Boxy/Peanut Bulges in Barred Galaxies and the Milky Way

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Galactic bulges can be built by mergers, by satellite accretion, or through internal evolution of disk galaxies. In this latter case, one of the well-studied mechanisms is the formation of bulges during the evolution of barred galaxies. Barred galaxies can produce both disk-like bulges and boxy/peanut bulges.

Observationally, bars are present in a large fraction of nearby disk galaxies, 72% when looking at H-band images. The fraction of strong bars is constant up to redshift z~1. Bars are long-lived structures with important dynamical effects on the galaxies which host them. Bars are responsible, for example, for the inflow of gas into the inner parts of the galaxy, which may lead to the formation of a nuclear bar and to fuelling of an AGN. Bars also trigger the formation of spiral arms and of stellar and gaseous rings via resonances.

Stellar bars in N-body simulations evolve with time from a flat bar to a boxy/peanut bulge through two buckling instabilities, as shown in the figure. This evolution is mainly determined by the exchange of angular momentum between the bar and the disk, bulge, and dark matter halo. We know now that also the Milky-Way and our neighbour galaxy M31 have such boxy/peanut bulges.

Our recent work has focussed on the angular momentum transfer from bars to preexisting bulges, and on characterizing the boxy/peanut bulge of our Milky Way Galaxy.

Edge-on view of galactic bulges produced by an evolving stellar bar. The boxy shape in bulges appears generally after the first buckling instability and the boxy/peanut bulge after the second buckling event (from Martinez-Valpuesta et al. 2006).

Recent Results
Mapping the Three-Dimensional Density of the Galactic Bulge with VVV Red Clump Stars

Chris Wegg & Ortwin Gerhard, 2013, MNRAS, 435, 1874 (to the paper)

Using red clump giant stars identified in the VVV survey we produced the most accurate and high resolution map of the inner regions of the Milky Way. Our density map covers the inner (2.2x1.4x1.1)kpc of the bulge/bar. Line-of-sight density distributions were estimated by deconvolving extinction and completeness corrected K-band magnitude distributions. In constructing our measurement, we assumed that the three-dimensional bulge is 8-fold mirror triaxially symmetric, but the map is otherwise completely non-parametric. In doing so we measure the angle of the bar-bulge to the line-of-sight to be (27+- 2)deg, where the dominant error is systematic arising from the details of the deconvolution process. Our density distribution shows a highly elongated bar with projected axis ratios ~(1:2.1) for isophotes reaching ~2kpc along the major axis. Along the bar axes the density falls off roughly exponentially, with axis ratios (10:6.3:2.6) and exponential scale-lengths (0.70:0.44:0.18)kpc. From about 400pc above the Galactic plane, the bulge density distribution displays a prominent X-structure. Overall, the density distribution of the Galactic bulge is characteristic for a strongly boxy/peanut shaped bulge within a barred galaxy.

Anyone is welcome to use the movie we made to vizualize the measured 3D structure. Just reference the original paper if you use the movie (Creative Commons attribution share-alike license).

longitude profiles
In the center we show an image of the 3D iso-density contours of the measured bulge density. Three projections are shown as surrounding plots: From above (i.e. from the North Galactic pole), from the side (i.e. along the intermediate axis of the bar) and the re-projected surface density from the sun. The three projections show the surface density of bulge red clump stars with isophotes spaced by 0.5 mag.


Inma Martinez-Valpuesta & Ortwin Gerhard, 2013, ApJL, 766, 3 (to the paper)

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.

longitude profiles

Evolution of the Jacobi energy of the particles. Although the bucking instabilities from which the boxy/peanut bulge forms are violent, particles retain knowledge of their initial conditions through their Jacobi energy.
We assign an initial radial metallicity gradient to the disk particles. After formation of the boxy bulge we project these metallicities and here show a predicted mean metallicity map of the Galactic bulge. This can be compared to the observations made by Gonzalez et al. (2013, A&A, 552, A110).


Kanak Saha & Ortwin Gerhard, 2013, MNRAS, 430, 2039 (to the paper)

We have explored the consequences of our previous work where we showed that a initial classical bulge (ICB) may be spun-up and "hidden" within a barred galaxies similar to the Milky Way. In this work we investigated the impact of an rotating ICB on the formation and secular evolution of the bar. We use a series of models containing both a spinning classical bulge, and a disk, which from a Boxy/peanut (BP) bulge. In all the models we show that a strong bar forms and transfers angular momentum to the ICB. 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.

We also find that deviations from the cylindrical rotation, which is typically characteristic of BP bulges, during the early phases of secular evolution. These 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.

