P. C. Hewett

University of Cambridge, Cambridge, England, United Kingdom

Are you P. C. Hewett?

Claim your profile

Publications (232)829.7 Total impact

  • [show abstract] [hide abstract]
    ABSTRACT: High ionization star forming galaxies are easily identified with strong emission line techniques such as the BPT diagram, but for ionization levels below log([O III]/Hβ) ~ 0.3 they become confused with low-ionization AGN, making their physical interpretation difficult. Mean field independent component analysis (MFICA) is a novel approach to processing emission line spectra that can disentangle the AGN and starlight contributions to emission lines allowing the properties of pure AGN and pure starburst galaxies to be interpreted over a wide range of ionization. We applied MFICA to large sample of low-z SDSS galaxies and created subsamples of pure star forming galaxies resulting in a sequence of varying ionization. We used a locally optimally emitting (LOC) cloud model to fit emission line ratios that constrain the excitation mechanism, spectral energy distribution, abundances and physical conditions. Preliminary results in fitting diagrams that constrain the excitation mechanism indicate that the variation of starburst galaxies is due to a change in the radial distribution of clouds, rather than differences in metallicity, ionization parameter or spectral energy distribution. This confirms that MFICA is a powerful tool to assess differences in emission line properties solely due to starbursts. We briefly discuss future work that will decipher other properties in star forming galaxies.
    01/2014;
  • [show abstract] [hide abstract]
    ABSTRACT: We present the analysis of optical and near-infrared spectra of the only four z>6.5 quasars known to date, discovered in the UKIDSS-LAS and VISTA-VIKING surveys. Our data-set consists of VLT/X-Shooter and Magellan/FIRE observations of the z>6.5 quasars, and includes new deep VLT/X-Shooter observations of the highest redshift quasar known to date (z=7.1). These are the best optical/NIR spectroscopic data that are likely to be obtained for the z>6.5 sample using current 6-10 m facilities. We estimate the black hole mass, the Eddington ratio, and the SiIV/CIV, CIII]/CIV, and FeII/MgII emission-line flux ratios. We perform spectral modeling using a procedure that allows us to derive a probability distribution for the continuum components and to obtain the quasar properties weighted upon the underlying distribution of continuum models. The z>6.5 quasars show the same emission properties as their counterparts at lower redshifts. The z>6.5 quasars host black holes with masses of ~10^9 M_sun that are accreting close to the Eddington luminosity (<log(L_Bol/L_Edd)>= -0.4+/-0.2), in agreement with what has been observed for a sample of 4.0<z<6.5 quasars. By comparing the SiIV/CIV and CIII]/CIV flux ratios with the results obtained from luminosity-matched samples at z~6 and 2<z<4.5, we find no evidence of evolution of the line ratios with cosmic time. We compare the measured FeII/MgII flux ratios with those obtained for a sample of 4.0<z<6.4 sources. The two samples are analyzed using a consistent procedure. The FeII/MgII flux ratio does not show any evolution in the 4.0<z<7.1 redshift range. Under the assumption that the FeII/MgII traces the Fe/Mg abundance ratio, this implies the presence of major episodes of chemical enrichment in the quasar hosts in the first ~0.8 Gyr after the Big Bang.
    11/2013;
  • Source
    [show abstract] [hide abstract]
    ABSTRACT: We present X-ray imaging and spectroscopy of the redshift z=7.084 quasar ULAS J112001.48+064124.3 obtained with Chandra and XMM-Newton. The quasar is detected as a point source with both observatories. The Chandra observation provides a precise position, confirming the association of the X-ray source and the quasar, while sufficient photons are detected in the XMM-Newton observation to yield a meaningful X-ray spectrum. In the XMM-Newton observation the quasar has a 2-10 keV luminosity of 4.7 +- 0.9 times 10^44 ergs/s and a spectral slope alpha = 1.6 +0.4 -0.3 (where f_nu propto nu^-alpha). The very soft spectrum suggests that the quasar is accreting above the Eddington rate, which would help to reduce the discrepancy between the age of the quasar implied by the small size of the ionized near zone in which it sits (<10^7 years), and the characteristic e-folding time (2.5 times 10^7 years if L/L_Edd=2). Such super-Eddington accretion would also alleviate the challenging constraints on the seed black hole mass, and the remnant of an individual population III star is a plausible progenitor if an average L/L_Edd>1.46 has been maintained over the quasar's lifetime.
