The relationship between galaxies and the state/chemical enrichment of the warm-hot intergalactic medium (WHIM) expected to dominate the baryon budget at low-z provides sensitive constraints on structure formation and galaxy evolution models. We present a deep redshift survey in the field of 1ES1553+113, a blazar with a unique combination of ultraviolet (UV)+X-ray spectra for surveys of the circumgalactic/intergalactic medium (CGM/IGM). Nicastro et al. reported the detection of two O VII WHIM absorbers at z = 0.4339 and 0.3551 in its spectrum, suggesting that the WHIM is metal rich and sufficient to close the missing baryons problem. Our survey indicates that the blazar is a member of a z = 0.433 group and that the higher-z O VII candidate arises from its intragroup medium. The resulting bias precludes its use in baryon censuses. The z = 0.3551 candidate occurs in an isolated environment 630 kpc from the nearest galaxy (with stellar mass log M-*/M-circle dot approximate to 9.7), which we show is unexpected for the WHIM. Finally, we characterize the galactic environments of broad H I Ly alpha absorbers (Doppler widths of b = 40-80 km s(-1); T less than or similar to 4 x10(5) K) that provide metallicity-independent WHIM probes. On average, broad Ly alpha absorbers are approximate to 2x closer to the nearest luminous (L > 0.25L(*)) galaxy (700 kpc) than narrow (b < 30 km s(-1); T less than or similar to 4 x 10(5) K) ones (1300 kpc) but approximate to 2x further than O VI absorbers (350 kpc). These observations suggest that gravitational collapse heats portions of the IGM to form the WHIM, but with feedback that does not enrich the IGM far beyond galaxy/group halos to levels currently observable in UV/X-ray metal lines.

We present the analysis of deep optical imaging of the galaxy cluster A133 with the IMACS instrument on Magellan. Our multi-band photometry enables stellar-mass measurements in the cluster member galaxies down to a mass limit of M = 3 x 108 M (0.1 of the Large Magellanic Cloud stellar mass). We observe a clear difference in the spatial distribution of large and dwarf galaxies within the cluster. Modeling these galaxy populations separately, we can confidently track the distribution of stellar mass locked in the galaxies to the cluster's virial radius. The extended envelope of the cluster's brightest galaxy can be tracked to 200 kpc. The central galaxy contributes 1/3 of the total cluster stellar mass within the radius r(500c).

Despite significant progress both observationally and theoretically, the origin of high-ionization nebular He If emission in galaxies dominated by stellar photoionization remains unclear. Accretion-powered radiation from high-mass X-ray binaries (HMXBs) is still one of the leading proposed explanations for the missing He-ionizing photons, but this scenario has yet to be conclusively tested. In this paper, we present nebular line predictions from a grid of photoionization models with input spectral energy distributions containing the joint contribution of both stellar atmospheres and a multicolour disc model for HMXBs. This grid demonstrates that HMXBs are inefficient producers of the photons necessary to power He II, and can only boost this line substantially in galaxies with HMXB populations large enough to power X-ray luminosities of 10(42) erg s(-1) per unit star formation rate (SFR). To test this, we assemble a sample of 11 low-redshift star-forming galaxies with high-quality constraints on both X-ray emission from Chandra and He II emission from deep optical spectra, including new observations with the MMT. These data reveal that the HMXB populations of these nearby systems are insufficient to account for the observed He It strengths, with typical X-ray luminosities or upper limits thereon of only 10(40)-10(41) erg s(-1) per SFR. This indicates that HMXBs are not the dominant source of He+ ionization in these metal-poor star-forming galaxies. We suggest that the solution may instead reside in revisions to stellar wind predictions, softer X-ray sources, or very hot products of binary evolution at low metallicity.

