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Using instruments that take up to a million spectra in a single field, we can see line-of-sight velocities throughout a churning nebula or cosmic outburst. The universe is not always [...]

Keck/LRIS multi-object spectroscopy has been carried out on 140 of some of the lowest and highest surface brightness faint (19 < R < 22) dwarf galaxy candidates in the core region of the Coma Cluster. These spectra are used to measure redshifts and establish membership for these faint dwarf populations. The primary goal of the low surface brightness sample is to test our ability to use morphological and surface brightness criteria to distinguish between Coma Cluster members and background galaxies using high resolution Hubble Space Telescope/Advanced Camera for Surveys images. Candidates were rated as expected members, uncertain, or expected background. From 93 spectra, 51 dwarf galaxy members and 20 background galaxies are identified. Our morphological membership estimation success rate is [?]100% for objects expected to be members and better than [?]90% for galaxies expected to be in the background. We confirm that low surface brightness is a very good indicator of cluster membership. High surface brightness galaxies are almost always background with confusion arising only from the cases of the rare compact elliptical (cE) galaxies. The more problematic cases occur at intermediate surface brightness. Many of these galaxies are given uncertain membership ratings, and these were found to be members about half of the time. Including color information will improve membership determination but will fail for some of the same objects that are already misidentified when using only surface brightness and morphology criteria. cE galaxies with B-V colors [?]0.2 mag redward of the red sequence in particular require spectroscopic follow up. In a sample of 47 high surface brightness, ultracompact dwarf candidates, 19 objects have redshifts which place them in the Coma Cluster, while another 6 have questionable redshift measurements but may also prove to be members. Redshift measurements are presented and the use of indirect means for establishing cluster membership is discussed.

We generate the peculiar velocity field for the Two Micron All-Sky Redshift Survey (2MRS) catalog using an orbit-reconstruction algorithm. The reconstructed velocities of individual objects in 2MRS are well correlated with the peculiar velocities obtained from high-precision observed distances within 3000 km s-1. We estimate the mean matter density to be Om = 0.31 +- 0.05 by comparing observed to reconstructed velocities in this volume. The reconstructed motion of the Local Group in the rest frame established by distances within 3000 km s-1 agrees with the observed motion and is generated by fluctuations within this volume, in agreement with observations. Having tested our method against observed distances, we reconstruct the velocity field of 2MRS in successively larger radii, to study the problem of convergence toward the cosmic microwave background (CMB) dipole. We find that less than half of the amplitude of the CMB dipole is generated within a volume enclosing the Hydra-Centaurus-Norma supercluster at around 40 h -1 Mpc. Although most of the amplitude of the CMB dipole seems to be recovered by 120 h -1 Mpc, the direction does not agree and hence we observe no convergence up to this scale. Due to dominant superclusters such as Shapley or Horologium-Reticulum in the southern hemisphere at scales above 120 h -1 Mpc, one might need to go well beyond 200 h -1 Mpc to fully recover the dipole vector. We develop a statistical model which allows us to estimate cosmological parameters from the reconstructed growth of convergence of the velocity of the Local Group toward the CMB dipole motion. For scales up to 60 h -1 Mpc, assuming a Local Group velocity of 627 km s-1, we estimate Om h 2 = 0.11 +- 0.06 and s8 = 0.9 +- 0.4, in agreement with WMAP5 measurements at the 1s level. However, for scales up to 100 h -1 Mpc, we obtain Om h 2 = 0.08 +- 0.03 and s8 = 1.0 +- 0.4, which agrees at the 1s to 2s level with WMAP5 results.

