arXiv subjects

Seb Oliver

Publications and source records attributed to Seb Oliver.

At least 55 records · Page 3Linked to original sources

arXiv2007

High-Redshift QSOs in the SWIRE Survey and the z~3 QSO Luminosity Function

We use a simple optical/infrared (IR) photometric selection of high-redshift QSOs that identifies a Lyman Break in the optical photometry and requires a red IR color to distinguish QSOs from common interlopers. The search yields 100 z~3 (U-dropout) QSO candidates with 19<r'<22 over 11.7 deg^2 in the ELAIS-N1 (EN1) and ELAIS-N2 (EN2) fields of the Spitzer Wide-area Infrared Extragalactic (SWIRE) Legacy Survey. The z~3 selection is reliable, with spectroscopic follow-up of 10 candidates confirming they are all QSOs at 2.83<z<3.44. We find that our z~4$ (g'-dropout) sample suffers from both unreliability and incompleteness but present 7 previously unidentified QSOs at 3.50<z<3.89. Detailed simulations show our z~3 completeness to be ~80-90% from 3.0<z<3.5, significantly better than the ~30-80% completeness of the SDSS at these redshifts. The resulting luminosity function extends two magnitudes fainter than SDSS and has a faint end slope of beta=-1.42 +- 0.15, consistent with values measured at lower redshift. Therefore, we see no evidence for evolution of the faint end slope of the QSO luminosity function. Including the SDSS QSO sample, we have now directly measured the space density of QSOs responsible for ~70% of the QSO UV luminosity density at z~3. We derive a maximum rate of HI photoionization from QSOs at z~3.2, Gamma = 4.8x10^-13 s^-1, about half of the total rate inferred through studies of the Ly-alpha forest. Therefore, star-forming galaxies and QSOs must contribute comparably to the photoionization of HI in the intergalactic medium at z~3.

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arXiv2007

Bayesian methods of astronomical source extraction

We present two new source extraction methods, based on Bayesian model selection and using the Bayesian Information Criterion (BIC). The first is a source detection filter, able to simultaneously detect point sources and estimate the image background. The second is an advanced photometry technique, which measures the flux, position (to sub-pixel accuracy), local background and point spread function. We apply the source detection filter to simulated Herschel-SPIRE data and show the filter's ability to both detect point sources and also simultaneously estimate the image background. We use the photometry method to analyse a simple simulated image containing a source of unknown flux, position and point spread function; we not only accurately measure these parameters, but also determine their uncertainties (using Markov-Chain Monte Carlo sampling). The method also characterises the nature of the source (distinguishing between a point source and extended source). We demonstrate the effect of including additional prior knowledge. Prior knowledge of the point spread function increase the precision of the flux measurement, while prior knowledge of the background has onlya small impact. In the presence of higher noise levels, we show that prior positional knowledge (such as might arise from a strong detection in another waveband) allows us to accurately measure the source flux even when the source is too faint to be detected directly. These methods are incorporated in SUSSEXtractor, the source extraction pipeline for the forthcoming Akari FIS far-infrared all-sky survey. They are also implemented in a stand-alone, beta-version public tool that can be obtained at http://astronomy.sussex.ac.uk/$\sim$rss23/sourceMiner\_v0.1.2.0.tar.gz

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arXiv2006

Optical and Infrared Diagnostics of SDSS galaxies in the SWIRE Survey

We present the rest-frame optical and infrared colours of a complete sample of 1114 z<0.3 galaxies from the Spitzer Wide-area InfraRed Extragalactic Legacy Survey (SWIRE) and the Sloan Digital Sky Survey (SDSS). We discuss the optical and infrared colours of our sample and analyse in detail the contribution of dusty star-forming galaxies and AGN to optically selected red sequence galaxies. We propose that the optical (g-r) colour and infrared log(L_{24}/L_{3.6}) colour of galaxies in our sample are determined primarily by a bulge-to-disk ratio. The (g-r) colour is found to be sensitive to the bulge-to-disk ratio for disk-dominated galaxies, whereas the log(L_{24}/L_{3.6}) colour is more sensitive for bulge-dominated systems. We identify ~18% (195 sources) of our sample as having red optical colours and infrared excess. Typically, the infrared luminosities of these galaxies are found to be at the high end of star-forming galaxies with blue optical colours. Using emission line diagnostic diagrams, 78 are found to have an AGN contribution, and 117 are identified as star-forming systems. The red (g-r) colour of the star-forming galaxies could be explained by extinction. However, their high optical luminosities cannot. We conclude that they have a significant bulge component. The number densities of optically red star-forming galaxies are found to correspond to ~13% of the total number density of our sample. In addition, these systems contribute ~13% of the total optical luminosity density, and 28% of the total infrared luminosity density of our SWIRE/SDSS sample. These objects may reduce the need for "dry-mergers".

