AzTEC Millimetre Survey of the COSMOS field – II. Source count overdensity and correlations with large-scale structure

We report an overdensity of bright submillimetre galaxies (SMGs) in the 0.15 deg² AzTEC/COSMOS survey and a spatial correlation between the SMGs and the optical-IR galaxy density at   z ≲ 1.1  . This portion of the COSMOS field shows a  ∼3σ  overdensity of robust SMG detections when compared to a background, or 'blank-field', population model that is consistent with SMG surveys of fields with no extragalactic bias. The SMG overdensity is most significant in the number of very bright detections (14 sources with measured fluxes   S _1.1 mm > 6  mJy), which is entirely incompatible with sample variance within our adopted blank-field number densities and infers an overdensity significance of  ≫ 4σ  . We find that the overdensity and spatial correlation to optical-IR galaxy density are most consistent with lensing of a background SMG population by foreground mass structures along the line of sight, rather than physical association of the SMGs with the   z ≲ 1.1  galaxies/clusters. The SMG positions are only weakly correlated with weak-lensing maps, suggesting that the dominant sources of correlation are individual galaxies and the more tenuous structures in the survey region, and not the massive and compact clusters. These results highlight the important roles cosmic variance and large-scale structure can play in the study of SMGs.     

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.     

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.     

Unresolved emission and ionized gas in the bulge of M31

We study the origin of unresolved X-ray emission from the bulge of M31 based on archival Chandra and XMM–Newton observations. We demonstrate that three different components are present. (i) Broad-band emission from a large number of faint sources – mainly accreting white dwarfs and active binaries, associated with the old stellar population, similar to the Galactic ridge X-ray emission of the Milky Way. The X-ray to K -band luminosity ratios are compatible with those for the Milky Way and for M32; in the 2–10 keV band, the ratio is  (3.6 ± 0.2) × 10²⁷ erg s⁻¹ L⁻¹_⊙  . (ii) Soft emission from ionized gas with a temperature of about ∼300 eV and a mass of  ∼2 × 10⁶ M_⊙  . The gas distribution is significantly extended along the minor axis of the galaxy, suggesting that it may be outflowing in the direction perpendicular to the galactic disc. The mass and energy supply from evolved stars and Type Ia supernovae is sufficient to sustain the outflow. We also detect a shadow cast on the gas emission by spiral arms and the 10-kpc star-forming ring, confirming significant extent of the gas in the 'vertical' direction. (iii) Hard extended emission from spiral arms, most likely associated with young stellar objects and young stars located in the star-forming regions. The   L _X/SFR  (star formation rate) ratio equals  ∼9 × 10³⁸ (erg s⁻¹)(M_⊙ yr⁻¹)⁻¹  , which is about ∼1/3 of the high-mass X-ray binary contribution, determined earlier from Chandra observations of other nearby galaxies.     

A disturbed molecular disc in the Circinus galaxy

We have used the Swedish ESO Submillimeter Telescope to observe the molecular gas in the Circinus galaxy using the CO(1 → 0) transition as a tracer. The central region and major axis have been mapped and several other points were also observed. The gas in the galaxy is concentrated towards the nucleus, the peak being coincident with the radio/optical core. The inclination of the molecular galactic disc is more comparable to that of the radio continuum than to that of the large-scale H  i emission. Evidence for an anomalous spur structure pointing radially away from the galactic centre is presented, and may indicate a causal link between it and similar features seen in optical lines and radio continuum. Our data suggest the presence of a central molecular ring or disc with radius 300 ± 50 pc and a rotation velocity of about 200 km s⁻¹ (assuming i  = 73°). The dynamical mass of the nucleus is estimated to be no greater than 3.9 × 10⁹ M_⊙. Assuming that the distribution of gas varies smoothly in the outer regions, we calculate the mass of molecular gas in the galaxy to be at least M _mol = 1.1 × 10⁹ M_⊙, and the star-forming efficiency to be 11 ± 2 L_⊙ M_⊙⁻¹. These results imply that Circinus is undergoing a massive central starburst which may be, at least partially, responsible for its extended minor axis emission seen in several wavebands.     

