Bulges of Spiral Galaxies: Stellar Populations, Structure, Kinematics, and Dynamics

Bhasker Moorthy

I present the results of a study aimed at understanding the formation mechanims of bulges through optical long-slit spectroscopy and imaging. Long-slit spectra of 38 bulges taken with the ARC 3.5m telescope were used to obtain luminosity-weighted age and metallicity gradients as well as line-of-sight velocity distributions. Bulge-to-disk decomposition was performed using images obtained with the same telescope to study the structural properties and to separate the bulge and disk contributions. Several bulges that were previously found to have similar colors as their disks were included to see if their spectral properties reveal evidence for secular evolution.

We find that red bulges of all Hubble types are similar to luminous ellipticals in their central stellar populations. They have large luminosity-weighted ages, metallicities, and alpha/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 alpha/Fe ratios are sensitive to central velocity dispersion and maximum disk rotational velocity. Red bulges and ellipticals follow the same scaling relations. We see differences in some scaling relations between blue and red bulges and between bulges of barred and unbarred galaxies. Most bulges have decreasing metallicity with increasing radius; galaxies with larger central metallicities have steeper gradients. Where positive age gradients (with the central regions being younger) are present, they are invariably in barred galaxies. The metallicities of bulges are correlated with those of their disks. Nearly all our bulges rotate fast with no discernable difference in the rotational velocities of the stellar and ionized gas components. The ratio of maximum rotational velocity to velocity dispersion ranges from 0.5 to 1 in a majority of our bulges, with a handful of bulges lying outside these two extremes. We see no correlation between a large ratio of rotational velocity to velocity dispersion do not show and disk-like (young and/or metal-poor) stellar populations. Neither do we see any correlation between kinematics and morphology; bulges of barred galaxies and bulges that are structurally disk-like do not appear distinct from other bulges in their kinematics. While some of our observations indicate that secular evolution cannot be ignored, our results are generally consistent with the hypothesis that bulge formation was dominated by a single mechanism, presumably mergers.