We present the global group properties of two samples of galaxy groups containing 39 high-quality X-ray-selected systems and 38 optically (spectroscopically) selected systems in coincident spatial regions at 0.12 < z < 0.79. The total mass range of the combined sample is similar to(10(12)-5) x 10(14) M-circle dot. Only nine optical systems are associable with X-ray systems. We discuss the confusion inherent in the matching of both galaxies to extended X-ray emission and of X-ray emission to already identified optical systems. Extensive spectroscopy has been obtained and the resultant redshift catalog and group membership are provided here. X-ray, dynamical, and total stellar masses of the groups are also derived and presented. We explore the effects of utilizing different centers and applying three different kinds of radial cut to our systems: a constant cut of 1 Mpc and two r(200) cuts, one based on the velocity dispersion of the system and the other on the X-ray emission. We find that an X-ray-based r(200) results in less scatter in scaling relations and less dynamical complexity as evidenced by results of the Anderson-Darling and Dressler-Schectman tests, indicating that this radius tends to isolate the virialized part of the system. The constant and velocity dispersion based cuts can overestimate membership and can work to inflate velocity dispersion and dynamical and stellar mass. We find L-X-sigma and M-stellar-L-X scaling relations for X-ray and optically selected systems are not dissimilar. The mean fraction of mass found in stars, excluding intracluster light, for our systems is similar to 0.014 with a logarithmic standard deviation of 0.398 dex. We also define and investigate a sample of groups which are X-ray underluminous given the total group stellar mass. For these systems the fraction of stellar mass contributed by the most massive galaxy is typically lower than that found for the total population of groups implying that there may be less intragroup medium contributed from the most massive member in these systems. Eighty percent of 15 underluminous groups have less than 40% of their stellar mass in the most massive galaxy which happens in less than 1% of cases with samples matched in stellar mass, taken from the combined group catalog.

We present the global group properties of two samples of galaxy groups containing 39 high-quality X-ray-selected systems and 38 optically (spectroscopically) selected systems in coincident spatial regions at 0.12 < z < 0.79. The total mass range of the combined sample is similar to(10(12)-5) x 10(14) M-circle dot. Only nine optical systems are associable with X-ray systems. We discuss the confusion inherent in the matching of both galaxies to extended X-ray emission and of X-ray emission to already identified optical systems. Extensive spectroscopy has been obtained and the resultant redshift catalog and group membership are provided here. X-ray, dynamical, and total stellar masses of the groups are also derived and presented. We explore the effects of utilizing different centers and applying three different kinds of radial cut to our systems: a constant cut of 1 Mpc and two r(200) cuts, one based on the velocity dispersion of the system and the other on the X-ray emission. We find that an X-ray-based r(200) results in less scatter in scaling relations and less dynamical complexity as evidenced by results of the Anderson-Darling and Dressler-Schectman tests, indicating that this radius tends to isolate the virialized part of the system. The constant and velocity dispersion based cuts can overestimate membership and can work to inflate velocity dispersion and dynamical and stellar mass. We find L-X-sigma and M-stellar-L-X scaling relations for X-ray and optically selected systems are not dissimilar. The mean fraction of mass found in stars, excluding intracluster light, for our systems is similar to 0.014 with a logarithmic standard deviation of 0.398 dex. We also define and investigate a sample of groups which are X-ray underluminous given the total group stellar mass. For these systems the fraction of stellar mass contributed by the most massive galaxy is typically lower than that found for the total population of groups implying that there may be less intragroup medium contributed from the most massive member in these systems. Eighty percent of 15 underluminous groups have less than 40% of their stellar mass in the most massive galaxy which happens in less than 1% of cases with samples matched in stellar mass, taken from the combined group catalog.

