We report the discovery of a small galaxy system in the vicinity of the Ne VIII absorber at z = 0.20701 toward HE0226 - 4110. The galaxy system consists of two 0.25 L(*) disk galaxies and a 0.05 L(*) galaxy all within Delta v < 300 km s(-1) and rho <= 200 h(-1) physical kpc of the absorber. We consider various scenarios for the origin of the Ne VIII absorption, including photo-ionized gas from an active galactic nucleus, a starburst-driven wind, a hot intragroup medium, hot gas in a galaxy halo, and a conductive front produced by cool clouds moving at high speed through a hot medium. We argue that the conductive front scenario is most likely responsible for producing the Ne VIII feature, because it is consistent with the observed galactic environment around the absorber and because it naturally explains the multi-phase nature of the gas and the kinematic signatures of the absorption profiles. Although our preferred scenario suggests that Ne VIII may not be directly probing the warm-hot intergalactic medium, it does imply the existence of an extended hot confining medium around a disk galaxy that may contain a significant reservoir of baryons in the form of hot gas.

We present an imaging and spectroscopic survey of galaxies in fields around QSOs HE 0226-4110, PKS 0405-123, and PG 1216+069. The fields are selected to have ultraviolet echelle spectra available, which uncover 195 Ly alpha absorbers and 13 O VI absorbers along the three sightlines. We obtain robust redshifts for 1104 galaxies of rest-frame absolute magnitude M-R - 5 log h less than or similar to - 16 and at projected physical distances. rho less than or similar to 4 h(-1) Mpc from the QSOs. Hubble Space Telescope (HST)/WFPC2 images of the fields around PKS 0405-123 and PG 1216+069 are available for studying the optical morphologies of absorbing galaxies. Combining the absorber and galaxy data, we perform a cross-correlation study to understand the physical origin of Lya and O VI absorbers and to constrain the properties of extended gas around galaxies. The results of our study are: (1) both strong Ly alpha absorbers of log N(H I) >= 14 and O VI absorbers exhibit a comparable clustering amplitude as emission-line-dominated galaxies and a factor of approximate to 6 weaker amplitude than absorption-line-dominated galaxies on comoving projected distance scales of r(p) < 3 h(-1) Mpc; (2) weak Ly alpha absorbers of log N(H I) < 13.5 appear to cluster very weakly around galaxies; (3) none of the absorption-line-dominated galaxies at r(p) <= 250 h(-1) kpc has a corresponding O VI absorber to a sensitive upper limit of W(1031) less than or similar to 0.03 angstrom, while the covering fraction of O VI absorbing gas around emission-line-dominated galaxies is found to be kappa approximate to 64%; and (4) high-resolution images of five O VI absorbing galaxies show that these galaxies exhibit disk-like morphologies with mildly disturbed features on the edge. Together, the data indicate that O VI absorbers arise preferentially in gas-rich galaxies. In addition, tidal debris in groups/galaxy pairs may be principally responsible for the observed O VI absorbers, particularly those of W(1031) > 70 m angstrom.

We present an imaging and spectroscopic survey of galaxies in fields around QSOs HE 0226-4110, PKS 0405-123, and PG 1216+069. The fields are selected to have ultraviolet echelle spectra available, which uncover 195 Ly alpha absorbers and 13 O VI absorbers along the three sightlines. We obtain robust redshifts for 1104 galaxies of rest-frame absolute magnitude M-R - 5 log h less than or similar to - 16 and at projected physical distances. rho less than or similar to 4 h(-1) Mpc from the QSOs. Hubble Space Telescope (HST)/WFPC2 images of the fields around PKS 0405-123 and PG 1216+069 are available for studying the optical morphologies of absorbing galaxies. Combining the absorber and galaxy data, we perform a cross-correlation study to understand the physical origin of Lya and O VI absorbers and to constrain the properties of extended gas around galaxies. The results of our study are: (1) both strong Ly alpha absorbers of log N(H I) >= 14 and O VI absorbers exhibit a comparable clustering amplitude as emission-line-dominated galaxies and a factor of approximate to 6 weaker amplitude than absorption-line-dominated galaxies on comoving projected distance scales of r(p) < 3 h(-1) Mpc; (2) weak Ly alpha absorbers of log N(H I) < 13.5 appear to cluster very weakly around galaxies; (3) none of the absorption-line-dominated galaxies at r(p) <= 250 h(-1) kpc has a corresponding O VI absorber to a sensitive upper limit of W(1031) less than or similar to 0.03 angstrom, while the covering fraction of O VI absorbing gas around emission-line-dominated galaxies is found to be kappa approximate to 64%; and (4) high-resolution images of five O VI absorbing galaxies show that these galaxies exhibit disk-like morphologies with mildly disturbed features on the edge. Together, the data indicate that O VI absorbers arise preferentially in gas-rich galaxies. In addition, tidal debris in groups/galaxy pairs may be principally responsible for the observed O VI absorbers, particularly those of W(1031) > 70 m angstrom.

