arXiv subjects

O. Almaini

Publications and source records attributed to O. Almaini.

At least 91 records · Page 5Linked to original sources

arXiv2005

Correlations between bright submillimetre sources and low-redshift galaxies

We present evidence for a positive angular correlation between bright submillimetre sources and low-redshift galaxies. The study was conducted using 39 sources selected from 3 contiguous, flux-limited SCUBA surveys, cross-correlated with optical field galaxies with magnitudes R<23 (with a median redshift of z~0.5). We find that the angular distribution of submm sources is skewed towards overdensities in the galaxy population, consistent with 25+-12 per cent being associated with dense, low-redshift structure. The signal appears to be dominated by the brightest sources with a flux density S_850>10mJy. We conduct Monte-Carlo simulations of clustered submm populations, and find that the probability of obtaining these correlations by chance is less than 0.4 per cent. The results may suggest that a larger than expected fraction of submm sources lie at z~0.5. Alternatively, we argue that this signal is most likely caused by gravitational lensing bias, which may be entirely expected given the steep submm source counts. Implications for future submm surveys are discussed.

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arXiv2004

Short time-scale optical variability of the dwarf Seyfert nucleus in NGC 4395

We present optical spectroscopic observations of the least-luminous known Seyfert 1 galaxy, NGC 4395, which was monitored every half-hour over the course of 3 nights. The continuum emission varied by ~35 per cent over the course of 3 nights, and we find marginal evidence for greater variability in the blue continuum than the red. A number of diagnostic checks were performed on the data in order to constrain any systematic or aperture effects. No correlations were found that adequately explained the observed variability, hence we conclude that we have observed real intrinsic variability of the nuclear source. No simultaneous variability was measured in the broad H-beta line, although given the difficulty in deblending the broad and narrow components it is difficult to comment on the significance of this result. The observed short time-scale continuum variability is consistent with NGC 4395 having an intermediate-mass (~10^5 solar masses) central supermassive black hole, rather than a very low accretion rate. Comparison with the Seyfert 1 galaxy NGC 5548 shows that the observed variability seems to scale with black hole mass in roughly the manner expected in accretion models. However the absolute time-scale of variability differs by several orders of magnitude from that expected in simple accretion disc models in both cases.

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arXiv2004

Sub-mm detection of a high redshift Type 2 QSO

We report on the first SCUBA detection of a Type 2 QSO at z=3.660 in the Chandra Deep Field South. This source is X-ray absorbed, shows only narrow emission lines in the optical spectrum and is detected in the sub-mm: it is the ideal candidate in an evolution scheme for AGN (e.g. Fabian (1999); Page et al. (2004)) of an early phase corresponding to the main growth of the host galaxy and formation of the central black hole. The overall photometry (from the radio to the X-ray energy band) of this source is well reproduced by the spectral energy distribution (SED) of NGC 6240, while it is incompatible with the spectrum of a Type 1 QSO (3C273) or a starburst galaxy (Arp 220). Its sub-mm (850 μm) to X-ray (2 keV) spectral slope (alpha_SX) is close to the predicted value for a Compton-thick AGN in which only 1% of the nuclear emission emerges through scattering. Using the observed flux at 850 μm we have derived a SFR=550--680 M$_odot/yr and an estimate of the dust mass, M$_dust=4.2 10^8 M_odot

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arXiv2004

Mid-infrared sources in the ELAIS Deep X-ray Survey

We present a cross-correlation of the European Large Area ISO survey (ELAIS) with the ELAIS Deep X-ray Survey of the N1 and N2 fields. There are 7 Chandra point sources with matches in the ELAIS Final Analysis 15um catalogue, out of a total of 28 extragalactic ISO sources present in the Chandra fields. Five of these are consistent with AGN giving an AGN fraction of ~19 per cent in the 15um flux range 0.8-6 mJy. We have co-added the hard X-ray fluxes of the individually-undetected ISO sources and find a low significance detection consistent with star formation in the remaining population. We combine our point source cross-correlation fraction with the XMM-Newton observations of the Lockman Hole and Chandra observations of the Hubble Deep Field North to constrain source count models of the mid-infrared galaxy population. The low dust-enshrouded AGN fraction in ELAIS implied by the number of cross-identifications between the ELAIS mid-infrared sample and the Chandra point sources is encouraging for the use of mid-infrared surveys to constrain the cosmic star formation history, provided there are not further large undetected populations of Compton-thick AGN.

