The Bulge/Disk Connection in Late-type Spirals

Recent ground-based photometric investigations suggest that central regions of late-type spirals are closely coupled to the inner disk and probably formed via secular evolution. Evidence presented in support of this model includes the predominance of exponential bulges, the correlation of bulge and disk scale lengths, blueness of the bulge and small differences between bulge and central disk colors, detection of spiral structure into the core, and rapid rotation. Recent HST observations show that our own bulge and that of M31, M32, and M33 probably harbor both an old and intermediate-age populations in agreement with models of early collapse of the spheroid plus gas transfer from the disk. Secular evolution provides a mechanism to build-up central regions in late-type spirals; mergers or accretion of small satellites could explain the brighter, kinematically distinct bulges of Sa's and SO's

A review of elliptical and disc galaxy structure, and modern scaling laws

A century ago, in 1911 and 1913, Plummer and then Reynolds introduced their models to describe the radial distribution of stars in `nebulae'. This article reviews the progress since then, providing both an historical perspective and a contemporary review of the stellar structure of bulges, discs and elliptical galaxies. The quantification of galaxy nuclei, such as central mass deficits and excess nuclear light, plus the structure of dark matter halos and cD galaxy envelopes, are discussed. Issues pertaining to spiral galaxies including dust, bulge-to-disc ratios, bulgeless galaxies, bars and the identification of pseudobulges are also reviewed. An array of modern scaling relations involving sizes, luminosities, surface brightnesses and stellar concentrations are presented, many of which are shown to be curved. These 'redshift zero' relations not only quantify the behavior and nature of galaxies in the Universe today, but are the modern benchmark for evolutionary studies of galaxies, whether based on observations, N-body-simulations or semi-analytical modelling. For example, it is shown that some of the recently discovered compact elliptical galaxies at 1.5 < z < 2.5 may be the bulges of modern disc galaxies.Comment: Condensed version (due to Contract) of an invited review article to appear in "Planets, Stars and Stellar Systems"(www.springer.com/astronomy/book/978-90-481-8818-5). 500+ references incl. many somewhat forgotten, pioneer papers. Original submission to Springer: 07-June-201

On the Generation of the Hubble Sequence through an Internal Secular Dynamical Process

The secular evolution process, which slowly transforms the morphology of a galaxy over its lifetime, could naturally account for observed properties of the great majority of physical galaxies if both stellar and gaseous accretion processes are taken into account. As an emerging paradigm for galaxy evolution, its dynamical foundation had been established in the past few years, and its observational consequences are yet to be fully explored. The secular evolution picture provides a coherent framework for understanding the extraordinary regularity and the systematic variation of galaxy properties along the Hubble sequence.Comment: 10 pages, 1 figure, invited talk presented at the international conference on "Penetrating Bars through the Masks of Cosmic Dust: The Hubble Tuning Fork Strikes a New Note", June 2004, South Afric

