Cl 1205+44: A Fossil Group at z = 0.59

Publications of the Astronomical Society of Japan, 2013

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 the 4Msec Chandra data, the group is first identified as an extended X-ray source. We use 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 ∼ 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 L 0.1−2.4keV = (1.8 ± 0.6) × 10 43 erg s −1 , which then translates into a group mass of (3.2 ± 0.8) × 10 13 M ⊙. This is the lowest mass group ever confirmed at z > 1.5. The 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 the 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 similar to those in the local Universe. One possible difference is the high fraction of AGN-38 +23 −20 % of the bright group member candidates are AGN, which might indicate a role for AGN in 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.

Monthly Notices of the Royal Astronomical Society, 2004

We present the first detailed X-ray observations, using Chandra, of NGC 6482-the nearest known fossil group. The group is dominated by an optically luminous giant elliptical galaxy and all other known group members are at least two magnitudes fainter. The global X-ray properties (luminosity, temperature, extent) of NGC 6482 fall within the range of other groups, but the detailed properties show interesting differences. We derive the gas temperature and total mass profiles for the central 30 h −1 70 kpc (∼0.1 r 200) using ACIS spatially resolved spectroscopy. The unusually high L X /L opt ratio is found to result from a high central gas density. The temperature profile shows a continuous decrease outward, dropping to 0.63 of its central value at 0.1r 200. The derived total mass profile is strongly centrally peaked, suggesting an early formation epoch. These results support a picture in which fossil groups are old, giving time for the most massive galaxies to have merged (via the effects of dynamical friction) to produce a central giant elliptical galaxy. Although the cooling time within 0.1r 200 is less than a Hubble time, no decrease in central temperature is detected. The entropy of the system lies toward the low side of the distribution seen in poor groups and drops all the way into the centre of the system, reaching very low values. No isentropic core, such as those predicted in simple pre-heating models, is present. Given the lack of any central temperature drop in the system, it seems unlikely that radiative cooling can be invoked to explain this low central entropy. The lack of any signature of central cooling is especially striking in a system that appears to be old and relaxed, and to have a central cooling time 10 8 yr. We find that the centrally peaked temperature profile is consistent with a steady-state cooling-flow solution with an accretion rate of 2 M yr −1 , given the large P dV work arising from the cuspy mass profile. However, solutions involving distributed or non-steady heating cannot be ruled out.

Monthly Notices of the Royal Astronomical Society, 2011

We report on two fossil groups of galaxies at redshifts z = 0.425 and 0.372 discovered in the Cosmic Evolution Survey (COSMOS) area. Selected as X-ray extended sources, they have total masses (M 200 ) equal to 1.9(±0.41) × 10 13 and 9.5(±0.42) × 10 13 M , respectively, as obtained from a recent X-ray luminosity-mass scaling relation. The lower mass system appears isolated, whereas the other sits in a well-known large-scale structure (LSS) populated by 27 other X-ray emitting groups. The identification as fossil is based on the i-band photometry of all the galaxies with a photometric redshift consistent with that of the group at the 2σ confidence level and within a projected groupcentric distance equal to 0.5R 200 , and i AB ≤ 22.5 mag limited spectroscopy. Both fossil groups exhibit high stellar-to-total mass ratios compared to all the X-ray selected groups of similar mass at 0.3 ≤ z ≤ 0.5 in the COSMOS. At variance with the composite galaxy stellar mass functions (GSMFs) of similarly massive systems, both fossil group GSMFs are dominated by passively evolving galaxies down to M stars ∼ 10 10 M (according to the galaxy broad-band spectral energy distributions). The relative lack of starforming galaxies with 10 10 ≤ M stars ≤ 10 11 M is confirmed by the galaxy distribution in the b − r versus i colour-magnitude diagram. Hence, the two fossil groups appear as more mature than the coeval, similarly massive groups. Their overall star formation activity ended rapidly after an accelerated build up of the total stellar mass; no significant infall of galaxies with M stars ≥ 10 10 M took place in the last 3 to 6 Gyr. This similarity holds although the two fossil groups are embedded in two very different density environments of the LSS, which suggests that their galaxy populations were shaped by processes that do not depend on the LSS. However, their progenitors may do so. We discuss why the late merging of a compact group is favoured over the early assembly as a formation scenario for the isolated, low-mass fossil group.

