In this scientific overview, I discuss some of the important features of barred galaxies, and attempt to identify some of the outstanding problems.

In this review, the morphology of barred galaxies at optical wavelengths is discussed, focussing mainly on early to intermediate revised Hubble types in the range S0+ to Sbc. Previous work by Hubble, Sandage, de Vaucouleurs, Vorontsov-Velyaminov, and Kormendy is described. The primary emphasis is on ring phenomena, and morphological results from the Catalog of Southern Ringed Galaxies are summarized.

Previous optical surface photometry of barred spiral galaxies revealed that there are two distinct types of bars: large bars tend to have a nearly constant surface brightness (”flat“ bar), while smaller bars tend to have a decreasing surface brightness with a scale length similar to the disk (”exponential“ bar). Statistically, flat bars tend to occur in early Hubble types and exponential bars in later types. Studies of resonances in spirals indicate that flat bars end inside corotation, while exponential bars end between the inner Lindblad and 4:1 resonances. Near-infrared (JHK) surface photometry of bars is presented in order to compare the stellar distributions and bar potentials in flat and exponential barred galaxies. The presence of isophotal twists in some galaxies provides additional information on resonances. The grand design and fiocculent optical structures in the two types of barred galaxies will be compared and contrasted with their near-infrared light distributions.

Late-type Magellanic spiral galaxies possess high surface brightness stellar bars whose centers are often offset from the photometric and kinematic disk centers derived from stellar and HI distributions. Since these systems represent the transitional stage between the pure disk and irregular galaxies, and because the spiral arm and bar properties of these systems present many unique aspects, an understanding of their properties will benefit our understanding of the physics underlying the Hubble sequence as well as the mechanisms associated with bar formation. I review detailed photometric and kinematic studies of several important SBm galaxies. Statistical studies using large samples drawn from the Third Reference Catalog of Bright Galaxies support the idea that scenarios invoking the formation of offset bars and/or dominant spiral arms through some tidal interaction mechanism might be attractive, since a common trait among the Magellanic spirals appears to be the presence of a physical neighbor.

Global photometric properties of barred galaxies are reviewed based on the surface photometry of barred galaxies.

Star formation in barred spirals is discussed, including star formation rates, distribution of H ɪ regions, and H ɪ region luminosity functions. In general, global star formation rates and other properties are consistent with non-barred spirals of comparable Hubble class. In barred spirals of intermediate class (SBb-SBc), the effects of the bar on star formation is clearly seen in the distribution of star forming sites. Two patterns emerge, one that shows star formation concentrated in ring-like zones corresponding to inner and nuclear rings (earlier types) and one that shows star formation concentrated in the bar itself (later types). These properties appear to be well-correlated with both Hubble stage and bar type.

Amongst relatively undisturbed spiral galaxies of type ≤ Sc barred morphology is unquestionably associated with powerful mid- and Far-IR emission. On the other hand, even amongst early type galaxies, those with LFIR/LB < 1/3 exhibit no association of high relative FIR luminosity with barred morphology, but some association of IR colors resembling those of star formation regions (SFRs). Amongst systems with LFIR/LB < 0.1 this ratio may be anti-correlated with barredness. It appears that enhanced IR emission from those galaxies whose star formation rate is currently elevated by the the bar translates them into the group with higher FIR-to-optical luminosity ratios. Depletion of extended nearnuclear gas and dust, once the bar has swept up the currently-available supplies, may reduce the fraction of the background stellar radiation field which can be converted to FIR radiation in the inner, most luminous parts of the galaxy. Thus, after the starburst has subsided, such galaxies may be less FIR-luminous than unbarred systems. Several uncertainties remain: it is still not clear whether barred morphology is a necessary condition for the generation of a starburst in an otherwise undisturbed galaxy, while evidence as to the effect of differing bar strengths is conflicting.

I review the evidence for global abundance gradients in barred galaxies to be flatter when compared with those of normal galaxies. The finding that the abundance gradient becomes flatter as the strength of the bar increases, illustrates the effectiveness of radial flows induced by bars at transporting large amount of gas across galaxy disks.

A study of general properties of H ɪɪ regions along the bars of 11 spiral galaxies is presented. From Hα imaging, distributions, morphology and star formation rates are established. Physical conditions derived from spectrophotometry reveal that strong shocks are absent in H ɪɪ regions along bars and electron densities appear to be normal. Abundance distributions indicate that mixing of the ISM is taking place along bars.

