Metallicity gradients through disk Instability: A simple model for the Milky Way's boxy bulge

Observations show a clear vertical metallicity gradient in the Galactic bulge, which is often taken as a signature of dissipative processes in the formation of a classical bulge. Various evidence shows, however, that the Milky Way is a barred galaxy with a boxy bulge representing the inner three-dimensional part of the bar. Here we show with a secular evolution N-body model that a boxy bulge formed through bar and buckling instabilities can show vertical metallicity gradients similar to the observed gradient, if the initial axisymmetric disk had a comparable radial metallicity gradient. In this framework the range of metallicities in bulge fields constrains the chemical structure of the Galactic disk at early times, before bar formation. Our secular evolution model was previously shown to reproduce inner Galaxy star counts and we show here that it also has cylindrical rotation. We use it to predict a full mean metallicity map across the Galactic bulge from a simple metallicity model for the initial disk. This map shows a general outward gradient on the sky as well as longitudinal perspective asymmetries. We also briefly comment on interpreting metallicity gradient observations in external boxy bulges.Comment: Accepted to ApJ Letter

Boxy/peanut bulges : formation, evolution and properties

We discuss the formation and evolution of boxy/peanut bulges (B/Ps) and present new simulations results. Orbital structure studies show that B/Ps are parts of bars seen edge-on, they have their origin in vertical instabilities of the disc material and they are somewhat shorter in extent than bars. When the bar forms it is vertically thin, but after a time of the order of a Gyr it experiences a vertical instability and buckles. At that time the strength of the bar decreases, its inner part becomes thicker, so that, seen edge-on, it acquires a peanut or boxy shape. A second buckling episode is seen in simulations with strong bars, accompanied by a further thickening of the B/P and a weakening of the bar. Quantitatively, this evolution depends considerably on the properties of the halo and particularly on the extent of its core. This influences the amount of angular momentum exchanged within the galaxy, emitted by near-resonant material in the bar region and absorbed by near-resonant material in the halo and in the outer disc. Haloes with small cores generally harbour stronger bars and B/Ps and they often witness double buckling.Comment: 7 pages, 3 figures, contribution to the conference "Chaos in Astronomy", Athens, sept. 2007, eds. G. Contopoulos & P.A. Patsi

Boxy/Peanut bulges, vertical buckling and galactic bars

Boxy/peanut bulges in disk galaxies have been associated to stellar bars. In this talk, we discuss the different properties of such bulges and their relation with the corresponding bar, using a very large sample of a few hundred numerical N-body simulations. We present and inter-compare various methods of measuring the boxy/peanut bulge properties, namely its strength, shape and possible asymmetry. Some of these methods can be applied to both simulations and observations. Our final goal is to get correlations that will allow us to obtain information on the boxy/peanut bulge for a galaxy viewed face-on as well as information on the bars of galaxies viewed edge-on.Comment: 4 pages. To appear in the proceedings of IAU Symposium 245 "Formation and Evolution of Galaxy Bulges", M. Bureau, E. Athanassoula, and B. Barbuy, ed

Dynamical evolution of a bulge in an N-body model of the Milky Way

The detailed dynamical structure of the bulge in the Milky Way is currently under debate. Although kinematics of the bulge stars can be well reproduced by a boxy-bulge, the possible existence of a small embedded classical bulge can not be ruled out. We study the dynamical evolution of a small classical bulge in a model of the Milky Way using a self-consistent high resolution N-body simulation. Detailed kinematics and dynamical properties of such a bulge are presented.Comment: 2 pages, 2 figures, to appear in the proceedings of "Assembling the Puzzle of the Milky Way", Le Grand Bornand (April 17-22, 2011), C. Reyle, A. Robin, M. Schultheis (eds.

Spin-up of massive classical bulges during secular evolution

Classical bulges in spiral galaxies are known to rotate but the origin of this observed rotational motion is not well understood. It has been shown recently that a low-mass classical bulge (ClB) in a barred galaxy can acquire rotation from absorbing a significant fraction of the angular momentum emitted by the bar. Our aim here is to investigate whether bars can spin up also more massive ClBs during the secular evolution of the bar, and to study the kinematics and dynamics of these ClBs. We use a set of self-consistent N-body simulations to study the interaction of ClBs with a bar that forms self-consistently in the disk. We use orbital spectral analysis to investigate the angular momentum gain by the classical bulge stars. We show that the ClBs gain, on average, about 2 - 6% of the disk's initial angular momentum within the bar region. Most of this angular momentum gain occurs via low-order resonances, particularly 5:2 resonant orbits. A density wake forms in the ClB which corotates and aligns with the bar at the end of the evolution. The spin-up process creates a characteristic linear rotation profile and mild tangential anisotropy in the ClB. The induced rotation is small in the centre but significant beyond $\sim2$ bulge half mass radii, where it leads to mass-weighted $V/\sigma \sim 0.2$, and reaches a local $V_{max}/\sigma_{in} \sim 0.5$ at around the scale of the bar. The resulting $V/\sigma$ is tightly correlated with the ratio of the bulge size to the bar size. In all models, a box/peanut bulge forms suggesting that composite bulges may be common. Bar-bulge resonant interaction in barred galaxies can provide some spin up of massive ClBs, but the process appears to be less efficient than for low-mass ClBs. Further angular momentum transfer due to nuclear bars or gas inflow would be required to explain the observed rotation if it is not primordial.Comment: 11 Pages, 14 figures; accepted for publication by A &

