▪ 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

We review internal processes of secular evolution in galaxy disks, concentrating on the buildup of dense central features that look like classical, merger-built bulges but that were made slowly out of disk gas. We call these pseudobulges. As an existence proof, we review how bars rearrange disk gas into outer rings, inner rings, and gas dumped into the center. In simulations, this gas reaches high densities that plausibly feed star formation. In the observations, many SB and oval galaxies show central concentrations of gas and star formation. Star formation rates imply plausible pseudobulge growth times of a few billion years. If secular processes built dense central components that masquerade as bulges, can we distinguish them from merger-built bulges? Observations show that pseudobulges retain a memory of their disky origin. They have one or more characteristics of disks: (1) flatter shapes than those of classical bulges, (2) large ratios of ordered to random velocities indicative of disk dynamics, (3) small velocity dispersions, (4) spiral structure or nuclear bars in the bulge part of the light profile, (5) nearly exponential brightness profiles, and (6) starbursts. These structures occur preferentially in barred and oval galaxies in which secular evolution should be rapid. So the cleanest examples of pseudobulges are recognizable. Thus a large variety of observational and theoretical results contribute to a new picture of galaxy evolution that complements hierarchical clustering and merging.

A major problem in extragalactic astronomy is the inability to distinguish in a robust, physical, and modelindependent way how galaxy populations are physically related to each other and to their formation histories. A similar, but distinct, and also long-standing question is whether the structural appearances of galaxies, as seen through their stellar light distributions, contain enough physical information to offer this classification. We argue through the use of 240 images of nearby galaxies that three model-independent parameters measured on a single galaxy image reveal its major ongoing and past formation modes and can be used as a robust classification system. These parameters quantitatively measure: the concentration (C), asymmetry (A), and clumpiness (S) of a galaxy’s stellar light distribution. When combined into a threedimensional ‘‘ CAS ’’ volume all major classes of galaxies in various phases of evolution are cleanly distinguished. We argue that these three parameters correlate with important modes of galaxy evolution: star

https://iopscience.iop.org/article/10.1086/375001/pdf

The Relationship between Stellar Light Distributions of Galaxies and Their Formation Histories

We analyse the orbital distribution of elliptical (E) and lenticular (S0) galaxies using SAURON integral-field stellar kinematics within about one effective (half-light) radius. We construct the anisotropy diagram, which relates the ratio of the ordered and random motion in a galaxy (V/sigma) to its observed ellipticity (epsilon), for the 48 E/S0 galaxies from the SAURON survey. For a subsample of 24 galaxies consistent with axisymmetry, we use three-integral axisymmetric Schwarzschild dynamical models to recover the detailed orbital distribution, and we find good agreement with the anisotropy derived from the (V/sigma, epsilon) diagram. In a companion paper (Paper IX), we show that the early-type galaxies can be subdivided into two classes of systems with or without a significant amount of specific stellar angular momentum. Here, we show that the two classes have different distributions on the (V/sigma, epsilon) diagram. The slow rotators are more common among the most massive systems and are generally classified as E from photometry alone. Those in our sample tend to be fairly round(epsilon less than or similar to 0.3), but can have significant kinematical misalignments, indicating that as a class they are moderately triaxial, and span a range of anisotropies (delta less than or similar to 0.3). The fast rotators are generally fainter and are classified as either E or S0. They can appear quite flattened (epsilon less than or similar to 0.7), do not show significant kinematical misalignments (unless barred or interacting), indicating they are nearly axisymmetric and span an even larger range of anisotropies (delta less than or similar to 0.5). These results are confirmed when we extend our analysis to 18 additional E/S0 galaxies observed with SAURON. The dynamical models indicate that the anisotropy inferred from the (V/sigma, epsilon) diagram is due to a flattening of the velocity ellipsoid in the meridional plane (sigma(R) > sigma(z)), which we quantify with the beta anisotropy parameter. We find a trend of increasing beta for intrinsically flatter galaxies. A number of the fast rotators show evidence for containing a flattened, kinematically distinct component, which in some cases counter-rotates relative to the main galaxy body. These components are generally more metal rich than the galaxy body. All these results support the idea that fast rotators are nearly oblate and contain disc-like components. The role of gas must have been important for their formation. The slow rotators are weakly triaxial. Current collisionless merger models seem unable to explain their detailed observed properties.

/pdf/the-sauron-project-x-the-orbital-anisotropy-of-elliptical-4994gm3pwk.pdf

The SAURON project – X. The orbital anisotropy of elliptical and lenticular galaxies: revisiting the (V/σ, ɛ) diagram with integral‐field stellar kinematics