    11/2013;
  • Source
    [show abstract] [hide abstract]
    ABSTRACT: We investigate the physical cause of the great range in the ionization level seen in the spectra of narrow lined active galactic nuclei (AGN). Mean field independent component analysis identifies examples of individual SDSS galaxies whose spectra are not dominated by emission due to star formation (SF), which we designate as AGN. We assembled high S/N ratio composite spectra of a sequence of these AGN defined by the ionization level of their narrow-line regions (NLR), extending down to very low-ionization cases. We used a local optimally emitting cloud (LOC) model to fit emission-line ratios in this AGN sequence. These included the weak lines that can be measured only in the co-added spectra, providing consistency checks on strong line diagnostics. After integrating over a wide range of radii and densities our models indicate that the radial extent of the NLR is the major parameter in determining the position of high to moderate ionization AGN along our sequence, providing a physical interpretation for their systematic variation. Higher ionization AGN contain optimally emitting clouds that are more concentrated towards the central continuum source than in lower ionization AGN. Our LOC models indicate that for the objects that lie on our AGN sequence, the ionizing luminosity is anticorrelated with the NLR ionization level, and hence anticorrelated with the radial concentration and physical extent of the NLR. A possible interpretation that deserves further exploration is that the ionization sequence might be an age sequence where low ionization objects are older and have systematically cleared out their central regions by radiation pressure. We consider that our AGN sequence instead represents a mixing curve of SF and AGN spectra, but argue that while many galaxies do have this type of composite spectra, our AGN sequence appears to be a special set of objects with negligible SF excitation.
    Monthly Notices of the Royal Astronomical Society 10/2013; 437(3). · 5.52 Impact Factor
  • Source
    [show abstract] [hide abstract]
    ABSTRACT: We present an analysis of the optical spectra of narrow emission-line galaxies, based on mean field independent component analysis (MFICA). Samples of galaxies were drawn from the Sloan Digital Sky Survey (SDSS) and used to generate compact sets of `continuum' and `emission-line' component spectra. These components can be linearly combined to reconstruct the observed spectra of a wider sample of galaxies. Only 10 components - five continuum and five emission line - are required to produce accurate reconstructions of essentially all narrow emission-line galaxies; the median absolute deviations of the reconstructed emission-line fluxes, given the signal-to-noise ratio (S/N) of the observed spectra, are 1.2-1.8 sigma for the strong lines. After applying the MFICA components to a large sample of SDSS galaxies we identify the regions of parameter space that correspond to pure star formation and pure active galactic nucleus (AGN) emission-line spectra, and produce high S/N reconstructions of these spectra. The physical properties of the pure star formation and pure AGN spectra are investigated by means of a series of photoionization models, exploiting the faint emission lines that can be measured in the reconstructions. We are able to recreate the emission line strengths of the most extreme AGN case by assuming the central engine illuminates a large number of individual clouds with radial distance and density distributions, f(r) ~ r^gamma and g(n) ~ n^beta, respectively. The best fit is obtained with gamma = -0.75 and beta = -1.4. From the reconstructed star formation spectra we are able to estimate the starburst ages. These preliminary investigations serve to demonstrate the success of the MFICA-based technique in identifying distinct emission sources, and its potential as a tool for the detailed analysis of the physical properties of galaxies in large-scale surveys.
