arXiv subjects

P. Mazzotta

Publications and source records attributed to P. Mazzotta.

89 records · Page 5Linked to original sources

arXiv2001

Non-hydrostatic gas in the core of the relaxed galaxy cluster A1795

Chandra data on A1795 reveal a mild edge-shaped discontinuity in the gas density and temperature in the southern sector of the cluster at r=60/h kpc. The gas inside the edge is 1.3-1.5 times denser and cooler than outside, while the pressure is continuous, indicating that this is a "cold front", the surface of contact between two moving gases. The continuity of the pressure indicates that the current relative velocity of the gases is near zero, making the edge appear to be in hydrostatic equilibrium. However, a total mass profile derived from the data in this sector under the equilibrium assumption, exhibits an unphysical jump by a factor of 2, with the mass inside the edge being lower. We propose that the cooler gas is "sloshing" in the cluster gravitational potential well and is now near the point of maximum displacement, where it has zero velocity but nonzero centripetal acceleration. The distribution of this non-hydrostatic gas should reflect the reduced gravity force in the accelerating reference frame, resulting in the apparent mass discontinuity. Assuming that the gas outside the edge is hydrostatic, the acceleration of the moving gas can be estimated from the mass jump, a ~ 800 h km/s/(10^8 yr). The gravitational potential energy of this gas that is available for dissipation is about half of its current thermal energy. The length of the cool filament extending from the cD galaxy (Fabian et al.) may give the amplitude of the gas sloshing, 30-40/h kpc. Such gas bulk motion might be caused by a disturbance of the central gravitational potential by past subcluster infall.

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arXiv2001

Chandra Observations of Cold Fronts in Cluster of Galaxies

High-resolution Chandra images of several clusters of galaxies reveal sharp, edge-like discontinuities in their gas density. The gas temperature is higher in front of the edge where the density is low, corresponding to approximately continuous pressure across the edge. This new phenomenon was called ``cold fronts'', to contrast it to shock fronts that should look similar in X-ray images but where the temperature should jump in the opposite direction. The first cold fronts were discovered in merging clusters, where they appear to delineate the boundaries of dense cool subcluster remnants moving through and being stripped by the surrounding shock-heated gas. Later, Chandra revealed cold fronts in the central regions of several apparently relaxed clusters. To explain the gas bulk motion in these clusters, we propose either a peculiar cluster formation history that resulted in an oscillating core, or gas sloshing (without the involvement of the underlying dark matter peak) caused by past subcluster infall or central AGN activity. We review these observations and discuss their implications for the X-ray cluster mass estimates.

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arXiv2001

Chandra Observation of MS 1455.0+2232: cold fronts in a massive cooling flow cluster?

We present the Chandra observation of the cluster of galaxies MS 1455.0+2232. From previous ASCA and ROSAT observations, this cluster was identified as a ``relaxed'' cluster that hosts one of the most massive cooling flows detected. With higher angular resolution, the Chandra X-ray image shows the presence of two surface brightness edges on opposite sides of the X-ray peak: the first at 190 kpc to the north and the second at 450 kpc to the south. Even though the low exposure of this observation limits our ability to constrain the temperature jump across both edges, we show that the northern edge is likely to be a ``cold front'' similar to others observed recently by Chandra in the clusters A2142, A3667, RX J1720.1+2638, and A2256. The observed cold front is most likely produced by the motion, from south to north, of a group-size dark matter halo. The most natural explanation for the presence of this observed moving subclump is that MS 1455.0+2232 is a merger cluster in the very last stage before it becomes fully relaxed. This scenario, however, appears to be unlikely as the cluster shows no further sign of ongoing merger. Moreover, it is not clear if a massive cooling flow could have survived this kind of merger. We propose an alternative scenario in which, as for RX J1720.1+2638, MS 1455.0+2232 is the result of the hierarchical collapse of two co-located density perturbations, the first a group-scale perturbation collapse followed by a second cluster-scale perturbation collapse that surrounded, but did not destroy, the first. We suggest that a cooling flow may have begun inside the already collapsed group-scale perturbation and may have been later amplified by the gas compression induced by the infall of the overlying main cluster mass.

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arXiv2001

Evidence for a heated gas bubble inside the "cooling flow" region of MKW3s

We report on the deep Chandra observation of central r=200kpc region of the cluster of galaxies MKW3s which was previously identified as a moderate cooling flow cluster. The Chandra image reveals two striking features -- a 100kpc long and 21kpc wide filament, extending from the center to the south-west and a nearly circular, 50kpc diameter depression 90 kpc south of the X-ray peak. The temperature map shows that the filamentary structure is colder while the surface brightness depression is hotter than the average cluster temperature at any radius. The hot and the cold regions indicate that both cooling and heating processes are taking place in the center of MKW3s. We argue that the surface brightness depression is produced by a heated, low-density gas bubble along the line of sight. We suggest that the heated bubble is produced by short-lived nuclear outbursts from the central galaxy.

