The second data release of the Gaia mission has revealed a very rich structure in local velocity space. In terms of in-plane motions, this rich structure is also seen as multiple ridges in the ...actions of the axisymmetric background potential of the Galaxy. These ridges are probably related to a combination of effects from ongoing phase-mixing and resonances from the spiral arms and the bar. We have recently developed a method for capturing the behaviour of the stellar phase-space distribution function at a resonance by re-expressing it in terms of a new set of canonical actions and angles variables valid in the resonant region. Here, by properly treating the distribution function at resonances, and by using a realistic model for a slowly rotating large Galactic bar with pattern speed Ωb = 39 km s−1 kpc−1, we show that no fewer than six ridges in local action space can be related to resonances with the bar. Two of these ridges at low angular momentum correspond to the corotation resonance, and can be associated with the Hercules moving group in local velocity space. Another ridge at high angular momentum corresponds to the outer Lindblad resonance, and can tentatively be associated with the velocity structure seen as an arch at high azimuthal velocities in Gaia data. The other ridges are associated with the 3:1, 4:1, and 6:1 resonances. The last can be associated with the so-called “horn” of the local velocity distribution. While it is clear that effects from spiral arms and incomplete phase-mixing related to external perturbations also play a role in shaping the complex kinematics revealed by Gaia data, the present work demonstrates that, contrary to common misconceptions, the bar alone can create multiple prominent ridges in velocity and action space.
We construct dynamical models of the Milky Way's box/peanut (B/P) bulge, using the recently measured 3D density of red clump giants (RCGs) as well as kinematic data from the Bulge Radial Velocity ...Assay (BRAVA) survey. We match these data using the nmagic made-to-measure method, starting with N-body models for barred discs in different dark matter haloes. We determine the total mass in the bulge volume of the RCGs measurement ( ± 2.2 × ±1.4 × ±1.2 kpc) with unprecedented accuracy and robustness to be 1.84 ± 0.07 × 1010 M⊙. The stellar mass in this volume varies between 1.25 and 1.6 × 1010 M⊙, depending on the amount of dark matter in the bulge. We evaluate the mass-to-light and mass-to-clump ratios in the bulge and compare them to theoretical predictions from population synthesis models. We find a mass-to-light ratio in the K band in the range 0.8–1.1. The models are consistent with a Kroupa or Chabrier initial mass function (IMF), but a Salpeter IMF is ruled out for stellar ages of 10 Gyr. To match predictions from the Zoccali IMF derived from the bulge stellar luminosity function requires ∼40 per cent or ∼ 0.7 × 1010 M⊙ dark matter in the bulge region. The BRAVA data together with the RCGs 3D density imply a low pattern speed for the Galactic B/P bulge of Ωp = 25-30 km s− 1 kpc− 1. This would place the Galaxy among the slow rotators (
$\mathcal {R}\ge 1.5$
). Finally, we show that the Milky Way's B/P bulge has an off-centred X structure, and that the stellar mass involved in the peanut shape accounts for at least 20 per cent of the stellar mass of the bulge, significantly larger than previously thought.
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 σ
l
and σ
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) × 106 M⊙, M
• = (3.86 ± 0.14|stat ± 0.4|syst) × 106 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) × 106 M⊙ and R
0 = 8.33 ± 0.11 kpc.
(ProQuest: ... denotes formulae and/or non-USASCII text omitted)We obtain the basic properties of the nuclear cluster of the Milky Way. First, we investigate the structural properties by constructing ...a stellar density map of the central 1000'' using extinction-corrected old star counts from VISTA, WFC3/IR, and VLT/NACO data. We describe the data using two components. The inner, slightly flattened (axis ratio of q= 0.80 + or - 0.04) 7 component is the nuclear cluster, while the outer component corresponds to the stellar component of the circumnuclear zone. For the nuclear cluster, we measure a half-light radius of 178 + or - 0.04 51" thickapproximate + or - 2 pc and a luminosity of M sub(ks)= - 16.0 + or - 0.5. Second, we measure detailed dynamics out to 4 pc. We obtain 10,351 proper motions from AO data, and 2513 radial velocities from VLT/SINFONI data. We determine the cluster mass by means of isotropic spherical Jeans modeling. We fix the distance to the Galactic Center and the mass of the supermassive black hole. We model the cluster either with a constant M/L or with a power law. For the latter case, we obtain a slope of 1.18 + or - 0.06. We get a cluster mass within 100'' of ... for both modeling approaches. A model which includes the observed flattening gives a 47% larger mass (see Chatzopoulos et al.). Our results slightly favor a core over a cusp in the mass profile. By minimizing the number of unbound stars within 8'', we obtain a distance of ... kpc when using an R sub(0) supermassive black hole mass relation from stellar orbits. Combining our results, we obtain ... , which is roughly consistent with a Chabrier IMF.