We use the ATLAS3D sample of 260 early-type galaxies to study the apparent kinematic misalignment angle, Ψ, defined as the angle between the photometric and kinematic major axes. We find that 71 per cent of nearby early-type galaxies are strictly aligned systems (Ψ≤ 5°), an additional 14 per cent have 5° <Ψ≤ 10° and 90 per cent of galaxies have Ψ≤ 15°. Taking into account measurement uncertainties, 90 per cent of galaxies can be considered aligned to better than 5°, suggesting that only a small fraction of early-type galaxies (˜10 per cent) are not consistent with the axisymmetry within the projected half-light radius. We identify morphological features such as bars and rings (30 per cent), dust structures (16 per cent), blue nuclear colours (6 per cent) and evidence of interactions (8 per cent) visible on ATLAS3D galaxies. We use KINEMETRY to analyse the mean velocity maps and separate galaxies into two broad types of regular and non-regular rotators. We find 82 per cent of regular rotators and 17 per cent of non-regular rotators, with two galaxies that we were not able to classify due to the poor data quality. The non-regular rotators are typically found in dense regions and are massive. We characterize the specific features in the mean velocity and velocity dispersion maps. The majority of galaxies do not have any specific features, but we highlight here the frequency of the kinematically distinct cores (7 per cent of galaxies) and the aligned double peaks in the velocity dispersion maps (4 per cent of galaxies). We separate galaxies into five kinematic groups based on the kinemetric features, which are then used to interpret the (Ψ-ɛ) diagram. Most of the galaxies that are misaligned have complex kinematics and are non-regular rotators. In addition, some show evidence of the interaction and might not be in equilibrium, while some are barred. While the trends are weak, there is a tendency that large values of Ψ are found in galaxies at intermediate environmental densities and among the most massive galaxies in the sample. Taking into account the kinematic alignment and the kinemetric analysis, the majority of early-type galaxies have velocity maps more similar to that of the spiral discs than to that of the remnants of equal-mass mergers. We suggest that the most common formation mechanism for early-type galaxies preserves the axisymmetry of the disc progenitors and their general kinematic properties. Less commonly, the formation process results in a triaxial galaxy with much lower net angular momentum.

/pdf/the-atlas3d-project-ii-morphologies-kinemetric-features-and-11lij0p0ta.pdf

The ATLAS3D project - II. Morphologies, kinemetric features and alignment between photometric and kinematic axes of early-type galaxies

The rotation curve, up to 0/sup prime/.6 from the center, has been derived from Ca ii H and K lines for the elliptical galaxy NGC 4697. A maximum seems to be reached at 15'' with 65 km s$sup -1$. The same galaxy has been investigated by means of photographic photometry in the B system. The luminosity profile follows the r$sup 1$/$sup 4$ law, and the mean ellipticity is 0.4. The total apparent magnitude is B=10.30 mag, and the corresponding absolute magnitude reaches M/subB/=20.5 magn assuming a distance of 14.8 Mpc. The spatial velocity curve and the light density profile are derived. Mass and angular momentum have been computed on the hypothesis that the mass-to-light ratio is constant through the galaxy, leaving it as a free parameter. Under several assumptions it is found that the angular momentum of the elliptical NGC 4697 is lower than that of a spiral of comparable mass by a factor 5 to a factor 30, when the mass-to-light ratio varies from 5 to 50. (AIP)

Dynamics of early type galaxies. I. The rotation curve of the elliptical galaxy NGC 4697

The International Ultraviolet Explorer observations of the nuclei of the elliptical galaxies NGC 3379 and NGC 4472 show that the flux drops rapidly with decreasing wavelength until about 2300 A, below which the flux increases rapidly. The hot UV component of the radiation can be approximately represented by a blackbody at 30,000 K. The spectra of NGC 3379, NGC 4472, and M31 are very similar from 1260 to 8000 A. The hot UV component is abnormally weak in M 32 and abnormally strong in M 87. The UV spectrum from 1300 to 3200 A of the metal-rich globular cluster NGC 6624 resembles those of NGC 3379, NGC 4472, and M 31, and shows evidence for hot blue stars. A comparison of the UV fluxes with those measured in large-redshift normal galaxies shows good agreement, although allowance must be made for the line-strength differences found between the integrated galaxy spectra and the nuclear-region spectra.

IUE observations of two elliptical galaxies : NGC 3379 and NGC 4472.

A spectrum of M87 obtained in the 1150-2000 A spectral range by the IUE is discussed. The visual and UV energy distribution of the nucleus of M87 reveals an essentially continuous curve from 1300 to 10,500 A, and a turnup in flux below 2200 A. It is shown that this turnup is not a consequence of scattered light in the instrument, while a plot of the deconvolved M87 UV profile and the visual profile shows that the radiation in the 1300-2200 A spectral region is distributed in the same way as the visual radiation; it is not a point source. In addition, there is evidence in the UV for an emission line of C IV; the relative strengths of C IV, H beta, and the forbidden O III lines at 4959 and 5007 A are consistent with those seen in planetary nebulae, but the forbidden O II line at 3727 A is much too strong.

International Ultraviolet Explorer observations of M87

As in the previously studied elliptical galaxies, the flux f(lambda) in NGC 4649 has a minimum at 2500 A followed by a rapid increase towards shorter wavelengths. In NGC 4649, the flux level of this rising branch is the highest so far observed and seems not to be correlated with the luminosity or activity in the galaxy. The excess shortward of 4000 A, observed in M87, another high level rising branch galaxy, is not present in NGC 4649. The UV spectrum of NGC 4649 down to 2500 A matches closely those of NGC 3379 and NGC 4472. The implications of the UV properties of elliptical galaxies on the stellar content as well as on magnitudes and colors of distant galaxies are discussed

IUE observations of NGC 4649, an elliptical galaxy with a strong ultraviolet flux

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Dynamics of early type galaxies. I. The rotation curve of the elliptical galaxy NGC 4697

IUE observations of two elliptical galaxies : NGC 3379 and NGC 4472.

International Ultraviolet Explorer observations of M87

IUE observations of NGC 4649, an elliptical galaxy with a strong ultraviolet flux