X-ray analysis of the cluster 3C 129

The cluster 3C 129 is classified as a rich cluster. An analysis of the properties of the cluster 3C 129 from ROSAT PSPC and HRI, Einstein IPC, and EXOSAT ME observations is presented. The mean temperature from a joint fit of the ROSAT PSPC and EXOSAT ME data is 5.5(±0.2) keV. The luminosity is 0.6×10⁴⁴ erg s⁻¹ in 0.2–2.4 keV and 2.7×10⁴⁴ erg s⁻¹ in 0.2–10 keV. We find a cooling flow with a rate of ∼84 M_⊙ yr⁻¹. The central gas density is 6×10⁻³ cm⁻³, and the ICM mass is 3.6×10¹³ M_⊙. The total cluster mass is ∼5×10¹⁴ M_⊙. The X-ray morphology shows an east–west elongation, which is evidence for a recent merger event. The radio source 3C 129.1 is located near the X-ray centre. Another cluster member galaxy (the radio galaxy 3C 129) is a prototype of head-tailed radio galaxies, and is located in the west part of the cluster. The tail points along the gradient of intracluster gas pressure. There are no significant point X-ray sources associated with the AGNs of the two radio galaxies.     

Impact of stochastic gas motions on galaxy cluster abundance profiles

The impact of stochastic gas motions on the metal distribution in cluster cores is evaluated. Peaked abundance profiles are a characteristic feature of clusters with cool cores, and abundance peaks are probably associated with the brightest cluster galaxies (BCGs), which dwell in cluster cores. However, the width of the abundance peaks is significantly broader than the BCG light distribution, suggesting that some gas motions are transporting metals originating from within the BCG. Assuming that this process can be treated as diffusive, and using the brightest X-ray cluster A426 (Perseus) as an example, we estimate that a diffusion coefficient of the order of  2 × 10²⁹ cm² s⁻¹  is needed to explain the width of the observed abundance profiles. Much lower (higher) diffusion coefficients would result in too peaked (too shallow) profiles. Such diffusion could be produced by stochastic gas motions, and our analysis provides constraints on the product of their characteristic velocity and their spatial coherence scale. We speculate that the activity of the supermassive black hole of the BCG is driving the stochastic gas motions in cluster cores. When combined with the assumption that the dissipation of the same motions is a key gas heating mechanism, one can estimate both the velocity and the spatial scale of such diffusive processes.     

The weak shock in the core of the Perseus cluster

The dissipation of energy from sound waves and weak shocks is one of the most promising mechanisms for coupling active galactic nucleus (AGN) activity to the surrounding intracluster medium, and so offsetting cooling in cluster cores. We present a detailed analysis of the weak shock found in deep Chandra observations of the Perseus cluster core. A comparison of the spectra either side of the shock front shows that they are very similar. By performing a deprojection analysis of a sector containing the shock, we produce temperature and density profiles across the shock front. These show no evidence for a temperature jump coincident with the density jump. To understand this result, we model the shock formation using 1D hydrodynamic simulations including models with thermal conduction and  γ < 5/3  gas. These models do not agree well with the data, suggesting that further physics is needed to explain the shock structure. We suggest that an interaction between the shock and the Hα filaments could have a significant effect on cooling the post-shock gas.
We also calculate the thermal energy liberated by the weak shock. The total energy in the shocked region is about 3.5 times the work needed to inflate the bubbles adiabatically, and the power of the shock is around  6 × 10⁴⁴ erg s⁻¹  per bubble, just over  10⁴⁵ erg s⁻¹  in total.     

Light curves of jetted gamma-ray burst afterglows in circumstellar clouds

We present a Chandra observation of the powerful radio galaxy 3C 294 showing clear evidence for a surrounding intracluster medium. At a redshift of 1.786 this is the most distant cluster of galaxies yet detected in X-rays. The radio core is detected as a point source, which has a spectrum consistent with a heavily absorbed power law, implying an intrinsic 2–10 keV luminosity of ∼10⁴⁵ erg s⁻¹. A small excess of emission is associated with the southern radio hotspots. The soft, diffuse emission from the intracluster medium is centred on the radio source. It has an hourglass shape in the north–south direction, extending to radii of at least 100 kpc, well beyond the radio source. The X-ray spectrum of this extended component is fitted by a thermal model with temperature ∼5 keV, or by gas cooling from above 7 keV at rates of ∼ 400–700 M_⊙ yr⁻¹. The rest-frame 0.3–10 keV luminosity of the cluster is ∼ 4.5×10⁴⁴ erg s⁻¹. The existence of such a cluster is consistent with a low-density universe.     

