We present preliminary results from two observing campaigns where global photometric data for most dwarf galaxies in the M81 and M101 groups as well as some field dwarfs were obtained. The galaxies in the denser M81 group are more often of dwarf elliptical type and are redder and fainter than those of the M101 group and surrounding field, which are mostly of the dwarf irregular types. But both types follow the same total magnitude - central surface brightness relation, so there might be an evolutionary connection between the two classes.

We present initial results of a study of low surface brightness dwarf galaxies within galaxy clusters at z ≤ .03 as part of our program to determine the clustering properties, luminosity functions, and morphologies of dwarf galaxies in a wider range of cluster environments. In addition to deep V-band images covering up to 1 deg2 in each of 13 different clusters, we have obtained velocities from fiber spectroscopy for 235 galaxies in A3526. In A3526, we find a drop in cluster galaxy counts at intermediate magnitudes which is supported by our spectroscopic results.

I describe the properties of the interstellar medium in LSB galaxies, and conclude that apart from low density the other important factor determining the slow evolution of LSB galaxies is the metallicity, and (due to inefficient cooling) the corresponding lack of large amounts of molecular gas.

Because of the weak correlation between surface brightness and luminosity, as galaxy surveys reach to lower surface brightnesses, increasing numbers of dwarf galaxies are found. Thus, studies of dwarf galaxies are relevant to our understanding of the low surface brightness universe. In particular, studies of nearby LSB dwarfs may help to answer questions concerning: (1) star formation in low abundance and low mass surface density envrionments, (2) star formation histories of galaxies and the relative youth of LSB galaxies, and (3) the chemical evolution of LSB galaxies. HST WFPC2 photometry of the resolved stars in nearby dwarf galaxies is particularly useful for all three questions.

H II region abundances remain the best probe of current ISM abundances. Current evidence favors total mass (or luminosity) as the primary factor for determining the present day chemical abundance in both LSB and HSB galaxies. It remains possible that mass surface density (or surface brightness) may be a second controlling parameter.

I argue that low surface brightness disks evolve sufficiently slowly that, at any redshift, there should be a high space density of these objects. Cosmological dimming, in combination with the tenacity of Freeman’s Law, will make their direct detection very difficult. Given their abundance at z =0, it seems reasonable to suggest that such hidden galaxies will be important absorbers along the line of sight to distant QSOs.

Recent studies have demonstrated that many galaxy clusters have luminosity functions (LFs) which are steep at the faint end. However, it is equally clear that not all clusters have identical LFs. In this paper we explore whether the variation in LF shape correlates with other cluster or environmental properties.

An unexpected characteristic of low surface brightness galaxies (LSBGs) is that a significant number are massive and possess substantial amounts of atomic gas. We present preliminary results of an ongoing program to obtain BVRIJHK imaging, along with some nuclear spectroscopy, of a well-defined sample of LSBGs which are gas-rich and of similar size to giant, high surface brightness spiral galaxies (HSBGs). These LSBGs span the entire range of Hubble disk morphologies. While their disks are bluer, on average, than comparable HSBGs, the optical morphology of massive LSBGs indicates that many of these systems have undergone previous star formation episodes. They typically have long disk scale lengths, and range from MB = −16 to −22 (H0 = 75 km s−1 Mpc−1). About half of the LSBGs with bulges show evidence of nuclear activity, and ~30% appear to be barred. These massive, gas-rich LSBGs apparently have varied, and often complex, evolutionary histories.

The extreme outer regions of disk galaxies, lying at or beyond the classical optical radius defined by R25, present an opportunity to study star formation and chemical evolution under unique physical conditions, possibly reminscent of those which existed during the early stages of disk evolution. We present here some of the first results from a large study to measure star formation rates and metallicities in the extreme outer limits of a sample of nearby spiral galaxies. Despite their low gas column densities, massive star formation is often observed in these outer parts, but at an azimuthally–averaged rate much lower than that seen in the inner disk. Gas-phase O/H abundances of roughly 10% solar characterize the gas at 1.5–2 R25. The implications of our results for star formation ‘laws’ and models of disk evolution are discussed.