We present the global group properties of two samples of galaxy groups containing 39 high-quality X-ray-selected systems and 38 optically (spectroscopically) selected systems in coincident spatial regions at 0.12 < z < 0.79. The total mass range of the combined sample is similar to(10(12)-5) x 10(14) M-circle dot. Only nine optical systems are associable with X-ray systems. We discuss the confusion inherent in the matching of both galaxies to extended X-ray emission and of X-ray emission to already identified optical systems. Extensive spectroscopy has been obtained and the resultant redshift catalog and group membership are provided here. X-ray, dynamical, and total stellar masses of the groups are also derived and presented. We explore the effects of utilizing different centers and applying three different kinds of radial cut to our systems: a constant cut of 1 Mpc and two r(200) cuts, one based on the velocity dispersion of the system and the other on the X-ray emission. We find that an X-ray-based r(200) results in less scatter in scaling relations and less dynamical complexity as evidenced by results of the Anderson-Darling and Dressler-Schectman tests, indicating that this radius tends to isolate the virialized part of the system. The constant and velocity dispersion based cuts can overestimate membership and can work to inflate velocity dispersion and dynamical and stellar mass. We find L-X-sigma and M-stellar-L-X scaling relations for X-ray and optically selected systems are not dissimilar. The mean fraction of mass found in stars, excluding intracluster light, for our systems is similar to 0.014 with a logarithmic standard deviation of 0.398 dex. We also define and investigate a sample of groups which are X-ray underluminous given the total group stellar mass. For these systems the fraction of stellar mass contributed by the most massive galaxy is typically lower than that found for the total population of groups implying that there may be less intragroup medium contributed from the most massive member in these systems. Eighty percent of 15 underluminous groups have less than 40% of their stellar mass in the most massive galaxy which happens in less than 1% of cases with samples matched in stellar mass, taken from the combined group catalog.

We present the global group properties of two samples of galaxy groups containing 39 high-quality X-ray-selected systems and 38 optically (spectroscopically) selected systems in coincident spatial regions at 0.12 < z < 0.79. The total mass range of the combined sample is similar to(10(12)-5) x 10(14) M-circle dot. Only nine optical systems are associable with X-ray systems. We discuss the confusion inherent in the matching of both galaxies to extended X-ray emission and of X-ray emission to already identified optical systems. Extensive spectroscopy has been obtained and the resultant redshift catalog and group membership are provided here. X-ray, dynamical, and total stellar masses of the groups are also derived and presented. We explore the effects of utilizing different centers and applying three different kinds of radial cut to our systems: a constant cut of 1 Mpc and two r(200) cuts, one based on the velocity dispersion of the system and the other on the X-ray emission. We find that an X-ray-based r(200) results in less scatter in scaling relations and less dynamical complexity as evidenced by results of the Anderson-Darling and Dressler-Schectman tests, indicating that this radius tends to isolate the virialized part of the system. The constant and velocity dispersion based cuts can overestimate membership and can work to inflate velocity dispersion and dynamical and stellar mass. We find L-X-sigma and M-stellar-L-X scaling relations for X-ray and optically selected systems are not dissimilar. The mean fraction of mass found in stars, excluding intracluster light, for our systems is similar to 0.014 with a logarithmic standard deviation of 0.398 dex. We also define and investigate a sample of groups which are X-ray underluminous given the total group stellar mass. For these systems the fraction of stellar mass contributed by the most massive galaxy is typically lower than that found for the total population of groups implying that there may be less intragroup medium contributed from the most massive member in these systems. Eighty percent of 15 underluminous groups have less than 40% of their stellar mass in the most massive galaxy which happens in less than 1% of cases with samples matched in stellar mass, taken from the combined group catalog.

We report the discovery of an X-ray group of galaxies located at a high redshift of z = 1.61 in the Chandra Deep Field South. Based on 4 Ms Chandra data, the group is first identified as an extended X-ray source. We have used a wealth of deep multi-wavelength data to identify the optical counterpart-our red sequence finder detects a significant over-density of galaxies at z similar to 1.6. The brightest group galaxy is spectroscopically confirmed at z = 1.61, based on published spectroscopic redshifts. Using this as a central redshift of the group, we measure an X-ray luminosity of L0.1-2.4keV = (1.8 +/- 0.6) x 10(43) erg s(-1), which then translates into a group mass of (3.2 +/- 0.8) x 10(13) M-circle dot. This is the lowest-mass group ever confirmed at z > 1.5. Deep optical-nearIR images from CANDELS reveal that the group exhibits a surprisingly prominent red sequence, and most of the galaxies are consistent with a formation redshift of z(f) = 3. A detailed analysis of the spectral energy distributions of the group member candidates confirms that most of them are indeed passive galaxies. Furthermore, their structural parameters measured from near-IR CANDELS images show that they are morphologically early-type. The newly identified group at z = 1.61 is dominated by quiescent early-type galaxies, and the group appears to be similar to those in the local Universe. One possible difference is the high fraction of AGN-38(-20)(+23)% of the bright group member candidates are AGN, which might indicate a role for AGN in the quenching of star formation. However, a statistical sample of high-z groups is needed to draw a general picture of groups at this redshift. Such a sample will hopefully be available in near-future surveys.

