244 research outputs found
Determination of astrophysical parameters of quasars within the Gaia mission
We describe methods designed to determine the astrophysical parameters of
quasars based on spectra coming from the red and blue spectrophotometers of the
Gaia satellite. These methods principally rely on two already published
algorithms that are the weighted principal component analysis and the weighted
phase correlation. The presented approach benefits from a fast implementation;
an intuitive interpretation as well as strong diagnostic tools on the potential
errors that may arise during predictions. The production of a semi-empirical
library of spectra as they will be observed by Gaia is also covered and
subsequently used for validation purpose. We detail the pre-processing that is
necessary in order for these spectra to be fully exploitable by our algorithms
along with the procedures that are used in order to predict the redshifts of
the quasars; their continuum slopes; the total equivalent width of their
emission lines and whether these are broad absorption line (BAL) quasars or
not. Performances of these procedures were assessed in comparison with the
Extremely Randomized Trees learning method and were proven to provide better
results on the redshift predictions and on the ratio of correctly classified
observations though the probability of detection of BAL quasars remains
restricted by the low resolution of these spectra as well as by their limited
signal-to-noise ratio. Finally, the triggering of some warning flags allows us
to obtain an extremely pure subset of redshift predictions where approximately
99% of the observations come along with absolute errors that are below 0.1.Comment: 17 pages, 11 figures, 5 table
Gaia GraL: Gaia DR2 Gravitational Lens Systems. V. Doubly-imaged QSOs discovered from entropy and wavelets
The discovery of multiply-imaged gravitationally lensed QSOs is fundamental to many astronomical and cosmological studies. However, these objects are rare and challenging to discover due to requirements of high-angular resolution astrometric, multiwavelength photometric and spectroscopic data. This has limited the number of known systems to a few hundred objects. We aim to reduce the constraints on angular resolution and discover multiply-imaged QSO candidates by using new candidate selection principles based on unresolved photometric time-series and ground-based images from public surveys. We selected candidates for multiply-imaged QSOs based on low levels of entropy computed from Catalina unresolved photometric time-series or Euclidean similarity to known lenses in a space defined by the wavelet power spectra of Pan-STARSS DR2 or DECaLS DR7 images, combined with multiple {\it Gaia} DR2 sources or large astrometric errors and supervised and unsupervised learning methods. We then confirmed spectroscopically some candidates with the Palomar Hale, Keck-I, and ESO/NTT telescopes. Here we report the discovery and confirmation of seven doubly-imaged QSOs and one likely double quasar. This demonstrates the potential of combining space-astrometry, even if unresolved, with low spatial-resolution photometric time-series and/or low-spatial resolution multi-band imaging to discover multiply-imaged lensed QSOs
Ultracool dwarfs in Gaia DR3
Aims. In this work we use the Gaia DR3 set of ultracool dwarf candidates and
complement the Gaia spectrophotometry with additional photometry in order to
characterise its global properties. This includes the inference of the
distances, their locus in the Gaia colour-absolute magnitude diagram and the
(biased through selection) luminosity function in the faint end of the Main
Sequence. We study the overall changes in the Gaia RP spectra as a function of
spectral type. We study the UCDs in binary systems, attempt to identify
low-mass members of nearby young associations, star forming regions and
clusters, and analyse their variability properties. Results. We detect 57
young, kinematically homogeneous groups some of which are identified as well
known star forming regions, associations and clusters of different ages. We
find that the primary members of 880 binary systems with a UCD belong mainly to
the thin and thick disk components of the Milky Way. We identify 1109 variable
UCDs using the variability tables in the Gaia archive, 728 of which belong to
the star forming regions defined by HMAC. We define two groups of variable UCDs
with extreme bright or faint outliers. Conclusions. The set of sources
identified as UCDs in the Gaia archive contains a wealth of information that
will require focused follow-up studies and observations. It will help to
advance our understanding of the nature of the faint end of the Main Sequence
and the stellar/substellar transition.Comment: Accepted by Astronomy and Astrophysics. 29 pages, 20 figures plus 3
appendice
<i>Gaia</i> Data Release 1. Summary of the astrometric, photometric, and survey properties
Context. At about 1000 days after the launch of Gaia we present the first Gaia data release, Gaia DR1, consisting of astrometry and photometry for over 1 billion sources brighter than magnitude 20.7.
Aims. A summary of Gaia DR1 is presented along with illustrations of the scientific quality of the data, followed by a discussion of the limitations due to the preliminary nature of this release.
Methods. The raw data collected by Gaia during the first 14 months of the mission have been processed by the Gaia Data Processing and Analysis Consortium (DPAC) and turned into an astrometric and photometric catalogue.
