Cluster Mass Estimate and a Cusp of the Mass Density Distribution in Clusters of Galaxies

We study density cusps in the center of clusters of galaxies to reconcile X-ray mass estimates with gravitational lensing masses. For various mass density models with cusps we compute X-ray surface brightness distribution, and fit them to observations to measure the range of parameters in the density models. The Einstein radii estimated from these density models are compared with Einstein radii derived from the observed arcs for Abell 2163, Abell 2218, and RX J1347.5-1145. The X-ray masses and lensing masses corresponding to these Einstein radii are also compared. While steeper cusps give smaller ratios of lensing mass to X-ray mass, the X-ray surface brightnesses estimated from flatter cusps are better fits to the observations. For Abell 2163 and Abell 2218, although the isothermal sphere with a finite core cannot produce giant arc images, a density model with a central cusp can produce a finite Einstein radius, which is smaller than the observed radii. We find that a total mass density profile which declines as $\sim r^{-1.4}$ produces the largest radius in models which are consistent with the X-ray surface brightness profile. As the result, the extremely large ratio of the lensing mass to the X-ray mass is improved from 2.2 to 1.4 for Abell 2163, and from 3 to 2.4 for Abell 2218. For RX J1347.5-1145, which is a cooling flow cluster, we cannot reduce the mass discrepancy.Comment: 23 pages, 10 figures, Latex, uses aasms4.sty, accepted for publication in ApJ, Part

Discovery of X-ray emission rom the distant lensing cluster of galaxies CL2236-04 at z = 0.552

X-ray emission from the distant lensing cluster CL2236-04 at $z$ = 0.552 was discovered by ASCA and ROSAT/HRI observations. If the spherical symmetric mass distribution model of the cluster is assumed, the lensing estimate of the cluster mass is a factor of two higher than that obtained from X-ray observations as reported for many distant clusters. However, the elliptical and clumpy lens model proposed by Kneib et al.(1993) is surprisingly consistent with the X-ray observations assuming that the X-ray emitting hot gas is isothermal and in a hydrostatic equilibrium state. The existence of the cooling flow in the central region of the cluster is indicated by the short central cooling time and the excess flux detected by ROSAT/HRI compared to the ASCA flux. However, it is shown that even if the AXJ2239-0429 has a cooling flow in the central region, the temperature measured by ASCA which is the mean emission-weighted cluster temperature in this case, should not be cooler than and different from the virial temperature of the cluster. Therefore, we conclude that the effect of the clumpiness and non-zero ellipticity in the mass distribution of the cluster are essential to explain the observed feature of the giant luminous arc, and there is no discrepancy between strong lensing and X-ray estimation of the mass of the cluster in this cluster.Comment: 18 pages, including 4 postscripts figs, LaTex. To appear in Part 1 of The Astrophysical Journa

Dark Matter and Baryon Fraction at the Virial Radius in Abell 2256

We combine ASCA and ROSAT X-ray data to constrain the radial dark matter distribution in the primary cluster of A2256, free from the isothermality assumption. Both instruments indicate that the temperature declines with radius. The region including the central galaxy has a multicomponent spectrum, which results in a wide range of allowed central temperatures. We find that the secondary subcluster has a temperature and luminosity typical of a rich cluster; however, the ASCA temperature map shows no signs of an advanced merger. It is therefore assumed that the primary cluster is in hydrostatic equilibrium. The data then require dark matter density profiles steeper than rho ~ r^-2.5 in its outer part. Acceptable models have a total mass within r=1.5 Mpc (the virial radius) of 6.0+-1.5 10^14 Msun at the 90% confidence, about 1.6 times smaller than the mass derived assuming isothermality. Near the center, dark matter profiles with and without central cusps are consistent with the data. Total mass inside the X-ray core (r=0.26 Mpc) is 1.28+-0.08 10^14 Msun, which exceeds the isothermal value by a factor of 1.4. Although the confidence intervals above may be underestimates since they do not include possible asymmetry and departures from hydrostatic equilibrium, the behavior of the mass distribution, if applicable to other clusters, can bring into better agreement X-ray and lensing mass estimates, but aggravate the ``baryon catastrophe''. The observed considerable increase in the gas content with radius, not anticipated by simulations, may imply that a significant fraction of thermal gas energy comes from sources other than gravity and merger shocks.Comment: Added dynamic argument against advanced merger. Latex, 10 pages, 3 figures; uses emulateapj.sty. ApJ in pres