In this paper we present the spatial distribution of ionized hydrogen gas, Hα, in a set of 52 bright barred Shapley-Ames spiral galaxies. In summary we were able to detect Hα line emission from distinct regions in the galaxies as from compact nuclei, from circumnuclear rings, from the bar, from ends of bars, from inner rings, from spiral arms, from outer rings and from places or structures perpendicular either to the bar or to the spiral arms.

Resonance ring galaxies are barred (and sometimes nonbarred) spiral galaxies which display an inner, outer, or nuclear ring in the light distribution. This study is of the HII region properties in 32 probable resonance ring galaxies selected mainly from the Catalog of Southern Ringed Galaxies (CSRG; Buta 1995). The objective of the study is to illustrate the distributions of HII regions in classic examples of ringed galaxies, and to relate the observed properties to resonance theory. Here we present a brief summary of a more extensive paper on the results of this study (Crocker, Baugus, & Buta 1996).

NGC 7742 is well known for its prominent blue nuclear ring around an EO-like core, and so appears as a Hoag-type galaxy, an elliptical galaxy with an outer ring (Schweizer et al. 1987). The galaxy is classified as Sa(r!) in the Revised Shapley-Ames Catalog (Sandage and Tammann 1987) with an exclamation mark to emphasize the prominence of the ring. Its photographs are published in Laustsen et al. (1987), Wray (1988), and Sandage & Bedke (1994).

The ring has a diameter of 19″ = 1.6 kpc at a distance of 17.1 Mpc (Buta & Crocker 1993), and so should be a nuclear ring of the galaxy. Nuclear rings and pseudorings are often detected in strongly barred (SB) galaxies, and interpreted to be linked to the inner Lindblad resonance (Buta & Crocker 1993). These nuclear features are, however, also found in some weakly-barred (SAB) and non-barred (SA) galaxies. NGC 7742 is a galaxy of the highest circular symmetry in its core, ring, and main body, and so the best object for a detailed study of formation mechanisms of nuclear rings in non-barred galaxies.

We extract the bulge light profiles of a sample of early type spirals, using K-band photometry and a new 2-dimensional decomposition method that does not assume a priori any surface brightness laws. We find that the shape of the light profile shows a good correlation with the morphological type of the galaxy, in the sense that the profiles tend to fall off more steeply at large radii for the later types. This trend shows that the formation of or interaction with the disk has probably affected the density distribution of the bulge. The fact that the transition in shape is continuous might also imply that most of the bulges of late type spirals were not formed by different mechanisms than the ones of early types.

The nuclear region of the barred galaxy NGC 4314 was observed and modeled by VIJHK photometry, long-slit spectroscopy, and SPH simulations. Young stellar populations is supposed to be made from the gas which are accumulated in the nuclear ring and/or inner spiral arms by the strong bar potential.

An inner-truncated disk (ITD) brightness profile is one in which the inward extrapolation of the outer disk surface brightness significantly exceeds the observed surface brightness in a region near the galaxy center. Freeman (1970) identified these profiles as Type II profiles. Fourteen of Freeman’s (1970) sample of 36 external galaxies are designated as Type II (ITD); seven of the ITD’s are classified as barred galaxies and ITD’s are found at all morphological types.

Ohta, Hamabe, & Wakamatsu (1990) report that luminosity profiles perpendicular to the bars of six early-type galaxies all exhibit the Freeman Type II shape, while the feature is lost when azimuthally averaged profiles of the same galaxies are studied. They postulate that the bar formation resulted in an azimuthal redistribution of the stars, but no net radial redistribution.

We present new and archival Hubble Space Telescope (HST) images of circumnuclear star-forming rings in barred spiral galaxies. We find that nuclear rings in barred galaxies are composed of large numbers of super star clusters similar to those found recently in other types of starburst systems. In NGC 1097 and NGC 6951, the young clusters have absolute magnitudes of up to Mv = −14 or −15, depending on highly uncertain extinction corrections, and effective radii of 2–3 pc. The images (especially that of NGC 6951) also show intricate spiral dust lane structure interior to the rings.

We analyzed a sample of SBO galaxies in order to disentangle the structure of the bar and of the other components inside the galaxies. The disk and the bulge components, modeled by using the luminosity and geometrical profiles, have been subtracted from each galaxy image in order to obtain the image of the bar. This allowed us to analyze in detail the bar structure, in particular its shape and its luminosity profile. Both uniform and saddle-shaped bars have been detected.