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Analysis and Interpretation of Hard X-Ray Emission from the Bullet Cluster (1ES 0657-558), the Most Distant Cluster of Galaxies Observed by RXTE
Evidence for nonthermal activity in clusters of galaxies is wellestablished from radio observations of synchrotron emission byrelativistic electrons. New windows in the extreme-ultraviolet and hardX-ray ranges have provided more powerful tools for the investigation ofthis phenomenon. Detections of hard X-rays in the 20-100 keV range havebeen reported for several clusters of galaxies, notably from Coma andothers. Based on these earlier observations we identified the relativelyhigh-redshift cluster 1ES 0657-558 (also known as RX J0658-5557) as agood candidate for hard X-ray observations. This cluster, also known asthe Bullet Cluster, has many other interesting and unusual features,most notably that it is undergoing a merger, clearly visible in theX-ray images. Here we present results from successful RXTE observationsof this cluster. We summarize past observations and their theoreticalinterpretation that guided us in the selection process. We describe thenew observations and present the constraints we can set on the flux andspectrum of the hard X-rays. Finally, we discuss the constraints one canset on the characteristics of accelerated electrons that produce thehard X-rays and the radio radiation.
| Scaling Mass Profiles around Elliptical Galaxies Observed with Chandra and XMM-Newton
We investigated the dynamical structure of 53 elliptical galaxies usingthe Chandra archival X-ray data. In X-ray-luminous galaxies, temperatureincreases with radius and gas density is systematically higher at theoptical outskirts, indicating the presence of a significant amount ofthe group-scale hot gas. In contrast, X-ray-dim galaxies show a flat ordeclining temperature profile against radius and the gas density isrelatively lower at the optical outskirts. Thus, it is found thatX-ray-bright and faint elliptical galaxies are clearly distinguished bythe temperature and gas density profile. The mass profile is well scaledby a virial radius r200 rather than an optical half-radiusre, is quite similar at (0.001-0.03)r200 betweenX-ray-luminous and dim galaxies, and smoothly connects to those profilesof clusters of galaxies. At the inner region of(0.001-0.01)r200 or (0.1-1)re, the mass profilewell traces a stellar mass with a constant mass-to-light ratio ofM/LB=3-10 Msolar/Lsolar. TheM/LB ratio of X-ray-bright galaxies rises up steeply beyond0.01r200 and thus requires a presence of massive dark matterhalo. From the deprojection analysis combined with the XMM-Newton data,we found that X-ray-dim galaxies NGC 3923, NGC 720, and IC 1459 alsohave a high M/LB ratio of 20-30 at 20 kpc, comparable to thatof X-ray-luminous galaxies. Therefore, dark matter is indicated to becommon in elliptical galaxies; their dark matter distribution, as wellas that of galaxy clusters, almost follows the NFW profile.
| A Soft X-Ray Excess in the A754 Cluster
We have analyzed the Chandra, BeppoSAX, ASCA, and ROSAT PSPCobservations of A754 and report evidence of a soft, diffuse X-raycomponent. A radial analysis shows that it is detected within a regionthat extends out to 8' from the X-ray center and that the emission ishigher in the central region of the cluster. Fitting a thermal model tothe combined BeppoSAX and PSPC spectra show excess emission below 1 keVin the PSPC and above 100 keV in the BeppoSAX PDS. The source, 26W20, isin the field of view of the PDS. The addition of a power law, with thespectral parameters measured by Silverman et al. in 1998 for 26W20,successfully models the hard component in the PDS. The excess softemission can be attributed to a low-temperature, 0.77-1.21 keV,component. The soft excess is also modeled with a power law, althoughthe 90% uncertainty for the normalization of the power law is consistentwith zero. Either component added to a hot thermal component provides astatistically significant improvement over a single hot thermalcomponent. The Chandra temperature map provides a detailed descriptionof the thermal state of the gas on a scale of 100 kpc and larger anddoes not show any region cooler than 5.9 keV (90% confidence) within theregion where the cool component was detected. Calculations of theexpected emission from one or more groups randomly embedded in a hot gascomponent were performed that demonstrate that groups are a plausiblesource of ~1 keV emission, in that they can match the measuredcool-component luminosity without violating the spatial temperatureconstraints provided by the temperature map. The cool component iscentrally peaked in the cluster, and the gas density and temperature arerelatively high, arguing against the warm hot intergalactic medium asthe source of the X-ray emission. Furthermore, because the coolcomponent is centrally peaked, the groups are likely embedded in theintracluster gas, rather than in the intercluster gas. The typical X-rayemission from early-type galaxies is not high enough to provide thetotal cool-component luminosity, 2.1×1043 ergss-1. The peak of the cool component is located between thelow-frequency radio halos, thus arguing against a nonthermalinterpretation for the emission based on the synchrotron inverse Comptonmodel, which requires that the nonthermal X-ray and radio emission becospatial. Thus, we conclude that emission from embedded groups is themost likely origin of the cool component in A754.
