Well-calibrated scaling relations between the observable properties and the total masses of clusters of galaxies are important for understanding the physical processes that give rise to these ...relations. They are also a critical ingredient for studies that aim to constrain cosmological parameters using galaxy clusters. For this reason much effort has been spent during the last decade to better understand and interpret relations of the properties of the intra-cluster medium. Improved X-ray data have expanded the mass range down to galaxy groups, whereas SZ surveys have opened a new observational window on the intracluster medium. In addition, continued progress in the performance of cosmological simulations has allowed a better understanding of the physical processes and selection effects affecting the observed scaling relations. Here we review the recent literature on various scaling relations, focussing on the latest observational measurements and the progress in our understanding of the deviations from self similarity.
We present weak lensing and X-ray analysis of 12 low-mass clusters from the Canada–France–Hawaii Telescope Lensing Survey and XMM-CFHTLS surveys. We combine these systems with high-mass systems from ...Canadian Cluster Comparison Project and low-mass systems from Cosmic Evolution Survey to obtain a sample of 70 systems, spanning over two orders of magnitude in mass. We measure core-excised L
X–T
X, M–L
X and M–T
X scaling relations and include corrections for observational biases. By providing fully bias-corrected relations, we give the current limitations for L
X and T
X as cluster mass proxies. We demonstrate that T
X benefits from a significantly lower intrinsic scatter at fixed mass than L
X. By studying the residuals of the bias-corrected relations, we show for the first time using weak lensing masses that galaxy groups seem more luminous and warmer for their mass than clusters. This implies a steepening of the M–L
X and M–T
X relations at low masses. We verify the inferred steepening using a different high-mass sample from the literature and show that variance between samples is the dominant effect leading to discrepant scaling relations. We divide our sample into subsamples of merging and relaxed systems, and find that mergers may have enhanced scatter in lensing measurements, most likely due to stronger triaxiality and more substructure. For the L
X–T
X relation, which is unaffected by lensing measurements, we find the opposite trend in scatter. We also explore the effects of X-ray cross-calibration and find that Chandra calibration leads to flatter L
X–T
X and M–T
X relations than XMM–Newton.
ABSTRACT
We investigate if the discrepancy between estimates of the total baryon mass fraction obtained from observations of the cosmic microwave background (CMB) and of galaxy groups/clusters ...persists when a large sample of groups is considered. To this purpose, 91 candidate X-ray groups/poor clusters at redshift 0.1 ⩽
z
⩽ 1 are selected from the COSMOS 2 deg
2
survey, based only on their X-ray luminosity and extent. This sample is complemented by 27 nearby clusters with a robust, analogous determination of the total and stellar mass inside
R
500
. The total sample of 118 groups and clusters with
z
⩽ 1 spans a range in
M
500
of ∼10
13
–10
15
M
☉
. We find that the stellar mass fraction associated with galaxies at
R
500
decreases with increasing total mass as
M
−0.37 ± 0.04
500
, independent of redshift. Estimating the total gas mass fraction from a recently derived, high-quality scaling relation, the total baryon mass fraction (
f
stars+gas
500
=
f
stars
500
+
f
gas
500
) is found to increase by ∼25%, when
M
500
increases from 〈
M
〉 = 5 × 10
13
M
☉
to 〈
M
〉 = 7 × 10
14
M
☉
. After consideration of a plausible contribution due to intracluster light (11%–22% of the total stellar mass) and gas depletion through the hierarchical assembly process (10% of the gas mass), the estimated values of the total baryon mass fraction are still lower than the latest CMB measure of the same quantity (WMAP5), at a significance level of 3.3σ for groups of 〈
M
〉 = 5 × 10
13
M
☉
. The discrepancy decreases toward higher total masses, such that it is 1σ at 〈
M
〉 = 7 × 10
14
M
☉
. We discuss this result in terms of nongravitational processes such as feedback and filamentary heating.
We study the stellar mass distribution for galaxies in 160 X-ray detected groups of 10(13) \textless Log(M-200/M-circle dot) \textless 2 x 10(14) and compare it with that of galaxies in the field to ...investigate the action of environment on the build-up of the stellar mass. We highlight differences in the build-up of the passive population in the field, which imprint features in the distribution of stellar mass of passive galaxies at Log(M/M-circle dot) \textless 10.5. The gradual diminishing of the effect when moving to groups of increasing total masses indicates that the growing influence of the environment in bound structures is responsible for the build-up of a quenched component at Log(M/M-circle dot) \textless 10.5. Differently, the stellar mass distribution of star-forming galaxies is similar in shape in all environments, and can be described by a single Schechter function both in groups and in the field. Little evolution is seen up to redshift 1. Nevertheless at z = 0.2-0.4 groups with M-200 \textless 6 x 10(13) M-circle dot (low-mass groups) tend to have a characteristic mass for star-forming galaxies that is 50% higher than in higher mass groups; we interpret it as a reduced action of environmental processes in these systems. Furthermore, we analyse the distribution of sSFR-Log(M) in groups and in the field, and find that groups show on average a lower sSFR (by similar to 0.2 dex) at z \textless 0.8. Accordingly, we find that the fraction of star-forming galaxies is increasing with redshift in all environments, but at a faster pace in the denser ones. Finally, our analysis highlights that low-mass groups have a higher fraction (by 50%) of the stellar mass locked in star-forming galaxies than higher mass systems (i.e. 2/3 of their stellar mass).