The local star formation rate and radio luminosity density

We present a new determination of the local volume-averaged star formation rate from the 1.4-GHz luminosity function of star forming galaxies. Our sample, taken from the   B ≤12  Revised Shapley–Ames catalogue (231 normal spiral galaxies over an effective area of 7.1 sr) has ≃100 per cent complete radio detections and is insensitive to dust obscuration and cirrus contamination. After removal of known active galaxies, the best-fitting Schechter function has a faint-end slope of  −1.27±0.07  in agreement with the local H α luminosity function, characteristic luminosity   L ∗=(2.6±0.7)×10²² W Hz⁻¹  and density   φ ∗=(4.8±1.1)×10⁻⁴ Mpc⁻³.  The inferred local radio luminosity density of  (1.73±0.37±0.03)×10¹⁹ W Hz⁻¹ Mpc⁻³  (Poisson noise, large-scale structure fluctuations) implies a volume-averaged star formation rate ∼2 times larger than the Gallego et al. H α estimate, i.e.   ρ _1.4 GHz=(2.10±0.45±0.04)×10⁻² M_⊙ yr⁻¹ Mpc⁻³  for a Salpeter initial mass function from  0.1–125 M_⊙  and Hubble constant of 50 km s⁻¹ Mpc⁻¹. We demonstrate that the Balmer decrement is a highly unreliable extinction estimator, and argue that optical–ultraviolet (UV) star formation rates (SFRs) are easily underestimated, particularly at high redshift.     

The host galaxy of a narrow-line Seyfert 1 galaxy, RE J1034+396, with X-ray quasi-periodic oscillations

Using simple stellar population synthesis, we model the bulge stellar contribution in the optical spectrum of a narrow-line Seyfert 1 galaxy, RE J1034+396. We find that its bulge stellar velocity dispersion is  67.7 ± 8 km s⁻¹  . The supermassive black hole (SMBH) mass is about  (1–4) × 10⁶ M_⊙  if it follows the well-known   M _BH–σ_*  relation found in quiescent galaxies. We also derive the SMBH mass from the Hβ second moment, which is consistent with that from its bulge stellar velocity dispersion. The SMBH mass of (1–4)  × 10⁶ M_⊙  implies that the X-ray quasi-periodic oscillation (QPO) of RE J1034+396 can be scaled to a high-frequency QPO at 27–108 Hz found in Galactic black hole binaries with a  10-M_⊙  black hole. With the mass distribution in different age stellar populations, we find that the mean specific star formation rate (SSFR) over the past 0.1 Gyr is  0.0163 ± 0.0011  Gyr⁻¹, the stellar mass in the logarithm is  10.155 ± 0.06  in units of solar mass and the current star formation rate is  0.23 ± 0.016 M_⊙ yr⁻¹  . For RE J1034+396, there is no relation between the Eddington ratio and the SSFR as suggested by Chen et al., despite a larger scatter in their relation. We also suggest that about 7.0 per cent of the total Hα luminosity and 50 per cent of the total [O  ii ] luminosity come from the star formation process.     

Hyperluminous infrared galaxies

39 galaxies are now known, from follow-up of faint IRAS sources and from submillimetre observations of high-redshift AGN, with far-infrared luminosities >10¹³ L_⊙. 13 of these, which have been found in 60- or 850-μm surveys, form an important unbiased subsample. 12 have been found by comparison of 60-μm surveys with quasar or radio galaxy catalogues, or from infrared surveys with colour selection biased towards AGN, while a further 14 have been found through submillimetre observations of known high-redshift AGN. In this paper I argue, on the basis of detailed modelling of the spectral energy distributions of hyperluminous galaxies with accurate radiative transfer models, and from evidence of high gas mass in several cases, that the bulk of the emission from these galaxies at rest frame wavelengths ≥50 μm is caused by star formation. Even after correction for the effects of lensing, hyperluminous galaxies with emission peaking at rest frame wavelengths ≥50 μm are therefore undergoing star formation at rates >10³ M_⊙ yr⁻¹ and are strong candidates for being primeval galaxies, in the process of a major episode of star formation.     

