We use the largest complete sample of 64 galaxy clusters (HIghest X-ray FLUx Galaxy Cluster Sample) with available high-quality X-ray data from Chandra, and apply 16 cool-core diagnostics to them, some of them new. In order to identify the best parameter for characterizing cool-core clusters and quantify its relation to other parameters, we mainly use very high spatial resolution profiles of central gas density and temperature, and quantities derived from them. We also correlate optical properties of brightest cluster galaxies (BCGs) with X-ray properties. To segregate cool core and non-cool-core clusters, we find that central cooling time, t cool , is the best parameter for low redshift clusters with high quality data, and that cuspiness is the best parameter for high redshift clusters. 72% of clusters in our sample have a cool core (t cool < 7.7 h −1/2 71 Gyr) and 44% have strong cool cores (t cool < 1.0 h −1/2 71 Gyr). We find strong cool-core clusters are characterized as having low central entropy and a systematic central temperature drop. Weak cool-core clusters have enhanced central entropies and temperature profiles that are flat or decrease slightly towards the center. Non-cool-core clusters have high central entropies.For the first time we show quantitatively that the discrepancy in classical and spectroscopic mass deposition rates can not be explained with a recent formation of the cool cores, demonstrating the need for a heating mechanism to explain the cooling flow problem. We find that strong cool-core clusters have a distribution of central temperature drops, centered on 0.4T vir . However, the radius at which the temperature begins to drop varies. This lack of a universal inner temperature profile probably reflects the complex physics in cluster cores not directly related to the cluster as a whole. Our results suggest that the central temperature does not correlate with the mass of the BCGs and weakly correlates with the expected radiative cooling only for strong cool-core clusters. Since 88% of the clusters in our sample have a BCG within a projected distance of 50 h −1 71 kpc from the X-ray peak, we argue that it is easier to heat the gas (e.g. with mergers or non-gravitational processes) than to separate the dense core from the brightest cluster galaxy. Diffuse, Mpc-scale radio emission, believed to be associated with major mergers, has not been unambiguously detected in any of the strong cool-core clusters in our sample. Of the weak cool-core clusters and non-cool-core clusters, most of the clusters (seven out of eight) that have diffuse, Mpc-scale radio emission have a large (>50 h −1 71 kpc) projected separation between their BCG and X-ray peak. In contrast, only two of the 56 clusters with a small separation between the BCG and X-ray peak (<50 h −1 71 kpc) show large-scale radio emission. Based on this result, we argue that a large projected separation between the BCG and the X-ray peak is a good indicator of a major merger. The properties of weak cool-core clusters as an intermedi...

Active galactic nuclei (AGN) at the center of galaxy clusters with gas cooling times that are much shorter than the Hubble time have emerged as heating agents powerful enough to prevent further cooling of the intracluster medium (ICM). We carried out an intensive study of the AGN heating−ICM cooling network by comparing various cluster parameters to the integrated radio luminosity of the central AGN, L R , defined as the total synchrotron power between 10 MHz and 15 GHz. This study is based on the HIFLUGCS sample comprising the 64 X-ray brightest galaxy clusters. We adopted the central cooling time, t cool , as the diagnostic to ascertain cooling properties of the HIFLUGCS sample and classify clusters with t cool < 1 Gyr as strong cool-core (SCC) clusters, with 1 Gyr < t cool < 7.7 Gyr as weak cool-core (WCC) clusters and with t cool > 7.7 Gyr as non-cool-core (NCC) clusters. We find 48 out of 64 clusters (75%) contain cluster center radio sources (CCRS) cospatial with or within 50 h −1 71 kpc of the X-ray peak emission. Furthermore, we find that the probability of finding a CCRS increases from 45% to 67% to 100% for NCC, WCC, and SCC clusters, respectively.We use a total of ∼140 independent radio flux-density measurements, with data at more than two frequencies for more than 54% of the sources extending below 500 MHz, enabling the determination of accurate estimates of L R . We find that L R in SCC clusters depends strongly on the cluster scale such that more massive clusters harbor more powerful radio AGN. The same trend is observed between L R and the classical mass deposition rate,Ṁ classical in SCC and partly also in WCC clusters, and can be quantified as L R ∝Ṁ 1.69±0.25 classical . We also perform correlations of the luminosity for the brightest cluster galaxy, L BCG , close to the X-ray peak in all 64 clusters with L R and cluster parameters, such as the virial mass, M 500 , and the bolometric X-ray luminosity, L X . To this end, we use the 2MASS K-band magnitudes and invoke the near-infrared bulge luminosity-black hole mass relation to convert L BCG to supermassive black hole mass, M BH . We find a weak correlation between M BH and L R for SCC clusters, L R ∼ M 4.10±0.42 BH , although with a few outliers. We find an excellent correlation of L BCG with M 500 and L X for the entire sample, the SCC clusters showing a tighter trend in both the cases. We discuss the plausible reasons behind these scaling relations in the context of cooling flows and AGN feedback.Our results strongly suggest an AGN-feedback machinery in SCC clusters, which regulates the cooling in the central regions. Since the dispersion in these correlations, such as that between L R andṀ classical or L R and M BH , increases in going from SCC to WCC clusters, we conclude there must be secondary processes that work either in conjunction with the AGN heating or independently to counteract the radiative losses in WCC clusters.

