18 research outputs found
Understanding Dwarf Galaxies in order to Understand Dark Matter
Much progress has been made in recent years by the galaxy simulation
community in making realistic galaxies, mostly by more accurately capturing the
effects of baryons on the structural evolution of dark matter halos at high
resolutions. This progress has altered theoretical expectations for galaxy
evolution within a Cold Dark Matter (CDM) model, reconciling many earlier
discrepancies between theory and observations. Despite this reconciliation, CDM
may not be an accurate model for our Universe. Much more work must be done to
understand the predictions for galaxy formation within alternative dark matter
models.Comment: Refereed contribution to the Proceedings of the Simons Symposium on
Illuminating Dark Matter, to be published by Springe
Gaia Data Release 2. Kinematics of globular clusters and dwarf galaxies around the Milky Way
AIMS: The goal of this paper is to demonstrate the outstanding quality of the second data release of the Gaia mission and its power for constraining many different aspects of the dynamics of the satellites of the Milky Way. We focus here on determining the proper motions of 75 Galactic globular clusters, nine dwarf spheroidal galaxies, one ultra-faint system, and the Large and Small Magellanic Clouds. METHODS: Using data extracted from the Gaia archive, we derived the proper motions and parallaxes for these systems, as well as their uncertainties. We demonstrate that the errors, statistical and systematic, are relatively well understood. We integrated the orbits of these objects in three different Galactic potentials, and characterised their properties. We present the derived proper motions, space velocities, and characteristic orbital parameters in various tables to facilitate their use by the astronomical community. RESULTS: Our limited and straightforward analyses have allowed us for example to (i) determine absolute and very precise proper motions for globular clusters; (ii) detect clear rotation signatures in the proper motions of at least five globular clusters; (iii) show that the satellites of the Milky Way are all on high-inclination orbits, but that they do not share a single plane of motion; (iv) derive a lower limit for the mass of the Milky Way of 9.1{_₂.₆⁺⁶·²} x 10¹¹ M⊙ based on the assumption that the Leo I dwarf spheroidal is bound; (v) derive a rotation curve for the Large Magellanic Cloud based solely on proper motions that is competitive with line-of-sight velocity curves, now using many orders of magnitude more sources; and (vi) unveil the dynamical effect of the bar on the motions of stars in the Large Magellanic Cloud. CONCLUSIONS: All these results highlight the incredible power of the Gaia astrometric mission, and in particular of its second data release
Gaia Data Release 2 Variable stars in the colour-absolute magnitude diagram
Context. The ESA Gaia mission provides a unique time-domain survey for more than 1.6 billion sources with G ≲ 21 mag.
Aims. We showcase stellar variability in the Galactic colour-absolute magnitude diagram (CaMD). We focus on pulsating, eruptive, and cataclysmic variables, as well as on stars that exhibit variability that is due to rotation and eclipses.
Methods. We describe the locations of variable star classes, variable object fractions, and typical variability amplitudes throughout the CaMD and show how variability-related changes in colour and brightness induce “motions”. To do this, we use 22 months of calibrated photometric, spectro-photometric, and astrometric Gaia data of stars with a significant parallax. To ensure that a large variety of variable star classes populate the CaMD, we crossmatched Gaia sources with known variable stars. We also used the statistics and variability detection modules of the Gaia variability pipeline. Corrections for interstellar extinction are not implemented in this article.
Results. Gaia enables the first investigation of Galactic variable star populations in the CaMD on a similar, if not larger, scale as was previously done in the Magellanic Clouds. Although the observed colours are not corrected for reddening, distinct regions are visible in which variable stars occur. We determine variable star fractions to within the current detection thresholds of Gaia. Finally, we report the most complete description of variability-induced motion within the CaMD to date.
Conclusions. Gaia enables novel insights into variability phenomena for an unprecedented number of stars, which will benefit the understanding of stellar astrophysics. The CaMD of Galactic variable stars provides crucial information on physical origins of variability in a way that has previously only been accessible for Galactic star clusters or external galaxies. Future Gaia data releases will enable significant improvements over this preview by providing longer time series, more accurate astrometry, and additional data types (time series BP and RP spectra, RVS spectra, and radial velocities), all for much larger samples of stars
Distance determination for RAVE stars using stellar models
(Abridged) Aims:We develop a method for deriving distances from spectroscopic
data and obtaining full 6D phase-space coordinates for the RAVE survey's second
data release. Methods: We used stellar models combined with atmospheric
properties from RAVE (Teff, logg and [Fe/H]) and (J-Ks) photometry from
archival sources to derive absolute magnitudes. We are able to derive the full
6D phase-space coordinates for a large sample of RAVE stars. This method is
tested with artificial data, Hipparcos trigonometric parallaxes and
observations of the open cluster M67. Results: When we applied our method to a
set of 16 146 stars, we found that 25% (4 037) of the stars have relative
(statistical) distance errors of < 35%, while 50% (8 073) and 75% (12 110) have
relative (statistical) errors smaller than 45% and 50%, respectively. Our
various tests show that we can reliably estimate distances for main-sequence
stars, but there is an indication of potential systematic problems with giant
stars. For the main-sequence star sample (defined as those with log(g) > 4),
25% (1 744) have relative distance errors < 31%, while 50% (3 488) and 75% (5
231) have relative errors smaller than 36% and 42%, respectively. Our full
dataset shows the expected decrease in the metallicity of stars as a function
of distance from the Galactic plane. The known kinematic substructures in the U
and V velocity components of nearby dwarf stars are apparent in our dataset,
confirming the accuracy of our data and the reliability of our technique. We
provide independent measurements of the orientation of the UV velocity
ellipsoid and of the solar motion, and they are in very good agreement with
previous work. Conclusions: The distance catalogue for the RAVE second data
release is available at http://www.astro.rug.nl/~rav