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Register a new userGlobular Clusters in the Sagittarius stream. Revising members and candidates with Gaia DR2
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We reconsider the case for the association of Galactic globular clusters (GCs) to the tidal stream of the Sagittarius dwarf spheroidal galaxy (Sgr dSph), using Gaia DR2 data. We use RR Lyrae to trace the stream in 6D and we select clusters matching the observed stream in position and velocity. In addition to the clusters residing in the main body of the galaxy (M 54, Ter 8, Ter 7, Arp 2) we confirm the membership of Pal 12 and Whiting 1 to the portion of the trailing arm populated by stars lost during recent perigalactic passages. NGC 2419, NGC 5634 and NGC 4147 are very interesting candidates, possibly associated to more ancient wraps of the stream. We note that all these clusters, with the exception of M 54, that lies within the stellar nucleus of the galaxy, are found in the trailing arm of the stream. The selected clusters are fully consistent with the [Fe/H] vs. [Mg/Fe], [Ca/Fe] patterns and the age-metallicity relation displayed by field stars in the main body of Sgr dSph.
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We use Gaia DR2 data to show that the globular cluster NGC5634 is physically associated with an arm of the Sagittarius Stream, the huge system of tidal tails created by the ongoing disruption of the Sagittarius dwarf spheroidal galaxy (Sgr dSph). Two additional arms of the Stream are also detected along the same line of sight, at different distances. We show that the Sgr Stream stars surrounding NGC5634 are more metal-poor, on average, than those found in the more distant Stream arm lying behind the cluster and in the main body of Sgr~dSph, confirming that a significant metallicity (and, presumably, age) gradient is present along the Stream. This analysis demonstrates the potential of the Gaia DR2 catalogue to directly verify if a cluster is physically associated to the Stream or not, without the need to rely on models of the tidal disruption of this system. [Withdrawn: see comments]
The Sagittarius stream is one of the best tools that we currently have to estimate the mass and shape of our Galaxy. However, assigning membership and obtaining the phase-space distribution of the stars that form the tails is quite challenging. Our goal is to produce a catalogue of RR Lyrae stars of Sagittarius and obtain an empiric measurement of the trends along the stream in sky position, distance and tangential velocities. We generate two initial samples from the Gaia DR2 RR Lyrae catalogue: one, selecting only the stars within pm20deg of the orbital plane of Sagittarius (Strip) and the other, the result of applying the Pole Count Map (nGC3) algorithm. We then use the model-independent, deterministic method developed in this work to remove most of the contamination by detecting and isolating the stream in distance and proper motions. The output is two empiric catalogues: the Strip sample (higher-completeness, lower-purity) which contains 11 677 stars, and the nGC3 sample (higher-purity, lower-completeness) with 6 608 stars. We characterise the changes along the stream in all the available dimensions, the 5 astrometric ones plus the metallicity, covering more than 2pi rad in the sky and obtain new estimates for the apocentres and the mean [Fe/H] of the RR Lyrae population. Also, we show the first map of the two components of the tangential velocity, thanks to the combination of distances and proper motions. Finally, we detect the bifurcation in the leading arm and report no significant difference between the two branches, either in metallicity, kinematics or distance. We provide the largest sample of RR Lyrae candidates of Sagittarius, which can be used as an input for a spectroscopic follow-up or as a reference for the new generation of models of the stream through the interpolators in distance and velocity that we have constructed.
We present the first full six-dimensional panoramic portrait of the Sagittarius stream, obtained by searching for wide stellar streams in the Gaia DR2 dataset with the STREAMFINDER algorithm. We use the kinematic behavior of the sample to devise a selection of Gaia RR Lyrae, providing excellent distance measurements along the stream. The proper motion data are complemented with radial velocities from public surveys. We find that the global morphological and kinematic properties of the Sagittarius stream are still reasonably well reproduced by the simple Law & Majewski (2010) model (LM10), although the model overestimates the leading arm and trailing arm distances by up to $sim 15$%. The sample newly reveals the leading arm of the Sagittarius stream as it passes into very crowded regions of the Galactic disk towards the Galactic Anticenter direction. Fortuitously, this part of the stream is almost exactly at the diametrically opposite location from the Galactic Center to the progenitor, which should allow an assessment of the influence of dynamical friction and self-gravity in a way that is nearly independent of the underlying Galactic potential model.
Line-of-sight kinematic studies indicate that many Galactic globular clusters have a significant degree of internal rotation. However, three-dimensional kinematics from a combination of proper motions and line-of-sight velocities are needed to unveil the role of angular momentum in the formation and evolution of these old stellar systems. Here we present the first quantitative study of internal rotation on the plane-of-the-sky for a large sample of globular clusters using proper motions from Gaia DR2. We detect signatures of rotation in the tangential component of proper motions for 11 out of 51 clusters at a $>$3-sigma confidence level, confirming the detection reported in Gaia collaboration et al. (2018) for 8 clusters, and additionally identify 11 GCs with a 2-sigma rotation detection. For the clusters with a detected global rotation, we construct the two-dimensional rotation maps and proper motion rotation curves, and we assess the relevance of rotation with respect to random motions ($V/sigmasim0.08-0.51$). We find evidence of a correlation between the degree of internal rotation and relaxation time, highlighting the importance of long-term dynamical evolution in shaping the clusters current properties. This is a strong indication that angular momentum must have played a fundamental role in the earliest phases of cluster formation. Finally, exploiting the spatial information of the rotation maps and a comparison with line-of-sight data, we provide an estimate of the inclination of the rotation axis for a subset of 8 clusters. Our work demonstrates the potential of Gaia data for internal kinematic studies of globular clusters and provides the first step to reconstruct their intrinsic three-dimensional structure.
The treatment of crowded fields in Gaia data will only be a reality in a few years from now. In particular, for globular clusters, only the end-of-mission data (public in 2022-2023) will have the necessary full crowding treatment and will reach sufficient quality for the faintest stars. As a consequence, the work on the deblending and decontamination pipelines is still ongoing. We describe the present status of the pipelines for different Gaia instruments, and we model the end-of-mission crowding errors on the basis of available information. We then apply the nominal post-launch Gaia performances, appropriately worsened by the estimated crowding errors, to a set of 18 simulated globular clusters with different concentration, distance, and field contamination. We conclude that there will be 103-104 stars with astrometric performances virtually untouched by crowding (contaminated by <1 mmag) in the majority of clusters. The most limiting factor will be field crowding, not cluster crowding: the most contaminated clusters will only contain 10-100 clean stars. We also conclude that: (i) the systemic proper motions and parallaxes will be determined to 1% or better up to 15 kpc, and the nearby clusters will have radial velocities to a few km/s ; (ii) internal kinematics will be of unprecendented quality, cluster masses will be determined to 10% up to 15 kpc and beyond, and it will be possible to identify differences of a few km/s or less in the kinematics (if any) of cluster sub-populations up to 10 kpc and beyond; (iii) the brightest stars (V<17 mag) will have space-quality, wide-field photometry (mmag errors), and all Gaia photometry will have 1-3% errors on the absolute photometric calibration.
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