio on cosmological scales
So far the only seemingly significant indication of a cosmological variation
exists for the proton-to-electron mass ratio as stated by Reinhold et al.
(2006). The measured indication of variation is based on the combined analysis
of H2 absorption systems in the spectra of Q0405-443 and Q0347-383 at z=2.595
and z=3.025, respectively. The high resolution data of the latter is reanalyzed
in this work to examine the influence of different fitting procedures and
further potential nonconformities. This analysis cannot reproduce the
significance achieved by the previous detection.Comment: 10 pages, 8 figures, as to appear in EPJ special - ACFC 2007
conference proceeding
Bounds on the fine structure constant variability from FeII absorption lines in QSO spectra
The Single Ion Differential alpha Measurement (SIDAM) method for measuring
fine stucture variations (daa)and its figures of merit are illustrated together
with the results produced by means of FeII absorption lines of QSO intervening
systems. The method provides daa ~= -0.12(+/- 1.79) ppm (parts-per-million) at
zabs = 1.15 towards HE 0515--4414 and daa = 5.66(+/-2.67) ppm at zabs= 1.84
towards Q 1101--264, which are so far the most accurate measurements for single
systems. SIDAM analysis for 3 systems from the Chand et al. (2004) sample
provides inconsistent results which we interpret as due to calibration errors
of the Chand et al. data at the level of about 10 ppm. In one system evidence
for photo-ionization Doppler shift between MgII and FeII lines is found. This
evidence has important bearings on the Many Multiplet method where the signal
for daa variability is carried mainly by systems involving MgII absorbers. Some
correlations are also found in the Murphy et al. sample which suggest larger
errors than previously reported. Thus, we consider unlikely that both the Chand
et al. and Murphy et al. datasets could provide an estimate of daa with an
accuracy at the level of 1 ppm. A new spectrograph like the ESPRESSO project
will be crucial to make progress in the astronomical determination of daa.Comment: Talk given at ACFC 2007 "Atomic Clocks and Fundamental Constants"
conference, Bad Honnef, June 2007, Savely Karshenboim and Ekkehard Peik
editor
Searches for Metal-Poor Stars from the Hamburg/ESO Survey using the CH G-band
We describe a new method to search for metal-poor candidates from the
Hamburg/ESO objective-prism survey (HES) based on identifying stars with
apparently strong CH G-band strengths for their colors. The hypothesis we
exploit is that large over-abundances of carbon are common among metal-poor
stars. The selection was made by considering two line indices in the 4300A
region, applied directly to the low-resolution prism spectra. This work also
extends a previously published method by adding bright sources to the sample.
The spectra of these stars suffer from saturation effects, compromising the
index calculations and leading to an undersampling of the brighter candidates.
Visual inspection and classification of the spectra from the HES plates yielded
a list of 5,288 new metal-poor candidates, which are presently being used as
targets for medium-resolution spectroscopic follow-up. Estimates of the stellar
atmospheric parameters, as well as carbon abundances, are now available for 117
of the first candidates, based on follow-up medium-resolution spectra obtained
with the SOAR 4.1m and Gemini 8m telescopes. We demonstrate that our new method
improves the metal-poor star fractions found by our pilot study by up to a
factor of three in the same magnitude range, as compared with our pilot study
based on only one CH G-band index. Our selection scheme obtained roughly a 40%
success rate for identification of stars with [Fe/H] < -1.0; the primary
contaminant is late-type stars with near solar abundances and, often, emission
line cores that filled in the CaII K line on the prism spectrum. Because the
selection is based on carbon, we greatly increase the numbers of known CEMP
stars from the HES with intermediate metallicities -2.0 < [Fe/H] < -1.0, which
previous survey efforts undersampled. There are eight newly discovered stars
with [Fe/H] < -3.0 in our sample, including two with [Fe/H] < -3.5.Comment: 18 pages, 13 figures; accepted for publication in A
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