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.     

Chandra measurements of the distribution of mass in the luminous lensing cluster Abell 2390

We present spatially resolved X-ray spectroscopy of the luminous lensing cluster Abell 2390, using observations made with the Chandra observatory. The temperature of the X-ray gas rises with increasing radius within the central ∼ 200 kpc of the cluster, and then remains approximately isothermal, with kT =11.5_−1.6^+1.5 keV , out to the limits of the observations at r ∼1.0 Mpc . The total mass profile determined from the Chandra data has a form in good agreement with the predictions from numerical simulations. Using the parametrization of Navarro, Frenk and White, we measure a scale radius r _s∼0.8 Mpc and a concentration parameter c ∼3 . The best-fitting X-ray mass model is in good agreement with independent gravitational lensing results and optical measurements of the galaxy velocity dispersion in the cluster. The X-ray gas to total mass ratio rises with increasing radius with f _gas∼21 per cent at r =0.9 Mpc . The azimuthally averaged 0.3–7.0 keV surface brightness profile exhibits a small core radius and a clear 'break' at r ∼500 kpc , where the slope changes from S _X ∼^∝  r ^−1.5 to S _X ∼^∝  r ^−3.6 . The data for the central region of the cluster indicate the presence of a cooling flow with a mass deposition rate of 200–300 M_⊙ yr⁻¹ and an effective age of 2–3 Gyr .     

Quantifying the coexistence of massive black holes and dense nuclear star clusters

In large spheroidal stellar systems, such as elliptical galaxies, one invariably finds a  10⁶–10⁹ M_⊙  supermassive black hole at their centre. In contrast, within dwarf elliptical galaxies one predominantly observes a  10⁵–10⁷ M_⊙  nuclear star cluster. To date, few galaxies have been found with both types of nuclei coexisting and even less have had the masses determined for both central components. Here, we identify one dozen galaxies housing nuclear star clusters and supermassive black holes whose masses have been measured. This doubles the known number of such hermaphrodite nuclei – which are expected to be fruitful sources of gravitational radiation. Over the host spheroid (stellar) mass range  10⁸–10¹¹ M_⊙  , we find that a galaxy's nucleus-to-spheroid (baryon) mass ratio is not a constant value but decreases from a few per cent to ∼0.3 per cent such that  log[( M _BH+ M _NC)/ M _sph]=−(0.39 ± 0.07) log[ M _sph/10¹⁰ M_⊙]− (2.18 ± 0.07)  . Once dry merging commences and the nuclear star clusters disappear, this ratio is expected to become a constant value.
As a byproduct of our investigation, we have found that the projected flux from resolved nuclear star clusters is well approximated with Sérsic functions having a range of indices from ∼0.5 to ∼3, the latter index describing the Milky Way's nuclear star cluster.     

The SCUBA HAlf Degree Extragalactic Survey – VI. 350-μm mapping of submillimetre galaxies