    Monthly Notices of the Royal Astronomical Society 01/2013; 430(4). · 5.52 Impact Factor
  • Source
    [show abstract] [hide abstract]
    ABSTRACT: We present the first sample of spectroscopically confirmed heavily reddened broad-line quasars selected using the new near infra-red VISTA Hemisphere Survey and WISE All-Sky Survey. Observations of four candidates with (J-K)>2.5 and K<=16.5 over ~180 sq deg, leads to confirmation that two are highly dust-reddened broad-line Type 1 quasars at z~2. The typical dust extinctions are AV ~ 2-2.5 mags. We measure black-hole masses of ~10^9 M0 and extinction corrected bolometric luminosities of ~10^47 erg/s, making these among the brightest Type 1 quasars currently known. Despite this, these quasars lie well below the detection limits of wide-field optical surveys like the SDSS with iAB > 22. We also present WISE photometry at 3--22um, for our full sample of spectroscopically confirmed reddened quasars including those selected from the UKIDSS Large Area Survey (Banerji et al. 2012a). We demonstrate that the rest-frame infrared SEDs of these reddened quasars are similar to UV-luminous Type 1 quasars with significant hot dust emission and starburst quasar hosts like Mrk231. The average 12um flux density of our reddened quasars is similar to that of the recently discovered HyLIRG WISE1814+3412 (z=2.452) at similar redshifts, with two of our reddened quasars also having comparable 22um flux densities to this extreme HyLIRG. These optically faint, heavily reddened broad-line quasars are therefore among the most mid infrared luminous galaxies at z~2, now being discovered using WISE.
    Monthly Notices of the Royal Astronomical Society 10/2012; 429(1). · 5.52 Impact Factor
  • Source
    [show abstract] [hide abstract]
    ABSTRACT: The results of a large area, ~600 deg^2, K-band flux-limited spectroscopic survey for luminous quasars are presented. The survey utilises the UKIRT Infrared Deep Sky Survey (UKIDSS) Large Area Survey (LAS) in regions of sky within the Sloan Digital Sky Survey (SDSS) footprint. The K-band excess (KX) of all quasars with respect to Galactic stars is exploited in combination with a photometric redshift/classification scheme to identify quasar candidates for spectroscopic follow-up observations. The data contained within this investigation will be able to provide new constraints on the fraction of luminous quasars reddened by dust with E(B-V)<=0.5 mag. The spectroscopic sample is defined using the K-band, 14.0<=K<=16.6, and SDSS i-band limits of i=19.5, 19.7 and 22.0 over sky areas of 287, 150 and 196 deg^2, respectively. The survey includes >3200 known quasars from the SDSS and more than 250 additional confirmed quasars from the KX-selection. A well-defined sub-sample of quasars in the redshift interval 1.0<=z<=3.5 includes 1152 objects from the SDSS and 172 additional KX-selected quasars. The quasar selection is >95 per cent complete with respect to known SDSS quasars and >95 per cent efficient, largely independent of redshift and i-band magnitude. The properties of the new KX-selected quasars confirm the known redshift-dependent effectiveness of the SDSS quasar selection and provide a sample of luminous quasars experiencing intermediate levels of extinction by dust. The catalogue represents an important step towards the assembly of a well-defined sample of luminous quasars that may be used to investigate the properties of quasars experiencing intermediate levels of dust extinction within their host galaxies or due intervening absorption line systems.
    Monthly Notices of the Royal Astronomical Society 06/2012; 424(4). · 5.52 Impact Factor
  • Source
    [show abstract] [hide abstract]
    ABSTRACT: Using the IRAM Plateau de Bure Interferometer, we report the detection of the 158 {mu}m [C II] emission line and underlying dust continuum in the host galaxy of the quasar ULAS J112001.48+064124.3 (hereafter J1120+0641) at z 7.0842 {+-} 0.0004. This is the highest redshift detection of the [C II] line to date, and allows us to put the first constraints on the physical properties of the host galaxy of J1120+0641. The [C II] line luminosity is 1.2 {+-} 0.2 Multiplication-Sign 10{sup 9} L{sub Sun }, which is a factor {approx}4 lower than observed in a luminous quasar at z = 6.42 (SDSS J1148+5251). The underlying far-infrared (FIR) continuum has a flux density of 0.61 {+-} 0.16 mJy, similar to the average flux density of z {approx} 6 quasars that were not individually detected in the rest-frame FIR. Assuming that the FIR luminosity of L{sub FIR} = 5.8 Multiplication-Sign 10{sup 11}-1.8 Multiplication-Sign 10{sup 12} L{sub Sun} is mainly powered by star formation, we derive a star formation rate in the range 160-440 M{sub Sun} yr{sup -1} and a total dust mass in the host galaxy of 6.7 Multiplication-Sign 10{sup 7}-5.7 Multiplication-Sign 10{sup 8} M{sub Sun} (both numbers have significant uncertainties given the unknown nature of dust at these redshifts). The [C II] line width of {sigma}{sub V} = 100 {+-} 15 km s{sup -1} is among the smallest observed when compared to the molecular line widths detected in z {approx} 6 quasars. Both the [C II] and dust continuum emission are spatially unresolved at the current angular resolution of 2.0 Multiplication-Sign 1.7 arcsec{sup 2} (corresponding to 10 Multiplication-Sign 9 kpc{sup 2} at the redshift of J1120+0641).