The peculiar velocity of the Local Group of galaxies manifested in the cosmic microwave background dipole is found to decompose into three dominant components. The three components are clearly separated because they arise on distinct spatial scales and are fortuitously almost orthogonal in their influences. The nearest, which is distinguished by a velocity discontinuity at ~7 Mpc, arises from the evacuation of the Local Void. We lie in the Local Sheet that bounds the void. Random motions within the Local Sheet are small, and we advocate a reference frame with respect to the Local Sheet in preference to the Local Group. Our Galaxy participates in the bulk motion of the Local Sheet away from the Local Void. The component of our motion on an intermediate scale is attributed to the Virgo Cluster and its surroundings, 17 Mpc away. The third and largest component is an attraction on scales larger than 3000 km s[?]1 and centered near the direction of the Centaurus Cluster. The amplitudes of the three components are 259, 185, and 455 km s[?]1, respectively, adding collectively to 631 km s[?]1 in the reference frame of the Local Sheet. Taking the nearby influences into account, particularly that of the Local Void, causes the residual attributed to large scales to align with observed concentrations of distant galaxies and reduces somewhat the amplitude of motion attributed to their pull. Concerning the motion of ~260 km s[?]1 away from the Local Void, given the velocities expected from gravitational instability theory in the standard cosmological paradigm, the distance to the center of the Local Void must be at least 23 Mpc from our position. The Local Void is extremely large.

We present the latest results of the Clusters in the Zone of Avoidance (CIZA) survey, which is mapping large-scale structures behind the Milky Way by performing the first systematic search for galaxy clusters at low Galactic latitudes. The survey's approach, which uses X-ray emission to locate cluster candidates, minimizes the problems faced by optically selected cluster surveys, which have traditionally avoided this region of the sky due to the severe extinction present along the Galactic plane. We here present the second flux-limited CIZA cluster catalog, containing 57 X-ray-selected galaxy clusters, of which 88% are new discoveries. We use this sample to examine the degree to which known large-scale structures extend into the zone of avoidance and highlight newly discovered structures that have previously gone unnoticed. We note that the CIZA survey has thus far found fewer rich clusters in the Great Attractor region than would be expected given the region's proposed mass. Instead, we find a significant increase in the number of clusters behind the Great Attractor, with the most notable being an association of clusters near the Shapley supercluster. We propose that these clusters trace an extension of the large-scale filament network in which the Shapley concentration is embedded. We also highlight an association of clusters near the Galactic anticenter, which form the first supercluster found to be completely hidden by the Milky Way. Our preliminary findings of a less massive Great Attractor and the detection of significant structures behind the complex support studies that suggest the motion of nearby galaxies is due, in part, to a large-scale bulk flow, which is induced by overdensities beyond the Hydra-Centaurus region.

The luminosity of the tip of the red giant branch (TRGB) provides an excellent measure of galaxy distances and is easily determined in the resolved images of nearby galaxies observed with Hubble Space Telescope. There is now a large amount of archival data that are relevant to the TRGB methodology and offer comparisons with other distance estimators. Zero-point issues related to the TRGB distance scale are reviewed in this paper. Consideration is given to the metallicity dependence of the TRGB, the transformations between HST flight systems and Johnson-Cousins photometry, the absolute magnitude scale based on horizontal branch measurements, and the effects of reddening. The zero point of the TRGB is established with a statistical accuracy of 1%, modulo the uncertainty in the magnitude of the horizontal branch, with a typical rms uncertainty of 3% in individual galaxy distances at high Galactic latitude. The zero point is consistent with the Cepheid period-luminosity relation scale but invites reconsideration of the claimed metallicity dependence with that method. The maser distance to NGC 4258 is consistent with TRGB but presently has lower accuracy.

Large and well-defined variations exist between the distribution of mass and the light of stars on extragalactic scales. Mass concentrations in the range 1012-1013 M manifest the most light per unit mass. Group halos in this range are typically the hosts of spiral and irregular galaxies with ongoing star formation. On average M/LB ~ 90 M/L in these groups . More massive halos have less light per unit mass. Within a given mass range, halos that are dynamically old as measured by crossing times and galaxy morphologies have distinctly less light per unit mass. At the other end of the mass spectrum, below 1012 M, there is a cutoff in the manifestation of light. Group halos in the range 1011-1012 M can host dwarf galaxies but with such low luminosities that M/LB values can range from several hundred to several thousand. It is suspected that there must be completely dark halos at lower masses. Given the form of the halo mass function, the low relative luminosities of the high-mass halos have the greatest cosmological implications. Of order half the clustered mass may reside in halos with greater than 1014 M. By contrast, only 5%-10% of clustered mass would lie in entities with less than 1012 M.

