ALBERT

Description: We study the late-time evolution of the central regions of two Milky Way (MW)-like simulations of galaxies formed in a cosmological context, one hosting a fast bar and the other a slow one. We find that bar length, Rb, measurements fluctuate on a dynamical time-scale by up to 100 per cent, depending on the spiral structure strength and measurement threshold. The bar amplitude oscillates by about 15 per cent, correlating with Rb. The Tremaine–Weinberg method estimates of the bars’ instantaneous pattern speeds show variations around the mean of up to $sim !20{{ m per cent}}$, typically anticorrelating with the bar length and strength. Through power spectrum analyses, we establish that these bar pulsations, with a period in the range ∼60–200 Myr, result from its interaction with multiple spiral modes, which are coupled with the bar. Because of the presence of odd spiral modes, the two bar halves typically do not connect at exactly the same time to a spiral arm, and their individual lengths can be significantly offset. We estimated that in about 50 per cent of bar measurements in MW-mass external galaxies, the bar lengths of SBab-type galaxies are overestimated by $sim !15{{ m per cent}}$ and those of SBbc types by $sim !55{{ m per cent}}$. Consequently, bars longer than their corotation radius reported in the literature, dubbed ‘ultrafast bars’, may simply correspond to the largest biases. Given that the Scutum–Centaurus arm is likely connected to the near half of the MW bar, recent direct measurements may be overestimating its length by 1–1.5 kpc, while its present pattern speed may be 5–10 $ m km s^{-1} kpc^{-1}$ smaller than its time-averaged value.

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 investigate interstellar extinction curve variations towards ~4 deg 2 of the inner Milky Way in VIJK s photometry from the OGLE-III (third phase of the Optical Gravitational Lensing Experiment) and VVV (VISTA Variables in the Via Lactea) surveys, with supporting evidence from diffuse interstellar bands and F 435 W, F 625 W photometry. We obtain independent measurements towards ~2000 sightlines of A I , E ( V – I ), E ( I – J ) and E ( J – K s ), with median precision and accuracy of 2 per cent. We find that the variations in the extinction ratios A I / E ( V – I ), E ( I – J )/ E ( V – I ) and E ( J – K s )/ E ( V – I ) are large (exceeding 20 per cent), significant and positively correlated, as expected. However, both the mean values and the trends in these extinction ratios are drastically shifted from the predictions of Cardelli and Fitzpatrick, regardless of how R V is varied . Furthermore, we demonstrate that variations in the shape of the extinction curve have at least two degrees of freedom, and not one (e.g. R V ), which we confirm with a principal component analysis. We derive a median value of 〈 A V / A Ks 〉 = 13.44, which is ~60 per cent higher than the ‘standard’ value. We show that the Wesenheit magnitude W I  =  I – 1.61( I – J ) is relatively impervious to extinction curve variations. Given that these extinction curves are linchpins of observational cosmology, and that it is generally assumed that R V variations correctly capture variations in the extinction curve, we argue that systematic errors in the distance ladder from studies of Type Ia supernovae and Cepheids may have been underestimated. Moreover, the reddening maps from the Planck experiment are shown to systematically overestimate dust extinction by ~100 per cent and lack sensitivity to extinction curve variations.

Description: Microlensing provides a unique tool to break the stellar to dark matter degeneracy in the inner Milky Way. We combine N -body dynamical models fitted to the Milky Way's Boxy/Peanut bulge with exponential disc models outside this, and compute the microlensing properties. Considering the range of models consistent with the revised MOA-II data, we find low dark matter fractions in the inner Galaxy: at the peak of their stellar rotation curve a fraction f v  = (0.88 ± 0.07) of the circular velocity is baryonic (at 1, f v  〉 0.72 at 2). These results are in agreement with constraints from the EROS-II microlensing survey of brighter resolved stars, where we find f v  = (0.9 ± 0.1) at 1. Our fiducial model of a disc with scale length 2.6 kpc, and a bulge with a low dark matter fraction of 12 per cent, agrees with both the revised MOA-II and EROS-II microlensing data. The required baryonic fractions, and the resultant low contribution from dark matter, are consistent with the NFW profiles produced by dissipationless cosmological simulations in Milky Way mass galaxies. They are also consistent with recent prescriptions for the mild adiabatic contraction of Milky Way mass haloes without the need for strong feedback, but there is some tension with recent measurements of the local dark matter density. Microlensing optical depths from the larger OGLE-III sample could improve these constraints further when available.

