Intrinsic scatter of caustic masses and hydrostatic bias: An observational study

All estimates of cluster mass have some intrinsic scatter and perhaps some bias with true mass even in the absence of measurement errors for example caused by cluster triaxiality and large scale structure. Knowledge of the bias and scatter values is fundamental for both cluster cosmology and astrophysics. In this paper we show that the intrinsic scatter of a mass proxy can be constrained by measurements of the gas fraction because masses with higher values of intrinsic scatter with true mass produce more scattered gas fractions. Moreover, the relative bias of two mass estimates can be constrained by comparing the mean gas fraction at the same (nominal) cluster mass. Our observational study addresses the scatter between caustic (i.e., dynamically estimated) and true masses, and the relative bias of caustic and hydrostatic masses. For these purposes, we used the X-ray Unbiased Cluster Sample, a cluster sample selected independently from the intracluster medium content with reliable masses: 34 galaxy clusters in the nearby ($0.050<z<0.135$) Universe, mostly with $14<\log M_{500}/M_\odot \lesssim 14.5$, and with caustic masses. We found a 35\% scatter between caustic and true masses. Furthermore, we found that the relative bias between caustic and hydrostatic masses is small, $0.06\pm0.05$ dex, improving upon past measurements. The small scatter found confirms our previous measurements of a highly variable amount of feedback from cluster to cluster, which is the cause of the observed large variety of core-excised X-ray luminosities and gas masses.Comment: A&A, in press, minor language changes from previous versio

Variegate galaxy cluster gas content: Mean fraction, scatter, selection effects and covariance with X-ray luminosity

We use a cluster sample selected independently of the intracluster medium content with reliable masses to measure the mean gas mass fraction and its scatter, the biases of the X-ray selection on gas mass fraction, and the covariance between the X-ray luminosity and gas mass. The sample is formed by 34 galaxy clusters in the nearby ($0.050<z<0.135$) Universe, mostly with $14<\log M_{500}/M_\odot \lesssim 14.5$, and with masses calculated with the caustic technique. First, we found that integrated gas density profiles have similar shapes, extending earlier results based on subpopulations of clusters such as those that are relaxed or X-ray bright for their mass. Second, the X-ray unbiased selection of our sample allows us to unveil a variegate population of clusters; the gas mass fraction shows a scatter of $0.17\pm0.04$ dex, possibly indicating a quite variable amount of feedback from cluster to cluster, which is larger than is found in previous samples targeting subpopulations of galaxy clusters, such as relaxed or X-ray bright clusters. The similarity of the gas density profiles induces an almost scatterless relation between X-ray luminosity, gas mass, and halo mass, and modulates selection effects in the halo gas mass fraction: gas-rich clusters are preferentially included in X-ray selected samples. The almost scatterless relation also fixes the relative scatters and slopes of the $L_X-M$ and $M_{gas}-M$ relations and makes core-excised X-ray luminosities and gas masses fully covariant. Therefore, cosmological or astrophysical studies involving X-ray or SZ selected samples need to account for both selection effects and covariance of the studied quantities with X-ray luminosity/SZ strength.Comment: A&A, in press, minor language changes from previous versio

Evidence of unrelaxed IGM around IC1262

AIMS: A peculiar morphology of the hot gas was discovered at the center of IC1262 with the ROSAT HRI. Sensitive Chandra and XMM-Newton data were requested to investigate the characteristics of this structure to understand its nature. METHODS: We have exploited the high resolution and sensitivity of Chandra's ACIS-S to investigate the peculiar morphology and spectral characteristics of hot gas in the group around IC1262. XMM-Newton data are only partially usable due to very heavy high background contamination, but they are useful to confirm and strengthen the results from Chandra. RESULTS: The Chandra data show a quite dramatic view of the \object{IC1262} system: a sharp discontinuity east of the central galaxy, with steep drops and a relatively narrow feature over 100 kpc long, plus an arc/loop to the N, are all indicative of a turmoil in the high energy component. Their morphologies could suggest them to be tracers of shocked material caused either by peculiar motions in the system or by a recent merger process, but the spectral characteristics indicate that the structure is cooler than its surroundings. The lack of evidence of significant structures in the velocity distribution of the group members and the estimated scale of the phenomenon make the interpretation of its physical nature challenging. We review a few possible interpretations, in light of similar phenomena observed in clusters and groups. The ram pressure stripping of a bright spiral galaxy, now near the center of the group, is a promising interpretation for most of the features observed. The relation with the radio activity requires a better sampling of the radio parameters that can only be achieved with deeper and higher resolution observations.Comment: Accepted for pubblication in Astronomy & Astrophysics. Figs 1, 2, 3 and 9-12 are given as JPEG files due to the restrictions on space available on astro-p

Diffuse Gas and LMXBs in the Chandra Observation of the S0 Galaxy NGC 1553

We have spatially and spectrally resolved the sources of X-ray emission from the X-ray faint S0 galaxy NGC 1553 using an observation from the Chandra X-ray Observatory. The majority (70%) of the emission in the 0.3 - 10.0 keV band is diffuse, and the remaining 30% is resolved into 49 discrete sources. Most of the discrete sources associated with the galaxy appear to be low mass X-ray binaries (LMXBs). The luminosity function of the LMXB sources is well-fit by a broken power-law with a break luminosity comparable to the Eddington luminosity for a 1.4 solar mass neutron star. It is likely that those sources with luminosities above the break are accreting black holes and those below are mostly neutron stars in binary systems. Spectra were extracted for the total emission, diffuse emission, and sum of the resolved sources; the spectral fits for all require a model including both a soft and hard component. The diffuse emission is predominately soft while the emission from the sources is mostly hard. Approximately 24% of the diffuse emission arises from unresolved LMXBs, with the remainder resulting from thermal emission from hot gas. There is a very bright source at the projected position of the nucleus of the galaxy. The spectrum and luminosity derived from this central source are consistent with it being an AGN; the galaxy also is a weak radio source. Finally, the diffuse emission exhibits significant substructure with an intriguing spiral feature passing through the center of the galaxy. The X-ray spectrum and surface brightness of the spiral feature are consistent with adiabatic or shock compression of ambient gas, but not with cooling. This feature may be due to compression of the hot interstellar gas by radio lobes or jets associated with the AGN.Comment: 23 pages using emulateapj.sty; ApJ, in press; revised version includes correction to error in the L_X,src/L_B ratio as well as other revision