16 research outputs found
MOA-II Galactic Microlensing Constraints: The Inner Milky Way has a Low Dark Matter Fraction and a Near Maximal Disk
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 disk 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
of the circular velocity is baryonic (at , at ). These results are in agreement with constraints from the
EROS-II microlensing survey of brighter resolved stars, where we find
at . Our fiducial model of a disk with scale length
2.6kpc, and a bulge with a low dark matter fraction of 12%, 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.Comment: 14 pages, 13 figures, submitted to MNRA
The Structure of the Milky Way's Bar Outside the Bulge
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 UKIDSS, 2MASS, VVV,
and GLIMPSE. We match and combine these surveys to investigate a wide area in
latitude and longitude, |b|<9deg and |l|<40deg. We find: (1) The bar extends to
l~25deg at |b|~5deg from the Galactic plane, and to l~30deg at lower latitudes.
(2) The long bar has an angle to the line-of-sight in the range (28-33)deg,
consistent with studies of the bulge at |l|<10deg. (3) The scale-height of RCG
stars smoothly transitions from the bulge to the thinner long bar. (4) There is
evidence for two scale heights in the long bar. We find a ~180pc thin bar
component reminiscent of the old thin disk near the sun, and a ~45pc super-thin
bar component which exists predominantly towards the bar end. (5) Constructing
parametric models for the RC magnitude distributions, we find a bar half length
of 5.0+-0.2kpc for the 2-component bar, and 4.6+-0.3kpc 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.Comment: Accepted for publication by MNRA
The Initial Mass Function of the Inner Galaxy Measured From OGLE-III Microlensing Timescales
We use the timescale distribution of ~3000 microlensing events measured by
the OGLE-III survey, together with accurate new made-to-measure dynamical
models of the Galactic bulge/bar region, to measure the IMF in the inner Milky
Way. The timescale of each event depends on the mass of the lensing object,
together with the relative distances and velocities of the lens and source. The
dynamical model provides statistically these distances and velocities allowing
us to constrain the lens mass function, and from this to infer the IMF.
Parameterising the IMF as a broken power-law, we find slopes in the main
sequence and brown
dwarf region where we
use a fiducial 50% binary fraction, and the systematic uncertainty covers the
range of binary fractions 0-100%. Similarly for a log-normal IMF we conclude
and
. These values are very
similar to a Kroupa or Chabrier IMF respectively, showing that the IMF in the
bulge is indistinguishable from that measured locally, despite the lenses lying
in the inner Milky Way where the stars are mostly ~10Gyr old and formed on a
fast -element enhanced timescale. This therefore constrains models of
IMF variation that depend on the properties of the collapsing gas cloud.Comment: 6 pages, 3 figures. Accepted by ApJ
The Stellar Halo in the Inner Milky Way: Predicted Shape and Kinematics
We have used N-body simulations for the Milky Way to investigate the
kinematic and structural properties of the old metal-poor stellar halo in the
barred inner region of the Galaxy. We find that the extrapolation of the
density distribution for bulge RR Lyrae stars, , approximately
matches the number density of RR Lyrae in the nearby stellar halo. We follow
the evolution of such a tracer population through the formation and evolution
of the bar and box/peanut bulge in the N-body model. We find that its density
distribution changes from oblate to triaxial, and that it acquires slow
rotation in agreement with recent measurements. The maximum radial velocity is
km/s at , and the velocity dispersion is
km/s. Even though the simulated metal-poor halo in the bulge has a
barred shape, just of the orbits follow the bar, and it does not trace
the peanut/X structure. With these properties, the RR Lyrae population in the
Galactic bulge is consistent with being the inward extension of the Galactic
metal-poor stellar halo.Comment: 5 pages, 5 figures. Accepted for publication in MNRAS Letter
Revisiting the Tale of Hercules: how stars orbiting the Lagrange points visit the Sun
We propose a novel explanation for the Hercules stream consistent with recent
measurements of the extent and pattern speed of the Galactic bar. We have
