ALBERT

Description: The mean absolute extinction towards the central parsec of the Milky Way is A K ~= 3 mag, including both foreground and Galactic Centre dust. Here we present a measurement of dust extinction within the Galactic old nuclear star cluster (NSC), based on combining differential extinctions of NSC stars with their l proper motions along Galactic longitude. Extinction within the NSC preferentially affects stars at its far side, and because the NSC rotates, this causes higher extinctions for NSC stars with negative l , as well as an asymmetry in the l -histograms. We model these effects using an axisymmetric dynamical model of the NSC in combination with simple models for the dust distribution. Comparing the predicted asymmetry to data for ~7100 stars in several NSC fields, we find that dust associated with the Galactic Centre mini-spiral with extinction A K ~= 0.15–0.8 mag explains most of the data. The largest extinction A K ~= 0.8 mag is found in the region of the Western arm of the mini-spiral. Comparing with total A K determined from stellar colours, we determine the extinction in front of the NSC. Finally, we estimate that for a typical extinction of A K ~= 0.4 the statistical parallax of the NSC changes by ~0.4 per cent.

Description: We derive new constraints on the mass, rotation, orbit structure, and statistical parallax of the Galactic old nuclear star cluster and the mass of the supermassive black hole. We combine star counts and kinematic data from Fritz et al., including 2500 line-of-sight velocities and 10 000 proper motions obtained with VLT instruments. We show that the difference between the proper motion dispersions l and b cannot be explained by rotation, but is a consequence of the flattening of the nuclear cluster. We fit the surface density distribution of stars in the central 1000 arcsec by a superposition of a spheroidal cluster with scale ~100 arcsec and a much larger nuclear disc component. We compute the self-consistent two-integral distribution function f ( E , L z ) for this density model, and add rotation self-consistently. We find that (i) the orbit structure of the f ( E , L z ) gives an excellent match to the observed velocity dispersion profiles as well as the proper motion and line-of-sight velocity histograms, including the double-peak in the v l -histograms. (ii) This requires an axial ratio near q 1  = 0.7 consistent with our determination from star counts, q 1  = 0.73 ± 0.04 for r  〈 70 arcsec. (iii) The nuclear star cluster is approximately described by an isotropic rotator model. (iv) Using the corresponding Jeans equations to fit the proper motion and line-of-sight velocity dispersions, we obtain best estimates for the nuclear star cluster mass, black hole mass, and distance M * ( r  〈 100 arcsec) = (8.94 ± 0.31| stat  ± 0.9| syst ) 10 6 M , M •  = (3.86 ± 0.14| stat  ± 0.4| syst ) 10 6 M , and R 0  = 8.27 ± 0.09| stat  ± 0.1| syst  kpc, where the estimated systematic errors account for additional uncertainties in the dynamical modelling. (v) The combination of the cluster dynamics with the S-star orbits around Sgr A* strongly reduces the degeneracy between black hole mass and Galactic Centre distance present in previous S-star studies. A joint statistical analysis with the results of Gillessen et al., gives M •  = (4.23 ± 0.14) 10 6 M and R 0  = 8.33 ± 0.11 kpc.

