Projection effects in galaxy cluster samples: insights from X-ray redshifts

Up to now, the largest sample of galaxy clusters selected in X-rays comes from the ROSAT All-Sky Survey (RASS). Although there have been many interesting clusters discovered with the RASS data, the broad point spread function (PSF) of the ROSAT satellite limits the amount of spatial information of the detected objects. This leads to the discovery of new cluster features when a re-observation is performed with higher resolution X-ray satellites. Here we present the results from XMM-Newton observations of three clusters: RXCJ2306.6-1319, ZwCl1665 and RXCJ0034.6-0208, for which the observations reveal a double or triple system of extended components. These clusters belong to the extremely expanded HIghest X-ray FLUx Galaxy Cluster Sample (eeHIFLUGCS), which is a flux-limited cluster sample ($f_\textrm{X,500}\geq 5\times10^{-12}$ erg s$^{-1}$ cm$^{-2}$ in the $0.1-2.4$ keV energy band). For each structure in each cluster, we determine the redshift with the X-ray spectrum and find that the components are not part of the same cluster. This is confirmed by an optical spectroscopic analysis of the galaxy members. Therefore, the total number of clusters is actually 7 and not 3. We derive global cluster properties of each extended component. We compare the measured properties to lower-redshift group samples, and find a good agreement. Our flux measurements reveal that only one component of the ZwCl1665 cluster has a flux above the eeHIFLUGCS limit, while the other clusters will no longer be part of the sample. These examples demonstrate that cluster-cluster projections can bias X-ray cluster catalogues and that with high-resolution X-ray follow-up this bias can be corrected

X-Ray Studies of the Abell 3158 Galaxy Cluster with eROSITA

The most nearby clusters are the best places to study physical and enrichment effects in the faint cluster outskirts. A3158 located at z=0.059 is quite extended with a characteristic radius r$_{200}$=23.95 arcmin. In 2019, A3158 was observed as a calibration target in a pointed observation with the eROSITA telescope onboard the SRG mission. We determined 1d temperature, abundance and normalisation profiles from eROSITA and XMM-Newton and Chandra data as well as 2d maps of temperature distribution from eROSITA data. The velocity dispersion was determined and the cluster mass was calculated. The overall temperature was measured to be 4.725$\pm$ 0.035 keV. The profiles of eROSITA all agree on a ~10% level with those determined with XMM-Newton and Chandra data. From the temperature map we see that the cluster lacks a cool core, as noted before. The presence of a previously detected off-centre cool clump West of the central cluster region is observed. Furthermore there is a bow shaped edge near the location of the cool gas clump West of the cluster centre. An extension of gas is detected for the first time further out in the West. The velocity dispersion of the cluster was measured to be 1058$\pm$41 km s$^{-1}$. The total mass was determined as $M_{200}$=1.38$\pm$ 0.25x10$^{15}$ $M_{\odot}$. The mass estimate from the M-T relation is significantly lower at M$_{200}$=5.09$\pm$ 0.59x10$^{14}M_{\odot}$. An extended X-ray source located South of the cluster also coincides with a galaxy overdensity with redshifts in the range 0.05<z<0.07. These are indications that the cluster may be undergoing merger activity. Another extended source located North of the cluster is detected in X-rays and coincides with an overdensity of galaxies with redshifts in the range of 0.070<z<0.077. This is likely a background cluster not related to A3158. Additionally a known SPT cluster at z=0.53 was detected.Comment: 14 pages, 17 figures in the main text, and 3 figures in the appendix. Accepted by A&A for the Special Issue: The Early Data Release of eROSITA and Mikhail Pavlinsky ART-XC on the SRG Missio

EROSITA Spectro-Imaging Analysis of the Abell 3408 Galaxy Cluster

The X-ray telescope eROSITA onboard the newly launched SRG mission serendipitously observed the galaxy cluster A3408 ($z=0.0420$) during the PV observation of the AGN 1H0707-495. Despite its brightness and large extent, it has not been observed by any modern X-ray observatory. A neighbouring cluster in NW direction, A3407 ($z=0.0428$), appears to be close at least in projection ($\sim 1.7$ Mpc). This cluster pair could be in a pre- or post-merger state. We perform a detailed X-ray analysis of A3408. We construct particle background subtracted and exposure corrected images and surface brightness profiles in different sectors. The spectral analysis is performed out to $1.4r_{500}$. Additionally, a temperature map is presented depicting the distribution of the ICM temperature. Furthermore, we make use of data from the RASS to estimate some bulk properties of A3408 and A3407, using the growth curve analysis method and scaling relations. The imaging analysis shows a complex morphology of A3408 with a strong elongation in SE-NW direction. This is quantified by comparing the surface brightness profiles of the NW, SW, SE and NE directions, where the NW and SE directions show a significantly higher surface brightness compared to the other directions. We determine a gas temperature ${\rm k_B}T_{500}=(2.23\pm0.09)$ keV. The T-profile reveals a hot core within $2'$ of the emission peak, ${\rm k_B}T=3.04^{+0.29}_{-0.25}$ keV. Employing a M-T relation, we obtain $M_{500}=(9.27\pm0.75)\times 10^{13}M_{\odot}$ iteratively. The $r_{200}$ of A3407 and A3408 are found to overlap in projection which makes ongoing interactions plausible. The 2d T-map reveals higher temperatures in W than in E direction. A3407 and A3408 are likely in a pre-merger state, affecting the ICM properties, i.e., increased temperatures in the direction of A3407 indicate adiabatic compression or shocks due to the interaction.Comment: 10 pages, 7 figures (main text), 2 figures (appendix). Submitted to A&A for the Special Issue: The Early Data Release of eROSITA and Mikhail Pavlinsky ART-XC on the SRG Missio