Bar-Driven Evolution and 2D Spectroscopy of Bulges

A multi-faceted approach is described to constrain the importance of bar-driven evolution in disk galaxies, particularly bulge formation. N-body simulations are used to construct stellar kinematic bar diagnostics for edge-on systems and to quantify the expected vertical structure of bars, and they are compared to observations of 30 edge-on spirals, most with a boxy bulge. Long-slit spectra of the galaxies show characteristic double-hump rotation curves, dispersion profiles with secondary peaks and/or flat maxima, and correlated h3 and V profiles, indicating that most of them harbor edge-on bars. The presence of cold, quasi-axisymmetric central stellar disks is also suggested, presumably formed through bar-driven gaseous inflow and star formation. K-band imaging of the same galaxies spectacularly highlights radial variations of the bars' scaleheights, as expected from vertical disk instabilities. The light profiles also vary radially in shape but never approach a classic de Vaucouleurs law. Filtering of the images further isolates the specific orbit families at the origin of the boxy structure, which can be directly related to periodic orbit calculations in 3D barred potentials. Bars are thus shown to contribute substantially to the formation of both large-scale triaxial bulges and embedded central disks. Relevant results from the SAURON survey of the stellar/ionized-gas kinematics and stellar populations of spheroids are also described. Examples are used to illustrate the potential of coupling stellar kinematics and linestrengths (age and metallicity), here specifically to unravel the dynamical evolution and related chemical enrichment history of bars and bulges. [Abridged]Comment: 10 pages, including 4 figures (LaTeX, kapproc.cls, procps.sty). To appear in "Penetrating Bars through Masks of Cosmic Dust: the Hubble Tuning Fork Strikes a New Note", eds. D.L. Block, K.C. Freeman, I. Puerari, & R. Groess (Kluwer: Dordrecht). A version with full resolution PostScript figures is available at http://www.astro.columbia.edu/~bureau/Publications/peanut_sa_04.ps.g

Secular Evolution and the Growth of Pseudobulges in Disk Galaxies

Galaxy evolution is in transition from an early universe dominated by hierarchical clustering to a future dominated by secular processes. These result from interactions involving collective phenomena such as bars, oval disks, spiral structure, and triaxial dark halos. This paper summarizes a review by Kormendy & Kennicutt (2004) using, in part, illustrations of different galaxies. In simulations, bars rearrange disk gas into outer rings, inner rings, and galactic centers, where high gas densities feed starbursts. Consistent with this picture, many barred and oval galaxies have dense central concentrations of gas and star formation rates that can build bulge-like stellar densities on timescales of a few billion years. We conclude that secular evolution builds 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. Many pseudobulges can be recognized because they have characteristics of disks: (1) flatter shapes than those of classical bulges, (2) correspondingly large ratios of ordered to random velocities, (3) small velocity dispersions, (4) spiral structure or nuclear bars, (5) nearly exponential brightness profiles, and (6) starbursts. These structures occur preferentially in barred and oval galaxies in which secular evolution should be most rapid. Thus a variety of observational and theoretical results contribute to a new paradigm of secular evolution that complements hierarchical clustering.Comment: 19 pages, 9 Postscript figures; requires kapproc.cls and procps.sty; to appear in "Penetrating Bars Through Masks of Cosmic Dust: The Hubble Tuning Fork Strikes a New Note", ed. Block, Freeman, Puerari, Groess, and Block, Dordrecht: Kluwer, in press; for a version with full resolution figures, see http://chandra.as.utexas.edu/~kormendy/ar3ss.htm

The effects of a disc field on bulge surface brightness

Collisionless N-body simulations are used in an effort to reproduce the observed tendency of the surface brightness profile of bulges to change progressively from an R-1/4 law to an exponential, going from early-to late-type spirals. A possible cause for this is the formation of the disc, later in the history of the galaxy, and this is simulated by applying on the N-body bulge the force field of an exponential disc the surface density of which increases with time. It is shown that n, the index of the Sersic law Sigma(n)(r) proportional to exp[-(r/r(0))(1/n)] that best describes the surface brightness profile, does indeed decrease from 4 (de Vaucouleurs law) to smaller values; this decrease is larger for more massive and more compact discs. A large part of the observed trend of n with B/D ratio is explained, and many of the actual profiles can be matched exactly by the simulations. The correlation between the disc scalelength and bulge effective radius, used recently to support the 'secular evolution' origin for bulges, is also shown to arise naturally in a scenario like this. This mechanism, however, saturates at around n = 2 and exponential bulges cannot be produced; as n gets closer to 1,the profile becomes increasingly robust against a disc field. These results provide strong support to the old-bulge hypothesis for the early-type bulges. The exponential bulges, however, remain essentially unexplained; the results here suggest that they did not begin their Lives as R-1/4 spheroids, and hence were probably formed, at least in part, by different processes from those of early-type spirals