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.     

J0041+3224: a new double–double radio galaxy

We report the discovery of a double–double radio galaxy (DDRG), J0041+3224, with the Giant Metrewave Radio Telescope (GMRT) and subsequent high-frequency observations with the Very Large Array (VLA). The inner and outer doubles are aligned within ∼4° and are reasonably collinear with the parent optical galaxy. The outer double has a steeper radio spectrum compared to the inner one. Using an estimated redshift of 0.45, the projected linear sizes of the outer and inner doubles are 969 and 171 kpc, respectively. The time-scale of interruption of jet activity has been estimated to be ∼20 Myr, similar to other known DDRGs. We have compiled a sample of known DDRGs, and have re-examined the inverse correlation between the ratio of the luminosities of the outer to the inner double and the size of the inner double, l _in. Unlike the other DDRGs with   l _in≳ 50 kpc  , the inner double of J0041+3224 is marginally more luminous than the outer one. The two DDRGs with   l _in≲  few kpc have a more luminous inner double than the outer one, possibly due to a higher efficiency of conversion of beam energy as the jets propagate through the dense interstellar medium. We have examined the symmetry parameters and found that the inner doubles appear to be more asymmetric in both its armlength and its flux density ratios compared to the outer doubles, although they appear marginally more collinear with the core than the outer double. We discuss briefly the possible implications of these trends.     

Radio emission from the Sy 1.5 galaxy NGC 5033

We present new continuum VLA observations of the nearby Sy 1.5 galaxy NGC 5033, made at 4.9 and 8.4 GHz on 2003 April 8. Combined with VLA archival observations at 1.4- and 4.9-GHz made on 1993 August 7, 1999 August 29 and 1999 October 31, we sample the galaxy radio emission at scales ranging from the nuclear regions (≲100 pc) to the outer regions of the disc (∼40 kpc). The high-resolution VLA images show a core–jet structure for the Sy 1.5 nucleus. While the core has a moderately steep non-thermal radio spectrum ( S _ν∝ν^α; α^4.9_1.5≈−0.4), the inner kpc region shows a steeper spectrum (α^8.4_1.5≈−0.9). This latter spectrum is typical of galaxies where energy losses are high, indicating that the escape rate of cosmic ray electrons in NGC 5033 is low. The nucleus contributes little to the total 1.4-GHz radio power of NGC 5033 and, based on the radio to far-infrared (FIR) relation, it appears that the radio and FIR emission from NGC 5033 are dominated by a starburst that during the last 10 Myr produced stars at a rate of 2.8 M_⊙ yr⁻¹ yielding a supernova (type Ib/c and II) rate of 0.045 yr⁻¹. This supernova rate corresponds to about 1 SN event every 22 yr. Finally, from our deep 8.4-GHz VLA-D image, we suggest the existence of a radio spur in NGC 5033, which could have been due to a hot superbubble formed as a consequence of sequential supernova explosions occurring during the lifetime of a giant molecular cloud.     

LoCuSS: the connection between brightest cluster galaxy activity, gas cooling and dynamical disturbance of X-ray cluster cores

We study the distribution of projected offsets between the cluster X-ray centroid and the brightest cluster galaxy (BCG) for 65 X-ray-selected clusters from the Local Cluster Substructure Survey, with a median redshift of   z = 0.23  . We find a clear correlation between X-ray/BCG projected offset and the logarithmic slope of the cluster gas density profile at  0.04 r ₅₀₀(α  ), implying that more dynamically disturbed clusters have weaker cool cores. Furthermore, there is a close correspondence between the activity of the BCG, in terms of detected Hα and radio emission, and the X-ray/BCG offset, with the line-emitting galaxies all residing in clusters with X-ray/BCG offsets of ≤15 kpc. Of the BCGs with  α < −0.85  and an offset <0.02 r ₅₀₀, 96 per cent (23/24) have optical emission and 88 per cent (21/24) are radio active, while none has optical emission outside these criteria. We also study the cluster gas fraction ( f _gas) within r ₅₀₀ and find a significant correlation with X-ray/BCG projected offset. The mean f _gas of the 'small offset' clusters (<0.02 r ₅₀₀) is  0.106 ± 0.005 (σ= 0.03  ) compared to  0.145 ± 0.009 (σ= 0.04  ) for those with an offset >0.02 r ₅₀₀, indicating that the total mass may be systematically underestimated in clusters with larger X-ray/BCG offsets. Our results imply a link between cool core strength and cluster dynamical state consistent with the view that cluster mergers can significantly perturb cool cores, and set new constraints on models of the evolution of the intracluster medium.     