Molecular hydrogen emission in Cygnus A

We present J , H and K -band spectroscopy of Cygnus A, spanning 1.0–2.4 μm in the rest-frame and hence several rovibrational H₂, H recombination and [Fe  ii ] emission lines. The lines are spatially extended by up to 6 kpc from the nucleus, but their distinct kinematics indicate that the three groups (H, H₂ and [Fe  ii ]) are not wholly produced in the same gas. The broadest line, [Fe  ii ] λ 1.644, exhibits a non-Gaussian profile with a broad base (FWHM≃1040 km s⁻¹), perhaps because of the interaction with the radio source. Extinctions to the line-emitting regions substantially exceed earlier measurements based on optical H recombination lines.
Hard X-rays from the quasar nucleus are likely to dominate the excitation of the H₂ emission. The results of Maloney, Hollenbach & Tielens are thus used to infer the total mass of gas in H₂ v=1–0 S(1)-emitting clouds as a function of radius, for gas densities of 10³ and 10⁵ cm⁻³, and stopping column densities N _H=10²²–10²⁴ cm⁻². Assuming azimuthal symmetry, at least 2.3×10⁸ M_⊙ of such material is present within 5 kpc of the nucleus, if the line-emitting clouds see an unobscured quasar spectrum. Alternatively, if the bulk of the X-ray absorption to the nucleus inferred by Ueno et al. actually arises in a circumnuclear torus, the implied gas mass rises to ∼10¹⁰ M_⊙. The latter plausibly accounts for 10⁹ yr of mass deposition from the cluster cooling flow, for which within this radius.     

A Chandra detection of the radio hotspot of 3C 123

Chandra X-ray Observatory observations of the powerful, peculiar radio galaxy 3C 123 have resulted in an X-ray detection of the bright eastern hotspot, with a 1-keV flux density of ∼5 nJy. The X-ray flux and spectrum of the hotspot are consistent with the X-rays being inverse-Compton scattering of radio synchrotron photons by the population of electrons responsible for the radio emission ('synchrotron self-Compton emission') if the magnetic fields in the hotspot are close to their equipartition values. 3C 123 is thus the third radio galaxy to show X-ray emission from a hotspot which is consistent with being in equipartition. Chandra also detects emission from a moderately rich cluster surrounding 3C 123, with L _X(2–10 keV)=2×10⁴⁴ erg s⁻¹ and kT ∼5 keV, and absorbed emission from the active nucleus, with an inferred intrinsic column density of 1.7×10²² cm⁻² and an intrinsic 2–10 keV luminosity of 10⁴⁴ erg s⁻¹.     

The Norma cluster (ACO 3627) – I. A dynamical analysis of the most massive cluster in the Great Attractor

A detailed dynamical analysis of the nearby rich Norma cluster (ACO 3627) is presented. From radial velocities of 296 cluster members, we find a mean velocity of 4871 ± 54 km s⁻¹ and a velocity dispersion of 925 km s⁻¹. The mean velocity of the E/S0 population (4979 ± 85 km s⁻¹) is offset with respect to that of the S/Irr population (4812 ± 70 km s⁻¹) by  Δ v = 164 km s⁻¹  in the cluster rest frame. This offset increases towards the core of the cluster. The E/S0 population is free of any detectable substructure and appears relaxed. Its shape is clearly elongated with a position angle that is aligned along the dominant large-scale structures in this region, the so-called Norma wall. The central cD galaxy has a very large peculiar velocity of 561 km s⁻¹ which is most probably related to an ongoing merger at the core of the cluster. The spiral/irregular galaxies reveal a large amount of substructure; two dynamically distinct subgroups within the overall spiral population have been identified, located along the Norma wall elongation. The dynamical mass of the Norma cluster within its Abell radius is  1–1.1 × 10¹⁵  h ⁻¹₇₃ M_⊙  . One of the cluster members, the spiral galaxy WKK 6176 which recently was observed to have a 70 kpc X-ray tail, reveals numerous striking low-brightness filaments pointing away from the cluster centre suggesting strong interaction with the intracluster medium.     