Monthly Notices of the Royal Astronomical Society, 2014

We report on the X-ray and optical observations of galaxy groups selected from the 2dfGRS group catalog, to explore the possibility that galaxy groups hosting a giant elliptical galaxy and a large optical luminosity gap present between the two brightest group galaxies, can be associated with an extended X-ray emission, similar to that observed in fossil galaxy groups. The X-ray observations of 4 galaxy groups were carried out with Chandra telescope with 10-20 ksec exposure time. Combining the X-ray and the optical observations we find evidences for the presence of a diffuse extended X-ray emission beyond the optical size of the brightest group galaxy. Taking both the X-ray and the optical criteria, one of the groups is identified as a fossil group and one is ruled out because of the contamination in the earlier optical selection. For the two remaining systems, the X-ay luminosity threshold is close to the convention know for fossil groups. In all cases the X-ray luminosity is below the expected value from the X-ray selected fossils for a given optical luminosity of the group. A rough estimation for the comoving number density of fossil groups is obtained to be 4 to 8×10 −6 Mpc −3 , in broad agreement with the estimations from observations of X-ray selected fossils and predictions of cosmological simulations.

The Astronomical Journal, 2009

We use SDSS-DR4 photometric and spectroscopic data out to redshift z ∼ 0.1 combined with ROSAT All Sky Survey X-ray data to produce a sample of twenty-five fossil groups (FGs), defined as bound systems dominated by a single, luminous elliptical galaxy with extended X-ray emission. We examine possible biases introduced by varying the parameters used to define the sample and the main pitfalls are discussed. The spatial density of FGs, estimated via the V /V MAX test, is 2.83 ×10 −6 h 3 75 Mpc −3 for L X > 0.89 × 10 42 h −2 75 erg s −1 consistent with Vikhlinin et al. (1999), who examined an X-ray overluminous elliptical galaxy sample (OLEG). We compare the general properties of FGs identified here with a sample of bright field ellipticals generated from the same dataset. These two samples show no differences in the distribution of neighboring faint galaxy density excess, distance from the red sequence in the color-magnitude diagram, and structural parameters such as a 4 and internal color gradients. Furthermore, examination of stellar populations shows that our twenty-five FGs have similar ages, metallicities, and α-enhancement as the bright field ellipticals, undermining the idea that these systems represent fossils of a physical mechanism that occurred at high redshift. Our study reveals no difference between FGs and field ellipticals, suggesting that FGs might not be a distinct family of true fossils, but rather the final stage of mass assembly in the Universe.

2008

We use SDSS-DR4 photometric and spectroscopic data out to redshift z 0.1 combined with ROSAT All Sky Survey X-ray data to produce a sample of twenty-five fossil groups (FGs), defined as bound systems dominated by a single, luminous elliptical galaxy with extended X-ray emission. We examine possible biases introduced by varying the parameters used to define the sample and the main pitfalls are discussed. The spatial density of FGs, estimated via the V/V_ MAX test, is 2.83 x 10^-6 h_75^3 Mpc^-3 for L_x > 0.89 x 10^42 h_75^-2 erg/s consistent with Vikhlinin et al. (1999), who examined an X-ray overluminous elliptical galaxy sample (OLEG). We compare the general properties of FGs identified here with a sample of bright field ellipticals generated from the same dataset. These two samples show no differences in the distribution of neighboring faint galaxy density excess, distance from the red sequence in the color-magnitude diagram, and structural parameters such as a_4 and internal co...

2011

We report on two fossil groups of galaxies at redshifts z = 0.425 and 0.372 discovered in the Cosmic Evolution Survey (COSMOS ) area. Selected as X-ray extended sources, they have total masses (M 200 ) equal to 1.9 (±0.41) × 10 13 and 9.5 (±0.42) × 10 13 M ⊙ , respectively, as obtained from a recent X-ray luminosity-mass scaling relation. The lower mass system appears isolated, whereas the other sits in a well-known large-scale structure (LSS) populated by 27 other X-ray emitting groups. The identification as fossil is based on the i-band photometry of all the galaxies with a photometric redshift consistent with that of the group at the 2σ confidence level and within a projected group-centric distance equal to 0.5R 200 , and i AB 22.5 mag limited spectroscopy. Both fossil groups exhibit high stellar-to-total mass ratios compared to all the X-ray selected groups of similar mass at 0.3 z 0.5 in the COSMOS . At variance with the composite galaxy stellar mass functions (GSMFs) of similarly massive systems, both fossil group GSMFs are dominated by passively evolving galaxies down to M stars ∼ 10 10 M ⊙ (according to the galaxy broad-band spectral energy distributions). The relative lack of star-forming galaxies with 10 10 M stars 10 11 M ⊙ is confirmed by the galaxy distribution in the b − r vs i colour-magnitude diagram. Hence, the two fossil groups appear as more mature than the coeval, similarly massive groups. Their overall star formation activity ended rapidly after an accelerated build up of the total stellar mass; no significant infall of galaxies with M stars 10 10 M ⊙ took place in the last 3 to 6 Gyr. This similarity holds although the two fossil groups are embedded in two very different density environments of the LSS, which suggests that their galaxy populations were shaped by processes that do not depend on the LSS. However, their progenitors may do so. We discuss why the late merging of a compact group is favoured over the early assembly as a formation scenario for the isolated, low-mass fossil group.