We present an imaging and spectroscopic survey of galaxies in fields around QSOs HE 0226-4110, PKS 0405-123, and PG 1216+069. The fields are selected to have ultraviolet echelle spectra available, which uncover 195 Ly alpha absorbers and 13 O VI absorbers along the three sightlines. We obtain robust redshifts for 1104 galaxies of rest-frame absolute magnitude M-R - 5 log h less than or similar to - 16 and at projected physical distances. rho less than or similar to 4 h(-1) Mpc from the QSOs. Hubble Space Telescope (HST)/WFPC2 images of the fields around PKS 0405-123 and PG 1216+069 are available for studying the optical morphologies of absorbing galaxies. Combining the absorber and galaxy data, we perform a cross-correlation study to understand the physical origin of Lya and O VI absorbers and to constrain the properties of extended gas around galaxies. The results of our study are: (1) both strong Ly alpha absorbers of log N(H I) >= 14 and O VI absorbers exhibit a comparable clustering amplitude as emission-line-dominated galaxies and a factor of approximate to 6 weaker amplitude than absorption-line-dominated galaxies on comoving projected distance scales of r(p) < 3 h(-1) Mpc; (2) weak Ly alpha absorbers of log N(H I) < 13.5 appear to cluster very weakly around galaxies; (3) none of the absorption-line-dominated galaxies at r(p) <= 250 h(-1) kpc has a corresponding O VI absorber to a sensitive upper limit of W(1031) less than or similar to 0.03 angstrom, while the covering fraction of O VI absorbing gas around emission-line-dominated galaxies is found to be kappa approximate to 64%; and (4) high-resolution images of five O VI absorbing galaxies show that these galaxies exhibit disk-like morphologies with mildly disturbed features on the edge. Together, the data indicate that O VI absorbers arise preferentially in gas-rich galaxies. In addition, tidal debris in groups/galaxy pairs may be principally responsible for the observed O VI absorbers, particularly those of W(1031) > 70 m angstrom.

We present an imaging and spectroscopic survey of galaxies in fields around QSOs HE 0226-4110, PKS 0405-123, and PG 1216+069. The fields are selected to have ultraviolet echelle spectra available, which uncover 195 Ly alpha absorbers and 13 O VI absorbers along the three sightlines. We obtain robust redshifts for 1104 galaxies of rest-frame absolute magnitude M-R - 5 log h less than or similar to - 16 and at projected physical distances. rho less than or similar to 4 h(-1) Mpc from the QSOs. Hubble Space Telescope (HST)/WFPC2 images of the fields around PKS 0405-123 and PG 1216+069 are available for studying the optical morphologies of absorbing galaxies. Combining the absorber and galaxy data, we perform a cross-correlation study to understand the physical origin of Lya and O VI absorbers and to constrain the properties of extended gas around galaxies. The results of our study are: (1) both strong Ly alpha absorbers of log N(H I) >= 14 and O VI absorbers exhibit a comparable clustering amplitude as emission-line-dominated galaxies and a factor of approximate to 6 weaker amplitude than absorption-line-dominated galaxies on comoving projected distance scales of r(p) < 3 h(-1) Mpc; (2) weak Ly alpha absorbers of log N(H I) < 13.5 appear to cluster very weakly around galaxies; (3) none of the absorption-line-dominated galaxies at r(p) <= 250 h(-1) kpc has a corresponding O VI absorber to a sensitive upper limit of W(1031) less than or similar to 0.03 angstrom, while the covering fraction of O VI absorbing gas around emission-line-dominated galaxies is found to be kappa approximate to 64%; and (4) high-resolution images of five O VI absorbing galaxies show that these galaxies exhibit disk-like morphologies with mildly disturbed features on the edge. Together, the data indicate that O VI absorbers arise preferentially in gas-rich galaxies. In addition, tidal debris in groups/galaxy pairs may be principally responsible for the observed O VI absorbers, particularly those of W(1031) > 70 m angstrom.