A Chandra X-ray observation of the globular cluster Terzan 1

We present a ∼19-ks Chandra Advanced CCD Imaging Spectrometer (ACIS)-S observation of the globular cluster Terzan 1. 14 sources are detected within 1.4 arcmin of the cluster centre with two of these sources predicted to be not associated with the cluster (background active galactic nuclei or foreground objects). The neutron star X-ray transient, X1732−304, has previously been observed in outburst within this globular cluster with the outburst seen to last for at least 12 yr. Here, we find four sources that are consistent with the ROSAT position for this transient, but none of the sources are fully consistent with the position of a radio source detected with the Very Large Array that is likely associated with the transient. The most likely candidate for the quiescent counterpart of the transient has a relatively soft spectrum and an unabsorbed 0.5–10 keV luminosity of  2.6 × 10³² erg s⁻¹  , quite typical of other quiescent neutron stars. Assuming standard core cooling, from the quiescent flux of this source we predict long (>400 yr) quiescent episodes to allow the neutron star to cool. Alternatively, enhanced core cooling processes are needed to cool down the core. However, if we do not detect the quiescent counterpart of the transient this gives an unabsorbed 0.5–10 keV luminosity upper limit of  8 × 10³¹ erg s⁻¹  . We also discuss other X-ray sources within the globular cluster. From the estimated stellar encounter rate of this cluster we find that the number of sources we detect is significantly higher than expected by the relationship of Pooley et al.     

The dynamical state of RX J1347.5−1145 from a combined strong lensing and X-ray analysis

We perform a combined X-ray and strong lensing analysis of RX J1347.5−1145, one of the most luminous galaxy clusters at X-ray wavelengths. We show that evidence from strong lensing alone, based on published Very Large Telescope (VLT) and new Hubble Space Telescope ( HST ) data, strongly argues in favour of a complex structure. The analysis takes into account arc positions, shapes and orientations, and is done thoroughly in the image plane. The cluster inner regions are well fitted by a bimodal mass distribution, with a total projected mass of   M _tot= (9.9 ± 0.3) × 10¹⁴ M_⊙  h ⁻¹  within a radius of 360 kpc  h ⁻¹ (1.5 arcmin). Such a complex structure could be a signature of a recent major merger as further supported by X-ray data. A temperature map of the cluster, based on deep Chandra observations, reveals a hot front located between the first main component and an X-ray emitting south-eastern subclump. The map also unveils a filament of cold gas in the innermost regions of the cluster, most probably a cooling wake caused by the motion of the cD inside the cool core region. A merger scenario in the plane of the sky between two dark matter subclumps is consistent with both our lensing and X-ray analyses, and can explain previous discrepancies with mass estimates based on the virial theorem.     

A Chandra observation of the distant radio galaxy B2 0902+343: a powerful obscured active galaxy

We report a Chandra observation of the   z =3.395  radio galaxy B2 0902+343. The unresolved X-ray source is centred on the active nucleus. The spectrum is well fitted by a flat power law of photon index of  Γ∼1.1  with intrinsic absorption of  8×10²² cm^-2  , and an intrinsic  2–10 keV  luminosity of  3.3×10⁴⁵ erg s^-1  . More complex models that allow for a steeper spectral index cause the column density and intrinsic luminosity to increase. The data limit any thermal luminosity of the hot magnetized medium, assumed responsible for high Faraday rotation measures seen in the radio source, to less than ∼10⁴⁵ erg s⁻¹.     

X-ray active galactic nuclei in the core of the Perseus cluster

We present a study of the X-ray emission from the nuclei of galaxies observed in the core of the Perseus cluster in a deep exposure with Chandra . Point sources are found coincident with the nuclei of 13 early-type galaxies, as well as the central galaxy NGC 1275. This corresponds to all galaxies brighter than M _B > −18 in the Chandra field. All of these sources have a steep power-law spectral component and four have an additional thermal component. The unabsorbed power-law luminosities in the 0.5–7.0 keV band range from 8 × 10³⁸ to 5 × 10⁴⁰ erg s⁻¹. We find no simple correlations between the K -band luminosity, or the FUV and NUV AB magnitudes of these galaxies and their X-ray properties. We have estimated the black hole masses of the nuclei using the K -band   M _BH– L _{K bol}  relation and again find no correlation between black hole mass and the X-ray luminosity. Bondi accretion on to the black holes in the galaxies with minihaloes should make them much more luminous than observed.     