We present an analysis of Chandra observations of two high-redshift clusters of galaxies, ClJ1113.1−2615 at z = 0.725 and ClJ0152.7−1357 at z = 0.833. We find ClJ1113.1−2615 to be morphologically relaxed with a temperature of kT = 4.3 +0.5 −0.4 keV and a mass (within the virial radius) of 4.3 +0.8 −0.7 × 10 14 M ⊙ . ClJ0152.7−1357, by contrast, is resolved into a northern and southern subcluster, each massive and X-ray luminous, in the process of merging. The temperatures of the subclusters are found to be 5.5 +0.9 −0.8 keV and 5.2 +1.1 −0.9 keV respectively, and we estimate their respective masses to be 6.1 +1.7 −1.5 × 10 14 M ⊙ and 5.2 +1.8 −1.4 × 10 14 M ⊙ within the virial radii. A dynamical analysis of the system shows that the subclusters are likely to be gravitationally bound. If the subclusters merge they will form a system with a mass similar to that of the Coma cluster. Two-dimensional modelling of the X-ray surface brightness reveals excess emission between the subclusters; suggestive, but not conclusive evidence of a shock front.

Astronomy and Astrophysics, 2020

We present an analysis of archival Chandra data of the merging galaxy cluster ClG 0217+70. The Fe xxv Heα X-ray emission line is clearly visible in the 25 ks observation, allowing a precise determination of the redshift of the cluster as z = 0.180 ± 0.006. We measure kT 500 = 8.3 ± 0.4 keV and estimate M 500 = (1.06 ± 0.11) × 10 15 M based on existing scaling relations. Correcting both the radio and X-ray luminosities with the revised redshift reported here, which is much larger than previously inferred based on sparse optical data, this object is no longer an X-ray underluminous outlier in the L X − P radio scaling relation. The new redshift also means that, in terms of physical scale, ClG 0217+70 hosts one of the largest radio halos and one of the largest radio relics known to date. Most of the relic candidates lie in projection beyond r 200. The X-ray morphological parameters suggest that the intracluster medium is still dynamically disturbed. Two X-ray surface brightness discontinuities are confirmed in the northern and southern parts of the cluster, with density jumps of 1.40 ± 0.16 and 3.0 ± 0.6, respectively. We also find a 700 × 200 kpc X-ray faint channel in the western part of the cluster, which may correspond to compressed heated gas or increased non-thermal pressure due to turbulence or magnetic fields.

The Astrophysical Journal, 1996

We report the discovery of a group of galaxies at redshift 2.38. We imaged ∼ 10% of a claimed supercluster of QSO absorption-lines at z = 2.38 (Francis & Hewett 1993). In this small field (2 ′ radius) we detect two Ly-α emitting galaxies. The discovery of two such galaxies in our tiny field supports Francis & Hewett's interpretation of the absorption-line supercluster as a high redshift 'Great Wall'. One of the Ly-α galaxies lies 22 ′′ from a background QSO, and may be associated with a multi-component Ly-α absorption complex seen in the QSO spectrum. This galaxy has an extended (∼ 50kpc) lumpy Ly-α morphology, surrounding a compact IR-bright nucleus. The nucleus shows a pronounced break in its optical-UV colors at ∼ 4000Å (rest-frame), consistent with a stellar population of mass ∼ 7 × 10 11 M Sun , an age of > 500Myr, and little ongoing star-formation. C IV emission is detected, suggesting that a concealed AGN is present. The Ly-α emission is redshifted by ∼ 490km s −1 with respect to the C IV emission, probably due to absorption. Extended H-α emission is also detected; the ratio of Ly-α flux to H-α is abnormally low (∼ 0.7), probable evidence for extended dust. This galaxy is surrounded by a number of very red (B − K > 5) objects, some of which have colors suggesting that they too are at z = 2.38. We hypothesize that this galaxy, its neighbors and a surrounding lumpy gas cloud may be a giant elliptical galaxy in the act of bottom-up formation.