Context.
Early-type galaxies (ETGs) are found to follow a wide variety of merger and accretion histories in cosmological simulations.
Aims.
We characterize the photometric and kinematic properties of ...simulated ETG stellar halos, and compare them to the observations.
Methods.
We selected a sample of 1114 ETGs in the TNG100 simulation and 80 in the higher-resolution TNG50. These ETGs span a stellar mass range of 10
10.3
− 10
12
M
⊙
and they were selected within the range of
g
−
r
colour and
λ
-ellipticity diagram populated by observed ETGs. We determined photometric parameters, intrinsic shapes, and kinematic observables in their extended stellar halos. We compared the results with central IFU kinematics and ePN.S planetary nebula velocity fields at large radii, studying the variation in kinematics from center to halo, and connecting it to a change in the intrinsic shape of the galaxies.
Results.
We find that the simulated galaxy sample reproduces the diversity of kinematic properties observed in ETG halos. Simulated fast rotators (FRs) divide almost evenly in one third having flat
λ
profiles and high halo rotational support, a third with gently decreasing profiles, and another third with low halo rotation. However, the peak of rotation occurs at larger
R
than in observed ETG samples. Slow rotators (SRs) tend to have increased rotation in the outskirts, with half of them exceeding
λ
= 0.2. For
M
*
> 10
11.5
M
⊙
halo rotation is unimportant. A similar variety of properties is found for the stellar halo intrinsic shapes. Rotational support and shape are deeply related: the kinematic transition to lower rotational support is accompanied by a change towards rounder intrinsic shape. Triaxiality in the halos of FRs increases outwards and with stellar mass. Simulated SRs have relatively constant triaxiality profiles.
Conclusions.
Simulated stellar halos show a large variety of structural properties, with quantitative but no clear qualitative differences between FRs and SRs. At the same stellar mass, stellar halo properties show a more gradual transition and significant overlap between the two families, despite the clear bimodality in the central regions. This is in agreement with observations of extended photometry and kinematics.
We use a sample of 938 red clump giant stars located in the direction of the Galactic long bar to study the chemistry of Milky Way bar stars. Kinematically separating stars on bar orbits from stars ...with inner disc orbits, we find that stars on bar-like orbits are more metal rich with a mean iron abundance of ⟨Fe/H⟩ = +0.30 compared to ⟨Fe/H⟩ = +0.03 for the inner disc. Spatially selecting bar stars is complicated by a strong vertical metallicity gradient of −1.1 dex kpc−1, but we find the metallicity distribution varies in a manner consistent with our orbital selection. Our results have two possible interpretations. The first is that the most metal rich stars in the inner Galaxy pre-existed the bar, but were kinematically cold at the time of bar formation and therefore more easily captured onto bar orbits when the bar formed. The second is that the most metal rich stars formed after the bar, either directly onto the bar following orbits or were captured by the bar after their formation.
Aims. We present kinematic and photometric evidence for an accretion event in the halo of the cD galaxy M 87 in the last Gyr. Methods. Using velocities for ~300 planetary nebulas (PNs) in the M 87 ...halo, we identify a chevron-like substructure in the PN phase-space. We implement a probabilistic Gaussian mixture model to identify PNs that belong to the chevron. From analysis of deep V-band images of M 87, we find that the region with the highest density of chevron PNs is a crown-shaped substructure in the light. Results. We assign a total of NPN,sub = 54 to the substructure, which extends over ~50 kpc along the major axis where we also observe radial variations of the ellipticity profile and a colour gradient. The substructure has highest surface brightness in a 20 kpc × 60 kpc region around 70 kpc in radius. In this region, the substructure causes an increase in surface brightness by ≳60%. The accretion event is consistent with a progenitor galaxy with a V-band luminosity of L = 2.8±1.0×109 L⊙ ,V, a colour of (B − V) = 0.76±0.05, and a stellar mass of M = 6.4±2.3×109 M⊙. Conclusions. The accretion of this progenitor galaxy has caused an important modification of the outer halo of M 87 in the last Gyr. This result provides strong evidence that the galaxy’s cD halo is growing through the accretion of smaller galaxies as predicted by hierarchical galaxy evolution models.