Results. Gaia DR1 consists of three components: a primary astrometric data set which contains the positions, parallaxes, and mean proper motions for about 2 million of the brightest stars in common with the HIPPARCOS and Tycho-2 catalogues – a realisation of the Tycho-Gaia Astrometric Solution (TGAS) – and a secondary astrometric data set containing the positions for an additional 1.1 billion sources. The second component is the photometric data set, consisting of mean G-band magnitudes for all sources. The G-band light curves and the characteristics of ∼3000 Cepheid and RR-Lyrae stars, observed at high cadence around the south ecliptic pole, form the third component. For the primary astrometric data set the typical uncertainty is about 0.3 mas for the positions and parallaxes, and about 1 mas yr−1 for the proper motions. A systematic component of ∼0.3 mas should be added to the parallax uncertainties. For the subset of ∼94 000 HIPPARCOS stars in the primary data set, the proper motions are much more precise at about 0.06 mas yr−1. For the secondary astrometric data set, the typical uncertainty of the positions is ∼10 mas. The median uncertainties on the mean G-band magnitudes range from the mmag level to ∼0.03 mag over the magnitude range 5 to 20.7.
Conclusions. Gaia DR1 is an important milestone ahead of the next Gaia data release, which will feature five-parameter astrometry for all sources. Extensive validation shows that Gaia DR1 represents a major advance in the mapping of the heavens and the availability of basic stellar data that underpin observational astrophysics. Nevertheless, the very preliminary nature of this first Gaia data release does lead to a number of important limitations to the data quality which should be carefully considered before drawing conclusions from the data
Gaia Data Release 3: Processing and validation of BP/RP low-resolution spectral data
(Abridged) Blue (BP) and Red (RP) Photometer low-resolution spectral data is
one of the exciting new products in Gaia Data Release 3 (Gaia DR3). We
calibrate about 65 billion individual transit spectra onto the same mean BP/RP
instrument through a series of calibration steps, including background
subtraction, calibration of the CCD geometry and an iterative procedure for the
calibration of CCD efficiency as well as variations of the line-spread function
and dispersion across the focal plane and in time. The calibrated transit
spectra are then combined for each source in terms of an expansion into
continuous basis functions. Time-averaged mean spectra covering the optical to
near-infrared wavelength range [330, 1050] nm are published for approximately
220 million objects. Most of these are brighter than G = 17.65 but some BP/RP
spectra are published for sources down to G = 21.43. Their signal- to-noise
ratio varies significantly over the wavelength range covered and with magnitude
and colour of the observed objects, with sources around G = 15 having S/N above
100 in some wavelength ranges. The top-quality BP/RP spectra are achieved for
sources with magnitudes 9 < G < 12, having S/N reaching 1000 in the central
part of the RP wavelength range. Scientific validation suggests that the
internal calibration was generally successful. However, there is some evidence
for imperfect calibrations at the bright end G < 11, where calibrated BP/RP
spectra can exhibit systematic flux variations that exceed their estimated flux
uncertainties. We also report that due to long-range noise correlations, BP/RP
spectra can exhibit wiggles when sampled in pseudo-wavelength.Comment: Submitted to A&
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Gaia Data Release 2: The celestial reference frame (Gaia -CRF2)
Context. The second release of Gaia data (Gaia DR2) contains the astrometric parameters for more than half a million quasars. This set defines a kinematically non-rotating reference frame in the optical domain. A subset of these quasars have accurate VLBI positions that allow the axes of the reference frame to be aligned with the International Celestial Reference System (ICRF) radio frame. Aims. We describe the astrometric and photometric properties of the quasars that were selected to represent the celestial reference frame of Gaia DR2 (Gaia-CRF2), and to compare the optical and radio positions for sources with accurate VLBI positions. Methods. Descriptive statistics are used to characterise the overall properties of the quasar sample. Residual rotation and orientation errors and large-scale systematics are quantified by means of expansions in vector spherical harmonics. Positional differences are calculated relative to a prototype version of the forthcoming ICRF3. Results. Gaia-CRF2 consists of the positions of a sample of 556 869 sources in Gaia DR2, obtained from a positional cross-match with the ICRF3-prototype and AllWISE AGN catalogues. The sample constitutes a clean, dense, and homogeneous set of extragalactic point sources in the magnitude range G ≈ 16 to 21 mag with accurately known optical positions. The median positional uncertainty is 0.12 mas for G < 18 mag and 0.5 mas at G = mag. Large-scale systematics are estimated to be in the range 20 to 30 μas. The accuracy claims are supported by the parallaxes and proper motions of the quasars in Gaia DR2. The optical positions for a subset of 2820 sources in common with the ICRF3-prototype show very good overall agreement with the radio positions, but several tens of sources have significantly discrepant positions. Conclusions. Based on less than 40% of the data expected from the nominal Gaia mission, Gaia-CRF2 is the first realisation of a non-rotating global optical reference frame that meets the ICRS prescriptions, meaning that it is built only on extragalactic sources. Its accuracy matches the current radio frame of the ICRF, but the density of sources in all parts of the sky is much higher, except along the Galactic equator
