I discuss some of the interesting discoveries gradually emerging from the HIPASS HI survey. Why were so very few dark clouds and dark galaxies identified? Could that be partly due to optical misidentifications? In some cases yes. Will Arecibo overcome some of the deficiencies of HIPASS? I argue not because large telescopes are ill suited to blind surveys. I discuss the problem of Inchoate Galaxies which can be neither young nor old, and the constancy of HI column density found amongst all sources turning up in blind HI surveys. Could some of these unexpected phenomena be the result of Spin Temperature Freezeout? If so there is a lot more HI out there than we imagined.

One explanation for the disparity between Cold Dark Matter predictions of galaxy numbers and observations could be that there are numerous dark galaxies in the Universe. These galaxies may still contain baryons, but no stars, and may be detectable in the 21cm line of atomic hydrogen. In this paper we describe a numerical model of the galaxy population and predict what might be found in blind 21cm surveys. We describe the detection of a dark galaxy candidate (VIRGOHI21) and discuss a model of its origin.

Deep optical/near-IR surface photometry of galaxies outside the Local Group have revealed faint and very red halos around objects as diverse as disk galaxies and starbursting dwarf galaxies. The colours of these structures are too extreme to be reconciled with stellar populations similar to those seen in the stellar halos of the Milky Way or M31, and alternative explanations like dust reddening, high metallicities or nebular emission are also disfavoured. A stellar population obeying an extremely bottom-heavy initial mass function (IMF), is on the other hand consistent with all available data. Because of its high mass-to-light ratio, such a population would effectively behave as baryonic dark matter and could account for some of the baryons still missing in the low-redshift Universe. Here, we give an overview of current red halo detections, alternative explanations for the origin of the red colours and ongoing searches for red halos around types of galaxies for which this phenomenon has not yet been reported. A number of potential tests of the bottom-heavy IMF hypothesis are also discussed.

We give a non-exhaustive review of the use of strong gravitational lensing in placing constraints on the quantity of dark and visible mass in galaxies. We discuss development of the methodology and summarise some recent results.

We review our attempts to discover lost baryons at low redshift with the “X-ray forest” of absorption lines from the warm-hot intergalactic medium. We discuss the best evidence to date along the Mrk 421 sightline. We then discuss the missing baryons in the Local Group and the significance of the z = 0 absorption systems in X-ray spectra. We argue that the debate over the Galactic vs. extragalactic origin of the z = 0 systems is premature as these systems likely contain both components. Observations with next generation X-ray missions such as Constellation-X and XEUS will be crucial to map out the warm-hot intergalactic medium.

The nature of dark matter is one of the outstanding questions of astrophysics. The internal motions of member stars reveal that the lowest luminosity galaxies in the Local Group are the most dark-matter dominated. New large datasets allow one to go further, and determine systematic properties of their dark matter haloes. We summarise recent results, emphasising the critical role of the dwarf spheroidal galaxies in understanding both dark matter and baryonic processes that shape galaxy evolution.

In the past years a wealth of observations has allowed us to unravel the structural properties of the Dark and Luminous mass distribution in spirals. As result, it has been found that their rotation curves follow, out their virial radius, an Universal function (URC) made by two terms: one due to the gravitational potential of a Freeman stellar disk and the other due to that of a dark halo. The importance of the latter is found to decrease with galaxy mass. Individual objects reveal in detail that dark halos have a density core, whose size correlates with its central value. These properties will guide ΛCDM Cosmology to evolve to match the challenge that observations presently pose.

QSO absorption lines are sensitive to very low density gas, and consequently, QSO spectroscopy provides a powerful tool for measuring the distribution, physical conditions, and metal enrichment of baryons in the intergalactic medium and in “dark galaxies”. However, ultraviolet spectroscopy is required to study QSO absorbers in the nearby universe where the connections between the absorbing gas and galaxies/environment can be probed. This talk reviewed several recent studies of low-z QSO absorbers in order to demonstrate the value of combining high-resolution ultraviolet spectroscopy with complementary information on the absorber environment, e.g., mapping of 21 cm emission in the vicinity of the absorbers; some notes on these examples are presented in this paper. The absorbers probed range from high-N(H I) damped Lyman α absorbers down to very low-column Lyα forest clouds with log N(H I) < 13.5. The high-N(H I) systems are candidate “dark galaxies” – these systems are more metal enriched than the high-z IGM, possibly due to in-situ star formation, and some of the absorbers are highly metal enriched. However, we have obtained follow-up 21cm emission mapping with the VLA and deep optical imaging, and while we do find H I clouds associated with the absorbers in 21cm emission, we often do not find any evidence of in-situ stars. At this juncture, it seems more likely that these low-z absorbers were enriched with metals within galaxies and were subsequently transported out into the IGM, e.g., by tidal stripping or galactic winds. This contribution also summarizes some recent results on the search for lost baryons in the “warm-hot” (shock-heated) low-redshift IGM.

