Making use of a new high-resolution spiral galaxy simulation as well as
Gaia
DR2 and EDR3 data complemented by chemical abundances from the Galah DR3, APOGEE DR16, and LAMOST DR5 surveys, we explore ...the possible link between the Milky Way (MW) spiral arms, (
R
,
v
ϕ
) ridges, and moving groups in local
v
R
-
v
ϕ
space. We show that the tightly wound main spiral arms in the
N
-body simulation can be successfully identified using overdensities in angular momentum (AM) or guiding space and also in the distribution of dynamically cold stars close to their guiding centres. Stars in the AM overdensities that travel over many kiloparsec in radius trace extended density ridges in (
R
,
v
ϕ
) space and overdensities in the
v
R
-
v
ϕ
plane of a solar neighbourhood (SNd)-like region, similar to those observed in the
Gaia
data. Similarly, the AM space of the MW contains several overdensities that correlate with a wave-like radial velocity pattern; this pattern is also reproduced by stars well beyond the SNd. We find that the fraction of
Gaia
stars located near their guiding centres shows three large-scale structures that approximately coincide with the MW spiral arms traced by distributions of maser sources in the Sagittarius, Local, and Perseus arms. This approach does not work for the Scutum arm near the end of the bar. Similar to the simulation, the stars in the AM overdensities follow the main (
R
,
v
ϕ
) density ridges with nearly constant angular momentum. When these ridges cross the SNd, they can be matched with the main
v
R
-
v
ϕ
features. Thus we suggest that the Hat is the inner tail of the Perseus arm, one of the Hercules components is the Sagittarius arm, and the Arcturus stream is likely to be the outermost tail of the Scutum-Centaurus arm. Based on previous work, the bar corotation is suggested to coincide with the second,
v
ϕ
≈ −55 km s
−1
Hercules stream ridge, and the OLR with the Sirius stream. The latter is supported by a sharp decrease in mean metallicity beyond the Sirius stream, which is an expected behaviour of the OLR, limiting migration of the metal-rich stars from the inner MW. In various phase-space coordinates, the AM overdensity stars have a mean metallicity that is systematically higher by about 0.05 dex than the metallicity of the surrounding stars. This is a predicted behaviour of the spiral arms. We show that the wave-like metallicity pattern can be traced at least up to |
z
|≈1 kpc. It is linked to radial velocity variations seen even farther (|
z
|≈2 kpc) from the Galactic mid-plane.
A new model for the luminosity distribution in the inner Milky Way is found, using a non-parametric penalized maximum-likelihood algorithm to deproject a dereddened COBE/ DIRBE L-band map of the ...inner Galaxy. The model is also constrained by the apparent magnitude (line-of-sight) distributions of clump giant stars in certain bulge fields. An important new feature is the inclusion of a spiral arm model in the disc. Spiral arms make the model appear broader on the sky; thus our bar is more elongated than in previous eight-fold symmetric models. They also lead to a smoother disc model interior to the Sun. The bar length is ≈3.5 kpc, and its axis ratios are 1:(0.3–0.4):0.3, independent of whether the spiral arm model is four-armed or two-armed. The larger elongation in the plane makes it possible to reproduce the observed clump giant distributions as well. With only the surface brightness data, a small model degeneracy is found even for fixed orientation of the bar, amounting to about ±0.1 uncertainty in the in-plane axial ratio. Including the clump giant data removes most of this degeneracy and also places additional constraints on the orientation angle of the bar. We estimate 15°≲ϕbar≲30°, with the best models obtained for 20°≲ϕbar≲25°. We use our reference model to predict a microlensing optical depth map towards the bulge, normalizing its mass by the observed terminal velocity curve. For clump giant sources at (l,b)=(3°.9, −3°.8) we find τ−6≡τ/10−6=1.27, within 1.8σ of the new MACHO measurement given by Popowski et al. The value for all sources at (l,b)=(2°.68, −3°.35) is τ−6=1.1, still >3σ away from the published MACHO DIA value. The dispersion of these τ−6 values within our models is ≃10 per cent. Because the distribution of sources is well fitted by the near-infrared model, increasing the predicted optical depths by >20 per cent will be difficult. Thus the high value of the measured clump giant optical depth argues for a near-maximal disc in the Milky Way.