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arXiv2001

1WGAJ1226.9+3332: a high redshift cluster discovered by Chandra

We report the detection of 1WGAJ1226.9+3332 as an arcminute scale extended X-ray source with the Chandra X-ray Observatory. The Chandra observation and R and K band imaging strongly support the identification of 1WGAJ1226.9+3332 as a high redshift cluster of galaxies, most probably at z=0.85 +- 0.15, with an inferred temperature kT =10 (+4;-3) keV and an unabsorbed luminosity (in a r=120" aperture) of 1.3 (+0.16;-0.14) x 1e45 erg/s (0.5-10 keV). This indication of redshift is also supported by the K and R band imaging, and is in agreement with the spectroscopic redshift of 0.89 found by Ebeling et al. (2001). The surface brightness profile is consistent with a beta-model with beta=0.770 +- 0.025, rc=(18.1 +-0.9)" (corresponding to 101 +- 5 kpc at z=0.89), and S(0)=1.02 +- 0.08 counts/arcsec**2. 1WGAJ1226.9+3332 was selected as an extreme X-ray loud source with FX/FV>60; this selection method, thanks to the large area sampled, seems to be a highly efficient method for finding luminous high z clusters of galaxies.

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arXiv2001

Clusters of galaxies among ROSAT blank field sources

We present here an efficient method for selecting high luminosity and massive high redshift clusters of galaxies, crucially important tools in cosmology. By selecting bright and extremely X-ray loud (high F(X)/F(V)) sources, we were able to identify 2 high redshift (z=0.45 and z=0.89) clusters so far and we have evidence for at least one more candidate at 0.4<z<1.1 out of a total of 16 selected sources.

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arXiv2001

Chandra Observation of RXJ1720.1+2638: a Nearly Relaxed Cluster with a Fast Moving Core?

We have analyzed the Chandra observation of the distant (z=0.164) galaxy cluster RXJ1720.1+2638 in which we find sharp features in the X-ray surface brightness on opposite sides of the X-ray peak: an edge at about 250 h_50^{-1} kpc to the South-East and a plateau at about 130 h_50^{-1} kpc to the North-West. The surface brightness edge and the plateau can be modeled as a gas density discontinuity (jump) and a slope change (break). The temperature profiles suggest that the jump and the break are the boundaries of a central, group-size (d=380h_50^{-1} kpc), dense, cold (T=4 keV) gas cloud, embedded in a diffuse hot (T=10 keV) intracluster medium. The density jump and the temperature change across the discontinuity are similar to the ``cold fronts'' discovered by Chandra in A2142 and A3667, and suggest subsonic motion of this central gas cloud with respect to the cluster itself. The most natural explanation is that we are observing a merger in the very last stage before the cluster becomes fully relaxed. However, the data are also consistent with an alternative scenario in which RXJ1720.1+2638 is the result of the collapse of two co-located density perturbations, the first a group-scale perturbation collapse followed by a second cluster-scale perturbation collapse that surrounded, but did not destroy, the first one. We also show that, because of the core motion, the total mass inside the cluster core, derived under the assumption of hydrostatic equilibrium, may underestimate the true cluster mass. If widespread, such motion may partially explain the discrepancy between X-ray and the strong lensing mass determinations found in some clusters.

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arXiv2001

Temperature and total mass profiles of the A3571 cluster of galaxies

We present BeppoSAX results of a spatially resolved spectral analysis of A3571, a relaxed nearby cluster of galaxies. In the central 2' (130/h_50 kpc) radius the metal abundance is 0.49 +- 0.08 solar and the absorption (1.13 +-0.28) x 10^21 atom/cm^2 whereas elsewhere within an 8'(520/h_50 kpc) radius the abundance is 0.32 \+- 0.05 solar and the absorption consistent with the galactic value of 4.4 x 10^20 atom/cm2. The significant central metal abundance enhancement is consistent with the supernova enrichment scenario. The excess absorption may be attributed to the cooling flow, whose mass flow rate is 80 +- 40 M_Sun/yr from our spectral fit. The BeppoSAX and ASCA radial temperature profiles agree over the entire overlapping radial range r < 25' = 1.6/h_50 Mpc. The combined BeppoSAX and ASCA temperature profile exhibits a constant value out to a radius of 10' (650/h_50 kpc) and a significant decrease (T propto r^-0.55, corresponding to gamma=1.28) at larger radii. These temperature data are used to derive the total mass profile. The best fit NFW dark matter density model results in a temperature profile that is not convectively stable, but the model is acceptable within the uncertainties of the data. The temperature profile is acceptably modeled with a ``core'' model for the dark matter density, consisting of a core radius with a constant slope at larger radii. With this model the total mass and formal 90% confidence errors within the virial radius r_178 (2.5/h_50 Mpc) are 9.1+3.6-1.5 x 10^14 h_50^-1 M_Sun, by a factor of 1.4 smaller than the isothermal value. The gas mass fraction increases with radius, reaching f_gas(r_178) = 0.26+0.05-0.10 x h_50^-3/2. Assuming that the measured gas mass fraction is the lower limit to the primordial baryonic fraction gives Omega_m < 0.4 at 90% confidence.