We report on the design and status of the Clusters in the Zone of Avoidance (CIZA) project, the first systematic X-ray search for clusters of galaxies behind the Milky Way. We demonstrate that an X-ray survey can find galaxy clusters at low Galactic latitude where optical searches are inefficient because of massive obscuration and extinction problems. We discuss the rationale for such a survey in the context of large-scale structure studies and describe in detail the combined X-ray/optical/NIR approach used by CIZA to identify clusters of galaxies at |b| < 20deg, a region of the sky that has traditionally been excluded from earlier cluster catalogs. So far, CIZA has identified and spectroscopically confirmed 137 galaxy clusters in what used to be the zone of avoidance; additional clusters have been confirmed in imaging observations and await spectroscopic observation. We present a catalog of the 73 X-ray-brightest CIZA clusters, 53 (73%) of which are new discoveries. This sample is statistically complete out to at least z ~ 0.075 (cz ~ 22,500 km s-1). We reassess local large-scale structure in the Great Attractor (GA) region in light of this first X-ray-selected cluster sample at low Galactic latitude. Our survey discovered CIZA J1324.7-5736, a luminous cluster at l = 307.deg4, b = 5.deg0, cz = 5700 km s-1, and possibly as massive as A3627, but ruled out a hypothesized X-ray-luminous cluster around the radio galaxy PKS 1343-601. We argue that this region of the GA may be best described as a wall-like, triangular structure with CIZA J1324.7-5736, A3627, and the Centaurus Cluster (A3526) at its vertices. Our survey also discovered what could be a supercluster in the same region but at cz ~ 15,000 km s-1, i.e., behind the GA. We speculate that the latter system may be partly responsible for the large-scale flow observed in this direction out to about that distance.

There is accumulating evidence that the faint end of the galaxy luminosity function might be very different in different locations. The luminosity function might be rising in rich clusters and flat in regions of low density. If galaxies form according to the model of hierarchical clustering, then there should be many small halos compared to the number of big halos. If this theory is valid, then there must be a mechanism that eliminates at least the visible component of galaxies in low-density regions. A plausible mechanism is photoionization of the intergalactic medium at a time before the epoch that most dwarf galaxies form in low-density regions but after the epoch of formation for similar systems that ultimately end up in rich clusters. The dynamical timescales are found to accommodate this hypothesis in a flat universe with Om [?] 0.4. If small halos exist but simply cannot be located because they have never become the sites of significant star formation, they still might have dynamical manifestations. These manifestations are hard to identify in normal groups of galaxies because small halos do not make a significant contribution to the global mass budget. It could be entertained, however, that there are clusters of halos where there are only small systems, clusters that are at the low-mass end of the hierarchical tree. There may be places where only a few small galaxies managed to form, enough for us to identify and use as test probes of the potential. It turns out that such environments might be common. Four probable groups of dwarfs are identified within 5 Mpc, and the assumption that they are gravitationally bound suggests M/LB ~ 300-1200 M/L, a factor of 6 +- 2 times higher than typical values for groups with luminous galaxies.