We present a detailed study of a peculiar source detected in the COSMOS survey at z = 0.359. Source CXOC J100043.1+020637, also known as CID-42, has two compact optical sources embedded in the same ...galaxy. The distance between the two, measured in the HST/ACS image, is 0.495" ± 0.005" that, at the redshift of the source, corresponds to a projected separation of 2.46 ± 0.02 kpc. A large (~1200 km s-1) velocity offset between the narrow and broad components of Hβ has been measured in three different optical spectra from the VLT/VIMOS and Magellan/IMACS instruments. CID-42 is also the only X-ray source in COSMOS, having in its X-ray spectra a strong redshifted broad absorption iron line and an iron emission line, drawing an inverted P-Cygni profile. The Chandra and XMM-Newton data show that the absorption line is variable in energy by ΔE = 500 eV over four years and that the absorber has to be highly ionized in order not to leave a signature in the soft X-ray spectrum. That these features—the morphology, the velocity offset, and the inverted P-Cygni profile—occur in the same source is unlikely to be a coincidence. We envisage two possible explanations, both exceptional, for this system: (1) a gravitational wave (GW) recoiling black hole (BH), caught 1-10 Myr after merging; or (2) a Type 1/Type 2 system in the same galaxy where the Type 1 is recoiling due to the slingshot effect produced by a triple BH system. The first possibility gives us a candidate GW recoiling BH with both spectroscopic and imaging signatures. In the second case, the X-ray absorption line can be explained as a BAL-like outflow from the foreground nucleus (a Type 2 AGN) at the rearer one (a Type 1 AGN), which illuminates the otherwise undetectable wind, giving us the first opportunity to show that fast winds are present in obscured active galactic nuclei (AGNs), and possibly universal in AGNs.
Abstract
Previous clustering analysis of low-power radio active galactic nuclei (AGNs) has indicated that they preferentially live in massive groups. The X-ray surveys of the Cosmic Evolution Survey ...(COSMOS) field have achieved a sensitivity at which these groups are directly detected out to z = 1.3. Making use of the surveys Chandra, XMM–Newton and VLA-COSMOS, we identify radio AGN members 1023.6≲ L
1.4 GHz/(W Hz−1) ≲ 1025 of galaxy groups (1013.2≲ M
200/M⊙≲ 1014.4; 0.1 < z < 1.3) and study (i) the radio-AGN–X-ray group occupation statistics as a function of group mass; and (ii) the distribution of radio AGNs within the groups. We find that radio AGNs are preferentially associated with galaxies close to the centre (<0.2r
200). Compared to our control sample of group members matched in stellar mass and colour to the radio-AGN host galaxies, we find a significant enhancement of radio-AGN activity associated with 1013.6≲ M
200/M⊙≲ 1014 haloes. We present the first direct measurement of the halo occupation distribution (HOD) for radio AGNs, based on the total mass function of galaxy groups hosting radio AGNs. Our results suggest a possible deviation from the usually assumed power-law HOD model. We also find an overall increase in the fraction of radio AGNs in galaxy groups (<1r
200), relative to that in all environments.
We quantify the importance of the mechanical energy released by radio galaxies inside galaxy groups. We use scaling relations to estimate the mechanical energy released by 16 radio-active galactic ...nuclei located inside X-ray-detected galaxy groups in the COSMOS field. By comparing this energy output to the host groups' gravitational binding energy, we find that radio galaxies produce sufficient energy to unbind a significant fraction of the intragroup medium. This unbinding effect is negligible in massive galaxy clusters with deeper potential wells. Our results correctly reproduce the breaking of self-similarity observed in the scaling relation between entropy and temperature for galaxy groups.
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) × 1013 and 9.5(±0.42) × 1013 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∼ 1010 M⊙ (according to the galaxy broad-band spectral energy distributions). The relative lack of star-forming galaxies with 1010≤M
stars≤ 1011 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≥ 1010 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.
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