A follow-up survey using the Submillimetre High-Angular Resolution Camera (SHARC-II) at 350 μm has been carried out to map the regions around several 850-μm-selected sources from the Submillimetre HAlf Degree Extragalactic Survey (SHADES). These observations probe the infrared (IR) luminosities and hence star formation rates in the largest existing, most robust sample of submillimetre galaxies (SMGs). We measure 350-μm flux densities for 24 850-μm sources, seven of which are detected at ≥2.5σ within a 10 arcsec search radius of the 850-μm positions. When results from the literature are included the total number of 350-μm flux density constraints of SHADES SMGs is 31, with 15 detections. We fit a modified blackbody to the far-IR (FIR) photometry of each SMG, and confirm that typical SMGs are dust-rich  ( M _dust≃ 9 × 10⁸ M_⊙)  , luminous  ( L _FIR≃ 2 × 10¹² L_⊙)  star-forming galaxies with intrinsic dust temperatures of ≃35 K and star formation rates of  ≃400 M_⊙ yr⁻¹  . We have measured the temperature distribution of SMGs and find that the underlying distribution is slightly broader than implied by the error bars, and that most SMGs are at 28 K with a few hotter. We also place new constraints on the 350-μm source counts, N ₃₅₀(>25 mJy) ∼ 200–500 deg⁻².     

The census of nuclear activity of late-type galaxies in the Virgo cluster

The first spectroscopic census of active galactic nuclei (AGNs) associated with late-type galaxies in the Virgo cluster was carried out by observing 213 out of a complete set of 237 galaxies more massive than   M _dyn > 10^8.5 M_⊙  . Among them, 77 are classified as AGNs [including 21 transition objects, 47 low-ionization nuclear emission regions (LINERs) and nine Seyferts] and comprise 32 per cent of the late-type galaxies in Virgo. Due to spectroscopic incompleteness, at most 21 AGNs are missed in the survey, so that the fraction would increase up to 41 per cent. Using corollary near-infrared observations that enable us to estimate galaxy dynamical masses, it is found that AGNs are hosted exclusively in massive galaxies, i.e.   M _dyn≳ 10¹⁰ M_⊙  . Their frequency increases steeply with the dynamical mass from zero at   M _dyn≈ 10^9.5 M_⊙  to virtually 1 at   M _dyn > 10^11.5 M_⊙  . These frequencies are consistent with those of low-luminosity AGNs found in the general field by the Sloan Digital Sky Survey. Massive galaxies that harbour AGNs commonly show conspicuous r -band star-like nuclear enhancements. Conversely, they often, but not necessarily, contain massive bulges. A few well-known AGNs (e.g. M61, M100, NGC 4535) are found in massive Sc galaxies with little or no bulge. The AGN fraction seems to be only marginally sensitive to galaxy environment. We infer the black hole masses using the known scaling relations of quiescent black holes. No black holes lighter than  ∼10⁶ M_⊙  are found active in our sample.     

The XMM–Newton/2dF survey – VIII. The extended X-ray sources

We present a sample of eight extended X-ray sources detected in the wide-field (∼2.3 deg²), bright (2–10 ks) XMM–Newton /2dF survey, reaching a flux limit of  ∼2 × 10⁻¹⁴ erg s⁻¹ cm⁻²  . Of these, seven are identified as secure X-ray clusters in the soft 0.3–2 keV band using a standard wavelet algorithm on either the PN or the MOS images. Spectroscopic or photometric redshifts are available for five clusters, spanning a range between 0.12 and 0.68. The X-ray spectral fittings show temperatures between 1 and 4.6 keV, characteristic of poor clusters and groups of galaxies. We derive for the first time the XMM–Newton cluster number count  log  N –log  S   distribution albeit with poor statistics. Both the  log  N –log  S   and the luminosity–temperature relation are in good agreement with previous ROSAT results.     