Based on observations of the Seyfert nucleus in NGC 1068 with ASCA, RXTE, and BeppoSAX, we report the discovery of a are (increase in flux by a factor of similar to1.6) in the 6.7 keV Fe K line component between observations obtained 4 months apart, with no significant change in the other (6.21, 6.4, and 6.97 keV) Fe Kalpha line components. During this time, the continuum flux decreased by similar to20%. The RXTE spectrum requires an Fe K absorption edge near 8.6 keV (Fe XXIII-XXV). The spectral data indicate that the 2-10 keV continuum emission is dominated (similar to2/3 of the luminosity) by reflection from a previously unidentified region of warm, ionized gas located less than or similar to0.2 pc from the AGN. The remaining similar to1/3 of the observed X-ray emission is reflected from optically thick, neutral gas. The coronal gas in the inner narrow-line region and/or the cold gas at the inner surface of the obscuring "torus" are possible cold reflectors. The inferred properties of the warm reflector are size (diameter) less than or similar to0.2 pc, gas density ngreater than or similar to10(5.5) cm(-3), ionization parameter xiapproximate to10(3.5) ergs cm s(-1), and covering fraction 0.003(L-0/10(43.5) ergs s(-1))(-1) <(&UOmega;/4π)<0.024(L-0/10(43.5) ergs s(-1))(-1), where L-0 is the intrinsic 2-10 keV X-ray luminosity of the AGN. We suggest that the warm reflector gas is the source of the ( variable) 6.7 keV Fe line emission and the 6.97 keV Fe line emission. The 6.7 keV line are is assumed to be due to an increase in the emissivity of the warm reflector gas from a decrease (by 20%-30%) in L-0. The properties of the warm reflector are most consistent with an intrinsically X-ray weak AGN with L(0)approximate to10(43.0) ergs s(-1). The optical and UV emission that scatters from the warm reflector into our line of sight is required to suffer strong extinction, which can be reconciled if the line of sight skims the outer surface of the torus. Thermal brems-strahlung radio emission from the warm reflector may be detectable in Very Long Baseline Array radio maps of the NGC 1068 nucleus.

A search was conducted for a hot intragroup medium in 109 low-redshift galaxy groups observed with the ROSAT PSPC. Evidence for diffuse, extended X-ray emission is found in at least 61 groups. Approximately one-third of these detections have not been previously reported in the literature. Most of the groups are detected out to less than half of the virial radius with ROSAT. Although some spiral-rich groups do contain an intragroup medium, diffuse emission is restricted to groups that contain at least one early-type galaxy.

The detailed morphology of the interstellar medium (ISM) in the central kiloparsec of galaxies is controlled by pressure and gravitation. The combination of these forces shapes both circumnuclear star formation and the growth of the central, supermassive black hole. We present visible and near-infrared Hubble Space Telescope images and color maps of 123 nearby galaxies that show the distribution of the cold ISM, as traced by dust, with excellent spatial resolution. These observations reveal that nuclear dust spirals are found in the majority of active and inactive galaxies and they possess a wide range in coherence, symmetry, and pitch angle. We have used this large sample to develop a classification system for circumnuclear dust structures. In spite of the heterogeneous nature of the complete sample, we only find symmetric, two-arm nuclear dust spirals in galaxies with large-scale bars, and these dust lanes clearly connect to dust lanes along the leading edges of the large-scale bars. Not all dust lanes along large-scale bars form two-arm spirals, however, and several instead end in nuclear rings. We find that tightly wound, or low pitch angle, nuclear dust spirals are more common in unbarred galaxies than barred galaxies. Finally, the extended narrow-line region in several of the active galaxies is well resolved. The connection between the ionized gas and circumnuclear dust lanes in four of these galaxies provides additional evidence that a significant fraction of their extended narrow-line region is ambient gas photoionized in situ by the active nucleus. In a future paper we will use our classification system for circumnuclear dust to identify differences between active and inactive galaxies, as well as barred and unbarred galaxies, in well-matched subsamples of these data.

We present results from an analysis of FUSE spectroscopy of the z(em) = 0.57 quasar PKS 0405 - 123. We focus on the intervening metal-line systems identified along the sight line and investigate their ionization mechanism, ionization state, and chemical abundances. Including Hubble Space Telescope STIS spectroscopy, we survey the entire sight line and identify six O VI absorbers to a 3 sigma equivalent width (EW) limit of 60 mAngstrom. This implies an incidence dN/dz = 16(-6)(+9) consistent with previous O VI studies. In half of the O VI systems we report positive detections of C III, suggesting that the gas is predominantly photoionized, has multiple ionization phases, or is in a nonequilibrium state. This contrasts with the general description of the warm-hot intergalactic medium (WHIM) as described by numerical simulations in which the gas is predominantly in collisional ionization equilibrium. An appreciable fraction of O vi absorbers may therefore have a different origin. We have also searched the sight line for the Ne VIII doublet (a better probe of the WHIM at T > 10(6) K) over the redshift range 0.2 < z < z(em). We report no positive detections to an EW limit of 80 mAngstrom, giving dN/dz < 40 at 95% c. l. The photoionized metal-line systems exhibit a correlation between the ionization parameter (U = Phi/cn(H), with Phi the flux of hydrogen ionizing photons) and H I column density for N(H I) = 10(14)-10(16) cm(-2). Both the slope and normalization of this correlation match the prediction inferred from the results of Dave and Tripp for the low-z Ly alpha forest. In turn, the data show a tentative, unexpected result: five out of the six photoionized metal-line systems show a total hydrogen column density within a factor of 2 of 10(18.7) cm(-2). Finally, the median metallicity [M/H] of twelve z similar to 0.3 absorbers with N(H I)> 10(14) cm(-2) is [M/H] > - 1.5, with large scatter. This significantly exceeds the median metallicity of C IV and O VI systems at z similar to 3 and indicates enrichment of the intergalactic medium over the past approximate to 10 Gyr.

