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Register a new userNew X-ray Clusters in the EMSS II: Optical Properties
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Authors
Aaron D. Lewis
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We present optical images for 9 new clusters of galaxies we have found in a reanalysis of the Einstein IPC images comprising the Extended Medium Sensitivity Survey (EMSS). Based on the presence of a red sequence of galaxies in a color-magnitude (CM) diagram, a redshift is estimated for each cluster. Galaxy overdensities (cluster richnesses) are measured in each field using the B_gc statistic which allows their plausible identification with the X-ray emission. The nature of our X-ray detection algorithm suggests that most of these clusters have low X-ray surface brightness (LSB) compared to the previously known EMSS clusters. We compare the optical and X-ray observations of these clusters with the well-studied Canadian Network for Observational Cosmology (CNOC) subsample of the EMSS, and conclude that the new clusters exhibit a similar range of optical richnesses, X-ray luminosities, and, somewhat surprisingly, galaxy populations as the predominantly rich, relaxed EMSS/CNOC clusters.
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Using a sample of 63 AGNs extracted from the $Einstein$ Extended Medium Sensitivity Survey (EMSS), we study the X-ray spectral properties of X-ray selected AGN in the 0.1$-$2.4 keV ROSAT band. These objects are all the EMSS AGN detected with more than 300 net counts in ROSAT PSPC images available from the public archive (as of May 31, 1995). A Maximum-Likelihood analysis is used to find the mean power-law spectral index $<alpha_p>$ and the intrinsic dispersion $sigma_p$. We find $<alpha_p>$=1.42 with $sigma_p$=0.44. This value is significantly steeper ($Delta alpha sim$0.4) than the mean $Einstein$/IPC spectral index obtained applying the ML analysis on the whole sample of EMSS AGN. This result shows that the soft excess already noted in optically selected AGN is present also in X-ray selected AGN. The relatively high value obtained for the intrinsic dispersion confirms that in the soft band AGN are characterized by a variety of spectral indices and the increase with respect to results obtained from the analysis of Einstein data ($Delta sigma_p sim$0.16) suggests a further broadening of the spectral index distribution as one moves to softer energies. A comparison between the mean spectral index of Radio-quiet and Radio-loud subsamples shows that the mean index of the RL sample is flatter than that of RQ, both in the IPC ($Delta alpha sim$0.3) and in the PSPC ($Delta alpha sim$0.4) data. This suggests that the additional X-ray component in RL AGN dominates the X-ray emission of RL AGN over almost two decades of energy ($sim$0.1$-$10 keV).
We present a comparison of optical and X-ray properties of galaxy clusters in the northern sky. We determine the recovery rate of X-ray detected clusters in the optical as a function of richness, redshift and X-ray luminosity, showing that the missed clusters are typically low contrast systems when observed optically. We employ four different statistical tests to test for the presence of substructure using optical two-dimensional data, finding that approximately 35% of the clusters show strong signs of substructure. However, the results are test-dependent, with variations also due to the magnitude range and radius utilized.We have also performed a comparison of X-ray luminosity and temperature with optical galaxy counts (richness). We find that the slope and scatter of the relations between richness and the X-ray properties are heavily dependent on the density contrast of the clusters. The selection of substructure-free systems does not improve the correlation between X-ray luminosity and richness, but this comparison also shows much larger scatter than one obtained using the X-ray temperature. In the latter case, the sample is significantly reduced because temperature measurements are available only for the most massive (and thus high contrast) systems. However, the comparison between temperature and richness is very sensitive to the exclusion of clusters showing signs of substructure. The correlation of X-ray luminosity and richness is based on the largest sample to date ($sim$ 750 clusters), while tests involving temperature use a similar number of objects as previous works ($lsim$100). The results presented here are in good agreement with existing literature.
