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.

Ram-pressure stripping of galactic gas is generally assumed to be inefficient in galaxy groups due to the relatively low density of the intragroup medium (IGM) and the small velocity dispersions of groups. To test this assumption, we obtained Chandra X-ray data of the starbursting spiral NGC 2276 in the NGC 2300 group of galaxies, a candidate for a strong galaxy interaction with hot intragroup gas. The data reveal a shock-like feature along the western edge of the galaxy and a low surface brightness tail extending to the east, similar to the morphology seen in other wavebands. Spatially resolved spectroscopy shows that the data are consistent with intragroup gas being pressurized at the leading western edge of NGC 2276 due to the galaxy moving supersonically through the IGM at a velocity similar to 850 km s(-1). Detailed modelling of the gravitational potential of NGC 2276 shows that the resulting ram pressure could significantly affect the morphology of the outer gas disc but is probably insufficient to strip large amounts of cold gas from the disc. We estimate the mass-loss rates due to turbulent viscous stripping and starburst outflows being swept back by ram pressure, showing that both mechanisms could plausibly explain the presence of the X-ray tail. Comparison to existing H I measurements shows that most of the gas escaping the galaxy is in a hot phase. With a total mass-loss rate of similar to 5 M-circle dot yr(-1), the galaxy could be losing its entire present H I supply within a Gyr. This demonstrates that the removal of galactic gas through interactions with a hot IGM can occur rapidly enough to transform the morphology of galaxies in groups. Implications of this for galaxy evolution in groups and clusters are briefly discussed.

We use a cosmological numerical simulation to study the tidal features produced by a minor merger with an elliptical galaxy. We find that the simulated tidal features are quantitatively similar to the red tidal features, i.e., dry tidal features, recently found in deep images of elliptical galaxies at intermediate redshifts. The minor merger in our simulation does not trigger star formation due to active galactic nuclei heating. Therefore, both the tidal features and the host galaxy are red, i.e., a dry minor merger. The stellar mass of the infalling satellite galaxy is about 10(10) M-circle dot, and the tidal debris reach the surface brightness of mu(R) similar to 27 mag arcsec(-2). Thus, we conclude that tidal debris from minor mergers can explain the observed dry tidal features in elliptical galaxies at intermediate redshifts, although other mechanisms (such as major dry mergers) may also be important.

We have undertaken a multiwavelength project to study the relatively unknown properties of groups and poor clusters of galaxies at intermediate redshifts. In this paper, we describe the XMM-Newton observations of six X-ray selected groups with 0.2 < z < 0: 6. The X-ray properties of these systems are generally in good agreement with the properties of low-redshift groups. They appear to follow the scaling relations between luminosity, temperature, and velocity dispersion defined by low-redshift groups and clusters. The X-ray emission in four of the six groups is also centered on a dominant early-type galaxy. The lack of a bright elliptical galaxy at the peak of the group X-ray emission is rare at low redshifts, and the other two groups may be less dynamically evolved. We find indications of excess entropy in these systems over self-similar predictions out to large radii. We also confirm the presence of at least one X-ray-luminous AGN associated with a group member galaxy and find several other potential group AGNs.

X-ray observations of hot, intergalactic gas in galaxy groups provide a useful means of characterizing the global properties of groups. However, X-ray studies of large group samples have typically involved very shallow X-ray exposures or have been based on rather heterogeneous samples. Here we present the first results of the XI (XMM/IMACS) Groups Project, a study targeting, for the first time, a redshift-selected, statistically unbiased sample of galaxy groups using deep X-ray data. Combining this with radio observations of cold gas and optical imaging and spectroscopy of the galaxy population, the project aims to advance the understanding of how the properties and dynamics of group galaxies relate to global group properties. Here, X-ray and optical data of the first four galaxy groups observed as part of the project are presented. In two of the groups we detect diffuse emission with a luminosity of L-X approximate to 10(41) erg s(-1), among the lowest found for any X-ray detected group thus far, with a comparable upper limit for the other two. Compared to typical X-ray selected groups of similar velocity dispersion, these four systems are all surprisingly X-ray faint. We discuss possible explanations for the lack of significant X-ray emission in the groups, concluding that these systems are most likely collapsing for the first time. Our results strongly suggest that, unlike our current optically selected sample, previous X-ray selected group samples represented a biased picture of the group population. This underlines the necessity of a study of this kind, if one is to reach an unbiased census of the properties of galaxy groups and the distribution of baryons in the Universe.