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arXiv2004

The Search for AGN in Distant Galaxy Clusters

We are undertaking the first systematic study of the prevalence of AGN activity in a large sample of high redshift galaxy clusters. Local clusters contain mainly red elliptical galaxies, and have little or no luminous AGN activity. However, recent studies of some moderate to high redshift clusters have revealed significant numbers of luminous AGN within the cluster. This effect may parallel the Butcher-Oemler effect - the increase in the fraction of blue galaxies in distant clusters compared to local clusters. Our aim is to verify and quantify recent evidence that AGN activity in dense environments increases with redshift, and to evaluate the significance of this effect. As cluster AGN are far less prevalent than field sources, a large sample of over 120 cluster fields at z > 0.1 has been selected from the Chandra archives and is being analysed for excess point sources. The size of the excess, the radial distribution and flux of the sources and the dependence of these on cluster redshift and luminosity will reveal important information about the triggering and fueling of AGN.

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arXiv2003

The AGN content of the z=0.83 cluster MS1054-0321

We present a survey of X-ray point sources in a 91 ksec Chandra ACIS-S observation of the z=0.83 cluster MS1054-0321. We detect 47 X-ray sources within the 8.3 arcmin by 8.3 arcmin field, of which two are immediately confirmed from pre-existing spectroscopy to be at the redshift of the cluster. At fluxes brighter than S_(0.5 - 8 keV) = 5 x 10^{-15} erg s^{-1} cm^{-2} we find a ~ 2 sigma excess compared to predictions from field surveys, consistent with an excess of approximately 6 AGN. If these sources are associated with the cluster, they too are AGN with luminosities of order L_(0.5 - 8 keV) ~ 10^{43} erg s^{-1}. Combined with the identification of 7 cluster AGN from deep radio observations (Best et al. 2002), these observations suggest significantly enhanced AGN activity in MS1054-03 compared to local galaxy clusters. Interestingly, the excess of X-ray detected AGN is found at radial distances of between 1 and 2 Mpc, suggesting they may be associated with infalling galaxies. The radio AGN are seen within the inner Mpc of the cluster and are largely undetected in the X-ray, suggesting they are either intrinsically less luminous and/or heavily obscured.

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arXiv2003

The ELAIS Deep X-ray Survey I: Chandra Source Catalogue and First Results

We present an analysis of two deep (75 ks) Chandra observations of the European Large Area ISO Survey (ELAIS) fields N1 and N2 as the first results from the ELAIS deep X-ray survey. This survey is being conducted in well studied regions with extensive multi-wavelength coverage. Here we present the Chandra source catalogues along with an analysis of source counts, hardness ratios and optical classifications. A total of 233 X-ray point sources are detected in addition to 2 soft extended sources, which are found to be associated with galaxy clusters. An over-density of sources is found in N1 with 30% more sources than N2, which we attribute to large-scale structure. A similar variance is seen between other deep Chandra surveys. The source count statistics reveal an increasing fraction of hard sources at fainter fluxes. The number of galaxy-like counterparts also increases dramatically towards fainter fluxes, consistent with the emergence of a large population of obscured sources.

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arXiv2002

Do sub-mm sources and quasars form an evolutionary sequence?

