7,158 research outputs found
GINS: a new tool for VLBI Geodesy and Astrometry
In the framework of the "Groupe de Recherches de G\'eod\'esie Spatiale"
(GRGS), a rigorous combination of the data from five space geodetic techniques
(VLBI, GPS, SLR, LLR and DORIS) is routinely applied to simultaneously
determine a Terrestrial Reference Frame (TRF) and Earth Orientation Parameters
(EOP). This analysis is conducted with the software package GINS which has the
capability to process data from all five techniques together. Such a
combination at the observation level should ultimately facilitate fine
geophysical studies of the global Earth system. In this project, Bordeaux
Observatory is in charge of the VLBI data analysis, while satellite geodetic
data are processed by other groups. In this paper, we present (i) details about
the VLBI analysis undertaken with GINS, and (ii) the results obtained for the
EOP during the period 2005-2006. We also compare this EOP solution with the IVS
(International VLBI Service for geodesy and astrometry) analysis coordinator
combined results. The agreement is at the 0.2 mas level, comparable to that of
the other IVS analysis centers, which demonstrates the capability of the GINS
software for VLBI analysis
VLBI analyses with the GINS software for multi-technique combination at the observation level
A rigorous approach to simultaneously determine a Terrestrial Reference Frame
(TRF) and Earth Orientation Parameters (EOP) is now currently applied on a
routine basis in a coordinated project within the Groupe de Recherches de
G\'eod\'esie Spatiale (GRGS) in France. Observations of the various space
geodetic techniques (VLBI, SLR, LLR, DORIS and GPS) are separately processed by
different analysis centers with the software package GINS-DYNAMO, developed and
maintained at the GRGS/CNES (Toulouse). This project is aimed at facilitating
fine geophysical analyses of the global Earth system (GGOS project). In this
framework, Bordeaux Observatory is in charge of the VLBI (Very Long Baseline
Interferometry) analyses with GINS for combination with the data of the other
space geodetic techniques at the observation level. In this paper, we present
(i) the analyses undertaken with this new VLBI software, and (ii) the results
obtained for the EOP from beginning 2005 until 2007. Finally, we compare this
EOP solution with the IVS (International VLBI Service) Analysis Coordinator
combined results. The agreement is at the 0.2 mas level, comparable to that of
the other IVS Analysis Centers, which demonstrates the VLBI capability of the
GINS software
Theoretical Predictions for Surface Brightness Fluctuations and Implications for Stellar Populations of Elliptical Galaxies
(Abridged) We present new theoretical predictions for surface brightness
fluctuations (SBFs) using models optimized for this purpose. Our predictions
agree well with SBF data for globular clusters and elliptical galaxies. We
provide refined theoretical calibrations and k-corrections needed to use SBFs
as standard candles. We suggest that SBF distance measurements can be improved
by using a filter around 1 micron and calibrating I-band SBFs with the
integrated V-K galaxy color. We also show that current SBF data provide useful
constraints on population synthesis models, and we suggest SBF-based tests for
future models. The data favor specific choices of evolutionary tracks and
spectra in the models among the several choices allowed by comparisons based on
only integrated light. In addition, the tightness of the empirical I-band SBF
calibration suggests that model uncertainties in post-main sequence lifetimes
are less than +/-50% and that the IMF in ellipticals is not much steeper than
that in the solar neighborhood. Finally, we analyze the potential of SBFs for
probing unresolved stellar populations. We find that optical/near-IR SBFs are
much more sensitive to metallicity than to age. Therefore, SBF magnitudes and
colors are a valuable tool to break the age/metallicity degeneracy. Our initial
results suggest that the most luminous stellar populations of bright cluster
galaxies have roughly solar metallicities and about a factor of three spread in
age.Comment: Astrophysical Journal, in press (uses Apr 20, 2000 version of
emulateapj5.sty). Reposted version has a minor cosmetic change to Table
Multi-step VLBI observations of weak extragalactic radio sources to align the ICRF and the future GAIA frame
The space astrometry mission GAIA will construct a dense optical QSO-based
celestial reference frame. For consistency between optical and radio positions,
it will be important to align the GAIA frame and the International Celestial
Reference Frame (ICRF) with the highest accuracy. Currently, it is found that
only 10% of the ICRF sources are suitable to establish this link, either