For the last 25 years, the 21 cm line has been used productively to investigate the large–scale structure of the Universe, its peculiar velocity field and the measurement of cosmic parameters. In February 2005 a blind HI survey that will cover 7074 square degrees of the high latitude sky was started at Arecibo, using the 7-beam feed L-band feed array (ALFA). Known as the Arecibo Legacy Fast ALFA (ALFALFA) Survey, the program is producing a census of HI-bearing objects over a cosmologically significant volume of the local Universe. With respect to previous blind HI surveys, ALFALFA offers an improvement of about one order of magnitude in sensitivity, 4 times the angular resolution, 3 times the spectral resolution, and 1.6 times the total bandwidth of HIPASS. ALFALFA can detect 7×104D2M⊙ of HI, where D is the source distance in Mpc. As of mid 2007, 44% of the survey observations and 15% of the source extraction are completed. We discuss the status of the survey and present a few preliminary results, in particular with reference to the proposed “dark galaxy” VirgoHI21.

The Arecibo Legacy Fast ALFA (ALFALFA) survey is a second generation blind extragalactic HI survey currently in progess which is exploiting Arecibo's superior sensitivity, angular resolution and digital technology to derive a census of the local HI universe over a cosmologically significant volume. As of the time of this meeting, some 4500 good quality extragalactic HI line sources have been identified in about 15% of the final survey area. ALFALFA is detecting HI masses as low as 106M⊙ and as large as 1010.8M⊙ with positional accuracies typically better than 20″, allowing immediate identification of the most probable optical counterparts. Only 3% of all extragalactic HI sources and less than 1% of detections with MH I > 109.5M⊙ cannot be identified with a stellar component. Because ALFALFA is far from complete, the discussion here focuses on limitations of past surveys that ALFALFA will overcome because of its greater volume, sensitivity and reduced susceptibility to source confusion and on a sampling of illustrative preliminary results. First ALFALFA results already suggest, in agreement with previous studies, that there does not appear to be a cosmologically significant population of optically dark but HI rich galaxies. ALFALFA promises a wealthy dataset for the exploration of many issues in near-field cosmology and galaxy evolution studies, setting the stage for their extension to higher redshifts in the future with the Square Kilometer Array (SKA).

The Arecibo Legacy Fast ALFA survey is in the process of yielding a complete HI dataset of the Virgo Cluster and its environs (Giovanelli et al. 2007, Kent et al., in preparation). Assuming a distance to Virgo of 16.7 Mpc, the minimum detectable HI mass by ALFALFA is of order 2 × 107 M⊙. A number of the HI detections appear to have interesting properties. Some appear associated with, but offset from, low surface brightness optical counterparts; others, at larger spatial offsets, may be tidally related to optical counterparts. Yet another class includes detections which are not identifiable with any optical counterparts. We present the ALFALFA results on these objects in the Virgo region, as well as followup aperture synthesis observations obtained with the VLA.

I review the history of blind HI surveys and the evidence for cosmic variance in the HI mass function. Even the most sensitive current surveys are challenged by noise levels that make determining the shape of the mass function difficult, and this can lead to discrepant results. However, some differences in the mass function appear to be real environmental effects, and this suggest that there are locales where dark (or dim) HI-rich galaxies are more likely to be found. The requirements for significantly deeper surveys beyond the local supercluster to sample cosmic variance in a wider variety of environoments are modeled and discussed.

The Arecibo Galaxy Environment Survey is a blind neutral hydrogen survey using the ALFA multibeam receiver at Arecibo Observatory to reach unprecedented sensitivities in a number of selected fields in the local Universe. When completed the survey will cover 200 square degrees out to a distance of at least 270 Mpc. If a population of gas-rich dark galaxies exists, then this survey is in a prime position to uncover that population.

So far 20 square degrees have been covered in the regions of Abell 1367, the Virgo Cluster, the NGC 7332/9 galaxy pair and the isolated galaxy NGC 1156. Over 200 sources have been found, including a number that have no obvious optical counterparts. We discuss here the potential of AGES for uncovering more such objects and the characteristics of the dark sources identified to date.

Recent VLA observations pointed at dwarf spheroidal (dSph) galaxies in the M 81 group reveal a hitherto hidden population of extremely low mass (~ 105 M⊙) HI clouds with no obvious optical counterparts. We have searched 10 fields in the M 81 group totalling 2.2 square degree, both targeting known dwarf spheroidal galaxies and blank fields around the central triplet. Our observations show that the new population of low–mass HI clouds appears to be confined to a region toward the south–east of the central triplet (at distances of ~ 100 kpc from M 81). Possible explanations for these free–floating HI clouds are that they are related to the dSphs found to the South–East of M 81, that they belong to the galaxies of the M 81 triplet (equivalent to HVCs), that they are of primordial nature and provide fresh, unenriched material falling into the M 81 group, or that they are tidal debris from the 3–body interaction involving M 81–M 82–NGC 3077. Based on circumstantial evidence, we currently favour the latter explanation.