The high redshift sub-mm sources discovered by SCUBA are widely believed to represent the dust-enshrouded formation of massive elliptical galaxies. Given the strong evidence for a link between the formation of the spheroid and the growth of the central black hole, one might expect to see a luminous quasar at the nucleus of every SCUBA source. Somewhat surprisingly, however, only a very small fraction (~5%) are detected by Chandra with quasar luminosities. In this paper I discuss some of the implications of these results and discuss the accumulating evidence that sub-mm sources and quasars may represent different stages in the evolutionary sequence of a massive proto-spheroid.

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arXiv2002

Obscured AGN from the ELAIS Deep X-ray Survey

The sources discovered in deep hard X-ray surveys with 2-8 keV fluxes of 10^-14 erg cm^-2 s^-1 make up the bulk of the X-ray background at these energies. We present here detailed multi-wavelength observations of three such sources from the ELAIS Deep X-ray Survey. The observations include sensitive near-infrared spectroscopy with the Subaru Telescope and X-ray spectral information from the Chandra X-ray Observatory. The sources observed all have optical-to-near-IR colours redder than an unobscured quasar and comprise a reddened quasar, a radio galaxy and an optically-obscured AGN. The reddened quasar is at a redshift z=2.61 and shows a very large X-ray absorbing column of N_H approx 3.10^23 cm^-2. This contrasts with the relatively small amount of dust reddening, implying a gas-to-dust ratio along the line-of-sight a hundred times greater than that of the Milky Way. The radio galaxy at z=1.57 shows only narrow emission lines, but has a surprisingly soft X-ray spectrum. The softness of this spectrum either indicates an unusually low gas-to-dust ratio for the absorbing medium or X-ray emission related to the young radio source. The host galaxy is extremely red (R-K=6.4) and its optical/near-IR spectrum is best fit by a strongly reddened (A_V~2) starburst. The third X-ray source discussed is also extremely red (R-K=6.1) and lies in a close grouping of three other R-K>6 galaxies. No emission or absorption lines were detected from this object, but its redshift (and that of one of the nearby galaxies) are constrained by SED-fitting to be just greater than z=1. The extremely red colours of these two galaxies can be accounted for by old stellar populations. These observations illustrate the diverse properties of hard X-ray selected AGN.

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arXiv2002

The coincidence and angular clustering of Chandra and SCUBA sources

We explore the relationship between the hard X-ray and sub-mm populations using deep Chandra observations of a large, contiguous SCUBA survey. In agreement with other recent findings, we confirm that the direct overlap is small. Of the 17 sub-mm sources detected in this field at 850 microns, only one is coincident with a Chandra source. The resulting limits imply that the majority of SCUBA sources are not powered by AGN, unless the central engine is obscured by Compton-thick material with a low (<1 per cent) scattered component. Furthermore, since Chandra detects only ~5 per cent of SCUBA sources, the typical obscuration would need to be almost isotropic. The X-ray upper limits are so strong that in most cases we can also rule out a starburst SED at low redshift, suggesting that the majority of SCUBA sources lie at z>1 even if they are purely starburst galaxies. Despite the low detection rate, we find evidence for strong angular clustering between the X-ray and sub-mm populations. The implication is that AGN and SCUBA sources trace the same large-scale structure but do not generally coincide. If bright sub-mm sources represent massive elliptical galaxies in formation, we suggest that (for a given galaxy) the major episode of star-formation must be distinct from the period of observable quasar activity.

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arXiv2002

Discovery of the host galaxy of HDF850.1, the brightest sub-mm source in the Hubble Deep Field

