The results of a comparative study of star formation thresholds in gas-rich, low surface brightness, dwarf galaxies are presented. Approximately half the galaxies in the study were “high MH/LB” galaxies, which appear to have inefficient star formation properties. The comparison sample comprised of otherwise “normal” dwarf galaxies, with moderate current star formation rates. In all systems, sites of active star formation were associated with local peaks in the HI column density. For both types of galaxies, the azimuthally averaged gas column density is low. Similar to other LSB systems, the global gas densities are well below the critical threshold for star formation throughout the system. Thus, star formation is inhibited globally, but occurs locally in these gas-rich dwarf galaxies.

We analyze the star formation activity of an homogeneous LSB dwarf galaxy sample in the Virgo cluster, as a function of the radial velocity relative to the cluster mean velocity and the projected distance from the center of the cluster, using CCD images obtained at the Wise Observatory. The localized Hα emission in the HII regions of this sample is compared to that of an isolated gas-rich sample of LSB dwarf galaxies and that of a representative sample of Blue Compact Dwarf (BCD) galaxies in the cluster. We report preliminary results on the LSB dwarf star formation histories obtained from surface color distribution.

A thirteen-beam HI receiver has been constructed for the Parkes radio telescope. When this instrument is used in active scanning mode, it can rapidly survey large areas of sky, with a relatively uniform sensitivity. The Multibeam Working Group, comprising about 30 astronomers from more than a dozen institutions, is undertaking a blind HI survey of the entire southern sky. The status of the survey is described, with some of the first scientific results.

This is a report on HI observations of newly discovered nearby dwarf galaxies, most of which of low surface brightness, from the first section of the Karachentsev catalog. Observations were performed using the 100-m radiotelescope at Effelsberg, the Nançay radiotelescope, and the compact array of the Australia Telescope. We observed 220 galaxies with a detection rate of about 60%. 35 of the observed galaxies are located within the Local Volume (i.e., within 10 Mpc). The smallest detected galaxies have diameters around 0.3 kpc and HI-masses of a few times 106 solar masses. We confirm a correlation between HI column density and surface brightness of gas-rich disk galaxies.

We present preliminary results of a neutral hydrogen (HI) redshift survey to find Low Surface Brightness (LSB) galaxies in the very nearby universe. Our sample consists of all galaxies in the APM catalog (Maddox et al. 1990) with a mean surface brightness of μ ≥ 24 mag/arcsec2, down to a magnitude limit of bj ≤ 17. With the Parkes 64m radiotelescope 35 objects were detected at v < 4300 km s−1. The resulting luminosity function, HI mass function, and for the first time total mass function are presented. It is found that LSBs make a negligible contribution to the overall integrated luminosity, HI mass, and total mass contained in galaxies.

We report the detection, based on HST and Keck data, of two peculiar absorbers in the Lyα forest of the quasar PKS 0454+039. These clouds, at redshifts z = 0.6248 and 0.9315 respectively, display both Mg II and Fe II absorption lines in addition to the Lyα line. Based upon photoionization models, these are inferred to be photoionized by the intergalactic UV background, and to have H I column densities in the range 15.8 ≤ N(H I) ≤ 16.8. Furthermore, if one supposes that the relative abundances of heavy elements is similar to that of depleted clouds of our galaxy, the abundances of these two absorbers are greater than the solar value, which is a unique case for absorbers which are not associated to the quasar. We tentatively suggest that these absorbers may select giant low surface brightness galaxies.

Using simulations of the low redshift galaxy population based upon galaxy observations, it is shown (Linder 1998) that the majority of Lyα absorbers at low redshift could arise in low surface brightness (LSB) galaxies. The contribution to absorption from LSB galaxies is large for any galaxy surface brightness distribution which is currently supported by observations. Lyα absorbers should become powerful tools for studying the properties and evolution of galaxies, but first it will be necessary to establish observationally the nature of the Lyα absorbers at low redshift. Further simulations, in which the absorbing galaxy population is ‘observed’ with some selection criteria, are used to explore how easy it is for an observer to test for a scenario in which LSB galaxies give rise to most of the Lyα absorbers. It is shown that absorption arising in LSB galaxies is often likely to be attributed to high surface brightness galaxies at larger impact parameters from the quasar line of sight.

The HI Parkes Zone of Avoidance Survey is a 21 cm blind search with the multibeam receiver on the 64-m radiotelescope, looking for galaxies hidden behind the southern Milky Way. The first phase of the survey has uncovered 107 galaxies, two-thirds of which were previously unknown. The addition of these galaxies to existing extragalactic catalogs allows the connectivity of large-scale structures across the Zone of Avoidance within 3500 km s−1 to become evident. No nearby hidden “Andromeda-class” object was uncovered. Our census of the most dynamically important HI-bearing nearby galaxies is now complete, at least for those objects whose HI profiles are not totally buried in the Galactic HI signal. The full survey is ongoing, and is expected to produce a catalog of thousands of objects when it is finished.

