44 research outputs found
The context of the Local Volume: structures and motions in the nearby universe
The 6dF Galaxy Survey (6dFGS) and the 2MASS Redshift Survey (2MRS) provide
the most complete maps of the large-scale structures and motions in the nearby
universe. These maps have been used to reconstruct the density field in the
local volume, and to predict the corresponding velocity field and the dipole of
the Local Group motion.Comment: 4 pages, to appear in "Galaxies in the Local Volume", 2008, eds B.
Koribalski and H. Jerjen, Springer Astrophysics and Space Science Series
(proceedings of conference held in Sydney on 8-13 July 2007
The SuperCOSMOS Sky Survey. Paper III: Astrometry
In this, the third in a series of three papers concerning the SuperCOSMOS Sky
Survey, we describe the astrometric properties of the database. We describe the
algorithms employed in the derivation of the astrometric parameters of the
data, and demonstrate their accuracies by comparison with external datasets
using the first release of data, the South Galactic Cap survey. We show that
the celestial coordinates, which are tied to the International Celestial
Reference Frame via the Tycho-2 reference catalogue, are accurate to better
than +/- 0.2 arcsec at J,R=19,18 rising to +/- 0.3 arcsec at J,R=22,21 with
positional dependent systematic effects from bright to faint magnitudes at the
+/- 0.1 arcsec level. The proper motion measurements are shown to be accurate
to typically +/- 10 mas/yr at J,R=19,18 rising to +/- 50 mas/yr at J,R=22,21
and are tied to zero using the extragalactic reference frame. We show that the
zeropoint errors in the proper motions are 17 and are no
larger than 10 mas/yr for R < 17 mas/yr.Comment: 15 pages, 12 figures; accepted for publication in MNRA
The SuperCOSMOS Sky Survey. Paper II: Image detection, parameterisation, classification and photometry
In this, the second in a series of three papers concerning the SuperCOSMOS
Sky Survey, we describe the methods for image detection, parameterisation,
classification and photometry. We demonstrate the internal and external
accuracy of our object parameters. Using examples from the first release of
data, the South Galactic Cap survey, we show that our image detection
completeness is close to 100% to within 1.5 mag of the nominal plate limits. We
show that for the Bj survey data, the image classification is externally > 99%
reliable to Bj = 19.5. Internally, the image classification is reliable at a
level of > 90% to Bj=21, R=19. The photometric accuracy of our data is
typically 0.3 mag with respect to external data for m > 15. Internally, the
relative photometric accuracy in restricted position and magnitude ranges can
be as accurate as 5% for well exposed stellar images. Colours (B-R or R-I) are
externally accurate to 0.07 mag at Bj = 16.5 rising to 0.16 mag at Bj = 20.Comment: 22 pages, 16 figures; accepted for publication in MNRA
Preliminary results for RR Lyrae stars and Classical Cepheids from the Vista Magellanic Cloud (VMC) Survey
The Vista Magellanic Cloud (VMC, PI M.R. Cioni) survey is collecting
-band time series photometry of the system formed by the two Magellanic
Clouds (MC) and the "bridge" that connects them. These data are used to build
-band light curves of the MC RR Lyrae stars and Classical Cepheids and
determine absolute distances and the 3D geometry of the whole system using the
-band period luminosity (), the period - luminosity - color ()
and the Wesenhiet relations applicable to these types of variables. As an
example of the survey potential we present results from the VMC observations of
two fields centered respectively on the South Ecliptic Pole and the 30 Doradus
star forming region of the Large Magellanic Cloud. The VMC -band light
curves of the RR Lyrae stars in these two regions have very good photometric
quality with typical errors for the individual data points in the range of
0.02 to 0.05 mag. The Cepheids have excellent light curves (typical
errors of 0.01 mag). The average magnitudes derived for both types
of variables were used to derive relations that are in general good
agreement within the errors with the literature data, and show a smaller
scatter than previous studies.Comment: 7 pages, 6 figure. Accepted for publication in Astrophysics and Space
Science. Following a presentation at the conference "The Fundamental Cosmic
Distance Scale: State of the Art and the Gaia Perspective", Naples, May 201
Variation of the IMF
(abridged) The {stellar IMF} has been found to be essentially invariant.