We compare deep Magellan spectroscopy of 26 groups at 0.3 <= z <= 0.55, selected from the Canadian Network for Observational Cosmology 2 field survey, with a large sample of nearby groups from the 2PIGG catalogue. We find that the fraction of group galaxies with significant [O II]lambda 3727 emission (>= 5 angstrom) increases strongly with redshift, from similar to 29 per cent in 2dFGRS to 58 per cent in CNOC2, for all galaxies brighter than similar to M-* + 1.75. This trend is parallel to the evolution of field galaxies, where the equivalent fraction of emission-line galaxies increases from similar to 53 to similar to 75 per cent. The fraction of emission-line galaxies in groups is lower than in the field, across the full redshift range, indicating that the history of star formation in groups is influenced by their environment. We show that the evolution required to explain the data is inconsistent with a quiescent model of galaxy evolution; instead, discrete events in which galaxies cease forming stars (truncation events) are required. We constrain the probability of truncation (P-trunc) and find that a high value is required in a simple evolutionary scenario neglecting galaxy mergers (P-trunc greater than or similar to 0.3 Gyr(-1)). However, without assuming significant density evolution, P-trunc is not required to be larger in groups than in the field, suggesting that the environmental dependence of star formation was embedded at redshifts z greater than or similar to 0.45.

The evolution of galaxies in groups may have important implications for the evolution of the star formation history of the Universe, since many processes which operate in groups may suppress star formation and the fraction of galaxies in bound groups grows rapidly between z = 1 and the present day. In this paper, we present an investigation of the properties of galaxies in galaxy groups at intermediate redshift (z similar to 0.4). The groups were selected from the Canadian Network for Observational Cosmology Redshift Survey (CNOC2) redshift survey as described by Carlberg et al., with further spectroscopic follow-up undertaken at the Magellan telescope in order to improve the completeness and depth of the sample. We present the data for the individual groups, and find no clear trend in the fraction of passive galaxies with group velocity dispersion and group concentration. We stack the galaxy groups in order to compare the properties of group galaxies with those of field galaxies at the same redshift. The groups contain a larger fraction of passive galaxies than the field, this trend being particularly clear for galaxies brighter than M-BJ < -20 in the higher velocity dispersion groups. In addition, we see evidence for an excess of bright passive galaxies in the groups relative to the field. In contrast, the luminosity functions of the star-forming galaxies in the groups and the field are consistent. These trends are qualitatively consistent with the differences between group and field galaxies seen in the local Universe.

We present a galaxy survey of the field surrounding PKS 0405 - 123 performed with the WFCCD spectrometer at Las Campanas Observatory. The survey is comprised of two data sets: ( 1) a greater than 95% complete survey to R = 20 mag of the field centered on PKS 0405 - 123 with 10' radius (L approximate to 0.1L(*) and radius of 1 Mpc at z = 0.1); and (2) a set of four discontiguous ( i.e., nonoverlapping), flanking fields covering approximate to 1 deg(2) area with completeness approximate to 90% to R = 19.5 mag. With these data sets, one can examine the local and large-scale galactic environment of the absorption systems identified toward PKS 0405 - 123. In this paper, we focus on the O vi systems analyzed in Paper I. The results suggest that this gas arises in a diverse set of galactic environments including the halos of individual galaxies, galaxy groups, filamentary-like structures, and also regions devoid of luminous galaxies. In this small sample, there are no obvious trends between galactic environment and the physical properties of the gas. Furthermore, we find similar results for a set of absorption systems with comparable N-H I but no detectable metal lines. The observations indicate that metals are distributed throughout a wide range of environments in the local universe. Future papers in this series will address the distribution of galactic environments associated with metal-line systems and the Ly alpha forest based on data for over 10 additional fields. All of the data presented in this paper are made public at a dedicated web site.