Dynamical modelling of the galactic bulge and bar: the Milky Way's pattern speed, stellar and dark matter mass distribution (original) (raw)

Made-to-measure models of the Galactic box/peanut bulge: stellar and total mass in the bulge region

Monthly Notices of the Royal Astronomical Society, 2015

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-tomeasure 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 × 10 10 M. The stellar mass in this volume varies between 1.25 and 1.6 × 10 10 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 × 10 10 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 p = 25-30 km s −1 kpc −1. This would place the Galaxy among the slow rotators (R ≥ 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.

New insights on the Galactic Bulge Initial Mass Function

2015

We have derived the Galactic bulge initial mass function of the SWEEPS field in the mass range 0.15 M/M ⊙ 1.0, using deep photometry collected with the Advanced Camera for Surveys on the Hubble Space Telescope. Observations at several epochs, spread over 9 years, allowed us to separate the disk and bulge stars down to very faint magnitudes, F 814W ≈ 26 mag, with a proper-motion accuracy better than 0.5 mas/yr (20 km/s). This allowed us to determine the initial mass function of the pure bulge component uncontaminated by disk stars for this low-reddening field in the Sagittarius window. In deriving the mass function, we took into account the presence of unresolved binaries, errors in photometry, distance modulus and reddening, as well as the metallicity dispersion and the uncertainties caused by adopting different theoretical color-temperature relations. We found that the Galactic bulge initial mass function can be fitted with two power laws with a break at M ∼ 0.56 M ⊙ , the slope being steeper (α = −2.41±0.50) for the higher masses, and shallower (α = −1.25±0.20) for the lower masses. In the high-mass range, our derived mass function agrees well with the mass function derived for other regions of the bulge. In the low-mass range however, our mass function is slightly shallower, which suggests that separating the disk and bulge components is particularly important in the low-mass range. The slope of the bulge mass function is also similar to the slope of the mass function derived for the disk in the high-mass regime, but the bulge mass function is slightly steeper in the low-mass regime. We used our new mass function to derive stellar mass-to-light values for the Galactic bulge and we obtained 2.1 ≤ M/L F 814W ≤ 2.4 and 3.1 ≤ M/L F 606W ≤ 3.6 according to different assumptions on the slope of the IMF for masses larger than 1M ⊙ .

Theoretical Models of the Galactic Bulge

Astrophysics and Space Science Library, 2016

Near infrared images from the COBE satellite presented the first clear evidence that our Milky Way galaxy contains a boxy shaped bulge. Recent years have witnessed a gradual paradigm shift in the formation and evolution of the Galactic bulge. Bulges were commonly believed to form in the dynamical violence of galaxy mergers. However, it has become increasingly clear that the main body of the Milky Way bulge is not a classical bulge made by previous major mergers, instead it appears to be a bar seen somewhat end-on. The Milky Way bar can form naturally from a precursor disk and thicken vertically by the internal firehose/buckling instability, giving rise to the boxy appearance. This picture is supported by many lines of evidence, including the asymmetric parallelogram shape, the strong cylindrical rotation (i.e., nearly constant rotation regardless of the height above the disk plane), the existence of an intriguing X-shaped structure in the bulge, and perhaps the metallicity gradients. We review the major theoretical models and techniques to understand the Milky Way bulge. Despite the progresses in recent theoretical attempts, a complete bulge formation model that explains the full kinematics and metallicity distribution is still not fully understood. Upcoming large surveys are expected to shed new light on the formation history of the Galactic bulge. 1 A brief overview on the properties of the Galactic bulge Most spiral galaxies consist of three main components, an invisible dark matter halo, an embedded flat disk, and a central bulge. The Milky Way is no exception. The Milky Way bulge comprises about 15% of the total luminosity, and its stellar mass is about 1.2 − 1.6 × 10 10 M ⊙ (Portail et al., 2015). Galactic bulges contain crucial information about the galaxy formation and evolution. Major mergers between galax

The Initial Mass Function of the Galactic Bulge down to ∼0.15 M ⊙

The Astrophysical Journal, 2000

We present a luminosity function (LF) for lower main-sequence stars in the Galactic bulge near (l, b) \ (0¡, [6¡) to J \ 24, corresponding to This LF is derived from Hubble Space T elescope M J D 9.3. (HST) near infrared camera and multiobject spectrometer (NICMOS) observations of a region of 22A .5 with the F110W and F160W Ðlters. The main-sequence locus in the infrared shows a strong ] 22A .5, change in slope at J D 20.5 that is well Ðtted by new low-mass models that include water (M J D 5.75) and molecular hydrogen opacity. Our derived mass function (which is not corrected for binary companions) is the deepest measured to date in the bulge and extends to 0.15 with a power-law M _ , slope of a \ [1.33^0.07 ; a Salpeter mass function would have a \ [2.35. We also combine our J-band LF with previously published data for the evolved stars to produce a bulge LF spanning D15 mag. We show that this mass function has negligible dependence on the adopted bulge metallicity and distance modulus. Although shallower than the Salpeter slope, the slope of the bulge initial mass function (IMF) is steeper than that recently found for the Galactic disk (a \ [0.8 and a \ [0.54 from the data of Reid & Gizis and Gould et al., respectively, in the same mass interval) but is virtually identical to the disk IMF derived by Kroupa and coworkers. The bulge IMF is also quite similar to the mass functions derived for those globular clusters that are believed to have experienced little or no dynamical evolution. Finally, we derive the ratio of the bulge to be D0.9^0.1 and brieÑy discuss the impli-M/L J cations of this bulge IMF for the interpretation of the microlensing events observed in the direction of the Galactic bulge.