We report the discovery of an X-ray group of galaxies located at a high redshift of z = 1.61 in the Chandra Deep Field South. Based on 4 Ms Chandra data, the group is first identified as an extended X-ray source. We have used a wealth of deep multi-wavelength data to identify the optical counterpart-our red sequence finder detects a significant over-density of galaxies at z similar to 1.6. The brightest group galaxy is spectroscopically confirmed at z = 1.61, based on published spectroscopic redshifts. Using this as a central redshift of the group, we measure an X-ray luminosity of L0.1-2.4keV = (1.8 +/- 0.6) x 10(43) erg s(-1), which then translates into a group mass of (3.2 +/- 0.8) x 10(13) M-circle dot. This is the lowest-mass group ever confirmed at z > 1.5. Deep optical-nearIR images from CANDELS reveal that the group exhibits a surprisingly prominent red sequence, and most of the galaxies are consistent with a formation redshift of z(f) = 3. A detailed analysis of the spectral energy distributions of the group member candidates confirms that most of them are indeed passive galaxies. Furthermore, their structural parameters measured from near-IR CANDELS images show that they are morphologically early-type. The newly identified group at z = 1.61 is dominated by quiescent early-type galaxies, and the group appears to be similar to those in the local Universe. One possible difference is the high fraction of AGN-38(-20)(+23)% of the bright group member candidates are AGN, which might indicate a role for AGN in the quenching of star formation. However, a statistical sample of high-z groups is needed to draw a general picture of groups at this redshift. Such a sample will hopefully be available in near-future surveys.

We report the discovery of an X-ray group of galaxies located at a high redshift of z = 1.61 in the Chandra Deep Field South. Based on 4 Ms Chandra data, the group is first identified as an extended X-ray source. We have used a wealth of deep multi-wavelength data to identify the optical counterpart-our red sequence finder detects a significant over-density of galaxies at z similar to 1.6. The brightest group galaxy is spectroscopically confirmed at z = 1.61, based on published spectroscopic redshifts. Using this as a central redshift of the group, we measure an X-ray luminosity of L0.1-2.4keV = (1.8 +/- 0.6) x 10(43) erg s(-1), which then translates into a group mass of (3.2 +/- 0.8) x 10(13) M-circle dot. This is the lowest-mass group ever confirmed at z > 1.5. Deep optical-nearIR images from CANDELS reveal that the group exhibits a surprisingly prominent red sequence, and most of the galaxies are consistent with a formation redshift of z(f) = 3. A detailed analysis of the spectral energy distributions of the group member candidates confirms that most of them are indeed passive galaxies. Furthermore, their structural parameters measured from near-IR CANDELS images show that they are morphologically early-type. The newly identified group at z = 1.61 is dominated by quiescent early-type galaxies, and the group appears to be similar to those in the local Universe. One possible difference is the high fraction of AGN-38(-20)(+23)% of the bright group member candidates are AGN, which might indicate a role for AGN in the quenching of star formation. However, a statistical sample of high-z groups is needed to draw a general picture of groups at this redshift. Such a sample will hopefully be available in near-future surveys.