    The Astrophysical Journal Letters 06/2012; 751(2). · 6.35 Impact Factor
  • Source
    [show abstract] [hide abstract]
    ABSTRACT: Using the IRAM Plateau de Bure Interferometer, we report the detection of the 158 micron [CII] emission line and underlying dust continuum in the host galaxy of the quasar ULAS J112001.48+064124.3 (hereafter J1120+0641) at z=7.0842+/-0.0004. This is the highest redshift detection of the [CII] line to date, and allows us to put first constraints on the physical properties of the host galaxy. The [CII] line luminosity is (1.2+/-0.2)x10^9 Lsun, which is a factor ~4 lower than observed in a luminous quasar at z=6.42 (SDSS J1148+5251). The underlying far-infrared (FIR) continuum has a flux density of 0.61+/-0.16 mJy, similar to the average flux density of z~6 quasars that were not individually detected in the rest-frame FIR. Assuming the FIR luminosity of L_FIR = 5.8x10^11-1.8x10^12 Lsun is mainly powered by star-formation, we derive a star-formation rate in the range 160-440 Msun/yr and a total dust mass in the host galaxy of 6.7x10^7-5.7x10^8 Msun (both numbers have significant uncertainties given the unknown nature of dust at these redshifts). The [CII] line width of sigma_V=100+/-15 km/s is among the smallest observed when compared to the molecular line widths detected in z~6 quasars. Both the [CII] and dust continuum emission are spatially unresolved at the current angular resolution of 2.0x1.7 arcsec^2 (corresponding to 10x9 kpc^2 at the redshift of J1120+0641).
    03/2012;
  • Source
    [show abstract] [hide abstract]
    ABSTRACT: We present a new sample of purely near infra-red selected KVega<16.5 extremely red ((J-K)>2.5) quasar candidates at z~2 from 900 sq deg of data in the UKIDSS Large Area Survey (LAS). Five of these are spectroscopically confirmed to be heavily reddened Type 1 AGN with broad emission lines bringing our total sample of reddened quasars from the UKIDSS-LAS to 12 at z=1.4-2.7. At these redshifts, Ha (6563A) is in the K-band. However, the mean Ha equivalent width of the reddened quasars is only ten per cent larger than that of the optically selected population and cannot explain the extreme colours. Instead, dust extinction of Av~2-6 mags is required to reproduce the continuum colours of our sources. This is comparable to the dust extinctions seen in submillimetre galaxies at similar redshifts. We argue that the AGN are likely being observed in a relatively short-lived breakout phase when they are expelling gas and dust following a massive starburst, subsequently turning into UV-luminous quasars. Some of our quasars show direct evidence for strong outflows (v~800-1000 km/s) affecting the Ha line consistent with this scenario. We predict that a larger fraction of reddened quasar hosts are likely to be submillimetre bright compared to the UV-luminous quasar population. We use our sample to place new constraints on the fraction of obscured Type 1 AGN likely to be missed in optical surveys. Taken at face-value our findings suggest that the obscured fraction depends on quasar luminosity. The space density of obscured quasars is ~5x that inferred for UV-bright quasars from the SDSS luminosity function at Mi<-30 but seems to drop at lower luminosities even accounting for various sources of incompleteness in our sample (abridged).