Aims. We present a spectroscopic study of a sample of 287 planetary nebulas (PNs) around the brightest cluster galaxy (BCG) M 87 in Virgo A, of which 211 are located between 40 kpc and 150 kpc from ...the galaxy centre. With these data we can distinguish the stellar halo from the co-spatial intracluster light (ICL) and study both components separately. Methods. We obtained PN velocities with a high resolution FLAMES/VLT survey targeting eight fields in a total area of ~0.4 deg2. We identified PNs from their narrow and symmetric redshifted λ5007 Å OIII emission line, the presence of the second λ4959 Å OIII emission line, and the absence of significant continuum. We implement a robust technique to measure the halo velocity dispersion from the projected phase-space to identify PNs associated with the M 87 halo and ICL. Using photometric magnitudes, we construct PN luminosity functions (PNLFs), which are complete down to m5007 = 28.8. Results. The velocity distribution of the spectroscopically confirmed PNs is bimodal, containing a narrow component centred on the systemic velocity of the BCG and an off-centred broader component, which we identify as halo and ICL, respectively. We find that 243 PNs are part of the velocity distribution of the M 87 halo, while the remaining subsample of 44 PNs are intracluster PNs (ICPNs). Halo and ICPNs have different spatial distributions: the number density of halo PNs follow the galaxy’s surface brightness profile, whereas the ICPNs are characterised by a shallower power-law profile, IICL ∝ Rγ with γ in the range −0.34, −0.04 . No evidence is found for an asymmetry in the halo and ICPN density distributions when the NW and SE fields are studied separately. A study of the composite PN number density profile confirms the superposition of different PN populations associated with the M 87 halo and the ICL, characterised by different PN specific numbers α. We derive αhalo = 1.06 × 10-8NPN L⊙,bol-1 and αICL = 2.72 × 10-8NPN L⊙,bol-1, respectively. The M 87 halo PNLF has fewer bright PNs and a steeper slope towards faint magnitudes than the ICPNLF, and both are steeper than the standard PNLF for the M 31 bulge. Moreover, the ICPNLF has a dip at ~1−1.5 mag fainter than the bright cut-off, reminiscent of the PNLFs of systems with extended star formation history, such as M 33 or the Magellanic clouds. Conclusions. The BCG halo of M 87 and the Virgo ICL are dynamically distinct components with different density profiles and velocity distributions. Moreover, the different α-parameter values and PNLF shapes of the halo and ICL indicate distinct parent stellar populations, consistent with the existence of a gradient towards bluer colours at large radii. These results reflect the hierarchical build-up of the Virgo cluster.
We construct a large set of dynamical models of the galactic bulge, bar and inner disc using the made-to-measure method. Our models are constrained to match the red clump giant density from a ...combination of the VVV, UKIDSS and 2MASS infrared surveys together with stellar kinematics in the bulge from the BRAVA and OGLE surveys, and in the entire bar region from the ARGOS Survey. We are able to recover the bar pattern speed and the stellar and dark matter mass distributions in the bar region, thus recovering the entire galactic effective potential. We find a bar pattern speed of 39.0 plus or minus 3.5...km s super( - 1) kpc super( - 1), placing the bar corotation radius at 6.1 plus or minus 0.5...kpc and making the Milky Way bar a typical fast rotator. We evaluate the stellar mass of the long bar and bulge structure to be M sub( bar/bulge) = 1.88 plus or minus 0.12 x 10 super( 10)...M..., larger than the mass of disc in the bar region, M sub( inner disc) = 1.29 plus or minus 0.12 x 10 super( 10) M... The total dynamical mass in the bulge volume is 1.85 plus or minus 0.05 x 10 super( 10) M... Thanks to more extended kinematic data sets and recent measurement of the bulge initial mass function, our models have a low dark matter fraction in the bulge of 17 plus or minus 2 per cent. We find a dark matter density profile which flattens to a shallow cusp or core in the bulge region. Finally, we find dynamical evidence for an extra central mass of ~ 0.2 x 10 super( 10) M..., probably in a nuclear disc or discy pseudo-bulge. (ProQuest: ... denotes formulae/symbols omitted.)
Commissioning observations with the Apache Point Observatory Galactic Evolution Experiment (APOGEE), part of the Sloan Digital Sky Survey III, have produced radial velocities (RVs) for ~4700 ...K/M-giant stars in the Milky Way (MW) bulge. These high-resolution (R ~ 22,500), high-S/N (>100 per resolution element), near-infrared (NIR; 1.51-1.70 mu m) spectra provide accurate RVs ( epsilon sub(v) ~ 0.2 km s super(-1)) for the sample of stars in 18 Galactic bulge fields spanning -1degrees < l < 20degrees, b < 20degrees, and delta > -32degrees. This represents the largest NIR high-resolution spectroscopic sample of giant stars ever assembled in this region of the Galaxy. A cold (sigma sub(v) ~ 30 km s super(-1)), high-velocity peak (V sub(GSR) asymptotically = +200 km s super(-1)) is found to comprise a significant fraction (~10%) of stars in many of these fields. These high RVs have not been detected in previous MW surveys and are not expected for a simple, circularly rotating disk. Preliminary distance estimates rule out an origin from the background Sagittarius tidal stream or a new stream in the MW disk. Comparison to various Galactic models suggests that these high RVs are best explained by stars in orbits of the Galactic bar potential, although some observational features remain unexplained.