Gaia Data Release 1: Open cluster astrometry: Performance, limitations, and future prospects
Context. The first Gaia Data Release contains the Tycho-Gaia Astrometric
Solution (TGAS). This is a subset of about 2 million stars for which, besides
the position and photometry, the proper motion and parallax are calculated
using Hipparcos and Tycho-2 positions in 1991.25 as prior information. Aims. We
investigate the scientific potential and limitations of the TGAS component by
means of the astrometric data for open clusters. Methods. Mean cluster parallax
and proper motion values are derived taking into account the error correlations
within the astrometric solutions for individual stars, an estimate of the
internal velocity dispersion in the cluster, and, where relevant, the effects
of the depth of the cluster along the line of sight. Internal consistency of
the TGAS data is assessed. Results. Values given for standard uncertainties are
still inaccurate and may lead to unrealistic unit-weight standard deviations of
least squares solutions for cluster parameters. Reconstructed mean cluster
parallax and proper motion values are generally in very good agreement with
earlier Hipparcos-based determination, although the Gaia mean parallax for the
Pleiades is a significant exception. We have no current explanation for that
discrepancy. Most clusters are observed to extend to nearly 15 pc from the
cluster centre, and it will be up to future Gaia releases to establish whether
those potential cluster-member stars are still dynamically bound to the
clusters. Conclusions. The Gaia DR1 provides the means to examine open clusters
far beyond their more easily visible cores, and can provide membership
assessments based on proper motions and parallaxes. A combined HR diagram shows
the same features as observed before using the Hipparcos data, with clearly
increased luminosities for older A and F dwarfs
Gaia Data Release 3: The Galaxy in your preferred colours: Synthetic photometry from Gaia low-resolution spectra
Gaia Data Release 3 provides novel flux-calibrated low-resolution spectrophotometry for '220 million sources in the wavelength range 330 nm ≤ λ ≤ 1050 nm (XP spectra). Synthetic photometry directly tied to a flux in physical units can be obtained from these spectra for any passband fully enclosed in this wavelength range. We describe how synthetic photometry can be obtained from XP spectra, illustrating the performance that can be achieved under a range of different conditions - for example passband width and wavelength range - as well as the limits and the problems affecting it. Existing top-quality photometry can be reproduced within a few per cent over a wide range of magnitudes and colour, for wide and medium bands, and with up to millimag accuracy when synthetic photometry is standardised with respect to these external sources. Some examples of potential scientific application are presented, including the detection of multiple populations in globular clusters, the estimation of metallicity extended to the very metal-poor regime, and the classification of white dwarfs. A catalogue providing standardised photometry for 2.2×108sources in several wide bands of widely used photometric systems is provided (Gaia Synthetic Photometry Catalogue; GSPC) as well as a catalogue of '105 white dwarfs with DA/non-DA classification obtained with a Random Forest algorithm (Gaia Synthetic Photometry Catalogue for White Dwarfs; GSPC-WD)
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Gaia Data Release 1: Summary of the astrometric, photometric, and survey properties
At about 1000 days after the launch of Gaia we present the first Gaia data
release, Gaia DR1, consisting of astrometry and photometry for over 1 billion
sources brighter than magnitude 20.7. We summarize Gaia DR1 and provide
illustrations of the scientific quality of the data, followed by a discussion
of the limitations due to the preliminary nature of this release. Gaia DR1
consists of: a primary astrometric data set which contains the positions,
parallaxes, and mean proper motions for about 2 million of the brightest stars
in common with the Hipparcos and Tycho-2 catalogues and a secondary astrometric
data set containing the positions for an additional 1.1 billion sources. The
second component is the photometric data set,consisting of mean G-band
magnitudes for all sources. The G-band light curves and the characteristics of
~3000 Cepheid and RR Lyrae stars, observed at high cadence around the south
ecliptic pole, form the third component. For the primary astrometric data set
the typical uncertainty is about 0.3 mas for the positions and parallaxes, and
about 1 mas/yr for the proper motions. A systematic component of ~0.3 mas
should be added to the parallax uncertainties. For the subset of ~94000
Hipparcos stars in the primary data set, the proper motions are much more
precise at about 0.06 mas/yr. For the secondary astrometric data set, the
typical uncertainty of the positions is ~10 mas. The median uncertainties on
the mean G-band magnitudes range from the mmag level to ~0.03 mag over the
magnitude range 5 to 20.7. Gaia DR1 represents a major advance in the mapping
of the heavens and the availability of basic stellar data that underpin
observational astrophysics. Nevertheless, the very preliminary nature of this
first Gaia data release does lead to a number of important limitations to the
data quality which should be carefully considered before drawing conclusions
from the data
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