Iron line profiles including emission from within the innermost stable orbit of a black hole accretion disc

We propose a model for the source of the X-ray background (XRB) in which low-luminosity active nuclei ( L  ∼ 10⁴³ erg s⁻¹) are obscured ( N  ∼ 10²³ cm⁻²) by nuclear starbursts within the inner ∼ 100 pc. The obscuring material covers most of the sky as seen from the central source, rather than being distributed in a toroidal structure, and hardens the averaged X-ray spectrum by photoelectric absorption. The gas is turbulent with velocity dispersion ∼ few × 100 km s⁻¹ and cloud–cloud collisions lead to copious star formation. Although supernovae tend to produce outflows, most of the gas is trapped in the gravity field of the star-forming cluster itself and the central black hole. A hot ( T  ∼ 10⁶ − 10⁷ K) virialized phase of this gas, comprising a few per cent of the total obscuring material, feeds the central engine of ∼ 10⁷ M⊙ through Bondi accretion, at a sub-Eddington rate appropriate for the luminosity of these objects. If starburst-obscured objects give rise to the residual XRB, then only 10 per cent of the accretion in active galaxies occurs close to the Eddington limit in unabsorbed objects.     

Chandra measurements of the X-ray core and cluster of 3C 220.1

We report results of an 18-ks exposure with the ACIS instrument on Chandra of the powerful z =0.62 radio galaxy 3C 220.1. The X-ray emission separates into cluster gas of emission-weighted kT ∼5 keV , 0.7–12 keV luminosity (to a radius of 45 arcsec) of 5.6×10⁴⁴ erg s⁻¹ and unresolved emission (coincident with the radio core). While the extended X-ray emission is clearly thermal in nature, a straightforward cooling-flow model, even in conjunction with a point-source component, is a poor fit to the radial profile of the X-ray emission. This is despite the fact that the measured properties of the gas suggest a massive cooling flow of ∼130 M_⊙ yr⁻¹, and the data show weak evidence for a temperature gradient. The central unresolved X-ray emission has a power-law spectral energy index α ∼0.7 and 0.7–12 keV luminosity of 10⁴⁵ erg s⁻¹, and any intrinsic absorption is relatively small. The two-point spectrum of the core emission between radio and X-ray energies has α _rx=0.75 . Since this is a flatter spectrum than seen in other sources where the X-ray emission is presumed to be radio-related, regions close to the active galactic nucleus (AGN) in this source may dominate the central X-ray output, as is believed to be the case for lobe-dominated quasars. Simple unification models would be challenged if this were found to be the case for a large fraction of high-power radio galaxies.     

X-ray and radio observations of the poor cluster A3581 which hosts the radio galaxy PKS 1404 − 267

We present new X-ray and H  I 21-cm data on the poor cluster of galaxies Abell 3581. The ASCA spectrum requires a low temperature, has a strong requirement for excess absorption and shows evidence for multi-temperature components. The ROSAT HRI image shows the strongly peaked emission indicative of a cooling flow. Despite the low temperature (∼ 1.5–2.0 keV) and low luminosity (∼ 2 × 10⁴² erg s⁻¹ in the 2–10 keV band), Abell 3581 has a mass deposition rate ∼ 80 M⊙ yr⁻¹ which is larger than found for other nearby low-luminosity objects. VLA observations in the 21-cm band set velocity width and spin temperature dependent limits on the column density of atomic hydrogen.     

ROSAT PSPC observations of the outer regions of the Perseus cluster of galaxies

We present an analysis of four off-axis ROSAT Position Sensitive Proportional Counter (PSPC) observations of the Perseus cluster of galaxies (Abell 426). We detect the surface brightness profile to a radius of 80 arcmin (∼2.4 h⁻¹₅₀ Mpc) from the X-ray peak. The profile is measured in various sectors and in three different energy bands. First, a colour analysis highlights a slight variation of N _H over the region, and cool components in the core and in the eastern sector. We apply the β-model to the profiles from different sectors and present a solution to the, so-called, β-problem. The residuals from an azimuthally-averaged profile highlight extended emission both in the east and in the west, with estimated luminosities of about 8 and 1 ×10⁴³ erg s⁻¹, respectively. We fit several models to the surface brightness profile, including the one obtained from the Navarro, Frenk &38; White potential. We obtain the best fit with the gas distribution described by a power law in the inner, cooling region and a β-model for the extended emission. Through the best-fitting results and the constraints from the deprojection of the surface brightness profiles, we define the radius where the overdensity inside the cluster is 200 times the critical value, r ₂₀₀, at 2.7 h⁻¹₅₀ Mpc. Within 2.3  h⁻¹₅₀ Mpc (0.85 r ₂₀₀), the total mass in the Perseus cluster is 1.2 × 10¹⁵ M_⊙ and its gas fraction is about 30 per cent.     