because they are not bright enough at optical wavelengths or because they have
significant extended radio emission which precludes reaching the highest
astrometric accuracy. In order to improve the situation, we have initiated a
VLBI survey dedicated to finding additional suitable radio sources for aligning
the two frames. The sample consists of about 450 sources, typically 20 times
weaker than the current ICRF sources (down to the 20 mJy flux level), which
have been selected by cross-correlating optical and radio catalogues. This
paper presents the observing strategy to detect, image, and measure accurate
positions for these sources. It will also provide results about the VLBI
detectability of the sources, as derived from initial observations with the
European VLBI Network in June and October 2007. Based on these observations, an
excellent detection rate of 89% is found, which is very promising for the
continuation of this project
VLBI observations of weak extragalactic radio sources for the alignment of the future GAIA frame with the ICRF
The space astrometry mission GAIA will construct a dense optical QSO-based
celestial reference frame. For consistency between the optical and radio
positions, it will be important to align the GAIA frame and the International
Celestial Reference Frame (ICRF) with the highest accuracy. Currently, it is
found that only 10% of the ICRF sources are suitable to establish this link,
either because they are not bright enough at optical wavelengths or because
they have significant extended radio emission which precludes reaching the
highest astrometric accuracy. In order to improve the situation, we have
initiated a VLBI survey dedicated to finding additional high-quality radio
sources for aligning the two frames. The sample consists of about 450 sources,
typically 20 times weaker than the current ICRF sources, which have been
selected by cross-correlating optical and radio catalogues. This paper presents
the observing strategy and includes preliminary results of observation of 224
of these sources with the European VLBI Network in June 2007
Abundance patterns in early-type galaxies: is there a 'knee' in the [Fe/H] vs. [alpha/Fe] relation?
Early-type galaxies (ETGs) are known to be enhanced in alpha elements, in
accordance with their old ages and short formation timescales. In this
contribution we aim to resolve the enrichment histories of ETGs. This means we
study the abundance of Fe ([Fe/H]) and the alpha-element groups ([alpha/Fe])
separately for stars older than 9.5 Gyr ([Fe/H]o, [alpha/Fe]o) and for stars
between 1.5 and 9.5 Gyr ([Fe/H]i, [alpha/Fe]i). Through extensive simulation we
show that we can indeed recover the enrichment history per galaxy. We then
analyze a spectroscopic sample of 2286 early-type galaxies from the SDSS
selected to be ETGs. We separate out those galaxies for which the abundance of
iron in stars grows throughout the lifetime of the galaxy, i.e. in which
[Fe/H]o < [Fe/H]i. We confirm earlier work where the [Fe/H] and [alpha/Fe]
parameters are correlated with the mass and velocity dispersion of ETGs. We
emphasize that the strongest relation is between [alpha/Fe] and age. This
relation falls into two regimes, one with a steep slope for old galaxies and
one with a shallow slope for younger ETGs. The vast majority of ETGs in our
sample do not show the 'knee' in the plot of [Fe/H] vs. [alpha/Fe] commonly
observed in local group galaxies. This implies that for the vast majority of
ETGs, the stars younger than 9.5 Gyrs are likely to have been accreted or
formed from accreted gas. The properties of the intermediate-age stars in
accretion-dominated ETGs indicate that mass growth through late (minor) mergers
in ETGs is dominated by galaxies with low [Fe/H] and low [alpha/Fe]. The method
of reconstructing the stellar enrichment histories of ETGs introduced in this
paper promises to constrain the star formation and mass assembly histories of
large samples of galaxies in a unique way.Comment: 22 pages, 25 figures, accepted for publication by A&
Near-Infrared Microlensing of Stars by the Super-Massive Black Hole in the Galactic Center
We investigate microlensing amplification of faint stars in the dense stellar
cluster in the Galactic Center (GC) by the super-massive black hole (BH). Such
events would appear very close to the position of the radio source SgrA*, which
is thought to coincide with the BH, and could be observed during the monitoring
of stellar motions in the GC. We use the observed K-band (2.2 um) luminosity
function (KLF) in the GC and in Baade's Window, as well as stellar population
synthesis computations, to construct KLF models for the inner 300 pc of the
Galaxy. These, and the observed dynamical properties of this region, are used
to compute the rates of microlensing events, which amplify stars above