| Diffuse Nonthermal X-Ray Emission: Evidence of Cosmic-Ray Acceleration at the Shock Front in IC 1262
We report the first localization of diffuse, nonthermal, X-ray emissionfrom a nearby galaxy cluster. Using Chandra data, we have isolated adiffuse nonthermal X-ray component with a photon index ofΓX=2.21+0.14-0.15 and a flux of9.5+1.1-2.5×10-5 photonscm-2 s-1 keV-1 at 1 keV that extendsfrom ~1.5′ to ~2.5′ to the south of the X-ray flux peak.Comparison with simulations implies that the diffuse nonthermal emissionis produced by primary electrons, accelerated at shocks to relativisticvelocities. Using these results and the flux and hardness maps producedwith data from the Chandra Advanced CCD Imaging Spectrometer, weconclude that a smaller subclump coming from the north merged with IC1262. The offset of the cD galaxy from the X-ray peak and large peculiarvelocity indicate that the subclump's impact parameter was to the westand on the near side of IC 1262.
| A BeppoSAX Observation of the IC 1262 Galaxy Cluster
We present an analysis of BeppoSAX observations of the IC 1262 galaxycluster and report the first temperature and abundance measurements,along with preliminary indications of diffuse, nonthermal emission. Byfitting a 6' (~360 h-150 kpc) region with a singleMewe-Kaastra-Liedahl model with photoelectric absorption, we find atemperature of 2.1-2.3 keV and an abundance of 0.45-0.77 (both 90%confidence). We find that the addition of a power-law component providesa statistically significant improvement (F-test=90%) to the fit. Theaddition of a second thermal component also improves the fit, but weargue that it is physically implausible. The power-law component has aphoton index (ΓX) of 0.4-2.8 and a nonthermal flux of(4.1-56.7)×10-5 photons cm-2 s-1over the 1.5-10.5 keV range in the Medium Energy ConcentratorSpectrometer detector. An unidentified X-ray source found in the ROSATHigh Resolution Imager observation (~0.9′ from the center of thecluster) is a possible explanation for the nonthermal flux; however,additional evidence of diffuse, nonthermal emission comes from the NRAOVLA Sky Survey and the Westerbork Northern Sky Survey radiomeasurements, in which excess diffuse radio flux is observed afterpoint-source subtraction. The radio excess can be fitted to a simplepower law with a spectral index of ~1.8, which is consistent with thenonthermal X-ray emission spectral index. The steep spectrum is typicalof diffuse emission, and the size of the radio source implies that it islarger than the cD galaxy and not due to a discreet source.