Bulge particles density

Snapshot of the edge-on surface density (above) and velocity map (below) for the one of the simulated galaxies. Boxy/peanut (BP) bulged typically display cylindrical rotation. However, this model, containing an initial classical bulge, displays departures from cylindrical rotation beyond ≈ 0.5Rd.


Kanak Saha, Inma Martinez-Valpuesta & Ortwin Gerhard, 2012, MNRAS, 421, 333 (to the paper)

The secular processes driven by the bar may cause dramatic changes in the dynamical structure of a preexisting low-mass classical bulge, such as might be present in galaxies like the Milky Way. Such a bulge absorbs angular momentum emitted by the bar, mostly through resonances, particularly Lagrange point (-1:1) and ILR (2:1) orbits, but also retrograde non-resonant orbits absorb angular momentum while the bar  grows rapidly. Thus an initially non-rotating low-mass classical bulge transforms into a fast rotating, radially anisotropic and triaxial object, embedded in the similarly fast rotating boxy bulge formed from the disk. Towards the  end of the evolution, the classical bulge develops cylindrical rotation.

Bulge particles density
Bulge velocity

Edge-on surface density and velocity maps for the bulge particles alone at four different epochs during the secular evolution. Initially the bulge is non-rotating and flattened by the disc potential later on the classical bulge aquires the characteristic cylindrical rotation.


Ortwin Gerhard & Inma Martinez-Valpuesta, 2012, ApJL, 744, 8 (to the paper)

Starcount observations show evidence for a structural change in the Milky Way bulge inward of |l|~4◦. With an N-body barred galaxy simulation we showed that a boxy bulge formed through the bar and buckling instabilities matches these observations. The change in the slope of the model longitude profiles 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.

The same simulation snapshot was earlier used to clarify the apparent boxy bulge—long bar dichotomy. Furthermore, the nuclear star count map derived from this simulation snapshot displays a longitudinal asymmetry similar to that observed in the TwoMicron All Sky Survey (2MASS) data. These combined results
  • support the interpretation that the Galactic bulge originated from disk evolution,
  • and question arguments based on star count data for the existence of a secondary nuclear bar in the Milky Way.

Complete paper on ADS
Press Release
longitude profiles

Maxima of observed and modeled magnitude distributions for red clump (RC) giant stars in bulge fields as a function of longitude. Top: Simulated (black dots), compared with data from the VVV survey at b = ±1◦ (Gonzalez et al. 2011, A&A, 534, 14; open squares).
Face-on surface density of the particles with |z| < 300 pc (top) and at 3<b<4, with overplotted maxima of the line-of-sight density distributions (open circles) and maxima  of the simulated line-of-sight RC magnitude distributions (full circles)


Inma Martinez-Valpuesta & Ortwin Gerhard, 2011, ApJL, 734, 20 (to the paper)

We have known for sometime that the Milky Way is a barred disk galaxy. But 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. With a simulation we showed that these observations can be reproduced with a single boxy bulge and bar 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 calculated the star count distributions for this model at different longitudes and latitudes, in a similar way as observers have done for resolved star counts. We found good
quantitative agreement with the observations for a suitable model snapshot. The long bar signature in this model is partially is due to a volume effect in the star counts, and partially because of choosing a snapshot in which the planar bar has developed leading ends by interacting with the nearby spiral arm heads. We also calculated radial velocity predictions from this model for comparison with upcoming surveys.

Complete paper on ADS
Related publications
Press Release

Top panel: Face-on view of the simulation with bar rotating clockwise and its ends bend towards the leading side. Lower panel: edge-on  view of the same snapshot, as viewed from the  Sun. The boxy structure is noticeable.
Location of the star count maxima in the Galactic plane, for fields near the disk plane (black) and in the boxy bulge (pink). The top panel shows the maxima for the model with leading curved ends of the bar. The lower panel is for a model with straight bar ends. The thick solid line shows the true orientation of the model.


Related Publications

[1] Inma Martinez-Valpuesta & Ortwin Gerhard, 2013, ApJL, 766, 3. Metallicity gradients through disk Instability: A simple model for the Milky Way's boxy bulge . 2013ApJ...766L...3M

[2] Kanak Saha & Ortwin Gerhard, 2013, MNRAS, 430, 2039. Secular evolution and cylindrical rotation in boxy/peanut bulges: impact of initially rotating classical bulges . 2013MNRAS.430.2039S

[3] Saha, K., Martinez-Valpuesta, I., Gerhard, O. 2012, MNRAS, 421, 333. Spin-up of low-mass classical bulges in barred galaxies. 2012MNRAS.421..333S

[4] Gerhard, O., Martinez-Valpuesta, I. 2012, ApJ, 744, L8. The Inner Galactic Bulge: Evidence for a Nuclear Bar?. 2012ApJ...744L...8G

[5] Martinez-Valpuesta, I., Gerhard, O. 2011, ApJ, 734, L20. Unifying A Boxy Bulge and Planar Long Bar in the Milky Way. 2011ApJ...734L..20M

Previous publications.

