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

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 &

Gas Feedback on Stellar Bar Evolution

We analyze evolution of live disk-halo systems in the presence of various gas fractions, f_gas less than 8% in the disk. We addressed the issue of angular momentum (J) transfer from the gas to the bar and its effect on the bar evolution. We find that the weakening of the bar, reported in the literature, is not related to the J-exchange with the gas, but is caused by the vertical buckling instability in the gas-poor disks and by a steep heating of a stellar velocity dispersion by the central mass concentration (CMC) in the gas-rich disks. The gas has a profound effect on the onset of the buckling -- larger f_gas brings it forth due to the more massive CMCs. The former process leads to the well-known formation of the peanut-shaped bulges, while the latter results in the formation of progressively more elliptical bulges, for larger f_gas. The subsequent (secular) evolution of the bar differs -- the gas-poor models exhibit a growing bar while gas-rich models show a declining bar whose vertical swelling is driven by a secular resonance heating. The border line between the gas-poor and -rich models lies at f_gas ~ 3% in our models, but is model-dependent and will be affected by additional processes, like star formation and feedback from stellar evolution. The overall effect of the gas on the evolution of the bar is not in a direct J transfer to the stars, but in the loss of J by the gas and its influx to the center that increases the CMC. The more massive CMC damps the vertical buckling instability and depopulates orbits responsible for the appearance of peanut-shaped bulges. The action of resonant and non-resonant processes in gas-poor and gas-rich disks leads to a converging evolution in the vertical extent of the bar and its stellar dispersion velocities, and to a diverging evolution in the bulge properties.Comment: 12 pages, 12 figures, accepted for publication by the Astrophysical Journal. Minor corrections following the referee repor

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

A numerical study of interactions and stellar bars

For several decades it has been known that stellar bars in disc galaxies can be triggered by interactions, or by internal processes such as dynamical instabilities. In this work, we explore the differences between these two mechanisms using numerical simulations. We perform two groups of simulations based on isolated galaxies, one group in which a bar develops naturally, and another group in which the bar could not develop in isolation. The rest of the simulations recreate 1:1 coplanar fly-by interactions computed with the impulse approximation. The orbits we use for the interactions represent the fly-bys in groups or clusters of different masses accordingly to the velocity of the encounter. In the analysis we focus on bars' amplitude, size, pattern speed and their rotation parameter, ${\cal R}=R_{CR}/R_{bar}$. The latter is used to define fast (${\cal R}1.4$). Compared with equivalent isolated galaxies we find that bars affected or triggered by interactions: (i) remain in the slow regime for longer; (ii) are more boxy in face-on views; (iii) they host kinematically hotter discs. Within this set of simulations we do not see strong differences between retrograde or prograde fly-bys. We also show that slow interactions can trigger bar formation.Comment: 12 pages, 7 figures. Accepted for publication in MNRA

The mass and angular momentum distribution of simulated massive early-type galaxies to large radii

We study the dark and luminous mass distributions, circular velocity curves (CVC), line-of-sight kinematics, and angular momenta for a sample of 42 cosmological zoom simulations of massive galaxies. Using a temporal smoothing technique, we are able to reach large radii. We find that: (i)The dark matter halo density profiles outside a few kpc follow simple power-law models, with flat dark matter CVCs for lower-mass systems, and rising CVCs for high-mass haloes. The projected stellar density distributions at large radii can be fitted by Sersic functions with n>10, larger than for typical ETGs. (ii)The massive systems have nearly flat total CVCs at large radii, while the less massive systems have mildly decreasing CVCs. The slope of the CVC at large radii correlates with v_circ itself. (iii)The dark matter fractions within Re are in the range 15-30% and increase to 40-65% at 5Re. Larger and more massive galaxies have higher dark matter fractions. (iv)The short axes of simulated galaxies and their host dark matter haloes are well aligned and their short-to-long axis ratios are correlated. (v)The stellar vrms(R) profiles are slowly declining, in agreement with planetary nebulae observations in the outer haloes of most ETGs. (vi)The line-of-sight velocity fields v show that rotation properties at small and large radii are correlated. Most radial profiles for the cumulative specific angular momentum parameter lambda(R) are nearly flat or slightly rising, with values in [0.06,0.75] from 2Re to 5Re. (vii)Stellar mass, ellipticity at 5Re, and lambda(5Re) are correlated: the more massive systems have less angular momentum and are rounder, as for observed ETGs. (viii)More massive galaxies with a large fraction of accreted stars have radially anisotropic velocity distributions outside Re. Tangential anisotropy is seen only for galaxies with high fraction of in-situ stars. (Full abstract in PDF)Comment: 17 pages, 15 figures, 2 tables, accepted by MNRA