Radio observations of comet 9P/Tempel 1 with the Australia Telescope facilities during the Deep Impact encounter

We present radio observations of comet 9P/Tempel 1 associated with the Deep Impact spacecraft collision of 2005 July 4. Weak 18-cm OH emission was detected with the Parkes 64-m telescope, in data averaged over July 4–6, at a level of  12 ± 3 mJy km s⁻¹  , corresponding to OH production rate  2.8 × 10²⁸  molecules s⁻¹ (Despois et al. inversion model, or  1.0 × 10²⁸ s⁻¹  for the Schleicher & A'Hearn model). We did not detect the HCN 1–0 line with the Mopra 22-m telescope over the period July 2–6. The 3σ limit of 0.06 K km s⁻¹ for HCN on July 4 after the impact gives the limit to the HCN production rate of  <1.8 × 10²⁵ s⁻¹  . We did not detect the HCN 1–0 line, 6.7 GHz CH₃OH line or 3.4-mm continuum with the Australia Telescope Compact Array (ATCA) on July 4, giving further limits on any small-scale structure due to an outburst. The 3σ limit on HCN emission of 2.5 K km s⁻¹ from the ATCA around impact corresponds to limit < 4 × 10²⁹ HCN molecules released by the impact.     

Observations of the Sunyaev–Zel'dovich effect in the z=0.78 cluster MS 1137.5+6625

We have observed the   z =0.78  cluster MS 1137.5+6625 with the Ryle Telescope (RT) at 15 GHz. After subtraction of contaminating radio sources in the field, we find a Sunyaev–Zel'dovich flux decrement of  -421±60 μJy  on the ≈0.65 k λ baseline of the RT, spatially coincident with the optical and X-ray positions for the cluster core.
For a spherical King-profile cluster model, the best fit to our flux measurement has a core radius   θ _C=20 arcsec  , consistent with previous X-ray observations, and a central temperature decrement  Δ T =650±92 μK  .
Using this model, we calculate that the cluster has a gas mass inside a     radius of  2.9×10¹³ M_⊙  for an  Ω _M =1  universe and  1.6×10¹³ M_⊙  for  Ω _M =0.3  ,  Ω_Λ=0.7  . We compare this model with existing measurements of the total mass of the cluster, based on gravitational lensing, and estimate a gas fraction for MS 1137.5+6625 of ≈8 per cent.     

H 0 from an orientation-unbiased sample of Sunyaev–Zel'dovich and X-ray clusters

We have observed the Sunyaev–Zel'dovich (SZ) effect in a sample of five moderate-redshift clusters with the Ryle Telescope, and used them in conjunction with X-ray imaging and spectral data from ROSAT and ASCA to measure the Hubble constant. This sample was chosen with a strict X-ray flux limit using both the Bright Cluster Sample and the Northern ROSAT All-Sky Survey (RASS) cluster catalogues to be well above the surface brightness limit of the RASS, and hence to be unbiased with respect to the orientation of the cluster. This controls a major potential systematic effect in the SZ/X-ray method of measuring H ₀. Taking the weighted geometric mean of the results and including the main sources of error, namely the noise in the SZ measurement, the uncertainty in the X-ray temperatures and the unknown ellipticity and substructure of the clusters, we find   H ₀= 59⁺¹⁰₋₉ (random)⁺⁸₋₇(systematic) km s⁻¹ Mpc⁻¹  assuming a standard cold dark matter model with  Ω_M= 1.0, Ω_Λ= 0.0  or   H ₀= 66⁺¹¹₋₁₀ ⁺⁹₋₈ km  s⁻¹ Mpc⁻¹  if  Ω_M= 0.3, Ω_Λ= 0.7  .     

Radiative association of C and P, and Si and P atoms

The rate coefficients for the formation of carbon monophosphide (CP) and silicon monophosphide (SiP) by radiative association are estimated for temperatures ranging from 300 to 14 100 K. In this temperature range, the radiative association rate coefficients are found to vary from  1.14 × 10⁻¹⁸  to  1.62 × 10⁻¹⁸ cm³ s⁻¹  and from  3.73 × 10⁻²⁰  to  7.03 × 10⁻²⁰ cm³ s⁻¹  for CP and SiP, respectively. In both cases, rate coefficients increase slowly with the increase in temperature.