▪ Abstract The Universe is in transition. At early times, galactic evolution was dominated by hierarchical clustering and merging, processes that are violent and rapid. In the far future, evolution will mostly be secular—the slow rearrangement of energy and mass that results from interactions involving collective phenomena such as bars, oval disks, spiral structure, and triaxial dark halos. Both processes are important now. This review discusses internal secular evolution, concentrating on one important consequence, the buildup of dense central components in disk galaxies that look like classical, merger-built bulges but that were made slowly out of disk gas. We call these pseudobulges. We begin with an “existence proof”—a review of how bars rearrange disk gas into outer rings, inner rings, and stuff dumped onto the center. The results of numerical simulations correspond closely to the morphology of barred galaxies. In the simulations, gas is transported to small radii, where it reaches high densities and...

Secular Evolution and the Formation of Pseudobulges in Disk Galaxies

Objects designated as bulges in disc galaxies do not form a homogeneous class. I distinguish three types: the classical bulges, the properties of which are similar to those of ellipticals and which form by collapse or merging; boxy and peanut bulges, which are seen in near-edge-on galaxies and which are in fact just a part of the bar seen edge-on; and, finally, disc-like bulges, which result from the inflow of (mainly) gas to the centre-most parts, and subsequent star formation. I make a detailed comparison of the properties of boxy and peanut bulges with those of N-body bars seen edge-on, and answer previously voiced objections about the links between the two. I also present and analyse simulations where a boxy/peanut feature is present at the same time as a classical spheroidal bulge, and compare them with observations. Finally, I propose a nomenclature that can help to distinguish between the three types of bulges and avoid considerable confusion.

https://hal.archives-ouvertes.fr/hal-00008346/document

On the nature of bulges in general and of box/peanut bulges in particular: input from N-body simulations

We present a comprehensive series of simulations to study the secular evolution of disk galaxies expected in a ΛCDM universe. Our simulations are organized in a hierarchy of increasing complexity, ranging from rigid-halo collisionless simulations to fully live simulations with gas and star formation. Our goal is to examine which structural properties of disk galaxies may result from secular evolution rather than from hierarchical assembly. In the vertical direction, we find that various mechanisms lead to heating, the strongest of which is the buckling instability of a bar, which leads to peanut-shaped bulges; these can be recognized face-on even in the presence of gas. We find that bars are robust structures that survive buckling and require a large (~20% of the total mass of the disk) central mass concentration to be destroyed. This can occur in dissipative simulations, where bars induce strong gas inflows, but requires that radiative cooling overcome heating. We show how angular momentum redistribution leads to increasing central densities and disk scale lengths and to profile breaks at large radii. The breaks in these simulations are in excellent agreement with observations, even when the evolution is collisionless. Disk scale lengths increase even when the total disk angular momentum is conserved; thus, mapping halo angular momenta to scale lengths is nontrivial. A decomposition of the resulting profile into a bulge+disk gives structural parameters in reasonable agreement with observations although kinematics betrays their bar nature. These findings have important implications for galaxy formation models, which have so far ignored or introduced in a very simplified way the effects of nonaxisymmetric instabilities on the morphological evolution of disk galaxies.

https://iopscience.iop.org/article/10.1086/504147/pdf

The secular evolution of disk structural parameters

Evolution of stellar bars in disk galaxies is accompanied by dynamical instabilities and secular changes. Following the vertical buckling instability, the bars are known to weaken dramatically and develop a pronounced boxy/peanut shape when observed edge-on. Using high-resolution N-body simulations of stellar disks embedded in live axisymmetric dark matter halos, we have investigated the long-term changes in the bar morphology, specifically the evolution of the bar size, its vertical structure, and the exchange of angular momentum. We find that following the initial buckling, the bar resumes its growth from deep inside the corotation radius and follows the ultraharmonic resonance thereafter. We also find that this secular bar growth triggers a spectacular secondary vertical buckling instability that leads to the appearance of characteristic boxy/peanut/X-shaped bulges. The secular bar growth is crucial for the recurrent buckling to develop. Furthermore, the secondary buckling is milder, persists over a substantial period of time, ~3 Gyr, and can have observational counterparts. Overall, the stellar bars show recurrent behavior in their properties and evolve by increasing their linear and vertical extents, both dynamically and secularly. We also demonstrate explicitly that the prolonged growth of the bar is mediated by continuous angular momentum transfer from the disk to the surrounding halo and that this angular momentum redistribution is resonant in nature: a large number of lower resonances trap disk and halo particles, and this trapping is robust, in broad agreement with the earlier results in the literature.

