We present the first detailed X-ray observations of optically selected clusters at high redshift. Two clusters, Cl 1324 + 3011 at z = 0.76 and Cl 1604 + 4304 at z = 0.90, were observed with XMM-Newton. The optical center of each cluster is coincident with an extended X-ray source whose emission is detected out to a radius of similar to0.5 h(70)(-1) Mpc. The emission from each cluster appears reasonably circular, with some indication of asymmetries and more complex morphologies. Similar to other optically selected clusters at redshifts of z greater than or similar to0.4, both clusters are modest X-ray emitters with bolometric luminosities of only L-X(bol) = (1.4-2.0) x 10(44) h(70)(-2) ergs s(-1). We measure gas temperatures of T = 2.88(-0.49)(+0.71) keV for Cl 1324 + 3011 and 2.51(-0.69)(+1.05) keV for Cl 1604 + 4304. The X-ray properties of both clusters are consistent with the high-redshift L-X-T relation measured from X-ray-selected samples at z greater than or equal to 0.5. However, based on the local relations, their X-ray luminosities and temperatures are low for z = 0.5 their measured velocity dispersions (sigma). The clusters are cooler by a factor of 2-9 compared to the local sigma-T relation. We briefly discuss the possible explanations for these results.

We present the first detailed X-ray observations of optically selected clusters at high redshift. Two clusters, Cl 1324 + 3011 at z = 0.76 and Cl 1604 + 4304 at z = 0.90, were observed with XMM-Newton. The optical center of each cluster is coincident with an extended X-ray source whose emission is detected out to a radius of similar to0.5 h(70)(-1) Mpc. The emission from each cluster appears reasonably circular, with some indication of asymmetries and more complex morphologies. Similar to other optically selected clusters at redshifts of z greater than or similar to0.4, both clusters are modest X-ray emitters with bolometric luminosities of only L-X(bol) = (1.4-2.0) x 10(44) h(70)(-2) ergs s(-1). We measure gas temperatures of T = 2.88(-0.49)(+0.71) keV for Cl 1324 + 3011 and 2.51(-0.69)(+1.05) keV for Cl 1604 + 4304. The X-ray properties of both clusters are consistent with the high-redshift L-X-T relation measured from X-ray-selected samples at z greater than or equal to 0.5. However, based on the local relations, their X-ray luminosities and temperatures are low for z = 0.5 their measured velocity dispersions (sigma). The clusters are cooler by a factor of 2-9 compared to the local sigma-T relation. We briefly discuss the possible explanations for these results.

We present the first detailed X-ray observations of optically selected clusters at high redshift. Two clusters, Cl 1324 + 3011 at z = 0.76 and Cl 1604 + 4304 at z = 0.90, were observed with XMM-Newton. The optical center of each cluster is coincident with an extended X-ray source whose emission is detected out to a radius of similar to0.5 h(70)(-1) Mpc. The emission from each cluster appears reasonably circular, with some indication of asymmetries and more complex morphologies. Similar to other optically selected clusters at redshifts of z greater than or similar to0.4, both clusters are modest X-ray emitters with bolometric luminosities of only L-X(bol) = (1.4-2.0) x 10(44) h(70)(-2) ergs s(-1). We measure gas temperatures of T = 2.88(-0.49)(+0.71) keV for Cl 1324 + 3011 and 2.51(-0.69)(+1.05) keV for Cl 1604 + 4304. The X-ray properties of both clusters are consistent with the high-redshift L-X-T relation measured from X-ray-selected samples at z greater than or equal to 0.5. However, based on the local relations, their X-ray luminosities and temperatures are low for z = 0.5 their measured velocity dispersions (sigma). The clusters are cooler by a factor of 2-9 compared to the local sigma-T relation. We briefly discuss the possible explanations for these results.

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 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 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.

Deviations of galaxy groups from cluster scaling relations can be understood in terms of an excess of entropy in groups. The main effect of this excess is to reduce the density and thus the luminosity of the intragroup gas. Given this, groups should also show a steep relationship between X-ray luminosity and velocity dispersion. However, previous work suggests that this is not the case, with many measuring slopes flatter than the cluster relation. Examining the group L(X)-sigma relation shows that much of the flattening is caused by a small subset of groups that show very high X-ray luminosities for their velocity dispersions (or vice versa). Detailed Chandra study of two such groups shows that earlier ROSAT results were subject to significant (similar to30%-40%) point-source contamination but confirm that a significant hot intergalactic medium is present in these groups, although these are two of the coolest systems in which intergalactic X-ray emission has been detected. Their X-ray properties are shown to be broadly consistent with those of other galaxy groups, although the gas entropy in NGC 1587 is unusually low, and its X-ray luminosity is correspondingly high for its temperature when compared with most groups. This leads us to suggest that the velocity dispersion in these systems has been reduced in some way, and we consider how this might have come about.

Deviations of galaxy groups from cluster scaling relations can be understood in terms of an excess of entropy in groups. The main effect of this excess is to reduce the density and thus the luminosity of the intragroup gas. Given this, groups should also show a steep relationship between X-ray luminosity and velocity dispersion. However, previous work suggests that this is not the case, with many measuring slopes flatter than the cluster relation. Examining the group L(X)-sigma relation shows that much of the flattening is caused by a small subset of groups that show very high X-ray luminosities for their velocity dispersions (or vice versa). Detailed Chandra study of two such groups shows that earlier ROSAT results were subject to significant (similar to30%-40%) point-source contamination but confirm that a significant hot intergalactic medium is present in these groups, although these are two of the coolest systems in which intergalactic X-ray emission has been detected. Their X-ray properties are shown to be broadly consistent with those of other galaxy groups, although the gas entropy in NGC 1587 is unusually low, and its X-ray luminosity is correspondingly high for its temperature when compared with most groups. This leads us to suggest that the velocity dispersion in these systems has been reduced in some way, and we consider how this might have come about.

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.

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.