The low surface brightness galaxy HIPASS1126-72 was detected in the HI Parkes All Sky Survey (HIPASS). The galaxy was previously listed in the Southern Galaxy Catalogue under the name SGC1124.87221. This galaxy represents a class of galaxies that we will readily detect in the HIPASS survey, which have low surface brightness in the optical, but are easily detectable in neutral hydrogen.

UGC 7321 is an edge-on low surface brightness (LSB) spiral galaxy with a number of extraordinary properties. Its vertical scale height (~70 pc) is one of the smallest ever measured for a galaxy disk. Its disk also exhibits strong vertical and radial color gradients. UGC 7321 appears to be an extremely unevolved galaxy in both a dynamical and in a star-formation sense.

Between May, 1996 and January, 1997 HST WFPC-2 images of four nearby LSB galaxies were taken through both the 8140Å and 3000Å filters. The first galaxy imaged, UGC 12695, is a nearby (z~0.021) LSB disk galaxy. UGC 12695 has an unusual morphology, consisting of a Y-shaped nucleus surrounded by a faint spiral arm. Additionally, numerous HI regions are spread throughout the galaxy. One of the surprising discoveries with the WFPC2 imaging was that a number of what were previously believed to be structural peculiarities in the galaxy are actually background galaxies. Once the effects of these galaxies are removed, the resultant U—I color of UGC 12695 is only −0.2, making it possibly the bluest galaxy in the local universe. When combined with the metallicity studies of McGaugh (1992), these colors indicate UGC 12695 to be a highly unevolved galaxy.

The other three galaxies imaged - V1L4, V2L8, and V7L3 - are dwarf elliptical galaxies located in the Virgo cluster. The intent of their images was to determine the galaxies’ small scale structure and place limits on the density and type of giant branch stars within each galaxy. Placed at the distance of the Virgo cluster, luminosity fluctuations indicate the galaxies to contain only from 4 – 13 stars per pixel, coinciding with a K/M giant ratio ranging from 6 to ∞ (no M giant stars). Additionally, we found no evidence for stellar clumping in these galaxies although an extremely red, extremely small bulge was found at the core of V2L8.

High sensitivity H I observations now exist for six giant low surface brightness (LSB) disk galaxies including the two prototypes, Malin 1 (Bothun et al. 1987; Impey & Bothun 1989) and F568-6 (also known as Malin 2; Bothun et al. 1990). Their H I surface brightnesses are generally low, but proportionally not as low as their optical surface brightnesses. Their total H I masses and radial extents are quite large, however, with MHI ∼ 1010M⊙h−275 and with detectable H I out to 100 kpc h−175 or more in a couple of cases. The rotation curves of these systems rise slowly and are consistent with negligible disk contribution, similar to many previously observed dwarf galaxies. However, the peak rotation velocities of these galaxies are high (>200 km s−1) and infer high dynamical masses. These galaxies provide the some of the first examples of galaxies that are both massive and dark matter dominated.

We have carried out a deep, multi-color imaging study of Holmberg II (Ho II) and several other nearby LSB dwarf galaxies for which detailed HI maps exist. The formation of the HI holes in these galaxies has been attributed to multiple supernovae (SNe) occurring within windblown shells around young, massive star clusters. To search for evidence of the clusters, we have compared optical images with the HI maps and measured magnitudes and colors of objects in and around the HI holes.

Although the SN scenario requires that detectable star clusters should often be present in the centers of the HI holes, our observations have in most cases failed to reveal these clusters at the expected magnitudes. In fact, many of the HI holes are located in regions of very low optical surface brightness, which show no evidence of recent star formation.

