Bulges of spiral galaxies: Structure, kinematics, dynamics, and stellar populations

It is generally accepted that some bulges, referred to as classical bulges, are similar in their properties to elliptical galaxies while others, referred to as pseudobulges, are similar to disks. Arguments for disk-like behavior in some bulges include flatness, exponential light profiles, a large ratio of rotational to random velocities, young stellar populations, and morphological features such as inner bars, rings, and spiral structure. However there are hardly any known galaxies where all these properties are collectively seen. We have used long-slit spectra and images to explore the structure, morphology, kinematics, dynamics, and stellar populations of 38 bulges. Our galaxy sample was chosen to span a range of bulge properties and specifically targeted several galaxies with blue bulges and similar bulge/disk colors in an attempt to look for signatures of secular evolution.

The majority of the bulges we have studied appear elliptical-like in most of their properties: they are less elongated than their disks and appear featureless in images and unsharp masks; they have considerably larger velocity dispersions than their disks with the dynamically hot component being similar in extent to the photometrically-determined bulge-dominated region (the light in excess of the in- ward extrapolation of an exponential disk), and they are similar in stellar content to ellipticals of comparable s 0 . In their stellar content, red bulges are similar to luminous ellipticals while blue bulges are similar to low-luminosity ellipticals. Red bulges of all Hubble types have large luminosity-weighted ages, metallicities, and a/Fe ratios. Blue bulges can be separated into a metal-poor class that is restricted to late-types with small velocity dispersion and a young, metal-rich class that includes all Hubble types and velocity dispersions. Luminosity-weighted metallicities and a/ Fe ratios are sensitive to central velocity dispersion and maximum disk rotational velocity. Most bulges have decreasing metallicity with increasing radius: galaxies with larger central metallicities have steeper gradients.

Our results are generally consistent with the hypothesis that mergers have been the dominant mechanism responsible for bulge formation. However, there are a handful of galaxies that exhibit most of the properties that have traditionally been associated with pseudobulges. Furthermore, the following two results that are naturally explained by secular evolution pose challenges to the merger scenario: (1) The metallicities of bulges are correlated with those of their disks; and (2) At fixed s 0 , barred galaxies appear to have larger central metallicities than unbarred galaxies and are overrepresented among galaxies with age gradients.