specified detection thresholds. We present computations of the lensing rates
and amplifications as functions of the event durations (weeks to years), for a
range of detection thresholds. We find that short events dominate the total
rate and that long events tend to have large amplifications. For the current
detection limit of K=17 mag, the total microlensing rate is 0.003 1/yr, and the
rate of events with durations >1 yr is 0.001 1/yr. Recent GC proper motion
studies have revealed the possible presence of one or two variable K-band
sources very close to SgrA* (Genzel et al 97; Ghez et al 98). These sources may
have attained peak brightnesses of K~15 mag, about 1.5-2 mag above the
observational detection limits, and appear to have varied on a timescale of ~1
yr. This behavior is consistent with long-duration microlensing of faint stars
by the BH. However, we estimate that the probability that such an event could
have been detected during the course of the recent proper motion studies is
\~0.5%. A ten-fold improvement in the detection limit and 10 yr of monthly
monitoring would increase the total detection probability to ~20%. (Abridged)Comment: 29 p. with 5 figs. To appear in ApJ. Changed to reflect published
version. Short discussions of solar metallicity luminosity function and
star-star microlensing adde
The Ages of Elliptical Galaxies in a Merger Model
The tightness of the observed colour-magnitude and Mg- velocity
dispersion relations for elliptical galaxies has often been cited as an
argument against a picture in which ellipticals form by the merging of spiral
disks. A common view is that merging would mix together stars of disparate ages
and produce a large scatter in these relations. Here I use semi-analytic models
of galaxy formation to derive the distribution of the mean ages, colours and
metallicities of the stars in elliptical galaxies formed by mergers in a flat
CDM universe. It is seen that most of the stars in ellipticals form at
relatively high redshift (z > 1.9) and that the predicted scatter in the
colour-magnitude and Mg_2 - sigma relations falls within observational bounds.
I conclude that the apparent homogeneity in the properties of the stellar
populations of ellipticals is not inconsistent with a merger scenario for the
origin of these systems.Comment: latex file, figures available upon reques
Monitoring the Bi-Directional Relativistic Jets of the Radio Galaxy 1946+708
We report on a multi-frequency, multi-epoch campaign of Very Long Baseline
Interferometry observations of the radio galaxy 1946+708 using the VLBA and a
Global VLBI array. From these high-resolution observations we deduce the
kinematic age of the radio source to be 4000 years, comparable with the
ages of other Compact Symmetric Objects (CSOs). Ejections of pairs of jet
components appears to take place on time scales of 10 years and these
components in the jet travel outward at intrinsic velocities between 0.6 and
0.9 c. From the constraint that jet components cannot have intrinsic velocities
faster than light, we derive H_0 > 57 km s^-1 Mpc^-1 from the fastest pair of
components launched from the core. We provide strong evidence for the ejection
of a new pair of components in ~1997. From the trajectories of the jet
components we deduce that the jet is most likely to be helically confined,
rather than purely ballistic in nature.Comment: 20 pages, 8 figures, accepted to Ap
Radio astrometry with chromatic AGN core positions
Aims: The effect of frequency-dependent AGN core positions (``core-shifts'')
on radio Very Long Baseline Interferometry (VLBI) global astrometry
measurements is investigated.
Methods: The basic equations relating to VLBI astrometry are reviewed,
including the effects of source structure. A power-law representation of
core-shifts, based on both observations and theoretical considerations of jet
conditions, is incorporated.
Results: It is shown that, in the presence of core-shifts, phase and
group-delay astrometry measurements yield different positions. For a core
displacement from the jet base parametrized by Delta x (lambda) = k lambda^beta
group delays measure a ``reduced'' core-shift of (1-beta) Delta x (lambda). For
the astrophysically-significant case of beta = 1, group delays measure no shift
at all, giving the position of the jet base. At 8.4 GHz an estimated typical
offset between phase and group-delay positions of ~170 uas is smaller than the
current ~250 uas precision of group-delay positions of the sources used to
define the ICRF; however, this effect must be taken into account for future
measurements planned with improved accuracy when comparing with optical
positions of AGN to be obtained with the GAIA mission.Comment: Accepted for publication in Astronomy & Astrophysics, 4 page
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