    Monthly Notices of the Royal Astronomical Society 03/2012; 427(3). · 5.52 Impact Factor
  • [show abstract] [hide abstract]
    ABSTRACT: The UKIRT Infrared Deep Sky Survey (UKIDSS) is a large-scale near-IR survey which aim is to cover 7500 square degrees of the Northern sky. The survey is carried out using the Wide Field Camera (WFCAM), with a field of view of 0.21 square degrees, mounted on the 3.8m United Kingdom Infra-red Telescope (UKIRT) in Hawaii. The project comprises five surveys (LAS, GCS, DXS, GPS and UDS). The Large Area Survey (LAS) covers an area of 4000 square degrees in high Galactic latitudes (extragalactic) in the four bands Y(1.0um) J(1.2um) H(1.6um) and K(2.2um) to a depth of K=18.4. The Galactic Clusters Survey (GCS) aims to survey ten large open star clusters and star formation associations, covering a total of 1067 square degrees in the five bands Z (0.9um), Y(1.0um) J(1.2um) H(1.6um) and K(2.2um), plus a second pass in K for proper motions, to a depth of Z=20.4, Y=20.3, J=19.5, H=18.6, K=18.6. The Deep Extragalactic Survey (DXS) aims to map 35 square degrees of sky to a 5-σ point-source sensitivity of J=22.3 and K=20.8 in four carefully selected, multi-wavelength survey areas. The central regions of each field will also be mapped to H=21.8. The primary aim of the survey is to produce a photometric galaxy sample at a redshift of 1-2, within a volume comparable to that of the SDSS, selected in the same passband (rest frame optical). Details of the surveys can be found in the in the paper by Lawrence et al. (2007MNRAS.379.1599L), and at the UKIDSS Surveys site (http://www.ukidss.org/surveys/surveys.html). The data described here represent a subset of the UKIDSS data, limited to the public data and most representative columns. In the "Byte-by-byte Description" below the original names of the columns are given as bracketed names. (3 data files).
    VizieR Online Data Catalog. 03/2012;
  • Source
    [show abstract] [hide abstract]
    ABSTRACT: Euclid is a space-based survey mission from the European Space Agency designed to understand the origin of the Universe's accelerating expansion. It will use cosmological probes to investigate the nature of dark energy, dark matter and gravity by tracking their observational signatures on the geometry of the universe and on the cosmic history of structure formation. The mission is optimised for two independent primary cosmological probes: Weak gravitational Lensing (WL) and Baryonic Acoustic Oscillations (BAO). The Euclid payload consists of a 1.2 m Korsch telescope designed to provide a large field of view. It carries two instruments with a common field-of-view of ~0.54 deg2: the visual imager (VIS) and the near infrared instrument (NISP) which contains a slitless spectrometer and a three bands photometer. The Euclid wide survey will cover 15,000 deg2 of the extragalactic sky and is complemented by two 20 deg2 deep fields. For WL, Euclid measures the shapes of 30-40 resolved galaxies per arcmin2 in one broad visible R+I+Z band (550-920 nm). The photometric redshifts for these galaxies reach a precision of dz/(1+z) < 0.05. They are derived from three additional Euclid NIR bands (Y, J, H in the range 0.92-2.0 micron), complemented by ground based photometry in visible bands derived from public data or through engaged collaborations. The BAO are determined from a spectroscopic survey with a redshift accuracy dz/(1+z) =0.001. The slitless spectrometer, with spectral resolution ~250, predominantly detects Ha emission line galaxies. Euclid is a Medium Class mission of the ESA Cosmic Vision 2015-2025 programme, with a foreseen launch date in 2019. This report (also known as the Euclid Red Book) describes the outcome of the Phase A study.
    10/2011;
  • Source
    [show abstract] [hide abstract]
    ABSTRACT: In the restframe UV, two of the parameters that best characterize the range of emission-line properties in quasar broad emission-line regions are the equivalent width and the blueshift of the CIV line relative to the quasar rest frame. We explore the connection between these emission-line properties and the UV through X-ray spectral energy distribution (SED) for radio-quiet (RQ) quasars. Our sample consists of a heterogeneous compilation of 406 quasars from the Sloan Digital Sky Survey and Palomar-Green survey that have well-measured CIV emission-line and X-ray properties (including 164 objects with measured Gamma). We find that RQ quasars with both strong CIV emission and small CIV blueshifts can be classified as "hard-spectrum" sources that are (relatively) strong in the X-ray as compared to the UV. On the other hand, RQ quasars with both weak CIV emission and large CIV blueshifts are instead "soft-spectrum" sources that are (relatively) weak in the X-ray as compared to the UV. This work helps to further bridge optical/soft X-ray "Eigenvector 1" relationships to the UV and hard X-ray. Based on these findings, we argue that future work should consider systematic errors in bolometric corrections (and thus accretion rates) that are derived from a single mean SED. Detailed analysis of the CIV emission line may allow for SED-dependent corrections to these quantities.