Boxy/peanut (BP) bulges are believed to originate from galactic discs through secular processes. A little explored question is how this evolution would be modified if the initial disc was assembled ...around a preexisting classical bulge. Previously we showed that a low-mass initial classical bulge (ICB), as might have been present in Milky Way-like galaxies, can spin up significantly by gaining angular momentum from a bar formed through disc instability. Here we investigate how the disc instability and the kinematics of the final BP bulge depend on the angular momentum of such a low-mass ICB. We show that a strong bar forms and transfers angular momentum to the ICB in all our models. However, rotation in the ICB limits the emission of the bar's angular momentum, which in turn changes the size and growth of the bar, and of the BP bulge formed from the disc.
The final BP bulge in these models is a superposition of the BP bulge formed via the buckling instability and the spun-up ICB. We find that the long-term kinematics of the composite BP bulges in our simulations is independent of the rotation of the ICB, and is always described by cylindrical rotation. However, as a result of the co-evolution between bulge and bar, deviations from cylindrical rotation are seen during the early phases of secular evolution and may correspond to similar deviations observed in some bulges. We provide a simple criterion to quantify deviations from pure cylindrical rotation, apply it to all our model bulges, and also illustrate its use for two galaxies: NGC 7332 and NGC 4570.
ABSTRACT
We compare distance resolved, absolute proper motions in the Milky Way bar/bulge region to a grid of made-to-measure dynamical models with well-defined pattern speeds. The data are obtained ...by combining the relative VVV InfraRed Astrometric Catalogue (VIRAC) v1 proper motions with the Gaia Data Release 2 absolute reference frame. We undertake a comprehensive analysis of the various errors in our comparison, from both the data and the models, and allow for additional, unknown, contributions by using an outlier-tolerant likelihood function to evaluate the best-fitting model. We quantify systematic effects such as the region of data included in the comparison, the possible overlap from spiral arms, and the choice of synthetic luminosity function and bar angle used to predict the data from the models. Resulting variations in the best-fitting parameters are included in their final errors. We thus measure the bar pattern speed to be $\Omega _{\mathrm{b}}=33.29 \pm 1.81\, \mathrm{km\, s^{-1}\, kpc^{-1}}$ and the azimuthal solar velocity to be $V_{\phi ,\odot }=251.31 \pm 1.95\,\mathrm{km\, s}^{-1}$. These values, when combined with recent measurements of the Galactic rotation curve, yield the distance of corotation, $6.5 \lt R_\mathrm{CR}\, (\mathrm{kpc})\lt 7.5$, the outer Lindblad resonance (OLR), $10.7 \lt R_\mathrm{OLR}\, (\mathrm{kpc})\lt 12.4$, and the higher order, m = 4, OLR, $8.7 \lt R_\mathrm{OLR_4}\, (\mathrm{kpc})\lt 10.0$. The measured pattern speed provides strong evidence for the ‘long-slow’ bar scenario.
It has been known for sometime that the Milky Way is a barred disk galaxy. More recently several studies inferred from star count observations that the Galaxy must contain a separate, new, flat long ...bar component, twisted relative to the barred bulge. Here we use a simulation with a boxy bulge and bar to suggest that these observations can be reproduced with a single structure. In this simulation, a stellar bar evolved from the disk, and the boxy bulge originated from it through secular evolution and the buckling instability. We calculate star count distributions for this model at different longitudes and latitudes, in a similar way as observers have done for resolved star counts. Good agreement between the simulation and the observations can be achieved for a suitable snapshot, even though the simulation has a single boxy bulge and bar structure. In this model, part of the long bar signature is due to a volume effect in the star counts, and another part is due to choosing a snapshot in which the planar part of the boxy bulge and bar has developed leading ends through interaction with the adjacent spiral arm heads. We also provide predictions from this model for the line-of-sight velocity distributions at the longitudes with the long bar signature, for comparison with upcoming surveys.
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