Feedback through multiple outbursts in the cluster 2A 0335+096

We examine the core of the X-ray bright galaxy cluster 2A 0335+096 using deep Chandra X-ray imaging and spatially resolved spectroscopy, and include new radio observations. The set of around eight X-ray bright blobs in the core of the cluster, appearing like eggs in a bird's nest, contains multiphase gas from ∼0.5 to 2 keV. The morphology of the coolest X-ray emitting gas at 0.5 keV temperature is similar to the Hα emitting nebula known in this cluster, which surrounds the central galaxy. XMM–Newton grating spectra confirm the presence of material at these temperatures, showing reasonable agreement with Chandra emission measures. On scales of 80 to 250 kpc, there is a low temperature, high metallicity, swirl of intracluster medium as seen in other clusters. In the core, we find evidence for a further three X-ray cavities, in addition to the two previously discovered. Enhancements in 1.5 GHz radio emission are correlated with the X-ray cavities. The total  4 PV   enthalpy associated with the cavities is around  5 × 10⁵⁹ erg  . This energy would be enough to heat the cooling region for  ∼5 × 10⁷ yr  . We find a maximum pressure discontinuity of 26 per cent (2σ) across the surface brightness edge to the south-west of the cluster core. This corresponds to an upper limit on the Mach number of the cool core with respect to its surroundings of 0.55.     

Radio study of X-ray clusters of galaxies — I. A3528: a pre-merging cluster in the core of the Shapley Supercluster

As part of an extensive radio–IR–optical–X-ray study of ROSAT clusters of galaxies in the Hydra region we have observed the bimodal Abell cluster A3528, located in the core of the Shapley Supercluster ( z  ≃ 0.053), with the Molonglo Observatory Synthesis Telescope at 843 MHz and the Australia Telescope Compact Array at 1.4 and 2.4 GHz. This is part I in a series of papers which looks at the relationship between the radio and X-ray emission in samples of ROSAT selected clusters.   The radio source characteristics — tailed morphologies and steep spectra — are consistent with the effects of a dense intracluster medium and the pre-merging environment of A3528. In particular, we present evidence that the minor member of the radio-loud dumbbell galaxy located at the centre of the northern component of A3528 is on a plunging orbit. We speculate that this orbit may have been induced by the tidal interactions between the merging components of A3528. In addition, the radio source associated with the dominant member of the dumbbell galaxy exhibits many of the characteristics of compact steep spectrum sources. We argue that the radio emission from this source was triggered ∼ 10⁶ yr ago by tidal interactions between the two members of the dumbbell galaxy, strengthening the argument that compact steep spectrum (CSS) sources are young.   Re-analysis of archive pointed Position Sensitive Proportional Counter (PSPC) data using multiresolution filtering suggests the presence of an AGN and/or a cooling flow in the southern component of A3528.     

On the nature of EXO 0423.4−0840

We present a Chandra observation of the candidate BL Lac object EXO 0423.4−0840. The X-ray emission from EXO 0423.4−0840 is clearly extended, and is associated with an optical early-type galaxy, MCG-01-12-005, at the centre of cluster ClG 0422-09. We do not detect a point source that can be associated with a BL Lac, but we found a small radio source in the centre of MCG-01-12-005. The cluster gas temperature mapped by the Chandra observation drops continuously from 80 kpc towards the centre, and is locally single phase. We measure a metallicity profile that declines outwards with a value  0.8 Z_⊙  in the centre, dropping to  0.35 Z_⊙  at larger radius, which we interpret as a superposition of cluster gas and a dense interstellar medium (ISM) in the central galaxy. Although the temperature profile suggests that conduction is not efficient, the ISM and intra-cluster medium seem not to have mixed. The entropy profile declines continuously towards the centre, in agreement with recent results on groups and clusters. The radio source appears to have had some effect in terms of gas heating, as seen in the small-scale (∼10 kpc) entropy core, and the asymmetric hard emission on the same scale.     

