Introduction

Star formation (SF) is a local phenomenon which must find its explanation in the stability and fragmentation of dense molecular clouds. Studies in our own Galaxy have focussed on the structure of dense proto-stellar cores and the chemical and thermal balance of star-forming regions. These studies lend indirect support to a Schmidt (1959) law, but emphasize the need to include explicitly the structure of the multi-phase ISM to model accurately the most important heating and cooling processes. A large unknown in these investigations is the role of feedback. Supernova explosions and stellar radiation associated with the process of SF influence the global physical structure of the interstellar gas which supports this process.

Introduction

The nature of the X-ray emission of early-type, prominent-bulge spirals has been the subject of an on-going controversy, which has sought to establish if and how much of this emission can be ascribed to thermal emission of an optically thin hot gaseous medium. This is an important issue, because if it can be established that the X-ray emission is dominated by gravity-bound gaseous halos, the X-ray data may be used to measure the mass of these galaxies (see review in Fabbiano 1989).

In what follows, I give a summary of the work on this subject, and point out future opportunities.

A Brief History of X-ray Studies of Early-type Spirals

With the clear exception of M31, most of the bulges of early-type spirals could not be studied in detail with X-ray observatories, starting with the Einstein Observatory, in the early ʾ80s, and including all the facilities in orbit and operational at this time.

Introduction

HST UV images of nearby galaxies presented by Maoz et al. (1996) and Barth et al. (1998), as well as analogous space-borne optical images of early-type galaxies discussed by Lauer et al. (1995) and Carollo et al. (1997) have shown that about 15% of imaged galaxies show evidence of unresolved central spikes.

In the following we discuss two ‘prototype’ galactic spheroids, NGC 2681 and NGC 4552, that we properly monitored with HST–which host UV-bright, unresolved spikes at their center. While the early-spiral (Sa) galaxy NGC 2681 shows a nonvariable unresolved cusp, the UV spike which became visible at the center of the Virgo Elliptical NGC 4552 is a UV flare caught in mid-action, presumably related to a transient accretion event onto a central supermassive black hole (Renzini et al. 1995; Cappellari et al. 1998).

Introduction

NGC 4698 is classified Sa by Sandage & Tammann (1981) and Sab(s) by de Vaucouleurs et al. (1991; RC3). Sandage & Bedke (1994; CAG) present NGC 4698 as an example of the early-to-intermediate Sa type since it is characterized by a large central bulge and tightly wound spiral arms. In addition to a remarkable geometrical decoupling between the bulge and the disk (whose apparent major axes appear oriented in an orthogonal way upon simple visual inspection of galaxy plates; see Panels 78, 79 and 87 in CAG), a spectrum taken along the minor axis of the disk shows the presence of a stellar velocity gradient which could be ascribed to the bulge.

The innermost, denser regions of galaxies, i.e., the ‘bulges’, are a fundamental component of galaxies whose properties define the entire Hubble sequence. Understanding the origin of bulges is thus a required step toward understanding how such a sequence has come to place, i.e., toward deciphering how stars and galaxies condensed from the diffuse material in space into the structure that we observe today. Several decades of exploration of the Milky Way and Local Group bulges, and of nearby bulges external to the Local Group, have slowly built the orthodox view that bulges as a family should be reasonably old isotropic rotators with near-solar mean chemical abundances (although with a very wide abundance distribution function), i.e., nothing more than low-luminosity ellipticals. However, some major breakthroughs in the last few years concerning bulges in the local and early universe suggest that the time is ripe to perhaps reconsider this orthodoxy. The new picture that emerges from the most recent Hubble Space Telescope (HST) and 10m-class ground-based telescopes studies challenges the canonical beliefs about what bulges really are, how and when they form, and about the physical mechanisms that are important in determining their fundamental properties. Basic, and yet fundamental questions still need an answer: (i) Are bulges a one-parameter or a multi-parameter family? What are the average properties of bulges in terms of stellar populations and dynamics? What are the deviations from these averages?

Introduction

An important tool for galaxy research is the study of the surface brightness (SB) distribution. For spiral galaxies the determination of the scale length of the exponential disk has a long traditition (e.g. Courteau 1996). However, the errors in these results are still rather large (Knapen & van der Kruit 1991).

For a better understanding of spiral galaxies it is necessary to study the structure of both disk and bulge as well. In order to separate non-axisymmetric structures as bars or triaxial bulges from the axisymmetric disk, two-dimensional fits are advantageous (e.g. de Jong 1996). In the following I present a generalization of a nonlinear direct fit method to the two-dimensional SB distribution of near-infrared (NIR) images of spiral galaxies.

NIR Data

The aim of this project is the study of the distribution of the mass-carrying evolved stars in spiral galaxies of different Hubble types. For this purpose, NIR observations are advantageous since they have much less perturbations due to dust or especially bright young stars.

The observations were performed during several runs at the 2.2m telescope of the German-Spanish observatory on Calar Alto, Spain. The detector was the MAGIC-NIR-camera with a NICMOS chip of (0.67″) 256×256 pixels, for a total field of view of ≈ 3′ × 3′.

Introduction

Despite a great deal of progress in recent years, there still remain major uncertainties in our observational picture of the formation and evolution of galaxies in the high redshift Universe. Not least, the relationship between the star formation activity seen at high redshift and the present-day morphological components of the galaxy population, including the bulges that are the subject of this conference, remains unclear. The origin of the stars in the metal-rich spheroidal components of present-day galaxies, which constitute a half to two-thirds of all stars in the Universe (see Fukugita et al. 1998), is thus an unsolved observational question.

