The eROSITA mission will provide the largest sample of galaxy clusters detected in X-ray to date (one hundred thousand expected). This sample will be used to constrain cosmological models by measuring cluster masses. An important mass proxy is the el
ectron temperature of the hot plasma detected in X-rays. We want to understand the detection properties and possible bias in temperatures due to unresolved substructures in the cluster halos. We simulated a large number of galaxy cluster spectra with known temperature substructures and compared the results from analysing eROSITA simulated observations to earlier results from Chandra. We were able to constrain a bias in cluster temperatures and its impact on cluster masses as well as cosmological parameters derived from the survey. We found temperatures in the eROSITA survey to be biased low by about five per cent due to unresolved temperature substructures (compared to emission-weighted average temperatures from the Chandra maps). This bias would have a significant impact on the eROSITA cosmology constraints if not accounted for in the calibration. We isolated the bias effect that substructures in galaxy clusters have on temperature measurements and their impact on derived cosmological parameters in the eROSITA cluster survey.
We present new Chandra images of the X-ray emission from the core of the Perseus cluster of galaxies. The total observation time is now 1.4 Ms. New depressions in X-ray surface brightness are discovered to the north of NGC1275, which we interpret as
old rising bubbles. They imply that bubbles are long-lived and do not readily breakup when rising in the hot cluster atmosphere. The existence of a 300 kpc long NNW-SSW bubble axis means there cannot be significant transverse large scale flows exceeding 100 km/s. Interesting spatial correlations are seen along that axis in early deep radio maps. A semi-circular cold front about 100 kpc west of the nucleus is seen. It separates an inner disturbed region dominated by the activity of the active nucleus of NGC1275 from the outer region where a subcluster merger dominates.
The brightest galaxy in the nearby Perseus cluster, NGC1275, is surrounded by a network of filaments. These were first observed through their Halpha emission but are now known to have a large molecular component with a total mass approaching 10^11Msu
n of gas. The filaments are embedded in hot intracluster gas and stretch over 80 kpc. They have an unusual low excitation spectrum which is well modelled by collisional heating and ionization by secondary electrons. Here we note that the surface radiative flux from the outer filaments is close to the energy flux impacting on them from particles in the hot gas. We propose that the secondary electrons within the cold filaments, which excite the observed submillimetre through UV emission, are due to the hot surrounding gas efficiently penetrating the cold gas through reconnection diffusion. Some of the soft X-ray emission seen from the filaments is then due to charge exchange, although this is insufficient to account for all the observed X-ray flux. The filaments are complex with multiphase gas. Interpenetration of hot and cold gas leads to the filaments growing in mass, at a rate of up to 100Msunpyr. The lack of soft X-ray cooling emission in cool core clusters is then due to the non-radiative cooling of hot gas on mixing with cold gas around and within the central galaxy.
We present a new Chandra observation of the galaxy cluster Abell 2146 which has revealed a complex merging system with a gas structure that is remarkably similar to the Bullet cluster (eg. Markevitch et al. 2002). The X-ray image and temperature map
show a cool 2-3 keV subcluster with a ram pressure stripped tail of gas just exiting the disrupted 6-7 keV primary cluster. From the sharp jump in the temperature and density of the gas, we determine that the subcluster is preceded by a bow shock with a Mach number M=2.2+/-0.8, corresponding to a velocity v=2200^{+1000}_{-900} km/s relative to the main cluster. We estimate that the subcluster passed through the primary core only 0.1-0.3 Gyr ago. In addition, we observe a slower upstream shock propagating through the outer region of the primary cluster and calculate a Mach number M=1.7+/-0.3. Based on the measured shock Mach numbers M~2 and the strength of the upstream shock, we argue that the mass ratio between the two merging clusters is between 3 and 4 to one. By comparing the Chandra observation with an archival HST observation, we find that a group of galaxies is located in front of the X-ray subcluster core but the brightest cluster galaxy is located immediately behind the X-ray peak.
