We present here an optical and near-infrared (NIR) photometric study of the bulge of NGC 1371, an Sa galaxy in the Fornax cluster. The galaxy hosts a nuclear bar, from which two spiral arms depart, and a triaxial bulge and it is the most peculiar object in a sample of 17 isolated spiral galaxies studied here. The triaxial shape and the bar are apparent also in the H band, i.e. where the emission from the old (t > 107 yr) stellar population peaks (Grauer & Rieke, 1998). The implications of our findings for bulge formation and bar secular evolution models are discussed.

The kinematics of stars and ionized gas has been studied near the center of the SO galaxy NGC 4036. Dynamical models based both on stellar photometry and kinematics have been built in order to derive the gravitational potential in which the gas is expected to orbit. The observed gas rotation curve falls short of the circular velocity curve inferred from these models. Inside 10″ the observed gas velocity dispersion is found to be comparable to the predicted circular velocity, showing that the gas cannot be considered on circular orbits. The understanding of the observed gas kinematics is improved by models based on the Jeans Equations, which assume the ionized gas as an ensemble of collisionless cloudlets distributed in a spheroidal and in a disk component.

Using high resolution, two-dimensional hydrodynamical simulations, we investigate the evolution of a self-gravitating multi-phase interstellar medium in the central kiloparsec region of a galactic disk. We find that a gravitationally and thermally unstable disk evolves, in a self-stabilizing manner, into a globally quasi-stable disk that consists of cold (T < 100 K), dense clumps and filaments surrounded by hot (T > 104 K), diffuse medium. In the quasi-stable phase where cold and dense clouds are formed, the effective stability parameter, Q, has a value in the range 2-5. The dynamic range of our multi-phase calculations is 106 – 107 in both density and temperature. Phase diagrams for this turbulent medium are analyzed and discussed. We also succeeded in modeling star formation in the multi-phase ISM with 2 pc resolution. Massive stars formed in the dense, cold clouds are tracked for their life time, and finally explode as SNe. The filamentlike structure of the cold gas is stable for the SNe, although bubbles of the hot gas (T > 106 K) are formed. We observed recurrent burst-like SNe production.

Observations of the stellar content of the Milky Way's bulge helps us to understand the stellar content and evolution of distant galaxies. In this brief overview I will first highlight some recent work directed towards measuring the history of star formation and the chemical composition of the central few parsecs of the Galaxy. High resolution spectroscopic observations by Ramirez et al. (1998) of luminous M stars in this region yield a near solar value for [Fe/H] from direct measurements of iron lines. Then I will present some results from an ongoing program by my colleagues and myself which has the objective of delineating the star formation and chemical enrichment histories of the central 100 parsecs of the Galaxy, the ‘inner bulge’. We have found a small increase in mean [Fe/H] from Baade's Window to the Galactic Center and deduce a near solar value for stars at the center. For radial distances greater than 1° we fail to find a measurable population of stars that are significantly younger than those in Baade's Window. Within 1° of the Galactic Center we find a number of luminous M giants that most likely are the result of a star formation episode not more than one or two Gyr ago.

Within recent years, there has been a confluence of data that favors a large age for the bulges of the Milky Way and M31. A short formation timescale is required by the similarity in ages between the bulge and the old, metal-rich globular clusters. Detailed abundances of bulge giants are consistent with a short enrichment timescale. The bulge of M31 is similarly old and even more metal-rich than the Galactic bulge. There appears to be a strong connection between the M31 bulge and the halo, as metal-rich giants are found in M31 out to great distances. The stellar populations data support a rapid bulge formation timescale, perhaps even less than 1 Gyr.

“We must conclude, then, that in the central region of the Andromeda Nebula we have a metal-poor Population II, which reaches −3m for the brightest stars, and that underlying it there is a very much denser sheet of old stars, probably something like those in M67 or NGC 6752. We can be certain that these are enriched stars, because the cyanogen bands are strong, and so the metal/hydrogen ration is very much closer to what we observe in the Sun and in the present interstellar medium than to what is obwserved for Population II. And the process of enrichment probably has taken very little time. After the first generation of stars has formed, we can hardly speak of a ‘generation’, because the enrichment takes place so soon, and there is probably very little time difference.

The relevant dynamical processes for bulge formation are reviewed: collapse, accretion, bar formation, stochastic heating and external forcing. All of these processes take place at some level, but it appears hard to escape the conclusion that bulges formed quickly and early.