Saglia, R. P., Fabricius, M., Bender, R., Montalto, M., Lee, C.-H., Riffeser, A., et al. 2010, A&A, 509, A61. The old and heavy bulge of M 31 . I. Kinematics and stellar populations. 2010A&A...509A..61S

Comerón, S., Martínez-Valpuesta, I., Knapen, J. H., Beckman, J. E. 2009, ApJ, 706, L256. On the Curvature of Dust Lanes in Galactic Bars. 2009ApJ...706L.256C
Fathi, K., Beckman, J. E., Piñol-Ferrer, N., Hernandez, O., Martínez-Valpuesta, I., Carignan, C. 2009, ApJ, 704, 1657. Pattern Speeds of Bars and Spiral Arms from Hα Velocity Fields. 2009ApJ...704.1657F
de Lorenzo-Cáceres, A., Falcón-Barroso, J., Vazdekis, A., Martínez-Valpuesta, I. 2008, ApJ, 684, L83. Stellar Kinematics in Double-Barred Galaxies: The σ-Hollows. 2008ApJ...684L..83D
Berentzen, I., Shlosman, I., Martinez-Valpuesta, I., Heller, C. H. 2007, ApJ, 666, 189. Gas Feedback on Stellar Bar Evolution. 2007ApJ...666..189B
Rattenbury, N. J., Mao, S., Debattista, V. P., Sumi, T., Gerhard, O., de Lorenzi, F. 2007, MNRAS, 378, 1165. Proper motion dispersions of red clump giants in the galactic bulge: observations and model comparisons. 2007MNRAS.378.1165R
Merrett, H. R., Merrifield, M. R., Douglas, N. G., Kuijken, K., Romanowsky, A. J., Napolitano, N. R., et al. 2006, MNRAS, 369, 120. A deep kinematic survey of planetary nebulae in the Andromeda galaxy using the Planetary Nebula Spectrograph. 2006MNRAS.369..120M
Englmaier, P., Gerhard, O., 2006, CeMDA, 94, 369, Milky Way Gas Dynamics. 2006CeMDA..94..369E
Coccato, L., Sarzi, M., Pizzella, A., Corsini, E. M., Dalla Bontà, E., Bertola, F. 2006, MNRAS, 366, 1050. NGC 4435: a bulge-dominated galaxy with an unforeseen low-mass central black hole. 2006MNRAS.366.1050C
Martinez-Valpuesta, I., Shlosman, I., Heller, C. 2006, ApJ, 637, 214. Evolution of Stellar Bars in Live Axisymmetric Halos: Recurrent Buckling and Secular Growth. 2006ApJ...637..214M
López-Corredoira, M., Cabrera-Lavers, A., Gerhard, O. E. 2005, A&A, 439, 107. A boxy bulge in the Milky Way. Inversion of the stellar statistics equation with 2MASS data. 2005A&A...439..107L
López-Corredoira, M., Cabrera-Lavers, A., Gerhard, O. E., Garzón, F., 2004, A&A, 421, 953, Evidence for a deficit of young and old stars in the Milky Way inner in-plane disc. 2004A&A...421..953L
Martinez-Valpuesta, I., Shlosman, I. 2004, ApJ, 613, L29. Why Buckling Stellar Bars Weaken in Disk Galaxies. 2004ApJ...613L..29M
Bissantz, N., Debattista, V. P., Gerhard, O. 2004, ApJ, 601, L155. Large-Scale Model of the Milky Way: Stellar Kinematics and the Microlensing Event Timescale Distribution in the Galactic Bulge. 2004ApJ...601L.155B
Immeli, A., Samland, M., Westera, P., Gerhard, O. 2004, ApJ, 611, 20. Subgalactic Clumps at High Redshift: A Fragmentation Origin?. 2004ApJ...611...20I
Immeli, A., Samland, M., Gerhard, O., Westera, P. 2004, A&A, 413, 547. Gas physics, disk fragmentation, and bulge formation in young galaxies. 2004A&A...413..547I