| The peculiar cooling flow cluster RX J0820.9+0752
We present observations of the cluster of galaxies associated with theX-ray source RX J0820.9+0752 and its dramatic central cluster galaxy inthe optical and X-ray wavebands. Unlike other cooling flow centralcluster galaxies studied in detail, this system does not contain apowerful radio source at its core, and so provides us with an importantexample of where we expect to see only the processes caused directly bythe cooling flow itself. A 9.4-ks Chandra observation shows that the hotintracluster gas is cooling within a radius of 20 kpc at a rate of a fewtens of solar masses per year. The temperature profile is typical of acooling flow cluster and drops to below 1.8 keV in the core. Opticalimages taken with the AAT and Hubble Space Telescope show that thecentral galaxy is embedded in a luminous (LHα~ 5× 1042 erg s-1), extended line-emittingnebula that coincides spatially with a bright excess of X-ray emission,and separate, off-nucleus clumps of blue continuum that form part of apatchy structure arcing away from the main galaxy. The X-ray/Hαfeature is reminiscent of the 40-kpc long filament observed in A1795which is suggested to be a cooling wake, produced by the motion of thecentral cluster galaxy through the intracluster medium. We presentoptical spectra of the central cluster galaxy and its surroundings, andfind that the continuum blobs show stronger line emission, differingkinematic properties and more extreme ionization ratios than thesurrounding nebula. Accounting for the strong intrinsic reddening andits significant variation over the extent of the line-emitting region,we have fitted the continuum spectra of the blobs and the nucleus usingempirical stellar spectra from a library. We found that continuumemission from early main-sequence stars can account for the blue excesslight in the blobs. Kinematical properties associate the gas in thesystem with a nearby secondary galaxy, suggesting some kind of tidalinteraction between the two. We suggest that the secondary galaxy hasmoved through the cooling wake produced by the central cluster galaxy,dragging some of the gas out of the wake and triggering the starburstsfound in the blobs.
| Peculiar properties of 2 unusually X-ray bright early type galaxies
Two X-ray bright early type galaxies, selected for their unusually highX-ray to optical flux ratio, have been observed at high spatialresolution with the ROSAT HRI. Both sources are clearly extended (>250 kpc in radius), thus excluding a nuclear origin of the high X-rayemission. A small group of galaxies observed around IC 1262 could be themost natural explanation for the high X-ray luminosity observed. NGC6159 could be similarly explained, although there are to date nospectroscopic confirmations of a group of galaxies associated with it.The X-ray properties of these two sources are discussed in the contextof the properties of groups of galaxies. A peculiar very bright feature,in the shape of an arc, is detected at the center of the IC 1262 image.This could be the signature of a recent merger, or of peculiar highvelocities in the group.
| The peculiar motions of early-type galaxies in two distant regions - II. The spectroscopic data
We present the spectroscopic data for the galaxies studied in the EFARproject, which is designed to measure the properties and peculiarmotions of early-type galaxies in two distant regions. We have obtained1319 spectra of 714 early-type galaxies over 33 observing runs on 10different telescopes. We describe the observations and data reductionsused to measure redshifts, velocity dispersions and the Mgb and Mg_2Lick linestrength indices. Detailed simulations and intercomparison ofthe large number of repeat observations lead to reliable error estimatesfor all quantities. The measurements from different observing runs arecalibrated to a common zero-point or scale before being combined,yielding a total of 706 redshifts, 676 velocity dispersions, 676 Mgblinestrengths and 582 Mg_2 linestrengths. The median estimated errors inthe combined measurements are Delta cz=20 km s^-1, Delta sigma sigma=9.1 per cent, Delta Mgb Mgb=7.2 per cent and Delta Mg_2=0.015 mag.Comparison of our measurements with published data sets shows nosystematic errors in the redshifts or velocity dispersions, and onlysmall zero-point corrections to bring our linestrengths on to thestandard Lick system. We have assigned galaxies to physical clusters byexamining the line-of-sight velocity distributions based on EFAR andZCAT redshifts, together with the projected distributions on the sky. Wederive mean redshifts and velocity dispersions for these clusters, whichwill be used in estimating distances and peculiar velocities and to testfor trends in the galaxy population with cluster mass. The spectroscopicparameters presented here for 706 galaxies combine high-quality data,uniform reduction and measurement procedures, and detailed erroranalysis. They form the largest single set of velocity dispersions andlinestrengths for early-type galaxies published to date.