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arXiv2000

Temperature Structure of Four Merging Clusters Obtained with Chandra

We present preliminary Chandra results on z=0.2 clusters A665, A2163 and A2218, and a z=0.05 cluster A754. For A754, A665 and A2163, we have derived first high-resolution projected gas temperature maps. All three show strong spatial temperature variations in the inner r<0.5-1 Mpc regions, indicating ongoing mergers. The maps reveal a probable shock in front of a moving cluster core in A665, a rather complicated temperature distribution in the center of A2163, and possibly a merger of three subclusters in A754. At greater off-center distances, radial profiles for A2163 and A2218 show a temperature decline, in agreement with earlier ASCA results.

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arXiv2000

Chandra study of an overdensity of X-ray sources around two distant (z~0.5) clusters

We present results from a Chandra X-ray Observatory study of the field X-ray source populations in 4 different observations: two high-redshift (z~0.5) clusters of galaxies 3C295 and RXJ003033.2+261819; and two non-cluster fields with similar exposure time. Surprisingly, the 0.5-2 keV source surface densities (~900-1200 sources deg**-2 at a flux limit of 1.5x10**-15 erg cm**-2s**-1) measured in an ~8'x8' area surrounding each cluster exceed by a factor of ~2 the value expected on the basis of the ROSAT and Chandra logN-logS, with a significance of ~2 sigma each, or ~3.5 sigma when the 2 fields are combined (i.e. a probability to be a statistical fluctuation of <1% and <0.04%, respectively). The same analysis performed on the non-cluster fields and on the outer chips of the cluster fields does NOT show evidence of such an excess. In both cluster fields, the summed 0.5-10 keV spectrum of the detected objects is well fitted by a power-law with Gamma~1.7 similar to AGNs and shows no sign of intrinsic absorption. The few (~10 out of 35) optical identifications available to date confirm that most of them are, as expected, AGNs but the number of redshifts available is too small to allow conclusions on their nature. We discuss possible interpretations of the overdensity in terms of: a statistical variation of Cosmic Background sources; a concentration of AGNs and/or powerful starburst galaxies associated with the clusters; and g ravitational lensing of background QSO's by the galaxy clusters. All explanations are however difficult to reconcile with the large number of excess sources detected. Deeper X-ray observations and more redshifts measurements are clearly required to settle the issue.

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arXiv2000

An X-ray and optical study of the cluster A33

We report the first detailed X-ray and optical observations of the medium-distant cluster A33 obtained with the Beppo-SAX satellite and with the UH 2.2m and Keck II telescopes at Mauna Kea. The information deduced from X-ray and optical imaging and spectroscopic data allowed us to identify the X-ray source 1SAXJ0027.2-1930 as the X-ray counterpart of the A33 cluster. The faint, $F_{2-10 keV} \approx 2.4 \times 10^{-13} \ergscm2$, X-ray source 1SAXJ0027.2-1930, $\sim 2$ arcmin away from the optical position of the cluster as given in the Abell catalogue, is identified with the central region of A33. Based on six cluster galaxy redshifts, we determine the redshift of A33, $z=0.2409$; this is lower than the value derived by Leir and Van Den Bergh (1977). The source X-ray luminosity, $L_{2-10 keV} = 7.7 \times 10^{43} \ergs$, and intracluster gas temperature, $T = 2.9$ keV, make this cluster interesting for cosmological studies of the cluster $L_X-T$ relation at intermediate redshifts. Two other X-ray sources in the A33 field are identified. An AGN at z$=$0.2274, and an M-type star, whose emission are blended to form an extended X-ray emission $\sim 4$ arcmin north of the A33 cluster. A third possibly point-like X-ray source detected $\sim 3$ arcmin north-west of A33 lies close to a spiral galaxy at z$=$0.2863 and to an elliptical galaxy at the same redshift as the cluster.