Kinematics of the Galactic Bulge from Radial Velocity and Proper Motion Optical Surveys

Publications of the Astronomical Society of Australia, 2004

Large-scale surveys provide new constraints on the structure and evolution of the Milky Way. The population synthesis approach is a useful tool to interpret such data sets and to test scenarios of evolution of the Galaxy. New constraints on Galactic parameters have been obtained from the Besançon model of population synthesis, in particular in the inner regions, thanks to near-infrared surveys less affected by interstellar extinction than the optical ones. We present here a few preliminary comparisons between observed and simulated distributions of proper motions in the direction of the Galactic bulge. Next we discuss how bulge stars can be observed and separated from other populations with the RAVE and Gaia surveys.

Kinematics at the Edge of the Galactic Bulge: Evidence for Cylindrical Rotation

The Astrophysical Journal, 2009

We present new results from BRAVA, a large-scale radial velocity survey of the Galactic bulge, using M giant stars selected from the Two Micron All Sky Survey catalog as targets for the Cerro Tololo Inter-American Observatory 4m Hydra multi-object spectrograph. The purpose of this survey is to construct a new generation of self-consistent bar models that conform to these observations. We report the dynamics for fields at the edge of the Galactic bulge at latitudes b = −8 • and compare to the dynamics at b = −4 • . We find that the rotation curve V(r) is the same at b = −8 • as at b = −4 • . That is, the Galactic boxy bulge rotates cylindrically, as do boxy bulges of other galaxies. The summed line of sight velocity distribution at b = −8 • is Gaussian, and the binned longitude-velocity plot shows no evidence for either a (disk) population with cold dynamics or for a (classical bulge) population with hot dynamics. The observed kinematics are well modeled by an edge-on N-body bar, in agreement with published structural evidence. Our kinematic observations indicate that the Galactic bulge is a prototypical product of secular evolution in galaxy disks, in contrast with stellar population results that are most easily understood if major mergers were the dominant formation process.

Metallicity-dependent kinematics and morphology of the Milky Way bulge

Monthly Notices of the Royal Astronomical Society: Letters, 2016

We use N-body chemo-dynamic simulations to study the coupling between morphology, kinematics and metallicity of the bar/bulge region of our Galaxy. We make qualitative comparisons of our results with available observations and find very good agreement. We conclude that this region is complex, since it comprises several stellar components with different properties-i.e. a boxy/peanut bulge, thin and thick disc components, and, to lesser extents, a disky pseudobulge, a stellar halo and a small classical bulge-all cohabiting in dynamical equilibrium. Our models show strong links between kinematics and metallicity, or morphology and metallicity, as already suggested by a number of recent observations. We discuss and explain these links.

Separation of stellar populations by an evolving bar: implications for the bulge of the Milky Way

Monthly Notices of the Royal Astronomical Society, 2017

We present a novel interpretation of the previously puzzling different behaviours of stellar populations of the Milky Way's bulge. We first show, by means of pure N-body simulations, that initially co-spatial stellar populations with different in-plane random motions separate when a bar forms. The radially cooler populations form a strong bar, and are vertically thin and peanut-shaped, while the hotter populations form a weaker bar and become a vertically thicker box. We demonstrate that it is the radial, not the vertical, velocity dispersion that dominates this evolution. Assuming that early stellar discs heat rapidly as they form, then both the in-plane and vertical random motions correlate with stellar age and chemistry, leading to different density distributions for metal-rich and metal-poor stars. We then use a high-resolution simulation, in which all stars form out of gas, to demonstrate that this is what happens. When we apply these results to the Milky Way we show that a very broad range of observed trends for ages, densities, kinematics and chemistries, that have been presented as evidence for contradictory paths to the formation of the bulge, are in fact consistent with a bulge which formed from a continuum of disc stellar populations which were kinematically separated by the bar. For the first time, we are able to account for the bulge's main trends via a model in which the bulge formed largely in situ. Since the model is generic, we also predict the general appearance of stellar population maps of external edge-on galaxies.

The Milky Way Bulge: Observed properties and a comparison to external galaxies

The Milky Way bulge offers a unique opportunity to investigate in detail the role that different processes such as dynamical instabilities, hierarchical merging, and dissipational collapse may have played in the history of the Galaxy formation and evolution based on its resolved stellar population properties. Large observation programmes and surveys of the bulge are providing for the first time a look into the global view of the Milky Way bulge that can be compared with the bulges of other galaxies, and be used as a template for detailed comparison with models. The Milky Way has been shown to have a box/peanut (B/P) bulge and recent evidence seems to suggest the presence of an additional spheroidal component. In this review we summarise the global chemical abundances, kinematics and structural properties that allow us to disentangle these multiple components and provide constraints to understand their origin. The investigation of both detailed and global properties of the bulge now ...