A high-resolution radio study of neutral gas in the starburst galaxy NGC 520

We present subarcsec angular resolution observations of the neutral gas in the nearby starburst galaxy NGC 520. The central kpc region of NGC 520 contains an area of significantly enhanced star formation. The radio continuum structure of this region resolves into ∼10 continuum components. By comparing the flux densities of the brightest of these components at 1.4 GHz with published 15-GHz data we infer that these components detected at 1.4 and 1.6 GHz are related to the starburst and are most likely to be collections of several supernova remnants within the beam. None of these components is consistent with emission from an active galactic nuclei. Both neutral hydrogen (H  i ) and hydroxyl (OH) absorption lines are observed against the continuum emission, along with a weak OH maser feature probably related to the star formation activity in this galaxy. Strong H  i absorption  ( N _H∼ 10²² atoms cm⁻²)  traces a velocity gradient of 0.5 km s⁻¹ pc⁻¹ across the central kpc of NGC 520. The H  i absorption velocity structure is consistent with the velocity gradients observed in both the OH absorption and in CO emission observations. The neutral gas velocity structure observed within the central kpc of NGC 520 is attributed to a kpc-scale ring or disc. It is also noted that the velocity gradients observed for these neutral gas components appear to differ with the velocity gradients observed from optical ionized emission lines. This apparent disagreement is discussed and attributed to the extinction of the optical emission from the actual centre of this source hence implying that optical ionized emission lines are only detected from regions with significantly different radii to those sampled by the observations presented here.     

The growth and assembly of a massive galaxy at z∼ 2

We study the stellar mass assembly of the Spiderweb galaxy  (MRC 1138−262)  , a massive   z = 2.2  radio galaxy in a protocluster and the probable progenitor of a brightest cluster galaxy. Nearby protocluster galaxies are identified and their properties are determined by fitting stellar population models to their rest-frame ultraviolet to optical spectral energy distributions. We find that within 150 kpc of the radio galaxy the stellar mass is centrally concentrated in the radio galaxy, yet most of the dust-uncorrected, instantaneous star formation occurs in the surrounding low-mass satellite galaxies. We predict that most of the galaxies within 150 kpc of the radio galaxy will merge with the central radio galaxy by   z = 0  , increasing its stellar mass by up to a factor of ≃2. However, it will take several hundred Myr for the first mergers to occur, by which time the large star formation rates are likely to have exhausted the gas reservoirs in the satellite galaxies. The tidal radii of the satellite galaxies are small, suggesting that stars and gas are being stripped and deposited at distances of tens of kpc from the central radio galaxy. These stripped stars may become intracluster stars or form an extended stellar halo around the radio galaxy, such as those observed around cD galaxies in cluster cores.     

Long-term optical photometric monitoring of the quasar SDSS J153259.96−003944.1

We present results from an observation of the luminous cluster of galaxies Abell 2204 using the Chandra X-ray Observatory. We show the core of the cluster has a complex morphological structure, made up of a high-density core  ( n _e∼ 0.2 cm⁻³)  with flat surface brightness, a surrounding central plateau, a tail-like feature, wrapping around to the east, and an unusual radio source. A temperature map and deprojected profile shows that the temperature rises steeply outside these regions, until around ∼100 kpc where it drops, then rises again. Abundance maps and profiles show that there is a corresponding increase in abundance at the same radius as where the temperature drops. In addition, there are two cold fronts at radii of ∼28 and 54.5 kpc. The disturbed morphology indicates that the cluster core may have undergone a merger. However, despite this disruption, the mean radiative cooling time in the centre is short (∼230 Myr) and the morphology is regular on large scales.     

The K-band luminosity function at z = 1: a powerful constraint on galaxy formation theory

We report the first detection of an inverse Compton X-ray emission, spatially correlated with a very steep spectrum radio source (VSSRS), 0038-096, without any detected optical counterpart, in cluster Abell 85. The ROSAT PSPC data and its multiscale wavelet analysis reveal a large-scale (linear diameter of the order of 500 h ⁻¹₅₀ kpc), diffuse X-ray component, in addition to the thermal bremsstrahlung, overlapping an equally large-scale VSSRS. The primeval 3 K background photons, scattering off the relativistic electrons, can produce the X-rays at the detected level. The inverse Compton flux is estimated to be (6.5 ± 0.5) × 10⁻¹³ erg s⁻¹ cm⁻² in the 0.5–2.4 keV X-ray band. A new 327-MHz radio map is presented for the cluster field. The synchrotron emission flux is estimated to be (6.6 ± 0.90) × 10⁻¹⁴ erg s⁻¹ cm⁻² in the 10–100 MHz radio band. The positive detection of both radio and X-ray emission from a common ensemble of relativistic electrons leads to an estimate of (0.95 ± 0.10) × 10⁻⁶ G for the cluster-scale magnetic field strength. The estimated field is free of the 'equipartition' conjecture, the distance, and the emission volume. Further, the radiative fluxes and the estimated magnetic field imply the presence of 'relic' (radiative lifetime ≳ 10⁹ yr) relativistic electrons with Lorentz factors γ ≈ 700–1700; this would be a significant source of radio emission in the hitherto unexplored frequency range ν ≈ 2–10 MHz.     