We use SDSS data to investigate the scaling relations of 127 NoSOCS and 56 CIRS galaxy clusters at low redshift ($z le 0.10$). We show that richness and both optical and X-ray luminosities are reliable mass proxies. The scatter in mass at fixed observable is $sim$ 40%, depending on the aperture, sample and observable considered. For example, for the massive CIRS systems $sigma_{lnM500|N500}$ = 0.33 $pm$ 0.05 and $sigma_{lnM500|Lx}$ = 0.48 $pm$ 0.06. For the full sample $sigma_{lnM500|N500}$ = 0.43 $pm$ 0.03 and $sigma_{lnM500|Lx}$ = 0.56 $pm$ 0.06. We estimate substructure using two and three dimensional optical data, verifying that substructure has no significant effect on the cluster scaling relations (intercepts and slopes), independent of which substructure test we use. For a subset of twenty-one clusters, we estimate masses from the M-T$_X$ relation using temperature measures from BAX. The scaling relations derived from the optical and X-ray masses are indeed very similar, indicating that our method consistently estimates the cluster mass and yields equivalent results regardless of the wavelength from which we measure mass. For massive systems, we represent the mass-richness relation by a function with the form ${rm ln (M_{200}) = A + B times ln(N_{200}/60)}$, with M$_{200}$ being expressed in units of 10$^{14}$ M$_{odot}$. Using the virial mass, for CIRS clusters, we find A = (1.39 $pm$ 0.07) and B = (1.00 $pm$ 0.11). The relations based on the virial mass have a scatter of $sigma_{lnM200|N200}$ = 0.37 $pm$ 0.05, while $sigma_{lnM200|N200}$ = 0.77 $pm$ 0.22 for the caustic mass and $sigma_{lnM200|N200}$ = 0.34 $pm$ 0.08 for the temperature based mass (abridged).
Fifteen ROSAT PSPC observations available in the public archive are analyzed in order to study time and spectral variability of the 12 EMSS AGN detected by ROSAT with more than 2000 net counts. Time variability was investigated on 13 different time scales, ranging from 400 s to 1 year. Of the 12 sources analyzed, only two do not show a significant variability on any time scale. On short time scale about 20 percent of AGN are seen as variable sources while on time scale greater than 100.000 s the fraction becomes greater than 50 percent. However one should bare in mind that the visibility function for variability is far from being uniform and that small amplitude variations can be detected more often on long time scale than on short time scale. Spectral variability was detected in only two sources. MS1158.6-0323 shows an hardening of the spectrum with increasing intensity while MS2254.9-3712 shows a softening of the spectrum with increasing intensity. Finally, for one source (MS1416.3-1257), the observed variability is not due to an intrinsic flux variation but, instead, to a variation in the column density along the line of sight. Since this variability has been observed on a time scale of about 3.9 days, it is probably associated to the broad line clouds.
We present the X-ray properties and scaling relations of a large sample of clusters extracted from the Marenostrum MUltidark SImulations of galaxy Clusters (MUSIC) dataset. We focus on a sub-sample of 179 clusters at redshift z~0.11, with 3.2e14M_sun/h<M_vir<2e15Msun/h, complete in mass. We employed the X-ray photon simulator PHOX to obtain synthetic Chandra Observations and derive observable-like global properties of the intracluster medium (ICM), as X-ray temperature (T_X) and luminosity (L_X). T_X is found to slightly under-estimate the true mass-weighted temperature, although tracing fairly well the cluster total mass. We also study the effects of T_X on scaling relations with cluster intrinsic properties: total (M_500) and gas (M_g500) mass; integrated Compton parameter (Y_SZ) of the Sunyaev-Zeldovich (SZ) thermal effect; Y_X=M_g500 T_X. We confirm that Y_X is a very good mass proxy, with a scatter on M_500-Y_X and Y_SZ-Y_X lower than 5%. The study of scaling relations among X-ray, intrinsic and SZ properties indicates that MUSIC clusters reasonably resemble the self-similar prediction, especially for correlations involving T_X. The observational approach also allows for a more direct comparison with real clusters, from which we find deviations mainly due to the physical description of the ICM, affecting T_X and, particularly, L_X.
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