Despite extensive observational efforts, the brightest sub--mm source in the Hubble Deep Field, HDF850.1, has failed to yield a convincing optical/infrared identification almost 4 years after its discovery. This failure is all the more notable given the availability of supporting multi-frequency data of unparalleled depth, and sub-arcsec positional accuracy for the sub-mm/mm source. Consequently, HDF850.1 has become a test case of the possibility that the most violently star-forming objects in the universe are too red and/or distant to be seen in the deepest optical images. Here we report the discovery of the host galaxy of HDF850.1. This object has been revealed by careful analysis of a new, deep K-prime image of the HDF obtained with the Subaru 8.2-m telescope. Its reality is confirmed by a similar analysis of the HST NICMOS F160W image of the same region. This object is extremely faint (K=23.5), clumpy (on sub-arcsec scales) and very red (I-K > 5.2; H-K = 1.4 +- 0.35). The likelihood that it is the correct identification is strongly reinforced by a reanalysis of the combined MERLIN+VLA 1.4-GHz map of the field which yields a new radio detection of HDF850.1 only 0.1 arcsec from the new near-ir counterpart, and with sufficient positional accuracy to exclude all previously considered alternative optical candidates. We have calculated new confidence limits on the estimated redshift of HDF850.1 and find z = 4.1 +- 0.5. We also calculate that the flux density of HDF850.1 has been boosted by a factor of ~3 through lensing by the intervening elliptical 3-586.0, consistent with predictions that a small but significant fraction of blank-field sub-mm sources are lensed by foreground galaxies. We discuss the wider implications of these results for the sub-mm population and cosmic star-formation history.

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arXiv2002

The clustering of X-ray and sub-mm sources

It is becoming clear that luminous extragalactic X-ray and sub-mm sources are essentially distinct populations. Thus, if high redshift sub-mm sources represent massive spheroids in formation, there must be a time lag between the major epoch of star-formation and the appearance of a visible quasar. Despite this distinction, I find tentative evidence for a puzzling angular cross-correlation between X-ray sources and bright sub-mm sources in two independent fields. If this signal is due to large-scale structure it would argue for a low redshift (z<2) for many of the SCUBA sources. Alternatively, I suggest that the effect may be enhanced by gravitational lensing. The exceptionally steep slope of the bright sub-mm counts makes this population particularly prone to lensing bias. An apparent correlation may therefore be produced if X-ray sources trace the intervening large scale structure.

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arXiv2001

The SCUBA 8-mJy survey - II: Multiwavelength analysis of bright sub-mm sources

We present the results of a multi-wavelength study of the 19 most significant sub-mm sources detected in the SCUBA 8-mJy survey. As described in Scott et al. (2001), this survey covers ~260 arcmin^2 using the sub-millimetre camera SCUBA, to a limiting source detection limit S(850um) ~ 8 mJy. One advantage of this relatively bright flux-density limit is that accurate astrometric positions are potentially achievable for every source using existing radio and/or mm-wave interferometers. However, an associated advantage is that SED-based redshift constraints should be more powerful than in fainter sub-mm surveys. Here we therefore exploit the parallel SCUBA 450um data, in combination with existing radio and ISO data at longer and shorter wavelengths to set constraints on the redshift of each source. We also analyse new and existing optical and near-infrared imaging of our SCUBA survey fields to select potential identifications consistent with these constraints. Our derived SED-based redshift constraints, and the lack of statistically significant associations with even moderately bright galaxies allow us to conclude that all 19 sources lie at z > 1, and at least half of them apparently lie at z > 2.

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arXiv2001

The X-ray variability of high redshift QSOs

We present an analysis of X-ray variability in a sample of 156 radio quiet quasars taken from the ROSAT archive, covering a redshift range 0.1 < z < 4.1. A maximum likelihood method is used to constrain the amplitude of variability in these low signal to noise light curves. Through combining these data in ensembles we are able to identify trends in variability amplitude with luminosity and with redshift. The decline in variability amplitude with luminosity identified in local AGN (z < 0.1) is confirmed out to z=2. There is tentative evidence for an increase in QSO X-ray variability amplitude towards high redshifts (z > 2) in the sense that QSOs of the same X-ray luminosity are more variable at z > 2. We discuss possible explanations for this effect. The simplest explanation may be that high redshift QSOs are accreting at a larger fraction of the Eddington limit than local AGN.