In this paper we introduce a new method for analysing Milky Way phase-space which allows us to reveal the imprint left by the Milky Way bar and spiral arms on the stars with full phase-space data in
...Gaia
Data Release 2. The unprecedented quality and extended spatial coverage of these data allowed us to discover six prominent stellar density structures in the disc to a distance of 5 kpc from the Sun. Four of these structures correspond to the spiral arms detected previously in the gas and young stars (Scutum-Centaurus, Sagittarius, Local, and Perseus). The remaining two are associated with the main resonances of the Milky Way bar where corotation is placed at around 6.2 kpc and the outer Lindblad resonance beyond the solar radius, at around 9 kpc. For the first time we provide evidence of the imprint left by spiral arms and resonances in the stellar densities not relying on a specific tracer, through enhancing the signatures left by these asymmetries. Our method offers new avenues for studying how the stellar populations in our Galaxy are shaped.
Aims. We present a kinematic study of a sample of 298 planetary nebulas (PNs) in the outer halo of the central Virgo galaxy M 87 (NGC 4486). The line-of-sight velocities of these PNs are used to ...identify subcomponents, to measure the angular momentum content of the main M 87 halo, and to constrain the orbital distribution of the stars at these large radii. Methods. We use Gaussian mixture modelling to statistically separate distinct velocity components and identify the M 87 smooth halo component, its unrelaxed substructures, and the intra-cluster (IC) PNs. We compute probability weighted velocity and velocity dispersion maps for the smooth halo, and its specific angular momentum profile (λR) and velocity dispersion profile. Results. The classification of the PNs into smooth halo and ICPNs is supported by their different PN luminosity functions. Based on a Kolmogorov–Smirnov (K–S) test, we conclude that the ICPN line-of-sight velocity distribution (LOSVD) is consistent with the LOSVD of the galaxies in Virgo subcluster A. The surface density profile of the ICPNS at 100 kpc radii has a shallow logarithmic slope, −αICL ≃ −0.8, dominating the light at the largest radii. Previous B − V colour and resolved star metallicity data indicate masses for the ICPN progenitor galaxies of a few ×108 M⊙. The angular momentum-related λR profile for the smooth halo remains below 0.1, in the slow rotator regime, out to 135 kpc average ellipse radius (170 kpc major axis distance). Combining the PN velocity dispersion measurements for the M 87 halo with literature data in the central 15 kpc, we obtain a complete velocity dispersion profile out to Ravg = 135 kpc. The σhalo profile decreases from the central 400 km s−1 to about 270 km s−1 at 2–10 kpc, then rises again to ≃300 ± 50 km s−1 at 50–70 kpc, to finally decrease sharply to σhalo ∼ 100 km s−1 at Ravg = 135 kpc. The steeply decreasing outer σhalo profile and the surface density profile of the smooth halo can be reconciled with the circular velocity curve inferred from assuming hydrostatic equilibrium for the hot X-ray gas. Because this rises to νc,X ∼ km s−1 at 200 kpc, the orbit distribution of the smooth M 87 halo is required to change strongly from approximately isotropic within Ravg ∼ 60 kpc to very radially anisotropic at the largest distances probed. Conclusions. The extended LOSVD of the PNs in the M 87 halo allows the identification of several subcomponents: the ICPNs, the “crown” accretion event, and the smooth M 87 halo. In galaxies like M 87, the presence of these subcomponents needs to be taken into account to avoid systematic biases in estimating the total enclosed mass. The dynamical structure inferred from the velocity dispersion profile indicates that the smooth halo of M 87 steepens beyond Ravg = 60 kpc and becomes strongly radially anisotropic, and that the velocity dispersion profile is consistent with the X-ray circular velocity curve at these radii without non-thermal pressure effects.
In decomposing the H i rotation curves of disc galaxies, it is necessary to break a degeneracy between the gravitational fields of the disc and the dark halo by estimating the disc surface density. ...This is done by combining measurements of the vertical velocity dispersion of the disc with the disc scaleheight. The vertical velocity dispersion of the discs is measured from absorption lines (near the V band) of near-face-on spiral galaxies, with the light coming from a mixed population of giants of all ages. However, the scaleheights for these galaxies are estimated statistically from near-IR surface photometry of edge-on galaxies. The scaleheight estimate is therefore dominated by a population of older (>2 Gyr) red giants. In this paper, we demonstrate the importance of measuring the velocity dispersion for the same older population of stars that is used to estimate the vertical scaleheight. We present an analysis of the vertical kinematics of K-giants in the solar vicinity. We find the vertical velocity distribution best fitted by two components with dispersions of 9.6 ± 0.5 km s−1 and 18.6 ± 1.0 km s−1, which we interpret as the dispersions of the young and old disc populations, respectively. Combining the (single) measured velocity dispersion of the total young + old disc population (13.0 ± 0.1 km s−1) with the scaleheight estimated for the older population would underestimate the disc surface density by a factor of ∼2. Such a disc would have a peak rotational velocity that is only 70 per cent of that for the maximal disc, thus making it appear submaximal.
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