Description: While it is incontrovertible that the inner Galaxy contains a bar, its structure near the Galactic plane has remained uncertain, where extinction from intervening dust is greatest. We investigate here the Galactic bar outside the bulge, the long bar, using red clump giant (RCG) stars from United Kingdom Infrared Deep Sky Survey, Two Micron All Sky Survey, Vista Variables in the Via Lactea and Galactic Legacy Infrared Midplane Survey Extraordinaire. We match and combine these surveys to investigate a wide area in latitude and longitude, | b | ≤ 9° and | l | ≤ 40°. We find (i) the bar extends to l ~ 25° at | b | ~ 5° from the Galactic plane, and to l ~ 30° at lower latitudes; (ii) the long bar has an angle to the line-of-sight in the range (28°–33°), consistent with studies of the bulge at | l | 〈 10°; (iii) the scale height of RCG stars smoothly transitions from the bulge to the thinner long bar; (iv) there is evidence for two scale heights in the long bar; we find a ~180 pc thin bar component reminiscent of the old thin disc near the Sun, and a ~45 pc superthin bar components which exist predominantly towards the bar end; (v) constructing parametric models for the red clump magnitude distributions, we find a bar half-length of 5.0 ± 0.2 kpc for the two-component bar, and 4.6 ± 0.3 kpc for the thin bar component alone. We conclude that the Milky Way contains a central box/peanut bulge which is the vertical extension of a longer, flatter bar, similar as seen in both external galaxies and N -body models.

Description: Recent observations have discovered the presence of a box/peanut or X-shape structure in the Galactic bulge. Such box/peanut structures are common in external disc galaxies, and are well known in N -body simulations where they form following the buckling instability of a bar. From studies of analytical potentials and N -body models, it has been claimed in the past that box/peanut bulges are supported by ‘bananas’, or x 1 v 1 orbits. We present here a set of N -body models where instead the peanut bulge is mainly supported by brezel-like orbits, allowing strong peanuts to form with short extent relative to the bar length. This shows that stars in the X-shape do not necessarily stream along banana orbits which follow the arms of the X-shape. The brezel orbits are also found to be the main orbital component supporting the peanut shape in our recent made-to-measure dynamical models of the Galactic bulge. We also show that in these models the fraction of stellar orbits that contribute to the X-structure account for 40–45 per cent of the stellar mass.

Description: The inner Milky Way is dominated by a boxy, triaxial bulge which is believed to have formed through disc instability processes. Despite its proximity, its large-scale properties are still not very well known, due to our position in the obscuring Galactic disc. Here, we make a measurement of the three-dimensional density distribution of the Galactic bulge using red clump giants identified in DR1 of the Vista Variables in the Via Lactea survey. Our density map covers the inner (2.2 1.4 1.1) kpc of the bulge/bar. Line-of-sight density distributions are estimated by deconvolving extinction- and completeness-corrected K s -band magnitude distributions. In constructing our measurement, we assume that the three-dimensional bulge is eightfold mirror triaxially symmetric. In doing so, we measure the angle of the bar–bulge to the line of sight to be (27 ± 2)°, where the dominant error is systematic arising from the details of the deconvolution process. The resulting density distribution shows a highly elongated bar with projected axis ratios (1 : 2.1) for isophotes reaching ~2 kpc along the major axis. Along the bar axes the density falls off roughly exponentially, with axis ratios (10 : 6.3 : 2.6) and exponential scalelengths (0.70 : 0.44 : 0.18) kpc. From about 400 pc above the Galactic plane, the bulge density distribution displays a prominent X-structure. Overall, the density distribution of the Galactic bulge is characteristic for a strongly boxy/peanut-shaped bulge within a barred galaxy.

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: We consider the formation of extreme mass-ratio inspirals (EMRIs) sourced from a stellar cusp centred on a primary supermassive black hole (SMBH) and perturbed by an inspiraling less massive secondary SMBH. The problem is approached numerically, assuming the stars are non-interacting over these short time-scales and performing an ensemble of restricted three-body integrations. From these simulations, we see that not only can EMRIs be produced during this process, but the dynamics are also quite rich. In particular, most of the EMRIs are produced through a process akin to the Kozai–Lidov mechanism, but with strong effects due to the non-Keplerian stellar potential, general relativity and non-secular oscillations in the angular momentum on the orbital time-scale of the binary SMBH system.