/pdf/evolution-of-stellar-bars-in-live-axisymmetric-halos-51rix2qdlk.pdf

Evolution of Stellar Bars in Live Axisymmetric Halos: Recurrent Buckling and Secular Growth

We have analyzed the redshift-dependent fraction of galactic bars over 0:2 < z < 0:84 in 2157 luminous face-on spiral galaxies from the COSMOS 2 deg 2 field. Our sample is an order of magnitude larger than that used in any previous investigation,and is based onsubstantially deeper imaging data thanthat available from earlier wide-area studies of high-redshift galaxy morphology. We find that the fraction of barred spirals declines rapidly with redshift. Whereas in the local universe about 65% of luminous spiral galaxies contain bars (SB+SAB), at z � 0:84 this fraction drops to about20%.Overthisredshiftrangethefractionof strongbars(SBs)dropsfromabout30%tounder10%.Itisclearthat when the universe was half its present age, the census of galaxies on the Hubble sequence was fundamentally different fromthatof thepresentday.Amajorcluetounderstandingthisphenomenonhasalsoemergedfromouranalysis,which shows thatthe bar fractioninspiralgalaxiesisa strongfunctionof stellar mass, integratedcolor and bulge prominence. The bar fraction in very massive, luminous spirals is about constant out to z � 0:84, whereas for the low-mass, blue spirals it declines significantly with redshift beyond z ¼ 0:3. There is also a slight preference for bars in bulge-dominated systems at high redshifts that may be an important clue toward the coevolution of bars, bulges, and black holes. Our results thus haveimportant ramifications for the processes responsible forgalacticdownsizing, suggestingthatmassive galaxies matured early in a dynamical sense, and not just as a result of the regulation of their star formation rate. Subject headingg galaxies: evolution — galaxies: general — galaxies: high-redshift — galaxies: spiral — galaxies: structure

/pdf/evolution-of-the-bar-fraction-in-cosmos-quantifying-the-371ebe644w.pdf

Evolution of the Bar Fraction in COSMOS: Quantifying the Assembly of the Hubble Sequence

We present new high-resolution observations of the central region in the late-type spiral galaxy M100 (NGC 4321) supplemented by three-dimensional numerical modeling of stellar and gas dynamics, including star formation (SF). Near-infrared imaging has revealed a small bulge of 4'' effective diameter; a 60'' radial length stellar bar of moderate strength, previously inferred from optical and 21 cm observations; and an ovally shaped, ringlike structure in the plane of the disk between 10''-22'' from the center, whose major axis makes a large angle with the bar. The K isophotes become progressively elongated and skewed to the position angle of the bar both outside and inside the ''ring,'' forming an inner barlike region. The galaxy exhibits a mild circumnuclear starburst concentrated in the inner part of the K ''ring.'' This SF is prominent in Ha and the U, B, and V bands, forming an incomplete ring. In addition, two strong local maxima of K emission have been observed to lie remarkably symmetrically with respect to the galactic nucleus and equidistant from it at 7.''5, slightly leading the stellar bar. CO molecular emission is peaked in the dust lanes seen in the I - K color index image. We interpret the twists in K isophotes and the swinging of spiral arms through similar to 360 degrees inside the corotation radius as being indicative of the presence of a double inner Lindblad resonance (ILR) and test this hypothesis by modeling the gas flow in a self-consistent gas+stars disk embedded in a halo, with an overall NGC 4321-like mass distribution in the system. Both ILRs have been verified using nonlinear orbit analysis by determining the spatial extent of the family of orbits oriented along the minor axis of the bar. We have reproduced the basic morphology of the region inside corotation, including (1) the similar to 60 degrees bar; (2) the large-scale trailing shocks representing the offset dust lanes in the bar; (3) two symmetric K peaks corresponding to gas compression maxima which lie at the caustic formed by the interaction of a pair of trailing and leading shocks in the vicinity of the inner ILR, both peaks being sites of SF; and (4) two additional zones of SF corresponding to gas compression maxima at the bar's minor axis, where the large-scale shocks start to curl and which are referred in the literature as ''twin peaks.'' We argue further that the twisting of K isophotes in the neighborhood of the resonance region requires a population of red stars which are dynamically young and follow gas rather than stellar orbits, i.e., red supergiants. At the same time, a substantial contribution from OB stars to the K light is expected within the inner kiloparsec and especially in the symmetric K peaks. We also conclude that NGC 4321 hosts a single stellar bar which fuels the starburst activity within the circumnuclear ''ring'' by channeling gas there at the median rate of similar to 0.1-1 Mo yr(-1).