A cosmological context for compact massive galaxies

To provide a quantitative cosmological context to ongoing observational work on the formation histories and location of compact massive galaxies, we locate and study a sample of exceptionally compact systems in the Bolshoi simulation, using the dark matter structural parameters from a real, compact massive galaxy (NGC 1277) as a basis for our working criteria. We find that over 80% of objects in this nominal compact category are substructures of more massive groups or clusters, and that the probability of a given massive substructure being this compact increases significantly with the mass of the host structure; rising to ~30% for the most massive clusters in the simulation. Tracking the main progenitors of this subsample back to z=2, we find them all to be distinct structures with scale radii and densities representative of the population as a whole at this epoch. What does characterise their histories, in addition to mostly becoming substructures, is that they have almost all experienced below-average mass accretion since z=2; a third of them barely retaining, or even losing mass during the intervening 10 Gyr.Comment: 9 pages, 9 figure

Elliptical galaxies with rapidly decreasing velocity dispersion profiles: NMAGIC models and dark halo parameter estimates for NGC 4494

NGC 4494 is one of several intermediate-luminosity elliptical galaxies inferred to have an unusually diffuse dark matter halo. We use the chi^2-made-to-measure particle code NMAGIC to construct axisymmetric models of NGC 4494 from photometric and various kinematic data. The extended kinematics include light spectra in multiple slitlets out to 3.5 R_e, and hundreds of planetary nebulae velocities out to ~7 R_e, thus allowing us to probe the dark matter content and orbital structure in the halo. We use Monte Carlo simulations to estimate confidence boundaries for the halo parameters, given our data and modelling set-up. We find that the true potential of the dark matter halo is recovered within Delta G (merit function)<26 (Delta chi^2<59) at 70% confidence level (C.L.), and within Delta G<32 (Delta chi^2<70) at 90% C.L.. These numbers are much larger than the usually assumed Delta chi^2=2.3 (4.6) for 70% (90%) C.L. for two free parameters, perhaps case-dependent, but calling into question the general validity of the standard assumptions used for halo and black hole mass determinations. The best-fitting models for NGC 4494 have a dark matter fraction of about 0.6\pm0.1 at 5R_e (70% C.L.), and are embedded in a dark matter halo with circular velocity ~200 km/s. The total circular velocity curve (CVC) is approximately flat at v_c=220 km/s outside ~0.5R_e. The orbital anisotropy of the stars is moderately radial. These results are independent of the assumed inclination of the galaxy, and edge-on models are preferred. Comparing with the halos of NGC 3379 and NGC 4697, whose velocity dispersion profiles also decrease rapidly from the center outwards, the outer CVCs and dark matter halos are quite similar. NGC 4494 shows a particularly high dark matter fraction inside ~3R_e, and a strong concentration of baryons in the center.Comment: 21 pages, 23 figures, 1 table. Accepted for publication in MNRA

The inner Galactic bulge: evidence for a nuclear bar?

Recent data from the VVV survey have strengthened evidence for a structural change in the Galactic bulge inwards of |l|<=4 deg. Here we show with an N-body barred galaxy simulation that a boxy bulge formed through the bar and buckling instabilities effortlessly matches measured bulge longitude profiles for red clump stars. The same simulation snapshot was earlier used to clarify the apparent boxy bulge - long bar dichotomy, for the same orientation and scaling. The change in the slope of the model longitude profiles in the inner few degrees is caused by a transition from highly elongated to more nearly axisymmetric isodensity contours in the inner boxy bulge. This transition is confined to a few degrees from the Galactic plane, thus the change of slope is predicted to disappear at higher Galactic latitudes. We also show that the nuclear star count map derived from this simulation snapshot displays a longitudinal asymmetry similar to that observed in the 2MASS data, but is less flattened to the Galactic plane than the 2MASS map. These results support the interpretation that the Galactic bulge originated from disk evolution, and question the evidence advanced from star count data for the existence of a secondary nuclear bar in the Milky Way.Comment: ApJL in press, 4 figure