Description: We derive new constraints on the mass, rotation, orbit structure, and statistical parallax of the Galactic old nuclear star cluster and the mass of the supermassive black hole. We combine star counts and kinematic data from Fritz et al., including 2500 line-of-sight velocities and 10 000 proper motions obtained with VLT instruments. We show that the difference between the proper motion dispersions l and b cannot be explained by rotation, but is a consequence of the flattening of the nuclear cluster. We fit the surface density distribution of stars in the central 1000 arcsec by a superposition of a spheroidal cluster with scale ~100 arcsec and a much larger nuclear disc component. We compute the self-consistent two-integral distribution function f ( E , L z ) for this density model, and add rotation self-consistently. We find that (i) the orbit structure of the f ( E , L z ) gives an excellent match to the observed velocity dispersion profiles as well as the proper motion and line-of-sight velocity histograms, including the double-peak in the v l -histograms. (ii) This requires an axial ratio near q 1  = 0.7 consistent with our determination from star counts, q 1  = 0.73 ± 0.04 for r  〈 70 arcsec. (iii) The nuclear star cluster is approximately described by an isotropic rotator model. (iv) Using the corresponding Jeans equations to fit the proper motion and line-of-sight velocity dispersions, we obtain best estimates for the nuclear star cluster mass, black hole mass, and distance M * ( r  〈 100 arcsec) = (8.94 ± 0.31| stat  ± 0.9| syst ) 10 6 M , M •  = (3.86 ± 0.14| stat  ± 0.4| syst ) 10 6 M , and R 0  = 8.27 ± 0.09| stat  ± 0.1| syst  kpc, where the estimated systematic errors account for additional uncertainties in the dynamical modelling. (v) The combination of the cluster dynamics with the S-star orbits around Sgr A* strongly reduces the degeneracy between black hole mass and Galactic Centre distance present in previous S-star studies. A joint statistical analysis with the results of Gillessen et al., gives M •  = (4.23 ± 0.14) 10 6 M and R 0  = 8.33 ± 0.11 kpc.

Description: A proper understanding of the Milky Way (MW) dwarf galaxies in a cosmological context requires knowledge of their 3D velocities and orbits. However, proper motion (PM) measurements have generally been of limited accuracy and are available only for more massive dwarfs. We therefore present a new study of the kinematics of the MW dwarf galaxies. We use the Gaia DR2 for those dwarfs that have been spectroscopically observed in the literature. We derive systemic PMs for 39 galaxies and galaxy candidates out to 420 kpc, and generally find good consistency for the subset with measurements available from other studies. We derive the implied Galactocentric velocities, and calculate orbits in canonical MW halo potentials of low (0.8 × 1012 M⊙) and high mass (1.6 × 1012 M⊙). Comparison of the distributions of orbital apocenters and 3D velocities to the halo virial radius and escape velocity, respectively, suggests that the satellite kinematics are best explained in the high-mass halo. Tuc III, Crater II, and additional candidates have orbital pericenters small enough to imply significant tidal influences. Relevant to the missing satellite problem, the fact that fewer galaxies are observed to be near apocenter than near pericenter implies that there must be a population of distant dwarf galaxies yet to be discovered. Of the 39 dwarfs: 12 have orbital poles that do not align with the MW plane of satellites (given reasonable assumptions about its intrinsic thickness); 10 have insufficient PM accuracy to establish whether they align; and 17 satellites align, of which 11 are co-orbiting and (somewhat surprisingly, in view of prior knowledge) 6 are counter-orbiting. Group infall might have contributed to this, but no definitive association is found for the members of the Crater-Leo group.

Description: We use Gaia DR2 systemic proper motions of 45 satellite galaxies to constrain the mass of the Milky Way using the scale-free mass estimator of Watkins et al. (2010). We first determine the anisotropy parameter β, and the tracer satellites’ radial density index γ to be β = $-0.67^{+0.45}_{-0.62}$ and γ = 2.11 ± 0.23. When we exclude possible former satellites of the Large Magellanic Cloud, the anisotropy changes to β = $-0.21^{+0.37}_{-0.51}$. We find that the index of the Milky Way’s gravitational potential α, which is dependent on the mass itself, is the parameter with the largest impact on the mass determination. Via comparison with cosmological simulations of Milky Way-like galaxies, we carried out a detailed analysis of the estimation of the observational uncertainties and their impact on the mass estimator. We found that the mass estimator is biased when applied naively to the satellites of simulated Milky Way haloes. Correcting for this bias, we obtain for our Galaxy a mass of $0.58^{+0.15}_{-0.14}imes 10^{12}$ M⊙ within 64 kpc, as computed from the inner half of our observational sample, and $1.43^{+0.35}_{-0.32}imes 10^{12}$ M⊙ within 273 kpc, from the full sample; this latter value extrapolates to a virial mass of $M_mathrm{vir, Delta =97}=1.51^{+0.45}_{-0.40} imes 10^{12},{ m M}_{odot }$ corresponding to a virial radius of Rvir = 308 ± 29 kpc. This value of the Milky Way mass lies in-between other mass estimates reported in the literature, from various different methods.