| Arcsecond Positions of UGC Galaxies
We present accurate B1950 and J2000 positions for all confirmed galaxiesin the Uppsala General Catalog (UGC). The positions were measuredvisually from Digitized Sky Survey images with rms uncertaintiesσ<=[(1.2")2+(θ/100)2]1/2,where θ is the major-axis diameter. We compared each galaxymeasured with the original UGC description to ensure high reliability.The full position list is available in the electronic version only.
| Total magnitude, radius, colour indices, colour gradients and photometric type of galaxies
We present a catalogue of aperture photometry of galaxies, in UBVRI,assembled from three different origins: (i) an update of the catalogueof Buta et al. (1995) (ii) published photometric profiles and (iii)aperture photometry performed on CCD images. We explored different setsof growth curves to fit these data: (i) The Sersic law, (ii) The net ofgrowth curves used for the preparation of the RC3 and (iii) A linearinterpolation between the de Vaucouleurs (r(1/4) ) and exponential laws.Finally we adopted the latter solution. Fitting these growth curves, wederive (1) the total magnitude, (2) the effective radius, (3) the colourindices and (4) gradients and (5) the photometric type of 5169 galaxies.The photometric type is defined to statistically match the revisedmorphologic type and parametrizes the shape of the growth curve. It iscoded from -9, for very concentrated galaxies, to +10, for diffusegalaxies. Based in part on observations collected at the Haute-ProvenceObservatory.
| The peculiar motions of early-type galaxies in two distant regions. III - The photometric data
We present R-band CCD photometry for 776 galaxies observed in the EFARproject. The photometry is compared with photoelectric data, showingthat a common zero-point good to better than 1 per cent and a precisionof 0.03 mag per zero-point have been achieved. We give the circularlyaveraged surface brightness profiles and the photometric parameters ofthe 762 program galaxies, D(n) diameters, half-luminosity radii, totalmagnitudes, and average effective surface brightnesses. More than 80percent of the profiles have a global S/N ratio larger than 300. Theextrapolation needed to derive total magnitudes is less than 10 percentfor 80 percent of the fits. More than 80 percent of the galaxies havemean effective surface brightness larger than the observed skybrightness. In 90 percent of the profiles the estimate of thecontamination of the sky by the galaxy light is less than 1 percent. Wederive total magnitudes and half-luminosity radii to better than 0.15mag and 25 percent, respectively, for 90 percent of our sample. Incontrast, external comparisons show that data in the literature can bestrongly affected by systematic errors due to large extrapolations,small radial range, sky subtraction errors, seeing effects, and the useof a simple R exp 1/4 fit. The resulting errors can easily amount tomore than 0.5 mag in the total magnitudes and 50 percent in thehalf-luminosity radii.
| The Peculiar Motions of Early-Type Galaxies in Two Distant Regions. I. Cluster and Galaxy Selection
The EFAR project is a study of 736 candidate elliptical galaxies in 84clusters lying in two regions, toward Hercules-Corona Borealis andPerseus-Pisces-Cetus, at distances cz ~ 6000-15,000 km s^-1^. In thispaper (the first of a series), we present an introduction to the EFARproject and describe in detail the selection of the clusters andgalaxies in our sample. Fundamental data for the galaxies and clustersare given, including accurate new positions for each galaxy andredshifts for each cluster. The galaxy selection functions aredetermined by using diameters measured from Schmidt sky survey imagesfor 2185 galaxies in the cluster fields. Future papers in this serieswill present the spectroscopic and photometric observations of thissample, investigate the properties of the fundamental plane forelliptical galaxies, and determine the large- scale peculiar velocityfields in these two regions of the universe.