The central region in m100 - observations and modeling

We carry out a detailed orbit analysis of gravitational potentials selected at different times from an evolving self-consistent model galaxy consisting of a two-component disc (stars+gas) and a live halo. The results are compared with a pure stellar model, subject to nearly identical initial conditions, which are chosen so as to make the models develop a large-scale stellar bar. The bars are also subject to hose-pipe (buckling) instability which modifies the vertical structure of the disc. The diverging morphological evolution of both models is explained in terms of gas radial inflow, the resulting change in the gravitational potential at smaller radii, and the subsequent modification of the main families of orbits, both in and out of the disc plane.  We find that dynamical instabilities become milder in the presence of the gas component, and that the stability of planar and 3D stellar orbits is strongly affected by the related changes in the potential — both are destabilized, with the gas accumulation at the centre. This is reflected in the overall lower amplitude of the bar mode and in the substantial weakening of the bar, which appears to be a gradual process. The vertical buckling of the bar is much less pronounced and the characteristic peanut shape of the galactic bulge almost disappears when there is a substantial gas inflow towards the centre. Milder instability results in a smaller bulge, the basic parameters of which are in agreement with observations. We also find that the overall evolution in the model with a gas component is accelerated because of the larger central mass concentration and the resulting decrease in the characteristic dynamical time.

Gas‐driven evolution of stellar orbits in barred galaxies

New optical and near-infrared (NIR) K-band images of the inner 3 kpc region of the nearby Virgo spiral M100 (NGC 4321) display remarkable morphological changes with wavelength. While in the optical the light is dominated by circumnuclear zone of enhanced star formation, the morphological features in the 2.2 micrometer image correspond to a newly discovered kiloparsec-size stellar bar and a pair of leading arms situated inside an ovally shaped region. Analysis of the K image confirms its symmetry: only a very small percentage of the flux, some 5%, is emitted in antisymmetric structures. This indicates that the overall morphology observed in the NIR is dominated by a global density wave. Making a first-order correction for the presence of localized dust extinction in the K light using the I-K image, we find that the observed leading arm morphology is not caused or enhanced but in fact slightly hidden by dust. Possible mechanisms responsible for the optical and NIR morphology are discussed, and tests are proposed to discriminate between them. Our dynamical conclusions are supported with an evolutionary stellar population model reproducing the observed optical and NIR colors in a number of star-forming zones. We argue that the observed morphology is compatible with the presence of a pair of inner Lindblad resonances in the region and show this explicitly in an accompanying paper by detailed numerical modeling. The phenomena observed in NGC 4321 may provide insight into physical process leading to central activity in galaxies.

The striking near-infrared morphology of the inner region in m100

We present high angular and velocity resolution two-dimensional kinematic observations in the spectral lines of Hα and CO J = 1 → 0 of the circumnuclear starburst region in the barred spiral galaxy M100, and compare them with kinematics derived from our previously published numerical modeling. The Hα data, fully sampled and at subarcsecond resolution, show a rotation curve that is rapidly rising in the central ~140 pc, and stays roughly constant, at the main disk value, further out. Noncircular motions are studied from the Hα and CO data by detailed consideration of the velocity fields, residual velocity fields after subtraction of the rotation curve, and sets of position-velocity diagrams. These motions are interpreted as the kinematic signatures of gas streaming along the inner part of the bar, and of density wave streaming motions across a two-armed minispiral. Comparison with a two-dimensional velocity field and rotation curve derived from our 1995 dynamical model shows good qualitative and quantitative agreement for the circular and noncircular kinematic components. Both morphology and kinematics of this region require the presence of a double inner Lindblad resonance in order to explain the observed twisting of the near-infrared isophotes and the gas velocity field. These are compatible with the presence of a global density wave driven by the moderately strong stellar bar in this galaxy. We review recent observational and modeling results on the circumnuclear region in M100, and discuss the implications for bar structure and gas dynamics in the core of M100 and other disk galaxies.

/pdf/kinematics-of-ionized-and-molecular-hydrogen-in-the-core-of-37y42uyl9i.pdf

Kinematics of Ionized and Molecular Hydrogen in the Core of M100

We investigate the dynamical response of stellar orbits in a rotating barred galaxy potential to the perturbation by a nuclear gaseous ring. The change in three-dimensional periodic orbit families is examined as the gas accumulates between the inner Lindblad resonances. It is found that the phase space allowable to the x2 family of orbits is substantially increased, and a vertical instability strip appears with the growing mass of the ring. A significant distortion of the x1 orbits is observed in the vicinity of the ring, which leads to the intersection between orbits with different values of the Jacobi integral. We also examine the dependence of the orbital response to the eccentricity and alignment of the ring with the bar. Misalignment between an oval ring and a bar can leave observational footprints in the form of twisted near-infrared isophotes in the vicinity of the ring. It is inferred that a massive nuclear ring acts to weaken and dissolve the stellar bar exterior to the ring, whereas only weakly affecting the orbits interior to the inner Lindblad resonances. Consequences for gas evolution in the circumnuclear regions of barred galaxies are discussed as well.

C. H. Heller

Papers

The central region in m100 - observations and modeling

Gas‐driven evolution of stellar orbits in barred galaxies

The striking near-infrared morphology of the inner region in m100

Kinematics of Ionized and Molecular Hydrogen in the Core of M100

Dynamical Effects of Nuclear Rings in Disk Galaxies