adapted a made-to-measure dynamical model tailored for the Milky Way to
investigate the kinematics of the solar neighborhood (SNd). The model matches
the 3D density of the red clump giant stars (RCGs) in the bulge and bar as well
as stellar kinematics in the inner Galaxy, with a pattern speed of 39 km
s kpc. Cross-matching this model with the DR1 TGAS data
combined with RAVE and LAMOST radial velocities, we find that the model
naturally predicts a bimodality in the -velocity distribution for
nearby stars which is in good agreement with the Hercules stream. In the model,
the Hercules stream is made of stars orbiting the Lagrange points of the bar
which move outward from the bar's corotation radius to visit the SNd. While the
model is not yet a quantitative fit of the velocity distribution, the new
picture naturally predicts that the Hercules stream is more prominent inward
from the Sun and nearly absent only a few pc outward of the Sun, and
plausibly explains that Hercules is prominent in old and metal-rich stars.Comment: 7 pages, 5 figures. ApJ Letters, in pres
Structure and dynamics of the galactic bulge and bar
Understanding galaxy evolution is one of the most active research fields in astronomy today. The Milky Way, our home galaxy can be observed on a star-by-star basis, something impossible in other galaxies and is therefore a natural benchmark for testing in detail galaxy formation theories. Therefore, many recent and ongoing large scale surveys have been carried out, providing an unprecedented collection of data to analyze. It is however challenging from the Sun's perspective to infer the current state of the Galaxy. In the work presented here dynamical equilibrium models of the Galaxy in its current state are built, a key element for later inferring its formation history. The dynamics of stars and dark matter are modeled in a self-consistent way, reproducing as many datasets as possible using the flexible Made-to-Measure method. An inside-out approach is adopted, starting by focusing on the galactic bulge before moving out to the larger scales, the galactic bar and the nearby disk.
First a set of dynamical models of the galactic bulge with different dark matter fractions is made Chapter 2. Those models are fitted to reproduce both the 3D density of bulge stars, with their boxy/peanut shape, and the radial stellar kinematics in bulge fields measured by the BRAVA spectroscopic survey. Results from the modelling of different stellar and dark matter masses in the bulge lead to the most accurate measurement of the total dynamical mass of the galactic bulge up to date, of (1.84 \pm 0.07) \times 10^{10}\, \Msun in a volume of (\pm 2.2 \times \pm 1.4 \times \pm 1.2 )\kpc oriented along the bulge's principal axis. The orbital structure of the boxy/peanut shape in these dynamical models is then analyzed (Chapter 3). The boxy/peanut shape is found to be supported by novel brezel-like orbits, from which a strong peanut shape with a relatively short extension can be built, thus showing that boxy/peanut bulges are not necessarily supported by the so-called banana orbits as had been previously claimed in the literature.
Outside the central 2\kpc, the galactic bulge smoothly segues into the long bar. Taking advantage of recent new data, the modelling was extended to the entire long bar region (Chapter 4). Additional data were added to the previous bulge models, mainly the distribution of Red Clump Giants in the bar region from a combination of the VVV, UKIDSS and 2MASS photometric surveys together with stellar kinematics as a function of distance along the line of sight from the \argos survey. By modelling the dynamics of the bar region, the pattern speed of the galactic bulge and bar is found to be (39.0 \pm 3.5)\kmskpc. This places the bar corotation radius at (6.1 \pm 0.5 )\kpc, making the Milky Way bar a typical fast rotator. The stellar mass of the long bar and bulge structure is evaluated to , larger than the mass of disk in the bar region, . Thanks to more extended kinematic datasets and recent measurement of the bulge IMF, the dark matter is found to account for of the mass in the bulge, with a density profile that flattens from the solar neighborhood to a shallow cusp or core in the bulge region. Finally, dynamical evidence for an extra central mass of is found, probably in a nuclear disk or disky pseudobulge.