We present new optical and near-infrared imaging for a sample of 98 spectroscopically selected galaxy groups at 0.25 < z < 0.55, most of which have velocity dispersions Sigma < 500 km s(-1). We use point spread function matched aperture photometry to measure accurate colours for group members and the surrounding field population. The sample is statistically complete above a stellar mass limit of approximately M = 1 x 10(10) M(circle dot). The overall colour distribution is bimodal in both the field and group samples; but, at fixed luminosity the fraction of group galaxies populating the red peak is larger, by similar to 20 +/- 7 per cent, than that of the field. In particular, group members with early-type morphologies, as identified in Hubble Space Telescope imaging, exhibit a tight red sequence, similar to that seen for more massive clusters. Using optical and near-infrared colours, including data from the Spitzer Space Telescope, we show that approximately 20-30 per cent of galaxies on the red sequence may be dust-reddened galaxies with non-negligible star formation and early-spiral morphologies. This is true of both the field and group samples, and shows little dependence on near-infrared luminosity. Thus, the fraction of bright ((0.4)M(K) < -22) group members with no sign of star formation or active galactic nuclei activity, as identified by their colours or [O ii] emission, is 54 +/- 6 per cent. Our field sample, which includes galaxies in all environments, contains 35 +/- 3 per cent of such inactive galaxies, consistent with the amount expected if all such galaxies are located in groups and clusters. This reinforces our earlier conclusions that dense environments at z less than or similar to 0.5 are associated with a premature cessation of star formation in some galaxies; in particular, we find no evidence for significantly enhanced star formation in these environments. Simple galaxy formation models predict a quenching of star formation in groups that is too efficient, overpopulating the red sequence. Attempts to fix this by increasing the time-scale of this quenching equally for all group members distort the colour distribution in a way that is inconsistent with observations.

We present new optical and near-infrared imaging for a sample of 98 spectroscopically selected galaxy groups at 0.25 < z < 0.55, most of which have velocity dispersions Sigma < 500 km s(-1). We use point spread function matched aperture photometry to measure accurate colours for group members and the surrounding field population. The sample is statistically complete above a stellar mass limit of approximately M = 1 x 10(10) M(circle dot). The overall colour distribution is bimodal in both the field and group samples; but, at fixed luminosity the fraction of group galaxies populating the red peak is larger, by similar to 20 +/- 7 per cent, than that of the field. In particular, group members with early-type morphologies, as identified in Hubble Space Telescope imaging, exhibit a tight red sequence, similar to that seen for more massive clusters. Using optical and near-infrared colours, including data from the Spitzer Space Telescope, we show that approximately 20-30 per cent of galaxies on the red sequence may be dust-reddened galaxies with non-negligible star formation and early-spiral morphologies. This is true of both the field and group samples, and shows little dependence on near-infrared luminosity. Thus, the fraction of bright ((0.4)M(K) < -22) group members with no sign of star formation or active galactic nuclei activity, as identified by their colours or [O ii] emission, is 54 +/- 6 per cent. Our field sample, which includes galaxies in all environments, contains 35 +/- 3 per cent of such inactive galaxies, consistent with the amount expected if all such galaxies are located in groups and clusters. This reinforces our earlier conclusions that dense environments at z less than or similar to 0.5 are associated with a premature cessation of star formation in some galaxies; in particular, we find no evidence for significantly enhanced star formation in these environments. Simple galaxy formation models predict a quenching of star formation in groups that is too efficient, overpopulating the red sequence. Attempts to fix this by increasing the time-scale of this quenching equally for all group members distort the colour distribution in a way that is inconsistent with observations.