Multiwavelength observations of an evolved galaxy group: an end-point of galaxy merging?

The group of galaxies RXJ1340.6+4018 has approximately the same bolometric X-ray luminosity as other bright galaxy groups and poor clusters such as the Virgo cluster. However, 70 per cent of the optical luminosity of the group comes from a dominant giant elliptical galaxy, compared with 5 per cent from M87 in Virgo.The second brightest galaxy in RXJ1340.6+4018 is a factor of 10 fainter (Δ m ₁₂=2.5 mag) than the dominant elliptical, and the galaxy luminosity function has a gap at about L *.
We interpret the properties of the system as a result of galaxy merging within a galaxy group. We find that the central galaxy lies on the Fundamental Plane of ellipticals, has an undisturbed, non-cD morphology, and has no spectral features indicative of recent star formation, suggesting that the last major merger occurred ≳4 Gyr ago. The deviation of the system from the cluster L _X− T relation in the opposite sense to most groups may be caused by an early epoch of formation of the group or a strong cooling flow.
The unusual elongation of the X-ray isophotes and the similarity between the X-ray and optical ellipticities at large radii (∼230 kpc) suggest that both the X-ray gas and the outermost stars of the dominant galaxy are responding to an elongated dark matter distribution. RXJ1340.6+4018 may be part of a filamentary structure related to infall in the outskirts of the cluster A1774.     

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.     

Fabry–Perot imaging of NGC 6090 in H2 emission

We have obtained an H₂ v =1–0 S(1) image of a merging galaxy system, NGC 6090, by using a Fabry–Perot imager. The H₂ emission originates between the double nuclei of pre-merger galaxies, and exhibits an arc-like or ring-like structure almost connecting the double nuclei. This structure is similar to that suggested for Arp 220 from the velocity field measured by CO radio emission. The separation of the double nuclei in NGC 6090 is 5–6 arcsec, corresponding to a projected distance of 3–4 kpc. This is much larger than that of Arp 220 and suggests that the molecular gas distribution can form an organized shape between the nuclei, such as a ring, in a rather early phase of merging.     

Intermittent jet activity in the radio galaxy 4C 29.30?

We present radio observations at frequencies ranging from 240 to 8460 MHz of the radio galaxy 4C 29.30 (J0840+2949) using the Giant Metrewave Radio Telescope (GMRT), the Very Large Array (VLA) and the Effelsberg telescope. We report the existence of weak extended emission with an angular size of ∼520 arcsec (639 kpc) within which a compact edge-brightened double-lobed source with a size of 29 arcsec (36 kpc) is embedded. We determine the spectrum of the inner double from 240 to 8460 MHz and show that it has a single power-law spectrum with a spectral index of ∼0.8. Its spectral age is estimated to be ≲33 Myr. The extended diffuse emission has a steep spectrum with a spectral index of ∼1.3 and a break frequency ≲240 MHz. The spectral age is ≳200 Myr, suggesting that the extended diffuse emission is due to an earlier cycle of activity. We re-analyse archival X-ray data from Chandra and suggest that the X-ray emission from the hotspots consists of a mixture of non-thermal and thermal components, the latter being possibly due to gas which is shock heated by the jets from the host galaxy.     

The radio properties of the cD galaxy of Abell 2390

We present multifrequency, multi-epoch radio imaging of the complex radio source B2151+174 in the core of the cluster, Abell 2390  ( z ≃ 0.23)  . From new and literature data, we conclude that the Faranoff–Riley type II (FRII)-powerful radio source is the combination of a compact, core-dominated 'medium-sized symmetric object' (MSO) with a more extended, steeper spectrum mini-halo. B2151+174 is unusual in a number of important aspects. (i) It is one of the most compact and flat spectrum sources in a cluster core known; (ii) it shows a complex, compact twin-jet structure in a north–south orientation; (iii) the orientation of the jets is 45° misaligned with apparent structure (ionization cones and dust disc) of the host galaxy on larger scales. Since the twin-jet of the MSO has its northern half with an apparent 'twist', it might be that precession of the central supermassive black hole explains this misalignment. B2151+174 may be an example of the early stage (10³–10⁴ yr duration) of a 'bubble' being blown into the intracluster medium where the plasma has yet to expand.     