Type IIP supernova SN 2004et: a multiwavelength study in X-ray, optical and radio

We present X-ray, broad-band optical and low-frequency radio observations of the bright type IIP supernova SN 2004et. The Chandra X-ray Observatory observed the supernova at three epochs, and the optical coverage spans a period of ∼470 d since explosion. The X-ray emission softens with time, and we characterize the X-ray luminosity evolution as   L _X∝ t ^−0.4  . We use the observed X-ray luminosity to estimate a mass-loss rate for the progenitor star of  ∼2 × 10⁻⁶ M_⊙ yr⁻¹  . The optical light curve shows a pronounced plateau lasting for about 110 d. Temporal evolution of photospheric radius and colour temperature during the plateau phase is determined by making blackbody fits. We estimate the ejected mass of ⁵⁶Ni to be  0.06 ± 0.03 M_⊙  . Using the expressions of Litvinova & Nadëzhin we estimate an explosion energy of  (0.98 ± 0.25) × 10⁵¹ erg  . We also present a single epoch radio observation of SN 2004et. We compare this with the predictions of the model proposed by Chevalier, Fransson & Nymark. These multiwavelength studies suggest a main-sequence progenitor mass of  ∼20 M_⊙  for SN 2004et.     

L dwarfs in the Hyades

We present the results of a proper motion survey of the Hyades to search for brown dwarfs, based on UKIRT Deep Sky Survey (UKIDSS) and Two-Micron All Sky Survey (2MASS) data. This survey covers  ∼275 deg²  to a depth of   K ∼ 15  mag, equivalent to a mass of  ∼0.05 M_⊙  assuming a cluster age of 625 Myr. The discovery of 12 L dwarf Hyades members is reported. These members are also brown dwarfs, with masses between  0.05 < M < 0.075 M_⊙  . A high proportion of these L dwarfs appear to be photometric binaries.     

Two new large-separation gravitational lenses from SDSS

We present discovery images, together with follow-up imaging and spectroscopy, of two large-separation gravitational lenses found by our survey for wide arcs [the CAmbridge Sloan Survey Of Wide ARcs in the skY (CASSOWARY)]. The survey exploits the multicolour photometry of the Sloan Digital Sky Survey to find multiple blue components around red galaxies. CASSOWARY 2 (or 'the Cheshire Cat') is composed of two massive early-type galaxies at   z = 0.426  and 0.432, respectively, lensing two background sources, the first a star-forming galaxy at   z = 0.97  and the second a high -redshift galaxy  ( z > 1.4)  . There are at least three images of the former source and probably four or more of the latter, arranged in two giant arcs. The mass enclosed within the larger arc of radius ∼11 arcsec is  ∼33 × 10¹² M_⊙  . CASSOWARY 3 comprises an arc of three bright images of a   z = 0.725  source, lensed by a foreground elliptical at   z = 0.274  . The radius of the arc is ∼4 arcsec and the enclosed mass is  ∼2.5 × 10¹² M_⊙  . Together with earlier discoveries like the Cosmic Horseshoe and the 8 o'clock Arc, these new systems, with separations intermediate between the arcsecond-separation lenses of typical strong galaxy lensing and arcminute-separation cluster lenses, probe the very high end of the galaxy mass function.     

The X-ray luminosity function of AGN at z∼ 3

We combine Lyman-break colour selection with ultradeep (≳200 ks) Chandra X-ray imaging over a survey area of ∼0.35 deg² to select high-redshift active galactic nuclei (AGN). Applying careful corrections for both the optical and X-ray selection functions, the data allow us to make the most accurate determination to date of the faint end of the X-ray luminosity function (XLF) at   z ∼ 3  . Our methodology recovers a number density of X-ray sources at this redshift which is at least as high as previous surveys, demonstrating that it is an effective way of selecting high z AGN. Comparing to results at   z = 1  , we find no evidence that the faint slope of the XLF flattens at high z , but we do find significant (factor ∼3.6) negative evolution of the space density of low luminosity AGN. Combining with bright end data from very wide surveys we also see marginal evidence for continued positive evolution of the characteristic break luminosity   L _*  . Our data therefore support models of luminosity-dependent density evolution between   z = 1  and   z = 3  . A sharp upturn in the the XLF is seen at the very lowest luminosities  ( L _X≲ 10^42.5 erg s⁻¹)  , most likely due to the contribution of pure X-ray starburst galaxies at very faint fluxes.     

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.