Approximately 25% of the extragalactic sky is obscured by dust in our own Milky Way galaxy. Diligent optical and infrared surveys are successful at detecting galaxies through moderate Galactic dust extinction, but in the most heavily obscured regions near the Galactic plane, only radio surveys are effective.

The Dwingeloo Obscured Galaxies Survey (DOGS) is a 21-cm blind survey out to 4000 km s−1 in the northern “Zone of Avoidance” (ZOA). The DOGS project is designed to reveal hidden dynamically important nearby galaxies and to help “fill in the blanks” in the local large scale structure.

We present high resolution (~ 15″) VLA HI observations of one LSB dwarf galaxy and 5 Blue Compact Dwarfs. Previous works have emphasized the difference in the peak column density of the HI between the two types of galaxy. However, the peak column density is typically only a factor of 2—4 greater in BCDs than in LSB dwarfs, suggesting that the difference between the two is one of degree, not quality. The HI morphologies of the two types are often quite similar.

We present the first detection of the mean flux of the optical extragalactic background light (EBL) at 3000, 5500, and 8000Å, derived from coordinated data sets from HST and Las Campanas Observatory. To isolate the extragalactic component, we have measured and subtracted the flux from foreground sources explicitly. In addition to detections in all three bands, we identify the minimum surface brightness contributed by resolved galaxies (23 < V < 28 AB mag) using a non-standard method of aperture photometry to which these data are uniquely suited. Individually resolved galaxies account for ~ 30% of the mean EBL coming from galaxies fainter than V = 23 AB mag. Taking into account the effective surface brightness detection limits of the deepest galaxy counts, and the results of LSB surveys at low redshift, the EBL we detect can be explained by galaxy populations already cataloged.

I present here a review of the observed characteristics of the optical diffuse light in clusters, the possible sources of this light and some of the theories that try to explain the existence of big envelopes around the brightest cluster galaxies.

Diffuse ultraviolet background radiation may contain important information concerning the dark matter of the universe. I briefly review new Voyager observations of the diffuse background, which give a very low upper limit on the background radiation shortward of Lyman α, and I review the capabilities for detection and characterization of diffuse radiation that will be provided by a proposed new NASA mission. Low-surface-brightness radiation remains largely an unexplored frontier, particularly in the ultraviolet.

Individual sources are detected ideally to a confusion limit at a given wavelength, but there is still much information beyond that. Absolute background brightness measurements provide a crucial constraint to models describing the undetected population of sources in the distant – and/or low surface brightness – universe. We calculate how much low surface brightness galaxies (LSB) would contribute to the the overall extragalactic background light (EBL) and review the status of EBL measurements in the optical and infrared wavelengths. To be able to push deeper the detection limits on very faint sources a fluctuation analysis method is introduced. The use of these different approaches together is essential when studying the very faint and low surface brightness universe.

I comment on primordial abundances, their implications for cosmic baryon density (all are consistent with 4 ≤ η10 ≤ 5), the nature of baryonic dark matter (probably mainly hot intergalactic gas), its metallicity and implications for the overall yield. Recent estimates of the cosmic star formation rate as a function of red-shift are discussed in the light of the present-day densities of stars and heavy elements.

A brief review of primordial helium and deuterium abundances suggests a baryonic mass density of ΩB ≈ 0.04 – 0.045 (for Ho = 70). This mass may be dominated by intergalactic gas in clusters and groups of galaxies. The observed low chemical abundances in evolved dwarf galaxies might suggest that outflow was the origin for such gas, and we make general suggestions for the interpretation of the data from the next generation of X-ray spectroscopic satellites. The effects of both outflow and inflow on the chemical evolution of galaxies is discussed, particularly in the context of low surface brightness galaxies, and we comment on their dust content.

The high-quality long-exposure spectroscopic observations of the two most-metal deficient blue compact galaxies I Zw 18 and SBS 0335–052 are discussed. We confirm previous findings that underlying stellar absorption strongly influences the observed intensities of He I emission lines in the brightest NW component of I Zw 18, and hence this component should not be used for primordial He abundance determination. The effect of underlying stellar absorption, though present, is much smaller in the SE component. The extremely high signal-to-noise ratio spectrum (≥ 100 in the continuum) of the BCG SBS 0335–052 allows us to measure the helium mass fraction with precision better than 2% in nine different regions along the slit. The weighted mean of helium mass fraction in two most metal-deficient BCGs I Zw 18 and SBS 0335–052, Y=0.2462±0.0009, after correction for the He production in massive stars results in primordial He mass fraction Yp = 0.2452±0.0009.

Despite progress on many fronts in cosmology, outstanding questions remain. What is the nature of the dark matter? Is the inflationary big bang model viable? Must we accept a non-zero cosmological constant? Do we know the true population of galaxies? What is the range of star formation histories in the universe? Can gravity alone explain the large scale structure we observe? Studies of the low surface brightness universe may provide the answers to many of these questions.