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arXiv2000

Chandra Observation of Abell 2142: Survival of Dense Subcluster Cores in a Merger

We use Chandra data to map the gas temperature in the central region of the merging cluster A2142. The cluster is markedly nonisothermal; it appears that the central cooling flow has been disturbed but not destroyed by a merger. The X-ray image exhibits two sharp, bow-shaped, shock-like surface brightness edges or gas density discontinuities. However, temperature and pressure profiles across these edges indicate that these are not shock fronts. The pressure is reasonably continuous across these edges, while the entropy jumps in the opposite sense to that in a shock (i.e. the denser side of the edge has lower temperature, and hence lower entropy). Most plausibly, these edges delineate the dense subcluster cores that have survived a merger and ram pressure stripping by the surrounding shock-heated gas.

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arXiv1999

Chandra X-Ray Detection of the Radio Hotspots of 3C295

An observation of the radio galaxy 3C295 during the calibration phase of the Chandra X-ray Observatory reveals X-ray emission from the core of the galaxy, from each of the two prominent radio hotspots, and from the previously known cluster gas. We discuss the possible emission processes for the hotspots and argue that a synchrotron self-Compton model is preferred for most or all of the observed X-ray emission. SSC models with near equipartition fields thus explain the X-ray emission from the hotspots in the two highest surface brightness FRII radio galaxies, Cygnus A and 3C295. This lends weight to the assumption of equipartition and suggests that relativistic protons do not dominate the particle energy density.

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arXiv1999

The Planck Surveyor mission: astrophysical prospects

Although the Planck Surveyor mission is optimized to map the cosmic microwave background anisotropies, it will also provide extremely valuable information on astrophysical phenomena. We review our present understanding of Galactic and extragalactic foregrounds relevant to the mission and discuss on one side, Planck's impact on the study of their properties and, on the other side, to what extent foreground contamination may affect Planck's ability to accurately determine cosmological parameters. Planck's multifrequency surveys will be unique in their coverage of large areas of the sky (actually, of the full sky); this will extend by two or more orders of magnitude the flux density interval over which mm/sub-mm counts of extragalactic sources can be determined by instruments already available (like SCUBA) or planned for the next decade (like the LSA-MMA or the space mission FIRST), which go much deeper but over very limited areas. Planck will thus provide essential complementary information on the epoch-dependent luminosity functions. Bright radio sources will be studied over a poorly explored frequency range where spectral signatures, essential to understand the physical processes that are going on, show up. The Sunyaev-Zeldovich effect, with its extremely rich information content, will be observed in the direction of a large number of rich clusters of Galaxies. Thanks again to its all sky coverage, Planck will provide unique information on the structure and on the emission properties of the interstellar medium in the Galaxy. At the same time, the foregrounds are unlikely to substantially limit Planck's ability to measure the cosmological signals. Even measurements of polarization of the primordial Cosmic Microwave background fluctuations appear to be feasible.

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arXiv1998

Ionization balance for optically thin plasmas: rate coefficients for all atoms and ions of the elements H to Ni

We present in this paper new and updated calculations of the ionization equilibrium for all the elements from H to Ni. We collected for these elements all the data available in the literature for the ionization and radiative plus dielectronic recombination rates. In particular, the dielectronic rates have been fitted with a single formula and the related coefficients are tabulated. Our results are compared with previous works.

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arXiv1997

Is the Cluster Temperature Function a Reliable Test for $Ω_{0}$?

We discuss the evolution of the cluster temperature function (TF) in different scenarios for structure formation. We use the commonly adopted procedure of fitting the model parameters to the local TF data, finding the best fit values and, most of all, the associated statistical errors. These errors yield an uncertainty in the prediction of the TF evolution. We conclude that, at the moment, observations of cluster temperatures at z<=0.5 could provide only a weak test for $Ω_0$.

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arXiv1997

Intracluster Comptonization of the CMB: Mean Spectral Distrortion and Cluster Number Counts

The mean sky-averaged Comptonization parameter, y, describing the scattering of the CMB by hot gas in clusters of galaxies is calculated in an array of flat and open cosmological and dark matter models. The models are globally normalized to fit cluster X-ray data, and intracluster gas is assumed to have evolved in a manner consistent with current observations. We predict values of y lower than the COBE/FIRAS upper limit. The corresponding values of the overall optical thickness to Compton scattering are < 10^{-4} for relevant parameter values. Of more practical importance are number counts of clusters across which a net flux (with respect to the CMB) higher than some limiting value can be detected. Such number counts are specifically predicted for the COBRAS/SAMBA and BOOMERANG missions.

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