An analysis of stellar orbits in a doubly barred galaxy shows that the effect of a secondary bar is to destabilize the orbits, the process being accompanied by the appearance of vertical resonances which would enable stars to leave the galactic plane and move into the bulge. This phenomenon could contribute to bulge formation. Results of the orbital analysis are presented and their significance discussed.

Our new statistical study of bulges of disk galaxies reveals a frequency of almost 50% being boxy-or peanut-shaped. Therefore very common processes are required to explain this high fraction. In an analysis of a possible relation between this internal structure and the environment of galaxies with boxy/peanut-shaped bulge we find that on large scales there is no hint for a connection. However, galaxies with boxy- or peanut-shaped bulges have more companions and satellites and show more frequently interactions than a control sample. Thus we conclude that the small-scale environment is important for the existence of such bulges. The most likely reason responsible for the development of boxy/peanut-shaped bulges is a bar originating from galaxy interaction in stable disks or by an infalling satellite.

The gravitational clustering hierarchy and dissipative gas processes are both involved in the formation of bulges. Here I present a simple empirical model in which bulge material is assembled via gravitational accretion of visible companion galaxies. Assuming that merging leads to a starburst, I show that the resulting winds can be strong enough to self-regulate the accretion. A quasi-equilibrium accretion process naturally leads to the Kormendy relation between bulge density and size. Whether or not the winds are sufficiently strong and long lived to create the quasi-equilibrium must be tested with observations. To illustrate the model I use it to predict representative parameter-dependent star formation histories. The bulge building activity appears to peak around a redshift z ∼ 2, with tails to both higher and lower redshifts.

In the context of studying the properties of the mutual mass distribution of the bright and dark matter in bulges (or elliptical galaxies), the properties of the analytical phase–space distribution function (DF) of two–component spherical self–consistent stellar systems (where one density distribution follows the Hernquist profile, and the other a γ = 0 model, with different total masses and core radii [HO models]) are here summarized. A variable amount of radial Osipkov–Merritt (OM) orbital anisotropy is allowed in both components. The necessary and sufficient conditions that the model parameters must satisfy in order to correspond to a model where each one of the two distinct components has a positive DF (the so–called model consistency) are analytically derived, together with some results on the more general problem of the consistency of two–component γ1 + γ2 models. The possibility to add in a consistent way a black hole at the center of radially anisotropic γ-models is also discussed. In the particular case of HO models, it is proved that a globally isotropic Hernquist component is consistent for any mass and core radius of the superimposed γ = 0 halo. On the contrary, only a maximum value of the core radius is allowed to the γ = 0 component when a Hernquist halo is added. The combined effect of halo concentration and orbital anisotropy is successively investigated. […]

AGN are known to lie in galaxies, and both galaxies and AGN evolve similarly over cosmic time (e.g., Silk & Rees 1998). This suggests a close connection between the nuclear phenomena associated with black holes and the formation and evolution of ordinary galaxies. The host galaxies of AGN are a direct probe of the AGN-galaxy connection. Among AGN, BL Lac objects are know to reside mostly, if not systematically, in elliptical galaxies. BL Lac can therefore probe (massive) spheroids to large redshifts. Results from an HST WFPC2 survey of ∼ 100 BL Lac objects are here presented.

We present a study in B, I and H of a magnitude-limited sample of galactic bulges using WFPC2 and NICMOS. The high spatial resolution of HST allows us to study the dust contents near the center, and stellar populations in dust-free regions. We find extinction in 19/20 galaxies and infer an average central extinction of Av = 0.6 − 1.0 mag. For galactic bulges of types S0 to Sb, the tightness of the B − I vs I − H relation suggests that the age spread among bulges of early type galaxies is small, at most 2 Gyr independent of environment. Comparison with stellar population models shows that the bulges are old. Colors at 1 bulge effective-radius, where we expect extinction to be negligible, suggest that all of these bulges formed around at the same time as bright galaxies in the Coma cluster.

We present first results from an on-going survey of the stellar populations of the bulges and inner disks of spirals at various points along the Hubble sequence. In particular, we are investigating the hypotheses that bulges of early-type spirals are akin to (and may in fact originally have been) intermediate-luminosity ellipticals while bulges of late-type spirals are formed from dynamical instabilities in their disks. Absorption-line spectroscopy of the central regions of Sa–Sd spirals is combined with stellar population models to determine integrated mean ages and metallicities. These ages and metallicities are used to investigate stellar population differences both between the bulges and inner disks of these spirals and between bulges and ellipticals in an attempt to place observational constraints on the formation mechanisms of spiral bulges.