DAC-Less amplifier-less generation and transmission of QAM signals using sub-volt silicon-organic hybrid modulators

We demonstrate generation and transmission of optical signals by directly interfacing highly efficient silicon-organic hybrid (SOH) modulators to binary output ports of a field-programmable gate array. Using an SOH Mach-Zehnder modulator (MZM) and an SOH IQ modulator we generate ON-OFF- keying and binary phase-shift keying signals as well as quadrature phase-shift keying and 16-state quadrature amplitude modulation (16QAM) formats. Peak-to-peak voltages amount to only 0.27 V-pp for driving the MZM and 0.41 V-pp for the IQ modulator. Neither digital-to-analog converters nor drive amplifiers are required, and the RF energy consumption in the modulator amounts to record-low 18 fJ/bit for 16QAM signaling

Process tomography of ion trap quantum gates

A crucial building block for quantum information processing with trapped ions is a controlled-NOT quantum gate. In this paper, two different sequences of laser pulses implementing such a gate operation are analyzed using quantum process tomography. Fidelities of up to 92.6(6)% are achieved for single gate operations and up to 83.4(8)% for two concatenated gate operations. By process tomography we assess the performance of the gates for different experimental realizations and demonstrate the advantage of amplitude--shaped laser pulses over simple square pulses. We also investigate whether the performance of concatenated gates can be inferred from the analysis of the single gates

3C 295, a cluster and its cooling flow at z=0.46

We present ROSAT HRI data of the distant and X-ray luminous (L_x(bol)=2.6^ {+0.4}_{-0.2} 10^{45}erg/sec) cluster of galaxies 3C 295. We fit both a one-dimensional and a two-dimensional isothermal beta-model to the data, the latter one taking into account the effects of the point spread function (PSF). For the error analysis of the parameters of the two-dimensional model we introduce a Monte-Carlo technique. Applying a substructure analysis, by subtracting a cluster model from the data, we find no evidence for a merger, but we see a decrement in emission South-East of the center of the cluster, which might be due to absorption. We confirm previous results by Henry & Henriksen(1986) that 3C 295 hosts a cooling flow. The equations for the simple and idealized cooling flow analysis presented here are solely based on the isothermal beta-model, which fits the data very well, including the center of the cluster. We determine a cooling flow radius of 60-120kpc and mass accretion rates of dot{M}=400-900 Msun/y, depending on the applied model and temperature profile. We also investigate the effects of the ROSAT PSF on our estimate of dot{M}, which tends to lead to a small overestimate of this quantity if not taken into account. This increase of dot{M} (10-25%) can be explained by a shallower gravitational potential inferred by the broader overall profile caused by the PSF, which diminishes the efficiency of mass accretion. We also determine the total mass of the cluster using the hydrostatic approach. At a radius of 2.1 Mpc, we estimate the total mass of the cluster (M{tot}) to be (9.2 +/- 2.7) 10^{14}Msun. For the gas to total mass ratio we get M{gas}/M{tot} =0.17-0.31, in very good agreement with the results for other clusters of galaxies, giving strong evidence for a low density universe.Comment: 26 pages, 7 figures, accepted for publication in Ap

Stripped Spiral Galaxies as Promising Targets for the Determination of the Cepheid distance to the Virgo Cluster

The measurement of precise galaxy distances by Cepheid observations out to the distance of the Virgo cluster is important for the determination of the Hubble constant ($H_0$). The Virgo cluster is thereby often used as an important stepping stone. The first HST measurement of the distance of a Virgo galaxy (M100) using Cepheid variables provided a value for $H_0=80(\pm 17)$ km/s/Mpc (Freedman et al. 1994). This measurement was preceeded by a ground based study of the Virgo spiral NGC4571 (Pierce et al. 1994) formally providing $H_0= 87\pm7$ km/s/Mpc. These determinations rely on the accuracy with which the position of this observed spiral galaxy can be located with respect to the Virgo cluster center. This uncertainty introduces a major error in the determination of $H_0$, together with the uncertainty in the adopted Virgo infall velocity of the Local Group. Here we propose the use of spiral galaxies which show clear signs of being stripped off their interstellar medium by the intracluster gas of the Virgo cluster as targets for the Cepheid distance measurements. We show that the stripping process and the knowledge of the intracluster gas distribution from ROSAT X-ray observations allow us to locate these galaxies with an at least three times higher precision with respect to M87 than in the case of other spirals like M100. The X-ray observations further imply that M87 is well centered within the intracluster gas halo of the Virgo cluster and that M86 is associated with a group of galaxies and a larger dark matter halo. The combination of these informations could enable us to locate the two stripped spiral galaxies quite precisely within the Virgo cluster and could greatly improve the determination of the Virgo cluster distance.Comment: 21 pages, Latex(aaspp.sty), including 6 figures, accepted for publication in ApJL (shortened abstract:

Chandra Observations of Gas Stripping in the Elliptical Galaxy NGC 4552 in the Virgo Cluster

We use a 54.4 ks Chandra observation to study ram-pressure stripping in NGC4552 (M89), an elliptical galaxy in the Virgo Cluster. Chandra images in the 0.5-2 keV band show a sharp leading edge in the surface brightness 3.1 kpc north of the galaxy center, a cool (kT =0.51^{+0.09}_{-0.06} keV) tail with mean density n_e ~5.4 +/- 1.7 x 10^{-3} cm^{-3} extending ~10 kpc to the south of the galaxy, and two 3-4 kpc horns of emission extending southward away from the leading edge. These are all features characteristic of supersonic ram-pressure stripping of galaxy gas, due to NGC4552's motion through the surrounding Virgo ICM. Fitting the surface brightness profile and spectra across the leading edge, we find the galaxy gas inside the edge is cooler (kT = 0.43^{+0.03}_{-0.02} keV) and denser (n_e ~ 0.010 cm^{-3}) than the surrounding Virgo ICM (kT = 2.2^{+0.7}_{-0.4} keV and n_e = 3.0 +/- 0.3 x 10^{-4} cm^{-3}). The resulting pressure ratio between the free-streaming ICM and cluster gas at the stagnation point is ~7.6^{+3.4}_{-2.0} for galaxy gas metallicities of 0.5^{+0.5}_{-0.3} Zsolar, which suggests that NGC4552 is moving supersonically through the cluster with a velocity v ~ 1680^{+390}_{-220} km/s (Mach 2.2^{+0.5}_{-0.3}) at an angle xi ~ 35 +/- 7 degrees towards us with respect to the plane of the sky.Comment: 31 pages, 12 figures, ApJ, in press; paper split into 2 parts, Paper I(sec 1-3) here, added figs and discussion to conform to published version; Paper II (sec. 4) in astro-ph/060440

Constraining q_0 with Cluster Gas Mass Fractions: A Feasibility Study

As the largest gravitationally bound objects in the universe, clusters of galaxies may contain a fair sample of the baryonic mass fraction of the universe. Since the gas mass fraction from the hot ICM is believed to be constant in time, the value of the cosmological deceleration parameter $q_0$ can be determined by comparing the calculated gas mass fraction in nearby and distant clusters (Pen 1997). To test the potential of this method, we compare the gas fractions derived for a sample of luminous ($L_X > 10^{45}$erg s$^{-1}$), nearby clusters with those calculated for eight luminous, distant ($0.3 < z < 0.6$) clusters using ASCA and ROSAT observations. For consistency, we evaluate the gas mass fraction at a fixed physical radius of 1 $h_{50}^{-1}$ Mpc (assuming $q_0=0.0$). We find a best fit value of $q_0 = 0.07$ with -0.47 < q_0 < 0.67 at 95% confidence. We also determine the gas fraction using the method of Evrard, Metzler, & Navarro (1997) to find the total mass within $r_{500}$, the radius where the mean overdensity of matter is 500 times the critical density. In simulations, this method reduces the scatter in the determination of gravitational mass without biasing the mean. We find that it also reduces the scatter in actual observations for nearby clusters, but not as much as simulations suggest. Using this method, the best fit value is $q_0 = 0.04$ with -0.50 < q_0 < 0.64. The excellent agreement between these two methods suggests that this may be a useful technique for determining $q_0$. The constraints on $q_0$ should improve as more distant clusters are studied and precise temperature profiles are measured to large radii.Comment: 8 pages, 4 figures, uses emulateapj.sty, onecolfloat.st