Search

We present a small sample of Chandra X-ray sources selected from the fields of ACIS observations which probe fluxes around the break in the hard band source counts. The targets of these fields include nine nearby galaxy clusters, one distant cluster and two powerful radio galaxy fields. The follow-up of this serendipitous sample was biased towards X-ray hard and optically dim sources mostly not seen on the Digitized Sky Survey; for these, we present X-ray fluxes, optical and near-infrared photometry leading to 51 photometric redshifts in all and 18 independently measured spectroscopic redshifts. Few sources are associated with the target fields themselves. 56 of 58 sources imaged in the K-band are detected at with , and of these, 38 have hard X-ray count ratios and 24 of these are significantly hard with most of the counts emerging about 2 keV. We find that almost all are active galactic nuclei (AGN) hosted in massive early-type host galaxies with a photometric redshift distribution peaking at . Two type 2 quasars with intrinsic X-ray luminosity , Fe Kα emission lines and absorbing column density – and in one case – are discussed in detail; the sample contains at least 12 potential type 2 quasars in all. We discuss various detection strategies for type 2 quasars and calculate their inferred space density. This combines and extends a number of results from subsamples already published by us. [ABSTRACT FROM AUTHOR]

We present observations of a sample of optically faint, hard X-ray sources of the kind likely to be responsible for much of the hard X-ray background. We confirm that such sources are easily detected in the near-infrared, and find that they have a featureless continuum suggesting that the active nucleus is heavily obscured. The infrared colours of the majority of the targets observed are consistent with absorbed elliptical host galaxies at z=1–2. It is likely that we are observing some of the brighter members of the important new class of X-ray type II quasars. [ABSTRACT FROM AUTHOR]

Focuses on the discovery of a 40arcsec long X-ray filament in the core of the cluster of galaxies Abell 1795. Temperature of the X-ray emitting gas; Presentation of the temperature map of the core; Radiative cooling time of the X-ray emitting gas in the filament.

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 H[sub 2], 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, H[sub 2] and [Fe ii]) are not wholly produced in the same gas. The broadest line, [Fe ii]λ1.644, exhibits a non-Gaussian profile with a broad base (FWHM≃1040 km s[sup -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 H[sub 2] emission. The results of Maloney, Hollenbach & Tielens are thus used to infer the total mass of gas in H[sub 2] v=1–0 S(1)-emitting clouds as a function of radius, for gas densities of 10[sup 3] and 10[sup 5] cm[sup -3], and stopping column densities N[sub H]=10[sup 22]–10[sup 24] cm[sup -2]. Assuming azimuthal symmetry, at least 2.3×10[sup 8] M[sub ⊙] 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 ∼10[sup 10] M[sub ⊙]. The latter plausibly accounts for 10[sup 9] yr of mass deposition from the cluster cooling flow, for which M≃ 10M[sub ⊙]yr[sup -1] within this radius. [ABSTRACT FROM AUTHOR]

Investigates the chandra imaging of the complex X-ray core of the Perseus cluster. Appearance of the X-ray cooler gas and the outer optical parts of NGC 1275 galaxy; Interpretation of the low surface brightness; Determination of the energy content of the radio source.

We present near-infrared spectra of seven radio-loud quasars with a median redshift of 2.1, five of which were previously known to have Lyα nebulae. Extended [O iii] λ5007 and Hα emission are evident around six objects, at the level of a few times 10[sup -16] erg cm[sup -2] arcsec[sup -2] s[sup -1] within ≃2 arcsec of the nucleus (≡16 kpc in the adopted cosmology). Nuclear [O ii] λ3727 is detected in three of the five quasars studied at this wavelength and clearly extended in one of them. The extended [O iii] tends to be brighter on the side of the nucleus with the stronger, jet-like radio emission, indicating at least that the extranuclear gas is distributed anisotropically. It is also typically redshifted by several hundred km s[sup -1] from the nuclear [O iii], perhaps because of the latter being blueshifted from the host galaxy's systemic velocity. Alternatively, the velocity shifts could be due to infall (which is suggested by linewidths ∼1000 km s[sup -1] FWHM) in combination with a suitable dust geometry. Lyα/Hα ratios well below the case B value suggest that some dust is present. Photoionization modelling of the [O iii]/[O ii] ratios in the extended gas suggests that its pressure is around or less than a few times 10[sup 7] cm[sup -3] K; any confining intracluster medium is thus likely to host a strong cooling flow. A comparison with lower redshift work suggests that there has been little evolution in the nuclear emission-line properties of radio-loud quasars between redshifts 1 and 2. [ABSTRACT FROM AUTHOR]

We present integral field spectroscopy of a small sample of radio-loud quasars at intermediate redshift (0.26

We present a study of the morphology, kinematics and ionization structure of the extended emission-line regions in four intermediate-redshift (0.1181000 km s[sup -1] within 1 arcsec of the nucleus, suggesting that any merger is well-advanced. Emission-line intensity ratios point to active galactic nucleus (AGN) photoionization for the excitation of this gas at the systemic velocity. An isolated blob ∼8 kpc from the nucleus with a much smaller velocity dispersion may lie in a structure similar to the photoionization cones seen in lower-luminosity objects. A second, spatially unresolved, narrow-line component is also present on nucleus, blueshifted by ≃990 km s[sup -1] from the systemic and plausibly powered by photoionizing shocks. IRAS F23060+0505 has more ordered kinematics, with a region of increased FWHM coincident with the blue half of a dipolar velocity field. The systemic velocity rotation curve is asymmetric in appearance, as a result either of the on-going merger or of nuclear dust obscuration. From a higher-resolution ISIS spectrum, we attribute the blue asymmetry in the narrow-line profiles to a spatially resolved nuclear outflow. Emission-line intensity ratios suggest shock+precursor ionization for the systemic component, consistent with the X-ray view of a heavily obscured AGN. The lower-luminosity objects IRAS F01217+0122 and F01003-2238 complete the sample. The former has a featureless velocity field with a high FWHM, a high-ionization AGN spectrum and a ∼1 Gyr old starburst continuum. IRAS F01003-2238 has a dipolar velocity field and an H ii region emission-line spectrum with a strong blue continuum. After correction for intrinsic extinction, the latter can be reproduced with ∼10[sup 7] O5 stars, sufficient to power the bolometric luminosity of the entire galaxy. We accommodate this diversity within the merger-induced evolutionary scenario for ultraluminous infrared galaxies: the merger status is assessed from the kinematics in a way which is consistent with morphological and colour information on the galaxies, or with the inferred ages of the young stellar populations and the dominance of the AGN. [ABSTRACT FROM AUTHOR]