This dynamical model of the bar region provides both the gravitational potential and a consistent library of N-body orbits that can be used as a basis for more advanced modelling of the Galaxy. Recent and future spectroscopic surveys such as \apogee or GALAH will provide hundreds of thousands of stellar abundances of elements that can allow tracing back the formation history of the Galaxy. Chemodynamical models, a natural extension of the dynamical models to also include chemical information, will be vital to understand these new data. To this end, the Made-to-Measure method was extended to include the metallicity distribution of stars, hence constructing the first Made-to-Measure chemodynamical model (Chapter 5). This method was applied to the \argos and \apogee data to successfully fit with the dynamical model of the galactic bar the spatial and kinematic variations of the metallicity in the inner Galaxy. The resulting phase-space distribution of the different metallicity components in the inner Galaxy is then analyzed. The variations as a function of metallicity observed in the data are described and explained in term of differences in spatial, kinematic and orbital structure. This demonstrates that chemodynamical models of the barred inner Milky Way can be constructed using the Made-to-Measure method. Such models describe the present chemodynamical state of the Galaxy and will in the future be a valuable resource in confronting galactic evolution simulations
Sculpting Andromeda -- made-to-measure models for M31's bar and composite bulge: dynamics, stellar and dark matter mass
The Andromeda galaxy (M31) contains a box/peanut bulge (BPB) entangled with a
classical bulge (CB) requiring a triaxial modelling to determine the dynamics,
stellar and dark matter mass. We construct made-to-measure models fitting new
VIRUS-W IFU bulge stellar kinematic observations, the IRAC-3.6m
photometry, and the disc's HI rotation curve. We explore the parameter space
for the 3.6m mass-to-light ratio , the bar pattern speed
(), and the dark matter mass in the composite bulge ()
within 3.2kpc. Considering Einasto dark matter profiles, we find the best
models for ,
and
. These models have a dynamical bulge mass of
including a stellar mass
of (73%), of which the CB has
(28%) and the BPB
(45%). We also explore models with NFW
haloes finding that, while the Einasto models better fit the stellar
kinematics, the obtained parameters agree within the errors. The
values agree with adiabatically contracted cosmological NFW haloes with M31's
virial mass and radius. The best model has two bulge components with completely
different kinematics that only together successfully reproduce the observations
(). The modelling includes dust
absorption which reproduces the observed kinematic asymmetries. Our results
provide new constraints for the early formation of M31 given the lower mass
found for the classical bulge and the shallow dark matter profile, as well as
the secular evolution of M31 implied by the bar and its resonant interactions
with the classical bulge, stellar halo and disc.Comment: 32 pages, 32 Figures; Published in MNRA
Dynamical modelling of the inner Galactic barred disk
Understanding the present state of the Milky Way disk is a necessary first step towards learning about the formation history of our Galaxy. While it is clear from infrared photometry that the inner disk hosts a 5 kpc long bar with a central Box/Peanut bulge, the interplay between the bar and the inner disk remains poorly known. To this end we build N-body dynamical models of the inner Galaxy with the Made-to-Measure method, combining deep photometry from the VVV, UKIDSS and 2MASS surveys with kinematics from the BRAVA, OGLE and ARGOS surveys. We explore their stellar to dark matter fraction together with their bar pattern speed and constrain from the modelling the effective Galactic potential (gravitational potential + bar pattern speed) inside the solar radius. Our best model is able to reproduce simultaneously (i) the Box/Peanut shape of the bulge, (ii) the transition between bulge and long bar, (iii) the bulge line-of-sight kinematics and proper motion dispersions, (iv) the ARGOS velocity field in the bar region and (v) the rotation curve of the Galaxy inside 10 kpc. Our effective potential will be an important input to more detailed chemodynamical studies of the stellar populations in the inner Galaxy, as revealed by the ARGOS or APOGEE surveys