Definitions

What is a galactic bulge? A crucial pre-requisite to answering this question is to define the terminology. ‘Galactic’ seems innocuous, but is not. In discussion of bulges many people decree bulges to be synonymous with (small) ellipticals. Others note that a bulge is defined only relative to a disk. Are bulges simply small ellipticals? Did they form in the same way? Is the existence of a disk irrelevant to the history of a bulge?

Which leaves us to define a ‘bulge’. One must beware definitions which are self-fulfilling as much as one must beware definitions which are not restrictive. A consideration in a valuable definition must be the utility of bulges in defining the Hubble sequence. The invaluable Carnegie Atlas of Galaxies (Sandage & Bedke 1994) raises both a working definition and a clue to interpretation:

Introduction

Boxy/peanut-shaped bulges (hereafter referred to simply as boxy bulges) have, as their name indicates, excess light above the plane. They are thus easily identified in edge-on systems and display many interesting properties: their luminosity excess, an extreme three-dimensional structure, probable cylindrical rotation, etc. However, the main importance of boxy bulges resides in their incidence: at least 20-30% of all spiral galaxies possess a boxy or peanut-shaped bulge. They are thus essential to our understanding of bulge formation and evolution.

Early theories on the formation of boxy bulges were centered around accretion scenarios, where one or many satellites galaxies are accreted onto a preexisting bulge, and which lead to axisymmetric structures (e.g. Binney & Petrou 1985).

Introduction: Radio Emission from Spheroids

Radio emission is observed from the centers of both active spiral bulges and E/SO galaxies. There are distinct differences in the properties of the central radio emission from these classes of galaxies (Slee et al. 1994; Sadler et al. 1995). Spirals sometimes contain compact radio cores, possibly not related to starburst activity, but a large fraction of the emission originates from an extended region of several hundred parsecs. In earlytype galaxies the emission is always completely dominated by the unresolved core. The spectral index α of the core emission (with S ∼ να for flux density S and frequency ν) is typically around –1 for spirals and around 0.3 for ellipticals. These differences appear to hold for bulges and early-type galaxies of the same luminosity. Thus, rather surprisingly, it seems that radio cores ‘know’ what kind of host they reside in.

There is also a difference in radio emission between low- and high-luminosity ellipticals. Only in high-luminosity ellipticals do we see radio-jets on the scales of hundreds of kiloparsecs, i.e., galaxies classified as FRI or FRII (Fanaroff & Riley 1974, types I and II, respectively). As noted by Sadler (1997) this threshold roughly coincides with the break which marks differences in structural properties such as stellar rotation and central cusp slope (e.g., Faber et al. 1997).

Introduction

Radial transport of gas in galactic disks likely plays an important role in the formation and evolution of bulges. There are two aspects in the effect of gas transfer to bulges, in both of which stellar bars are involved.

Introduction

As shown by gravitational N-body simulations and observations of highly flattened giant galaxies including the Milky Way, the central parts of these systems at distances of, say, r ≲ 0.7-1 kpc from the center rotate slowly, and their local circular velocities of regular galactic rotation become less than (or comparable to) the residual (random) velocities. In such a thin, practically nonrotating (‘pressure-supported’) central disk, a typical star moves along the bending, perpendicular to the equatorial plane layer, under the action of two forces which act in opposite directions: the destabilizing centrifugal force, Fc, and the restoring gravitational attraction, Fg. Obviously, fierce instabilities of the buckling kind developing perpendicular to the plane may not be avoided if Fc > Fg. The latter condition is nothing else than the well-known condition of the so-called firehose electromagnetic instability in collisionless plasmas. The source of free energy in the instability is the intrinsic anisotropy of a velocity dispersion (‘temperature’).

Introduction

Bulge morphology has often been compared to that of elliptical galaxies, both of which were initially thought to be axisymmetric. Later it was discovered that elliptical galaxies are triaxial (see for instance de Zeeuw 1989; Bender 1988; and references therein) and soon afterward also triaxial bulges in spiral galaxies were found (Kormendy 1982; Zaritsky & Lo 1986; Bertola 1989, 1991; Shaw 1993; Varela et al. 1996). The radial surface brightness profile of a triaxial bulge usually follows a classic r¼ law and the distribution of triaxial bulges in barred and unbarred galaxies is similar (Pompei 1998), so in principle triaxiality and barred potentials are unrelated. It should be noted however that earlytype galaxies host the strongest bars, which are currently supposed to have formed a long (t > 108 yr) time ago (Noguchi 1996).

Introduction

NGC 4036 has been classified S03(8)/Sa in RSA (Sandage & Tammann 1981) and S0− in RC3 (de Vaucouleurs et al. 1991). Its total apparent magnitude is VT = 10.66 mag (RC3). This corresponds to a total luminosity Lv = 4.2 · 1010 at the assumed distance of d = V0/H0 = 30.2 Mpc, where V0 = 1509 ± 50 km s−1 (RSA) and assuming H0 = 50 km s−1 Mpc−1. At this distance the scale is 146 pc arcsec−1.

We measured the kinematics of stars and ionized gas along the galaxy major axis and derived their distribution in the nuclear regions by means of ground-based V-band and HST narrow-band imaging respectively.

Introduction

We model the multi-phase and inhomogeneous interstellar medium (ISM) in the inner region of a galactic disk including fundamental physical processes crucial for understanding star formation, global and local dynamics of the ISM in galaxies, and aspects of galaxy formation such as feedback. Most numerical simulations of the ISM and of star formation in galaxies have assumed simpler ISM models, e.g. an isothermal or nearly-isothermal equation of state, and either a smooth medium or discrete clouds.