VLA polarimetry observations of PKS 2322−123: estimating magnetic fields in the Abell 2597 cluster

We present 5-, 8-, and 15-GHz total intensity and polarimetric observations of the radio source PKS 2322−123 taken with the Very Large Array (VLA). This small (11 kpc) source is located at the centre of the cooling-core cluster Abell 2597. The inner X-ray structure, the radio morphology and the steep spectral index  (α=−1.8)  in the lobes all suggest that the radio emission is confined by the ambient X-ray gas. We detect a small region of polarized flux in the southern lobe and are able to calculate a Faraday rotation measure (RM) of 3620 rad m⁻² over this region. Based on these observations and Chandra X-ray data, we suggest that the southern lobe has been deflected from its original south-western orientation to the south and into our line of sight. Using the observed rotation measures (RMs) and our calculated electron density profiles, and assuming both a uniform and tangled magnetic field topology, we estimate a lower limit of the line-of-sight cluster magnetic field,   B _∥= 2.1  μG  .     

Chandra X-ray observations of the 3C 295 cluster core

We examine the properties of the X-ray gas in the central regions of the distant ( z =0.46) , X-ray luminous cluster of galaxies surrounding the powerful radio source 3C 295, using observations made with the Chandra Observatory . Between radii of 50 and 500 kpc, the cluster gas is approximately isothermal with an emission-weighted temperature, kT ∼5 keV . Within the central 50-kpc radius this value drops to kT ∼3.7 keV . The spectral and imaging Chandra data indicate the presence of a cooling flow within the central 50-kpc radius of the cluster, with a mass deposition rate of approximately 280 M_⊙ yr⁻¹. We estimate an age for the cooling flow of 1–2 Gyr , which is approximately 1000 times older than the central radio source. We find no evidence in the X-ray spectra or images for significant heating of the X-ray gas by the radio source. We report the detection of an edge-like absorption feature in the spectrum for the central 50-kpc region, which may be caused by oxygen-enriched dust grains. The implied mass in metals seen in absorption could have been accumulated by the cooling flow over its lifetime. Combining the results on the X-ray gas density profile with radio measurements of the Faraday rotation measure in 3C 295, we estimate the magnetic field strength in the region of the cluster core to be B ∼12 μG .     

Variable circular polarization associated with relativistic ejections from GRS 1915 + 105

We describe H α , SCUBA and MERLIN imaging of the interacting galaxy pair NGC 4490 and 4485. We detect an H α filament emerging from the disc of NGC 4490 to a projected distance of 3 kpc which has counterparts in both the radio continuum and H  i . The H  i counterpart extends to a projected distance of ∼30 kpc from NGC 4490 and we argue that this is evidence that the giant H  i envelope in this system has its origins in star formation. We use SCUBA and radio continuum data to attempt to place constraints on the distribution of dust with respect to the star forming regions. This analysis is limited by the lack of an independent estimate of the dust temperature, something that both 'SIRTF' and 'SOFIA' will be able to provide, however we find some evidence that most obscuring dust is not located within H  ii regions themselves.     

Deep inside the core of Abell 1795: the Chandra view

We present X-ray spatial and spectral analysis of the Chandra data from the central     of the cluster of galaxies Abell 1795. The plasma temperature rises outwards by a factor of 3, whereas the iron abundance decreases by a factor of 4. The spatial distribution of oxygen, neon, sulphur, silicon and iron shows that supernovae Type Ia dominate the metal enrichment process of the cluster plasma within the inner 150 kpc. Resolving both the gas density and temperature in nine radial bins, we recover the gravitational mass density profile and show that it flattens within 100 kpc as   ρ _DM∝ r ^-0.6  with a power-law index flatter than −1 at >3 σ level. The observed motion of the central galaxy and the presence of excesses and deficits along the north–south direction in the brightness distribution indicate that the central cluster region is not relaxed. In the absence of any non-gravitational heating source, the data from the inner ∼200 kpc indicate the presence of a cooling flow with an integrated mass deposition rate of about 100 M_⊙ yr⁻¹. Over the same cluster region, the observed rate of 74 M_⊙ yr⁻¹ is consistent with the recent XMM-Newton Reflection Grating Spectrometer limit of 150 M_⊙ yr⁻¹.