We present high resolution VLA-observations of the nearby Low Surface Brightness Dwarf Galaxy DDO 47. This object shows many hole-like structures in its neutral interstellar medium. The majority of the detected H I-shells are found to be expanding. Their origin is therefore believed to be due to stellar winds of the most massive stars and their subsequent supernova (SN type II) explosions within regions of recent star formation (SF). Current SF in DDO 47 is predominantly present on the rims of the HI-shells suggesting propagating SF. At a projected distance of 20 kpc (adopting a distance to DDO 47 of 4 Mpc), a companion galaxy was detected at almost the same systemic velocity (DDO 47 B). A search for an optical identification suggests that CGCG 087-033 is the optical counterpart of the companion. A preliminary dynamical analysis based on DDO 47’s rotation curve yields that it is dark matter dominated (about 80% of its dynamical mass is in some non-visible form). A simple mass model suggests that DDO 47 is one of the ‘thickest’ dwarf galaxies studied so far.

We follow the evolution of disk galaxies within a cluster that forms hierarchically in a standard cold dark matter N-body simulation. At a redshift z = 0.5 we select several dark matter halos that have quiet merger histories and are about to enter the newly forming cluster environment. The halos are replaced with equilibrium high resolution model spirals that are constructed to represent luminous examples of low surface brightness (LSB) and high surface brightness (HSB) galaxies. Whilst the models have the same total luminosity, ~ L*, they have very different internal mass profiles, core radii and disk scale lengths, however they all lie at the same place on the Tully-Fisher relation. Due to their “soft” central potentials, LSB galaxies evolve dramatically under the influence of rapid encounters with substructure and strong tidal shocks from the global cluster potential - galaxy harassment. As much as 90% of the LSB disk stars are tidally stripped and congregate in large diffuse tails that trace the orbital path of the galaxy and form the diffuse intra-cluster light. The bound stellar remnants closely resemble the dwarf spheroidals (dE’s) that populate nearby clusters, with large scale lengths and low central surface brightness.

The multi-phase structure of the interstellar medium in low surface brightness galaxies is investigated and compared to observations. It is found that the ambient pressure and metallicity very strongly influence the abundances of molecular hydrogen and carbon monoxide. The emissivity of the latter is computed and found to agree naturally with the upper limits measured for low surface brightness galaxies. The implications for star formation efficiency and galaxy evolution are discussed.

Near-infrared images of a sample of red, blue and giant low surface brightness disk galaxies (LSBGs) were combined with optical data with the aim of constraining their star formation histories. Most LSBGs have strong colour gradients consistent with mean stellar age gradients. We find that LSBGs have a large range of ages and metallicities, spanning those observed in normal disk galaxies. In particular, red and blue LSBGs have very different star formation histories and represent two independent routes to low B band surface brightness. Blue LSBGs are well described by models with low, roughly constant star formation rates, whereas red LSBGs are better described by a ‘faded disk’ scenario.

We analyze deep CCD images of nearby Blue Compact Dwarf (BCD) galaxies in an attempt to understand the nature of the progenitors which are hosting the current burst of star formation. In particular, we ask whether BCDs are hosted by normal or low-surface-brightness dI galaxies. We conclude that BCDs are in fact hosted by gas-rich galaxies which populate the extreme high-central-mass-density end of the dwarf galaxy distribution. Such galaxies are predisposed to having numerous strong bursts of star formation in their central regions. In this picture, BCDs can only occur in the minority of dwarf galaxies, rather than being a common phase experienced by all gas-rich dwarfs.

We studied the morphology of star formation in dwarf irregular galaxies and found that, in general, this takes place on one side of a galaxy and far from the center. This is mainly true for low surface brightness galaxies; in high surface brightness dwarf irregulars the star formation tends to be more centrally concentrated, as well as being more intense. We discuss possible star formation triggers in dwarf irregular galaxies, and evaluate the reasons for the peculiar distribution of star forming regions of these galaxies. Stochastic star formation, interactions with external gas, and tidal interactions appear to be ruled out as responsible for the asymmetric pattern of star formation. We conclude that asymmetry of a dark matter halo or of an asymmetric underlying stellar distribution may trigger an asymmetric pattern of star formation.

We combine specific globular cluster frequencies (SN) with newly measured surface brightness profiles to identify dEs that may be stripped dIs. Luminous dEs generally have higher surface brightnesses and steep central light profiles. Conversely, fainter dEs have low surface brightnesses and flatter central light profiles. The most likely candidates for stripped dIs have low SN and low central surface brightnesses.