We examine deep XMM-Newton Reflection Grating Spectrometer (RGS) spectra from the cores of three X-ray bright cool core galaxy clusters, Abell 262, Abell 3581 and HCG 62. Each of the RGS spectra show Fe XVII emission lines indicating the presence of
gas around 0.5 keV. There is no evidence for O VII emission which would imply gas at still cooler temperatures. The range in detected gas temperature in these objects is a factor of 3.7, 5.6 and 2 for Abell 262, Abell 3581 and HCG 62, respectively. The coolest detected gas only has a volume filling fraction of 6 and 3 per cent for Abell 262 and Abell 3581, but is likely to be volume filling in HCG 62. Chandra spatially resolved spectroscopy confirms the low volume filling fractions of the cool gas in Abell 262 and Abell 3581, indicating this cool gas exists as cold blobs. Any volume heating mechanism aiming to prevent cooling would overheat the surroundings of the cool gas by a factor of 4. If the gas is radiatively cooling below 0.5 keV, it is cooling at a rate at least an order of magnitude below that at higher temperatures in Abell 262 and Abell 3581 and two-orders of magnitude lower in HCG 62. The gas may be cooling non-radiatively through mixing in these cool blobs, where the energy released by cooling is emitted in the infrared. We find very good agreement between smooth particle inference modelling of the cluster and conventional spectral fitting. Comparing the temperature distribution from this analysis with that expected in a cooling flow, there appears to be a even larger break below 0.5 keV as compared with previous empirical descriptions of the deviations of cooling flow models.
We examine the core of the X-ray bright galaxy cluster 2A 0335+096 using deep Chandra X-ray imaging and spatially-resolved spectroscopy, and include new radio observations. The set of around eight X-ray bright blobs in the core of the cluster, appear
ing like eggs in a birds nest, contains multiphase gas from ~0.5 to 2 keV. The morphology of the coolest X-ray emitting gas at 0.5 keV temperature is similar to the Halpha emitting nebula known in this cluster, which surrounds the central galaxy. XMM-Newton grating spectra confirm the presence of material at these temperatures, showing excellent agreement with Chandra emission measures. On scales of 80 to 250 kpc there is a low temperature, high metallicity, swirl of intracluster medium as seen in other clusters. In the core we find evidence for a further three X-ray cavities, in addition to the two previously discovered. Enhancements in 1.5 GHz radio emission are correlated with the X-ray cavities. The total 4PV enthalpy associated with the cavities is around 5x10^59 erg. This energy would be enough to heat the cooling region for ~5x10^7 yr. We find a maximum pressure discontinuity of 26 per cent (2 sigma) across the surface brightness edge to the south-west of the cluster core. This corresponds to an upper limit on the Mach number of the cool core with respect to its surroundings of 0.55.
We present results from deep Chandra and XMM-Newton observations of the relaxed X-ray luminous galaxy cluster Abell 2204. We detect metallicity inhomogeneities in the intracluster medium on a variety of distance scales, from a ~12 kpc enhancement con
taining a few times 10^7 Msun of iron in the centre, to a region at 400 kpc radius with an excess of a few times 10^9 Msun. Subtracting an average surface brightness profile from the X-ray image yields two surface brightness depressions to the north and south of the cluster. Their morphology is similar to the cavities observed in cluster cores, but they have radii of 240 kpc and 160 kpc and have a total enthalpy of 2x10^62 erg. If they are fossil radio bubbles, their buoyancy timescales imply a total mechanical heating power of 5x10^46 erg/s, the largest such bubble heating power known. More likely, they result from the accumulation of many past bubbles. Energetically this is more feasible, as the enthalpy of these regions could combat X-ray cooling in this cluster to 500 kpc radius for around 2 Gyr. The core of the cluster also contains five to seven ~4 kpc radius surface brightness depressions that are not associated with the observed radio emission. If they are bubbles generated by the nucleus, they are too small to balance cooling in the core by an order of magnitude. However if the radio axis is close to the line of sight, projection effects may mask more normal bubbles. Using RGS spectra we detect a FeXVII line. Spectral fitting reveals temperatures down to ~0.7 keV; the cluster therefore shows a range in X-ray temperature of at least a factor of 15. The quantity of low temperature gas is consistent with a mass deposition rate of 65 Msun/yr.