The Chandra X-ray Observatory has revealed X-ray bubbles in the intracluster medium (ICM) of many nearby cooling flow clusters. The bubbles trace feedback that is thought to couple the central active galactic nucleus (AGN) to the ICM, helping to stabilize cooling flows and govern the evolution of massive galaxies. However, the prevalence and duty cycle of such AGN outbursts is not well understood. To this end, we study how cooling is balanced by bubble heating for complete samples of clusters (the Brightest 55 clusters of galaxies, hereafter B55, and the HIghest X-ray FLUx Galaxy Cluster Sample, HIFLUGCS). We find that the radio luminosity of the central galaxy only exceeds 2.5 x 10^30 erg s^-1 Hz^-1 in cooling flow clusters. This result implies a connection between the central radio source and the ICM, as expected if AGN feedback is operating. Additionally, we find a duty cycle for radio mode feedback, the fraction of time that a system possesses bubbles inflated by its central radio source, of > 69 per cent for B55 and > 63 per cent for HIFLUGCS. These duty cycles are lower limits since some bubbles are likely missed in existing images. We used simulations to constrain the bubble power that might be present and remain undetected in the cooling flow systems without detected bubbles. Among theses systems, almost all could have significant bubble power. Therefore, our results imply that the duty cycle of AGN outbursts with the potential to heat the gas significantly in cooling flow clusters is at least 60 per cent and could approach 100 per cent.Comment: Accepted to MNRAS, 24 pages, 5 figure

We present a multiwavelength morphological analysis of star forming clouds and filaments in the central (< ∼ 50 kpc) regions of 16 low redshift (z < 0.3) cool core brightest cluster galaxies (BCGs). The sample spans decades-wide ranges of X-ray mass deposition and star formation rates as well as active galactic nucleus (AGN) mechanical power, encompassing both high and low extremes of the supposed intracluster medium (ICM) cooling and AGN heating feedback cycle. New Hubble Space Telescope (HST) imaging of far ultraviolet continuum emission from young (< ∼ 10 Myr), massive (> ∼ 5 M ) stars reveals filamentary and clumpy morphologies, which we quantify by means of structural indices. The FUV data are compared with X-ray, Lyα, narrowband Hα, broadband optical/IR, and radio maps, providing a high spatial resolution atlas of star formation locales relative to the ambient hot (∼ 10 7−8 K) and warm ionised (∼ 10 4 K) gas phases, as well as the old stellar population and radio-bright AGN outflows. Nearly half of the sample possesses kpc-scale filaments that, in projection, extend toward and around radio lobes and/or X-ray cavities. These filaments may have been uplifted by the propagating jet or buoyant X-ray bubble, or may have formed in situ by cloud collapse at the interface of a radio lobe or rapid cooling in a cavity's compressed shell. Many other extended filaments, however, show no such spatial correlation, and the dominant driver of their morphology remains unclear. We nevertheless show that the morphological diversity of nearly the entire FUV sample is reproduced by recent hydrodynamical simulations in which the AGN powers a self-regulating rain of thermally unstable star forming clouds that precipitate from the hot atmosphere. In this model, precipitation triggers where the cooling-to-freefall time ratio is t cool /t ff ∼ 10. This condition is roughly met at the maxmial projected FUV radius for more than half of our sample, and clustering about this ratio is stronger for sources with higher star formation rates.

We present a detailed investigation of the X-ray luminosity (L X )-gas temperature (T vir ) relation of the complete X-ray flux-limited sample of the 64 brightest galaxy clusters in the sky (HIFLUGCS). We study the influence of two astrophysical processes, active galactic nuclei (AGN) heating and intracluster medium (ICM) cooling, on the L X −T vir relation, simultaneously for the first time. We employ homogeneously determined gas temperatures and central cooling times, measured with Chandra, and information about a central radio source from Mittal and collaborators. We determine best-fit relations for different subsamples using the cool-core strength and the presence of central radio activity as selection criteria. We find the strong cool-core clusters (SCCs) with short cooling times (<1 Gyr) to display the steepest relation (L X ∝ T 3.33±0.15 vir ) and the non-cool-core clusters (NCCs) with long cooling times (>7.7 Gyr) to display the shallowest (L X ∝ T 2.42±0.21 vir ). This has the simple implication that on the high-mass scale (T vir > 2.5 keV) the steepening of the L X −T vir relation is mainly due to the cooling of the intracluster medium gas. We propose that ICM cooling and AGN heating are both important in shaping the L X −T vir relation but on different length-scales. While our study indicates that ICM cooling dominates on cluster scales (T vir > 2.5 keV), we speculate that AGN heating dominates the scaling relation in poor clusters and groups (T vir < 2.5 keV). The intrinsic scatter about the L X −T vir relation in X-ray luminosity for the whole sample is 45.4% and varies from a minimum of 34.8% for weak cool-core clusters to a maximum of 59.4% for clusters with no central radio source. The scatter does not decrease if SCC clusters are excluded from the full sample. We find that the contribution of core luminosities within the cooling radius r cool , where the cooling time is 7.7 Gyr and gas cooling may be important, to the total X-ray luminosities amounts to 44% and 15% for the SCC and WCC clusters, respectively. We find that after excising the cooling region, the scatter in the L X −T vir relation drops from 45.4% to 39.1%, implying that the cooling region contributes ∼27% to the overall scatter. The remaining scatter is largely due to the NCCs. Lastly, the statistical completeness of the sample allows us to quantify and correct for selection effects individually for the subsamples. We find the true SCC fraction to be 25% lower than the observed one and the true normalizations of the L X −T vir relations to be lower by 12%, 7%, and 17% for SCC, WCC, and NCC clusters, respectively.