Analysis of the peculiar velocities of the galaxies should take account of the uncertainties in both redshifts and distances. We show how this can be done by a numerical application of the action principle. The method is applied to an improved catalog of the galaxies and tight systems of galaxies within 4himg1.gif Mpc, supplemented with a coarser sample of the major concentrations at 4himg1.gif Mpc to 20himg1.gif Mpc distance. Inclusion of this outer zone improves the fit of the mass tracers in the inner zone to their measured redshifts and distances, yielding best fits with reduced kh2 in redshift and distance in the range 1.5-2. These solutions are based on the assumption that the galaxies in and near the Local Group trace the mass, and a powerful test would be provided by observations of proper motions of the nearby galaxies. Predicted transverse galactocentric velocities of some of the nearby galaxies are confined to rather narrow ranges of values, and are on the order of 100 km s-1, large enough to be detected and tested by the proposed SIM and GAIA satellite missions.

The Hawaii Imaging Fabry-Perot Interferometer (HIFI) on the University of Hawaii 2.2 m telescope was used to map the Ha and [O III] l5007 emission-line profiles across the entire disk of the edge-on Sb galaxy NGC 4388. We confirm a rich complex of highly ionized gas that extends ~4 kpc above the disk of this galaxy. Low-ionization gas associated with star formation is also present in the disk. Evidence for bar streaming is detected in the disk component and is discussed in a companion paper. Nonrotational blueshifted velocities of 50-250 km s-1 are measured in the extraplanar gas northeast of the nucleus. The brighter features in this complex tend to have more blueshifted velocities. A redshifted cloud is also detected 2 kpc southwest of the nucleus. The velocity field of the extraplanar gas of NGC 4388 appears to be unaffected by the inferred supersonic (Mach number M [?] 3) motion of this galaxy through the ICM of the Virgo cluster. We argue that this is because the galaxy and the high-|z| gas lie behind a Mach cone with opening angle ~80deg. The shocked ICM that flows near the galaxy has a velocity of ~500 km s-1 and exerts insufficient ram pressure on the extraplanar gas to perturb its kinematics. We consider several explanations of the velocity field of the extraplanar gas. Velocities, especially blueshifted velocities on the north of the galaxy, are best explained as a bipolar outflow that is tilted by >12deg from the normal to the disk. The observed offset between the extraplanar gas and the radio structure may be due to buoyancy or refractive bending by density gradients in the halo gas. Velocity substructure in the outflowing gas also suggests an interaction with ambient halo gas.

The Ursa Major Cluster appears to be unevolved and made up of H I-rich spiral galaxies such as those one finds in the field. B, R, I, K' photometry has been obtained for 79 galaxies, including 62 in a complete sample with M b,iB<-16.[?]5 (with a distance to the cluster of 15.5 Mpc). The K' information is particularly important for the present discussion because it is not seriously affected by obscuration. There is reasonably convincing evidence that the distribution of exponential disk central surface brightnesses is bimodal. There is roughly an order of magnitude difference in the mean luminosity densities of high and low surface brightness disks. Disks avoid the domain between the high and low surface brightness zones. The few intermediate surface brightness examples in the sample all have significant neighbors within a projected distance of 80 kpc. The high surface brightness galaxies exhibit a range -21m $^{{&rm; m}}$ b,iB<-17m $^{{&rm; m}}$ , while the low surface brightness galaxies are found with -19m $^{{&rm; m}}$ b,iB down to the completion limit. High and low surface brightness galaxies in the overlap regime -19m $^{{&rm; m}}$ b,iB<-17m $^{{&rm; m}}$ that are indistinguishable in luminosity-line width plots have very distinct locations in surface brightness-scale length plots. The existence of separate high and low surface brightness families suggests that there are discrete radial configurations that are stable. Galaxies are driven into one of these regimes. The high surface brightness state has a lower luminosity cutoff. It is likely that the high surface brightness galaxies are dominated by dissipational matter at their centers, while the low surface brightness galaxies are dark matter dominated. The high surface brightness family subdivides into those with and without substantial bulges. In either case, these galaxies have essentially the same exponential disk central surface brightnesses. Evidently, there are two thresholds, probably controlled by angular momentum content or transfer. Passing from high to low specific angular momentum, there is first the transition from low surface brightness to high surface brightness regimes, and then the transition from exponential disk to disk plus bulge regimes.