We report the discovery of an X-ray group of galaxies located at a high redshift of z = 1.61 in the Chandra Deep Field South. Based on 4 Ms Chandra data, the group is first identified as an extended X-ray source. We have used a wealth of deep multi-wavelength data to identify the optical counterpart-our red sequence finder detects a significant over-density of galaxies at z similar to 1.6. The brightest group galaxy is spectroscopically confirmed at z = 1.61, based on published spectroscopic redshifts. Using this as a central redshift of the group, we measure an X-ray luminosity of L0.1-2.4keV = (1.8 +/- 0.6) x 10(43) erg s(-1), which then translates into a group mass of (3.2 +/- 0.8) x 10(13) M-circle dot. This is the lowest-mass group ever confirmed at z > 1.5. Deep optical-nearIR images from CANDELS reveal that the group exhibits a surprisingly prominent red sequence, and most of the galaxies are consistent with a formation redshift of z(f) = 3. A detailed analysis of the spectral energy distributions of the group member candidates confirms that most of them are indeed passive galaxies. Furthermore, their structural parameters measured from near-IR CANDELS images show that they are morphologically early-type. The newly identified group at z = 1.61 is dominated by quiescent early-type galaxies, and the group appears to be similar to those in the local Universe. One possible difference is the high fraction of AGN-38(-20)(+23)% of the bright group member candidates are AGN, which might indicate a role for AGN in the quenching of star formation. However, a statistical sample of high-z groups is needed to draw a general picture of groups at this redshift. Such a sample will hopefully be available in near-future surveys.

We present deep Gemini Multi-Object Spectrograph-South spectroscopy for 11 galaxy groups at 0.8 < z < 1.0, for galaxies with r(AB) < 24.75. Our sample is highly complete (> 66 per cent) for eight of the 11 groups. Using an optical-near-infrared colour-colour diagram, the galaxies in the sample were separated with a dust insensitive method into three categories: passive (red), star-forming (blue) and intermediate (green). The strongest environmental dependence is observed in the fraction of passive galaxies, which make up only similar to 20 per cent of the field in the mass range 10(10.3) < M-star/M-circle dot < 10(11.0), but are the dominant component of groups. If we assume that the properties of the field are similar to those of the 'pre-accreted' population, the environment quenching efficiency (is an element of(rho)) is defined as the fraction of field galaxies required to be quenched in order to match the observed red fraction inside groups. The efficiency obtained is similar to 0.4, similar to its value in intermediate-density environments locally. While green (intermediate) galaxies represent similar to 20 per cent of the star-forming population in both the group and field, at all stellar masses, the average specific star formation rate of the group population is lower by a factor of similar to 3. The green population does not show strong H delta absorption that is characteristic of starburst galaxies. Finally, the high fraction of passive galaxies in groups, when combined with satellite accretion models, require that most accreted galaxies have been affected by their environment. Thus, any delay between accretion and the onset of truncation of star formation (tau) must be <= 2 Gyr, shorter than the 3-7 Gyr required to fit data at z = 0. The relatively small fraction of intermediate galaxies require that the actual quenching process occurs quickly, with an exponential decay time-scale of tau(q) <= 1Gyr.

We present deep Gemini Multi-Object Spectrograph-South spectroscopy for 11 galaxy groups at 0.8 < z < 1.0, for galaxies with r(AB) < 24.75. Our sample is highly complete (> 66 per cent) for eight of the 11 groups. Using an optical-near-infrared colour-colour diagram, the galaxies in the sample were separated with a dust insensitive method into three categories: passive (red), star-forming (blue) and intermediate (green). The strongest environmental dependence is observed in the fraction of passive galaxies, which make up only similar to 20 per cent of the field in the mass range 10(10.3) < M-star/M-circle dot < 10(11.0), but are the dominant component of groups. If we assume that the properties of the field are similar to those of the 'pre-accreted' population, the environment quenching efficiency (is an element of(rho)) is defined as the fraction of field galaxies required to be quenched in order to match the observed red fraction inside groups. The efficiency obtained is similar to 0.4, similar to its value in intermediate-density environments locally. While green (intermediate) galaxies represent similar to 20 per cent of the star-forming population in both the group and field, at all stellar masses, the average specific star formation rate of the group population is lower by a factor of similar to 3. The green population does not show strong H delta absorption that is characteristic of starburst galaxies. Finally, the high fraction of passive galaxies in groups, when combined with satellite accretion models, require that most accreted galaxies have been affected by their environment. Thus, any delay between accretion and the onset of truncation of star formation (tau) must be <= 2 Gyr, shorter than the 3-7 Gyr required to fit data at z = 0. The relatively small fraction of intermediate galaxies require that the actual quenching process occurs quickly, with an exponential decay time-scale of tau(q) <= 1Gyr.