While some apparent differences are seen, the uncertainties inherent to this
game do not allow a firm conclusion to be made that the IMF varies
systematically with conditions. The IMF integrated over entire galaxies,
however, is another matter. Chemical and photometric properties of various
galaxies do hint at {galaxial IMFs} being steeper than the stellar IMF, as is
also deduced from direct star-count analysis in the MW. These results are
sensitive to the modelling of stellar populations and to corrections for
stellar evolution, and are thus also uncertain. However, by realising that
galaxies are made from dissolving star clusters, star clusters being viewed as
{the fundamental building blocks of galaxies}, the result is found that
galaxial IMFs must be significantly steeper than the stellar IMF, because the
former results from a folding of the latter with the star-cluster mass
function. Furthermore, this notion leads to the important insight that galaxial
IMFs must vary with galaxy mass, and that the galaxial IMF is a strongly
varying function of the star-formation history for galaxies that have assembled
only a small mass in stars. Cosmological implications of this are discussed.Comment: 13 pages, to appear in IMFat50: The Initial Mass Function 50 years
later, ed: E. Corbelli, F. Palla, and H. Zinnecker, Kluwer Academic
Publishers; a meeting held at the Abbazia di Spineto, Tuscany, Italy -- May
16-20, 200
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Gaia Early Data Release 3: Gaia photometric science alerts
Context. Since July 2014, the Gaia mission has been engaged in a high-spatial-resolution, time-resolved, precise, accurate astrometric, and photometric survey of the entire sky. Aims. We present the Gaia Science Alerts project, which has been in operation since 1 June 2016. We describe the system which has been developed to enable the discovery and publication of transient photometric events as seen by Gaia. Methods. We outline the data handling, timings, and performances, and we describe the transient detection algorithms and filtering procedures needed to manage the high false alarm rate. We identify two classes of events: (1) sources which are new to Gaia and (2) Gaia sources which have undergone a significant brightening or fading. Validation of the Gaia transit astrometry and photometry was performed, followed by testing of the source environment to minimise contamination from Solar System objects, bright stars, and fainter near-neighbours. Results. We show that the Gaia Science Alerts project suffers from very low contamination, that is there are very few false-positives. We find that the external completeness for supernovae, CE = 0.46, is dominated by the Gaia scanning law and the requirement of detections from both fields-of-view. Where we have two or more scans the internal completeness is CI = 0.79 at 3 arcsec or larger from the centres of galaxies, but it drops closer in, especially within 1 arcsec. Conclusions. The per-Transit photometry for Gaia transients is precise to 1% at G = 13, and 3% at G = 19. The per-Transit astrometry is accurate to 55 mas when compared to Gaia DR2. The Gaia Science Alerts project is one of the most homogeneous and productive transient surveys in operation, and it is the only survey which covers the whole sky at high spatial resolution (subarcsecond), including the Galactic plane and bulge. © S. T. Hodgkin et al. 2021
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Gaia Early Data Release 3: The celestial reference frame (Gaia-CRF3)
Context. Gaia-CRF3 is the celestial reference frame for positions and proper motions in the third release of data from the Gaia mission, Gaia DR3 (and for the early third release, Gaia EDR3, which contains identical astrometric results). The reference frame is defined by the positions and proper motions at epoch 2016.0 for a specific set of extragalactic sources in the (E)DR3 catalogue. Aims. We describe the construction of Gaia-CRF3 and its properties in terms of the distributions in magnitude, colour, and astrometric quality. Methods. Compact extragalactic sources in Gaia DR3 were identified by positional cross-matching with 17 external catalogues of quasi-stellar objects (QSO) and active galactic nuclei (AGN), followed by astrometric filtering designed to remove stellar contaminants. Selecting a clean sample was favoured over including a higher number of extragalactic sources. For the final sample, the random and systematic errors in the proper motions are analysed, as well as the radio-optical offsets in position for sources in the third realisation of the International Celestial Reference Frame (ICRF3). Results. Gaia-CRF3 comprises about 1.6 million QSO-like sources, of which 1.2 million have five-parameter astrometric solutions in Gaia DR3 and 0.4 million have six-parameter solutions. The sources span the magnitude range G = 13-21 with a peak density at 20.6 mag, at which the typical positional uncertainty is about 1 mas. The proper motions show systematic errors on the level of 12 ÎŒas yr-1 on angular scales greater than 15 deg. For the 3142 optical counterparts of ICRF3 sources in the S/X frequency bands, the median offset from the radio positions is about 0.5 mas, but it exceeds 4 mas in either coordinate for 127 sources. We outline the future of Gaia-CRF in the next Gaia data releases. Appendices give further details on the external catalogues used, how to extract information about the Gaia-CRF3 sources, potential (Galactic) confusion sources, and the estimation of the spin and orientation of an astrometric solution
Gaia early data release 3: summary of the contents and survey properties (Corrigendum)
ERRATUMThis article is an erratum for:[https://doi.org/10.1051/0004-6361/202039657]âââââââInstrumentatio
Gaia Early Data Release 3: acceleration of the solar system from Gaia astrometry
Stars and planetary system