We report the detection of extended X-ray emission around several powerful 3CR quasars with redshifts out to 0.73. The ROSAT HRI images of the quasars have been corrected for spacecraft wobble and compared with an empirical point-spread function. All the quasars examined show excess emission at radii of 15 arcsec and more, the evidence being strong for the more distant objects and weak only for the two nearest ones, which are known from other wavelengths not to lie in strongly clustered environments. The spatial profile of the extended component is consistent with thermal emission from the intracluster medium of moderately rich host clusters to the quasars. The total luminosities of the clusters are in the range ∼4×10[sup 44]–3×10[sup 45] erg s[sup -1], assuming a temperature of 4 keV. The inner regions of the intracluster medium are, in all cases, dense enough to be part of a cooling flow. [ABSTRACT FROM AUTHOR]

ABSTRACT We present new spectra of dominant galaxies in X-ray-selected clusters of galaxies, which combine with our previously published spectra to form a sample of 256 dominant galaxies in 215 clusters. 177 of the clusters are members of the ROSAT Brightest Cluster Sample (BCS; Ebeling et al.), and 17 have no previous measured redshift. This is the first paper in a series correlating the properties of brightest cluster galaxies and their host clusters in the radio, optical and X-ray wavebands. 27 per cent of the central dominant galaxies have emission-line spectra, all but five with line intensity ratios typical of cooling flow nebulae. A further 6 per cent show only [N ii]λλ6548,6584 with Hα in absorption. We find no evidence for an increase in the frequency of line emission with X-ray luminosity. Purely X-ray-selected clusters at low redshift have a higher probability of containing line emission. The projected separation between the optical position of the dominant galaxy and its host cluster X-ray centroid is less for the line-emitting galaxies than for those without line emission, consistent with a closer association of the central galaxy and the gravitational centre in cooling flow clusters. The more Hα-luminous galaxies have larger emission-line regions and show a higher ratio of Balmer to forbidden line emission, although there is a continuous trend of ionization behaviour across four decades in Hα luminosity. Galaxies with the more luminous line emission [L(Hα)> 10[sup 41]ergs[sup -1]] show a significantly bluer continuum, whereas lower luminosity and [N ii]-only line emitters have continua that differ little from those of non-line-emitting dominant galaxies. Values of the Balmer decrement in the more luminous systems commonly imply intrinsic reddening of E(B-V)∼0.3 and, when this is corrected for, the excess blue light can be characterized by a population of massive young stars. Several of the galaxies require a large population of O stars, which also provide sufficient photoionization to produce the observed Hα luminosity. The large number of lower mass stars relative to the O-star population suggests that this anomalous population is caused by a series of starbursts in the central galaxy. The lower Hα-luminosity systems show a higher ionization state and few massive stars, requiring instead the introduction of a harder source of photoionization, such as turbulent mixing layers, or low-level nuclear activity. The line emission from the systems showing only [N ii] is very similar to low-level LINER activity commonly found in many normal elliptical galaxies. [ABSTRACT FROM AUTHOR]

Presents the topics of discussion in the May 2002 issue of an astronomical journal. Formation mechanism of brown stars; Rotation of dwarf elliptical galaxies; X-ray luminosity in cluster cooling flows.

ABSTRACT NGC 1275 in the centre of the Perseus cluster of galaxies, Abell 426, is surrounded by a spectacular filamentary Hα nebula. Deep Chandra X-ray imaging has revealed that the brighter outer filaments are also detected in soft X-rays. This can be due to conduction and mixing of the cold gas in the filaments with the hot, dense intracluster medium. We show the correspondence of the filaments in both wavebands and draw attention to the relationship of two prominent curved north-west filaments to an outer, buoyant radio bubble seen as a hole in the X-ray image. There is a strong resemblance in the shape of the hole and the disposition of the filaments to the behaviour of a large air bubble rising in water. If this is a correct analogy, then the flow is laminar and the intracluster gas around this radio source is not turbulent. We obtain a limit on the viscosity of this gas. [ABSTRACT FROM AUTHOR]

ABSTRACT We present preliminary results from a deep observation lasting almost 200 ks of the centre of the Perseus cluster of galaxies around NGC 1275. The X-ray surface brightness of the intracluster gas beyond the inner 20 kpc, which contains the inner radio bubbles, is very smooth apart from some low-amplitude quasi-periodic ripples. A clear density jump at a radius of 24 kpc to the north-east, about 10 kpc out from the bubble rim, appears to be due to a weak shock driven by the northern radio bubble. A similar front may exist around both inner bubbles but is masked elsewhere by rim emission from bright cooler gas. The continuous blowing of bubbles by the central radio source, leading to the propagation of weak shocks and viscously dissipating sound waves seen as the observed fronts and ripples, gives a rate of working which balances the radiative cooling within the inner 50 kpc of the cluster core. [ABSTRACT FROM AUTHOR]