Blind HI surveys provide a census of galaxies in the local universe that is unbiased by their optical properties. Even the Arecibo Dual-Beam Survey with a sample of only 265 galaxies discovered many low surface brightness galaxies and one galaxy with no obvious stellar component. Overall the galaxies in this survey display a diverse range of gas-to-stellar properties. The environment in which a galaxy resides is shown to be one of the factors responsible for this diversity, but it is not the only one. Clearly there are other factors affecting the complex processes responsible for the conversion of gas into stars rapidly in some galaxies, slowly in others, and rapidly in the center while slowly in the outskirts in still other galaxies. Nevertheless, even the inefficient star formation observed in a large fraction of the gas-rich galaxies appears to be a significant contributor to the overall star-formation rate density locally and therefore an important driver of galaxy evolution that must be understood. We focus on a discussion of the stellar and star formation properties in a 21 cm selected sample of galaxies because it is these measurements that contain the most information about the nature of star formation in galaxies.

The global missing baryon problem – that the sum of observed baryons falls short of the number expected form BBN – is well known. In addition to this, there is also a local missing baryon problem that applies to individual dark matter halos. This halo by halo missing baryon problem is such that the observed mass fraction of baryons in individual galaxies falls short of the cosmic baryon fraction. This deficit is a strong function of circular velocity. I give an empirical estimate of this function, and note the presence of a critical scale of ~ 900 km s−1 therein. I also briefly review Ωb from BBN, highlighting the persistent tension between lithium and the CMB, and discuss some pros and cons of individual galaxies and clusters of galaxies as potential reservoirs of dark baryons.

Are voids in the distribution of galaxies only places with reduced matter density and low star formation efficiency or are they empty of matter? There is now compelling evidence of expansion away from the Local Void at very high velocities. The motion is most reasonably interpreted as an evacuation of the void, which requires that the void be very large and very empty.

Current explanation of the overabundance of dark matter subhalos in the Local Group (LG) indicates that there maybe a limit on mass of a halo, which can host a galaxy. This idea can be tested using voids in the distribution of galaxies: at some level small voids should not contain any (even dwarf) galaxies. We use observational samples complete to MB = −12 with distances less than 8 Mpc to construct the void function (VF): the distribution of sizes of voids empty of any galaxies. There are ~ 30 voids with sizes ranging from 1 to 5 Mpc. We then study the distribution of dark matter halos in very high resolution simulations of the LCDM model. The theoretical VF matches the observations remarkably well only if we use halos with circular velocities larger than 45 ± 10 km/s. This agrees with the Local Group predictions. There are smaller halos in the voids, but they should not produce any luminous matter. Small voids look quite similar to their giant cousins: the density has a minimum at the center of a void and it increases as we get closer to the border. Small nonluminous halos inside the void form a web of tiny filaments. Thus, both the Local Group data and the nearby voids indicate that isolated halos below 45 ± 10 km/s must not host galaxies and that small (few Mpc) voids are truly dark.

The distribution of baryons beyond galaxies is descibed. The majority of the baryons, which represent 4% of the cosmic mass and energy budget, lie far from individual galaxies in the diffuse intergalactic medium (IGM). Many of these baryons are in a warm phase that can be probed by quasar absorption in the Lyman-α line of hydrogen. The mature field of quasar spectroscopy can diagnose the location, physical state, metallicity, and general geometry of this gas, which is called the “cosmic web.” The remainder of the gas is kept very hot by infall and shocks and is mostly in higher density regions such as filaments, groups and clusters. The hot gas is only detectable via X-rays and the absorption of highly ionized species of heavy elements. The baryons in low density regions of space are excellent tracers of underlying dark matter. The evolution of the cosmic web indicates where to look for the baryons in collapsed objects but the overall inefficiency of galaxy formation has conspired to keep most baryons dark.

While the baryon fraction in galaxy groups and clusters may be expected to reflect the universal value, observations of cluster baryon fractions have generally fallen short of this expectation and indicated a possible correlation with cluster mass. We present a new determination of the total baryon budget in nearby galaxy groups and clusters that includes the contributions from stars in galaxies, intracluster stars, and the intracluster medium. We find that the baryon mass fraction within r500 is independent of system mass and lower than the WMAP value. We conclude however that the present shortfall provides no compelling evidence for additional missing baryons, since it may arise due to a theoretically predicted deficit of baryons within r500 and systematic uncertainties associated with the mass determinations. With the addition of the ICL to the stellar mass in galaxies, the increase in X-ray gas mass fraction with increasing total mass is entirely accounted for by a decrease in the total stellar mass fraction, supporting the argument that the behavior of both the stellar and X-ray gas components is dominated by a decrease in star formation efficiency in more massive environments. Within just the stellar component, the fraction of the total stellar luminosity in the central, giant brightest cluster galaxy (BCG) and ICL (hereafter the BCG+ICL component) decreases as velocity dispersion (σ) increases, suggesting that ICL may grow less efficiently in higher mass environments. The identification of low mass groups with large BCG+ICL components also demonstrates that the massive cluster environment is not required to form intracluster stars. These proceedings are a condensed version of the work presented in Gonzalez, Zaritsky & Zabludoff (2007), and we refer the reader to that paper for a more complete discussion.