| Large-Scale Structure at Low Galactic Latitude
We have extended the CfA Redshift Survey to low galactic latitudes toinvestigate the relation between the Great Wall in the North GalacticCap and the Perseus-Pisces chain in the South Galactic Cap. We presentredshifts for 2020 galaxies in the Catalogue of Galaxies and of Clustersof Galaxies (Zwicky et al. 1961-68, CGCG) in the following regions: 4^h^<= α <= 8^h^, 17^h^ <= α <= 20^h^, 0^deg^ <=δ <= 45^deg^. In these regions, the redshift catalogue includes1664 galaxies with B(0) <= 15.5 (of which 820 are newly measured) andis 97% complete. We also include redshifts for an additional 356galaxies in these regions with B(O) > 15.5; of these, 148 werepreviously unmeasured. The CGCG samples the galaxy distribution down tob_II_ = 10^deg^. In this paper, we discuss the acquisition and reductionof the spectra, and we examine the qualitative features of the redshiftdistribution. The Great Wall and the Perseus-Pisces chain are not simplyconnected across the Zone of Avoidance. These structures, which at firstappear to be coherent on scales of ~100 h^-1^ Mpc or more, actually formthe boundaries of neighboring voids of considerably smaller scale,approximately 50h^-1^ Mpc. The structures delineated by ouroptically-selected sample are qualitatively similar to those detected bythe far-infrared-selected IRAS 1.2 Jansky Survey (Fisher et al. 1995).Although the IRAS survey probes more deeply into the Zone of Avoidance,our optically-selected survey provides better sampling of structures atb_II_ >= 10^deg^.
| An image database. II. Catalogue between δ=-30deg and δ=70deg.
A preliminary list of 68.040 galaxies was built from extraction of35.841 digitized images of the Palomar Sky Survey (Paper I). For eachgalaxy, the basic parameters are obtained: coordinates, diameter, axisratio, total magnitude, position angle. On this preliminary list, weapply severe selection rules to get a catalog of 28.000 galaxies, wellidentified and well documented. For each parameter, a comparison is madewith standard measurements. The accuracy of the raw photometricparameters is quite good despite of the simplicity of the method.Without any local correction, the standard error on the total magnitudeis about 0.5 magnitude up to a total magnitude of B_T_=17. Significantsecondary effects are detected concerning the magnitudes: distance toplate center effect and air-mass effect.
| Integrated photoelectric magnitudes and color indices of bright galaxies in the Johnson UBV system
The photoelectric total magnitudes and color indices published in theThird Reference Catalogue of Bright Galaxies (RC3) are based on ananalysis of approximately equals 26,000 B, 25,000 B-V, and 17,000 U-Bmultiaperture measurements available up to mid 1987 from nearly 350sources. This paper provides the full details of the analysis andestimates of internal and external errors in the parameters. Thederivation of the parameters is based on techniques described by theVaucouleurs & Corwin (1977) whereby photoelectric multiaperture dataare fitted by mean Hubble-type-dependent curves which describe theintegral of the B-band flux and the typical B-V and U-B integrated colorgradients. A sophisticated analysis of the residuals of thesemeasurements from the curves was made to allow for the random andsystematic errors that effect such data. The result is a homogeneous setof total magnitudes BTA total colors(B-V)T and (U-B)T, and effective colors(B-V)e and (U-B)e for more than 3000 brightgalaxies in RC3.
| Photoelectric and CCD photometry of E and S0 galaxies
We present BR photoelectric photometry for 352 E and S0 galaxies thatare part of a large survey of the properties and peculiar motions ofgalaxies in distant clusters. Repeat measurements show our internalerrors to be 2-3 percent in B and R and 1-2 percent in B-R. Comparisonsof BR and BVR reductions for 10 galaxies also observed in V show smallsystematic errors due to differences between the spectral energydistributions of stars and galaxies. External comparisons with B-Vcolors in the literature confirm that these colors are good to 1percent. We also describe R-band CCD observations for 95 of the galaxiesand place these on a BR photometric system for photoelectric and CCDphotomerry, with a common zero-point good to better than 1 percent. Wefind the rms precision of both our photoelectric and CCD R magnitudes tobe 2-3 percent for galaxies as faint as R = 15.
| Identifications of radio sources with bright galaxies
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1967MNRAS.135..231C&db_key=AST
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