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arXiv2001

The extended counterpart of submm source Lockman850.1

The IRAM Plateau de Bure mm interferometer and deep K-band imaging have been used to identify the brightest submm source detected in the Lockman field of the UK 8mJy SCUBA survey. The near infrared counterpart is an extended (20-30kpc), clumpy, and extremely red object. The spectral energy distribution suggests it to be a dusty star forming object at a redshift of about 3 (2-4). Its star formation rate and near-infrared properties are consistent with Lockman850.1 being a massive elliptical in formation.

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arXiv2001

The ELAIS Deep X-ray Survey

We present initial follow-up results of the ELAIS Deep X-ray Survey which is being undertaken with the Chandra and XMM-Newton Observatories. 235 X-ray sources are detected in our two 75 ks ACIS-I observations in the well-studied ELAIS N1 and N2 areas. 90% of the X-ray sources are identified optically to R=26 with a median magnitude of R=24. We show that objects which are unresolved optically (i.e. quasars) follow a correlation between their optical and X-ray fluxes, whereas galaxies do not. We also find that the quasars with fainter optical counterparts have harder X-ray spectra, consistent with absorption at both wavebands. Initial spectroscopic follow-up has revealed a large fraction of high-luminosity Type 2 quasars. The prospects for studying the evolution of the host galaxies of X-ray selected Type 2 AGN are considered.

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arXiv2000

Starburst activity in a ROSAT Narrow Emission-Line Galaxy

We present multiwaveband photometric and optical spectropolarimetric observations of the R=15.9 narrow emission line galaxy R117_A which lies on the edge of the error circle of the ROSAT X-ray source R117 (from McHardy et al 1998). The overall spectral energy distribution of the galaxy is well modelled by a combination of a normal spiral galaxy and a moderate-strength burst of star formation. The far infra-red and radio emission is extended along the major axis of the galaxy, indicating an extended starburst. On positional grounds, the galaxy is a good candidate for the identification of R117 and the observed X-ray flux is very close to what would be expected from a starburst of the observed far infra-red and radio fluxes. Although an obscured high redshift QSO cannot be entirely ruled out as contributing some fraction of the X-ray flux, we find no candidates to K=20.8 within the X-ray errorbox and so conclude that R117_A is responsible for a large fraction, if not all, of the X-ray emission from R117. Searches for indicators of an obscured AGN in R117_A have so far proven negative; deep spectropolarimetric observations show no signs of broad lines to a limit of one per cent and, for the observed far infra-red and radio emission, we would expect a ten times greater X-ray flux if the overall emission were powered by an AGN. We therefore conclude that the X-ray emission from R117 is dominated by starburst emission from the galaxy R117_A.

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arXiv2000

X-ray absorption and rapid variability of the dwarf Seyfert nucleus of NGC4395

We report the detection of an absorbed central X-ray source and its strong, rapid variability in NGC4395, the least luminous Seyfert nucleus known. The X-ray source exhibits a number of flares with factors of 3-4 flux changes during a half day ASCA observation. Such X-ray variability is in constrast to the behaviour of other low luminosity active galaxies. It provides further support for an accreting black hole model rather than an extreme stellar process in accounting for the nuclear activity of NGC4395. The soft X-ray emission below 3 keV is strongly attenuated by absorption. The energy spectrum in this absorption band shows a dramatic change in response to the variation in continuum luminosity. A variable warm absorber appears to be an explanation for the spectral change. The absorption-corrected 2-10 keV luminosity is 4e39 erg/s for a source distance of 2.6 Mpc, and at 1 keV is one order of magnitude above previous ROSAT estimates. Our X-ray results infer the nuclear source of NGC4395 to be a scaled-down version of higher luminosity Seyfert nuclei, with an intermediate mass (10^4-10^5 Msun) black hole, unlike the nearby low luminosity active galaxies in which underfed massive black holes are suspected to reside.

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