Description: Context. The Milky Way nuclear star cluster (MWNSC) is a crucial laboratory for studying the galactic nuclei of other galaxies, but its properties have not been determined unambiguously until now. Aims. We aim to study the size and spatial structure of the MWNSC. Methods. This study uses data and methods that address potential shortcomings of previous studies on the topic. We use 0.2″ angular resolution Ks data to create a stellar density map in the central 86.4 pc × 21 pc at the Galactic center. We include data from selected adaptive-optics-assisted images obtained for the inner parsecs. In addition, we use Spitzer/IRAC mid-infrared (MIR) images. We model the Galactic bulge and the nuclear stellar disk in order to subtract them from the MWNSC. Finally, we fit a Sérsic model to the MWNSC and investigate its symmetry. Results. Our results are consistent with previous work. The MWNSC is flattened with an axis ratio of q = 0.71 ± 0.10, an effective radius of Re = (5.1 ± 1.0) pc, and a Sérsic index of n = 2.2 ± 0.7. Its major axis may be tilted out of the Galactic plane by up to −10°. The distribution of the giants brighter than the Red Clump (RC) is found to be significantly flatter than the distribution of the faint stars. We investigate the 3D structure of the central stellar cusp using our results on the MWNSC structure on large scales to constrain the deprojection of the measured stellar surface number density, obtaining a value of the 3D inner power law of γ = 1.38 ± 0.06sys ± 0.01stat. Conclusions. The MWNSC shares its main properties with other extragalactic NSCs found in spiral galaxies. The differences in the structure between bright giants and RC stars might be related to the existence of not completely mixed populations of different ages. This may hint at recent growth of the MWNSC through star formation or cluster accretion.

Description: A wealth of tiny galactic systems populates the surroundings of the Milky Way. However, some of these objects might have originated as former satellites of the Magellanic Clouds, in particular of the Large Magellanic Cloud (LMC). Examples of the importance of understanding how many systems are genuine satellites of the Milky Way or the LMC are the implications that the number and luminosity-mass function of satellites around hosts of different mass have for dark matter theories and the treatment of baryonic physics in simulations of structure formation. Here we aim at deriving the bulk motions and estimates of the internal velocity dispersion and metallicity properties in four recently discovered distant southern dwarf galaxy candidates, Columba I, Reticulum III, Phoenix II, and Horologium II. We combined Gaia DR2 astrometric measurements, photometry, and new FLAMES/GIRAFFE intermediate-resolution spectroscopic data in the region of the near-IR Ca II triplet lines; this combination is essential for finding potential member stars in these low-luminosity systems. We find very likely member stars in all four satellites and are able to determine (or place limits on) the bulk motions and average internal properties of the systems. The systems are found to be very metal poor, in agreement with dwarf galaxies and dwarf galaxy candidates of similar luminosity. Of these four objects, we can only firmly place Phoenix II in the category of dwarf galaxies because of its resolved high velocity dispersion (9.5 −4.4+6.8 km s−1) and intrinsic metallicity spread (0.33 dex). For Columba I we also measure a clear metallicity spread. The orbital pole of Phoenix II is well constrained and close to that of the LMC, suggesting a prior association. The uncertainty on the orbital poles of the other systems is currently very large, so that an association cannot be excluded, except for Columba I. Using the numbers of potential former satellites of the LMC identified here and in the literature, we obtain for the LMC a dark matter mass of M200 = 1.9 −0.9+1.3 × 1011 M⊙.