We present new optical and near-infrared imaging for a sample of 98 spectroscopically selected galaxy groups at 0.25 < z < 0.55, most of which have velocity dispersions Sigma < 500 km s(-1). We use point spread function matched aperture photometry to measure accurate colours for group members and the surrounding field population. The sample is statistically complete above a stellar mass limit of approximately M = 1 x 10(10) M(circle dot). The overall colour distribution is bimodal in both the field and group samples; but, at fixed luminosity the fraction of group galaxies populating the red peak is larger, by similar to 20 +/- 7 per cent, than that of the field. In particular, group members with early-type morphologies, as identified in Hubble Space Telescope imaging, exhibit a tight red sequence, similar to that seen for more massive clusters. Using optical and near-infrared colours, including data from the Spitzer Space Telescope, we show that approximately 20-30 per cent of galaxies on the red sequence may be dust-reddened galaxies with non-negligible star formation and early-spiral morphologies. This is true of both the field and group samples, and shows little dependence on near-infrared luminosity. Thus, the fraction of bright ((0.4)M(K) < -22) group members with no sign of star formation or active galactic nuclei activity, as identified by their colours or [O ii] emission, is 54 +/- 6 per cent. Our field sample, which includes galaxies in all environments, contains 35 +/- 3 per cent of such inactive galaxies, consistent with the amount expected if all such galaxies are located in groups and clusters. This reinforces our earlier conclusions that dense environments at z less than or similar to 0.5 are associated with a premature cessation of star formation in some galaxies; in particular, we find no evidence for significantly enhanced star formation in these environments. Simple galaxy formation models predict a quenching of star formation in groups that is too efficient, overpopulating the red sequence. Attempts to fix this by increasing the time-scale of this quenching equally for all group members distort the colour distribution in a way that is inconsistent with observations.

X-ray properties of galaxy groups can unlock some of the most challenging research topics in modern extragalactic astronomy: the growth of structure and its influence on galaxy formation. Only with the advent of the Chandra and XMM-Newton facilities have X-ray observations reached the depths required to address these questions in a satisfactory manner. Here we present an X-ray imaging study of two patches from the CNOC2 spectroscopic galaxy survey using combined Chandra and XMM-Newton data. A state of the art extended source finding algorithm has been applied, and the resultant source catalog, including redshifts from a spectroscopic follow-up program, is presented. The total number of spectroscopically identified groups is 25 spanning a redshift range 0.04-0.79. Approximately 50% of CNOC2 spectroscopically selected groups in the deeper X-ray ( RA14h) field are likely X-ray detections, compared to 20% in the shallower ( RA21h) field. Statistical modeling shows that this is consistent with expectations, assuming an expected evolution of the LX-M relation. A significant detection of a stacked shear signal for both spectroscopic and X-ray groups indicates that both samples contain real groups of about the expected mass. We conclude that the current area and depth of X-ray and spectroscopic facilities provide a unique window of opportunity at z similar to 0.4 to test the X-ray appearance of galaxy groups selected in various ways. There is at present no evidence that the correlation between X-ray luminosity and velocity dispersion evolves significantly with redshift, which implies that catalogs based on either method can be fairly compared and modeled.

X-ray properties of galaxy groups can unlock some of the most challenging research topics in modern extragalactic astronomy: the growth of structure and its influence on galaxy formation. Only with the advent of the Chandra and XMM-Newton facilities have X-ray observations reached the depths required to address these questions in a satisfactory manner. Here we present an X-ray imaging study of two patches from the CNOC2 spectroscopic galaxy survey using combined Chandra and XMM-Newton data. A state of the art extended source finding algorithm has been applied, and the resultant source catalog, including redshifts from a spectroscopic follow-up program, is presented. The total number of spectroscopically identified groups is 25 spanning a redshift range 0.04-0.79. Approximately 50% of CNOC2 spectroscopically selected groups in the deeper X-ray ( RA14h) field are likely X-ray detections, compared to 20% in the shallower ( RA21h) field. Statistical modeling shows that this is consistent with expectations, assuming an expected evolution of the LX-M relation. A significant detection of a stacked shear signal for both spectroscopic and X-ray groups indicates that both samples contain real groups of about the expected mass. We conclude that the current area and depth of X-ray and spectroscopic facilities provide a unique window of opportunity at z similar to 0.4 to test the X-ray appearance of galaxy groups selected in various ways. There is at present no evidence that the correlation between X-ray luminosity and velocity dispersion evolves significantly with redshift, which implies that catalogs based on either method can be fairly compared and modeled.