Brightest cluster galaxies (BCGs) in the cores of galaxy clusters have distinctly different properties from other low-redshift massive ellipticals. The majority of the BCGs in coolcore clusters show signs of active star formation. We present observations of NGC 4696, the BCG of the Centaurus galaxy cluster, at far-infrared (FIR) wavelengths with the Herschel space telescope. Using the PACS spectrometer, we detect the two strongest coolants of the interstellar medium, [C II] We also detect dust emission using the PACS and SPIRE photometers at all six wavebands. We perform a detailed spectral energy distribution fitting using a two-component modified blackbody function and find a cold 19-K dust component with mass 1.6 × 10 6 M and a warm 46-K dust component with mass 4.0 × 10 3 M . The total FIR luminosity between 8 and 1000 μm is 7.5 × 10 8 L , which using Kennicutt relation yields a low star formation rate of 0.13 M yr −1 . This value is consistent with values derived from other tracers, such as ultraviolet emission. Combining the spectroscopic and photometric results together with optical Hα, we model emitting clouds consisting of photodissociation regions adjacent to ionized regions. We show that in addition to old and young stellar populations, there is anotherHerschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.

We present multi-frequency observations of the radio galaxy Hydra-A (3C218) located in the core of a massive, X-ray luminous galaxy cluster. IFU spectroscopy is used to trace the kinematics of the ionised and warm molecular hydrogen which are consistent with a ∼ 5 kpc rotating disc. Broad, double-peaked lines of CO(2-1), [CII]157µm and [OI]63µm are detected. We estimate the mass of the cold gas within the disc to be M gas = 2.3 ± 0.3 × 10 9 M ⊙ . These observations demonstrate that the complex line profiles found in the cold atomic and molecular gas are related to the rotating disc or ring of gas. Finally, an HST image of the galaxy shows that this gas disc contains a substantial mass of dust. The large gas mass, SFR and kinematics are consistent with the levels of gas cooling from the ICM. We conclude that the cold gas originates from the continual quiescent accumulation of cooled ICM gas. The rotation is in a plane perpendicular to the projected orientation of the radio jets and ICM cavities hinting at a possible connection between the kpc-scale cooling gas and the accretion of material onto the black hole. We discuss the implications of these observations for models of cold accretion, AGN feedback and cooling flows.

We present Herschel observations of the core of the Perseus cluster of galaxies. Especially intriguing is the network of filaments that surround the brightest cluster galaxy, NGC 1275, previously imaged extensively in Hα and CO. In this work, we report detections of farinfrared (FIR) lines, in particular, [C II] 158, [O I] 63, [N II] 122, [O IB] 145 and [O III] 88 µm, with Herschel. All lines are spatially extended, except [O III], with the [C II] line emission extending up to 25 kpc from the core. [C II] emission is found to be co-spatial with Hα and CO. Furthermore, [C II] shows a similar velocity distribution to CO, which has been shown in previous studies to display a close association with the Hα kinematics. The spatial and kinematical correlation among [C II], Hα and CO gives us confidence to model the different components of the gas with a common heating model.With the help of FIR continuum Herschel measurements, together with a suite of coeval radio, sub-millimetre and IR data from other observatories, we performed a spectral energy distribution fitting of NGC 1275 using a model that contains contributions from dust emission as well as synchrotron active galactic nucleus emission. This has allowed us to accurately estimate the dust parameters. The data indicate a low dust emissivity index, β ≈ 1, a total dust mass close to 10 7 M , a cold dust component with temperature 38 ± 2 K and a warm dust component with temperature 116 ± 9 K. The FIR-derived star formation rate is 24 ± 1 M yr −1 , which is in agreement with the far-ultraviolet-derived star formation rate in the core, determined after applying corrections for both Galactic and internal reddening. The total IR luminosity in the range 8-1000 µm is inferred to be 1.5 × 10 11 L , making NGC 1275 a luminous IR galaxy.Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.