    The Astronomical Journal 09/2011; 142(4). · 4.97 Impact Factor
  • [show abstract] [hide abstract]
    ABSTRACT: We describe the application of non-negative matrix factorization to generate compact reconstructions of quasar spectra from the Sloan Digital Sky Survey (SDSS), with particular reference to broad absorption line quasars (BALQSOs). BAL properties are measured for SiIVlambda1400, CIVlambda1550, AlIIIlambda1860 and MgIIlambda2800, resulting in a catalogue of 3547 BALQSOs. Two corrections, based on extensive testing of synthetic BALQSO spectra, are applied in order to estimate the intrinsic fraction of CIV BALQSOs. First, the probability of an observed BALQSO spectrum being identified as such by our algorithm is calculated as a function of redshift, signal-to-noise ratio and BAL properties. Secondly, the different completenesses of the SDSS target selection algorithm for BALQSOs and non-BAL quasars are quantified. (1 data file).
    VizieR Online Data Catalog. 08/2011;
  • 07/2011;
  • [show abstract] [hide abstract]
    ABSTRACT: This paper discusses benchmark brown dwarfs in various environments, and focuses on those in wide binary systems. We present a summary of the recently discovered T dwarf population from the UKIDSS Large Area Survey, and describe the constraints that it places on our knowledge of the sub-stellar initial mass function. We also present some exciting results from our ongoing search for wide companions to this sample, that has so far revealed an M4-T8.5 binary system at ˜12 parsecs and also the first ever Tdwarf-white dwarf binary system. The T dwarfs in these binaries have their properties constrained by the primary object and are thus benchmark objects that are already testing the predictions of theoretical model atmospheres.
    06/2011; 16:06002.
  • Source
    [show abstract] [hide abstract]
    ABSTRACT: The quasar ULAS J1120+0641 at redshift z=7.085 has a highly ionised near zone which is smaller than those around quasars of similar luminosity at z~6. The spectrum also exhibits evidence for a damping wing extending redward of the systemic Lya redshift. We use radiative transfer simulations in a cosmological context to investigate the implications for the ionisation state of the inhomogeneous IGM surrounding this quasar. Our simulations show that the transmission profile is consistent with an IGM in the vicinity of the quasar with a volume averaged HI fraction of f_HI>0.1 and that ULAS J1120+0641 has been bright for 10^6--10^7 yr. The observed spectrum is also consistent with smaller IGM neutral fractions, f_HI ~ 10^-3--10-4, if a damped Lya system in an otherwise highly ionised IGM lies within 5 proper Mpc of the quasar. This is, however, predicted to occur in only ~5 per cent of our simulated sight-lines for a bright phase of 10^6--10^7 yr. Unless ULAS J1120+0641 grows during a previous optically obscured phase, the low age inferred for the quasar adds to the theoretical challenge of forming a 2x10^9 M_sol black hole at this high redshift.
    Monthly Notices of the Royal Astronomical Society 06/2011; 416(1). · 5.52 Impact Factor
  • [show abstract] [hide abstract]
    ABSTRACT: Numerous surveys are underway, or planned, to discover quasars of redshift z>6.5 in order to explore the epoch of cosmic reionisation. In UKIDSS we recently discovered the bright quasar ULAS J1120+0641, of redshift z=7.08, the highest redshift quasar known by a large margin. We propose photometry of this source in Channels 1 and 2 at S/N=30. The Spitzer observations form part of a campaign to measure the multiwavelength SED of the source which will be used to seek evidence for evolution at this very early time. The SED will also provide a benchmark for future surveys for high-redshift quasars to explore the epoch of reionisation at even higher redshifts. Additionally these deep confusion-limited observations will complement any future deep optical and near-ir observations of the field to detect companion galaxies and faint AGN.
    Spitzer Proposal. 05/2011;
  • [show abstract] [hide abstract]
    ABSTRACT: At high redshift two parameters span the range of emission line properties in quasar broad emission line regions: the CIV equivalent width and the blueshift of the CIV line relative to the quasar rest frame. We explore the connection between these emission line features and the UV through X-ray spectral energy distribution (SED) for radio-quiet quasars. We find that radio-quiet quasars with both strong CIV emission and small CIV blueshifts can be classified as ``hard-spectrum'' sources that are (relatively) strong the in X-ray as compared to the UV. The low-energy X-ray upturn (``soft excess'') in some quasars may be consistent with the high-energy extension of the ``big blue bump'' in hard-spectrum sources. On the other hand, RQ quasars with both weak CIV emission and large CIV blueshifts are instead ``soft-spectrum'' sources. The nature of sources with weak CIV and small blueshifts is less clear and needs further study. We argue that bolometric corrections for quasars may have systematic errors if a single mean SED is assumed for all objects, leading to additional systematic errors in accretion rates as determined by L_bol/M_vir. Bolometric corrections can instead be considered as a function of the ionizing spectrum, which can be inferred from the properties of the broad emission lines, particularly CIV.
    05/2011;
  • Source
    [show abstract] [hide abstract]
    ABSTRACT: Using a sample of ~30,000 quasars from the 7th Data Release of the Sloan Digital Sky Survey, we explore the range of properties exhibited by high-ionization, broad emission lines, such as C IV λ1549. Specifically, we investigate the anti-correlation between continuum luminosity and emission-line equivalent width (the Baldwin Effect (BEff)) and the "blueshifting" of the high-ionization emission lines with respect to low-ionization emission lines. Employing improved redshift determinations from Hewett & Wild, the blueshift of the C IV emission line is found to be nearly ubiquitous, with a mean shift of ~810 km s–1 for radio-quiet (RQ) quasars and ~360 km s–1 for radio-loud (RL) quasars. The BEff is present in both RQ and RL samples. We consider these phenomena within the context of an accretion disk-wind model that is modulated by the nonlinear correlation between ultraviolet and X-ray continuum luminosity. Composite spectra are constructed as a function of C IV emission-line properties in an attempt to reveal empirical relationships between different line species and the continuum. Within a two-component disk+wind model of the broad emission-line region (BELR), where the wind filters the continuum seen by the disk component, we find that RL quasars are consistent with being dominated by the disk component, while broad absorption line quasars are consistent with being dominated by the wind component. Some RQ objects have emission-line features similar to RL quasars; they may simply have insufficient black hole (BH) spin to form radio jets. Our results suggest that there could be significant systematic errors in the determination of L bol and BH mass that make it difficult to place these findings in a more physical context. However, it is possible to classify quasars in a paradigm where the diversity of BELR parameters is due to differences in an accretion disk wind between quasars (and over time); these differences are underlain primarily by the spectral energy distribution, which ultimately must be tied to BH mass and accretion rate.
    The Astronomical Journal 04/2011; 141(5):167. · 4.97 Impact Factor

Publication Stats

2k Citations
829.70 Total Impact Points

Institutions

  • 2001–2012
    • University of Cambridge
      • Institute of Astronomy
      Cambridge, England, United Kingdom
  • 1994–2011
    • Cancer Research UK Cambridge Institute
      Cambridge, England, United Kingdom
    • European Southern Observatory
      Arching, Bavaria, Germany
  • 2007
    • Universität Heidelberg
      • Centre for Astronomy (ZAH)
      Heidelburg, Baden-Württemberg, Germany
  • 2006
    • University of Wyoming
      Laramie, Wyoming, United States
  • 2005
    • Cambridge Healthtech Institute
      Needham, Massachusetts, United States
    • Imperial College London
      Londinium, England, United Kingdom
  • 2004
    • Cambridge College
      Cambridge, Massachusetts, United States
  • 1998
    • University of Sussex
      Brighton, England, United Kingdom
  • 1994–1995
    • The University of Arizona
      • Department of Astronomy
      Tucson, Arizona, United States
  • 1992
    • Dartmouth College
      Hanover, New Hampshire, United States
  • 1991
    • National Optical Astronomy Observatory
      Tucson, Arizona, United States
  • 1987
    • Space Telescope Science Institute
      Baltimore, Maryland, United States
  • 1985
    • Beijing Normal University
      Peping, Beijing, China