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arXiv2006

The spatial clustering of ultraluminous infrared galaxies over 1.5<z<3

We present measurements of the spatial clustering of galaxies with stellar masses >10^11Msun, infrared luminosities >10^12 Lsun, and star formation rates >200Msun per year in two redshift intervals; 1.5 1, both starbursts and AGN. Adopting plausible models for the growth of DM haloes with redshift, then the haloes hosting the 2 1 signpost stellar buildup in galaxies that will reside in clusters at z=0, with ULIRGs at increasing redshifts signposting the buildup of stars in galaxies that will reside in increasingly rich clusters.

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arXiv2006

Parametric modelling of the 3.6um to 8um colour distributions of galaxies in the SWIRE Survey

We fit a parametric model comprising a mixture of multi-dimensional Gaussian functions to the 3.6 to 8um colour and optical photo-z distribution of galaxy populations in the ELAIS-N1 and Lockman Fields of SWIRE. For 16,698 sources in ELAIS-N1 we find our data are best modelled (in the sense of the Bayesian Information Criterion) by the sum of four Gaussian distributions or modes (C_a, C_b, C_c and C_d). We compare the fit of our empirical model with predictions from existing semi-analytic and phenomological models. We infer that our empirical model provides a better description of the mid-infrared colour distribution of the SWIRE survey than these existing models. This colour distribution test is thus a powerful model discriminator and complementary to comparisons of number counts. We use our model to provide a galaxy classification scheme and explore the nature of the galaxies in the different modes of the model. C_a consists of dusty star-forming systems such as ULIRG's. Low redshift late-type spirals are found in C_b, where PAH emission dominates at 8um. C_c consists of dusty starburst systems at intermediate redshifts. Low redshift early-type spirals and ellipticals dominate C_d. We thus find a greater variety of galaxy types than one can with optical photometry alone. Finally we develop a new technique to identify unusual objects, and find a selection of outliers with very red IRAC colours. These objects are not detected in the optical, but have very strong detections in the mid-infrared. These sources are modelled as dust-enshrouded, strongly obscured AGN, where the high mid-infrared emission may either be attributed to dust heated by the AGN or substantial star-formation. These sources have z_ph ~ 2-4, making them incredibly infrared luminous, with a L_IR ~ 10^(12.6-14.1) L_sun.

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arXiv2006

Understanding Infrared Galaxy Populations: the SWIRE Legacy Survey

We discuss spectral energy distributions, photometric redshifts, redshift distributions, luminosity functions, source-counts and the far infrared to optical luminosity ratio for sources in the SWIRE Legacy Survey. The spectral energy distributions of selected SWIRE sources are modelled in terms of a simple set of galaxy and quasar templates in the optical and near infrared, and with a set of dust emission templates (cirrus, M82 starburst, Arp 220 starburst, and AGN dust torus) in the mid infrared. The optical data, together with the IRAC 3.6 and 4.5 mu data, have been used to determine photometric redshifts. For galaxies with known spectroscopic redshifts there is a notable improvement in the photometric redshift when the IRAC data are used, with a reduction in the rms scatter from 10% in (1+z) to 5%. While further spectroscopic data are needed to confirm this result, the prospect of determining good photometric redshifts for the 2 million extragalactic objects in SWIRE is excellent. The distribution of the different infrared sed types in the L{ir}/L{opt} versus L{ir} plane, where L{ir} and L{opt} are the infrared and optical bolometric luminosities, is discussed. Source-counts at 24, 70 and 160 mu are discussed, and luminosity functions at 3.6 and 24 mu are presented.

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arXiv2005

Statistical constraints on the IR galaxy number counts and cosmic IR background from the Spitzer GOODS survey

We perform fluctuation analyses on the data from the Spitzer GOODS survey (epoch one) in the Hubble Deep Field North (HDF-N). We fit a parameterised power-law number count model of the form dN/dS = N_o S^{-δ} to data from each of the four Spitzer IRAC bands, using Markov Chain Monte Carlo (MCMC) sampling to explore the posterior probability distribution in each case. We obtain best-fit reduced chi-squared values of (3.43 0.86 1.14 1.13) in the four IRAC bands. From this analysis we determine the likely differential faint source counts down to $10^{-8} Jy$, over two orders of magnitude in flux fainter than has been previously determined. From these constrained number count models, we estimate a lower bound on the contribution to the Infra-Red (IR) background light arising from faint galaxies. We estimate the total integrated background IR light in the Spitzer GOODS HDF-N field due to faint sources. By adding the estimates of integrated light given by Fazio et al (2004), we calculate the total integrated background light in the four IRAC bands. We compare our 3.6 micron results with previous background estimates in similar bands and conclude that, subject to our assumptions about the noise characteristics, our analyses are able to account for the vast majority of the 3.6 micron background. Our analyses are sensitive to a number of potential systematic effects; we discuss our assumptions with regards to noise characteristics, flux calibration and flat-fielding artifacts.

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arXiv2005

The extragalactic sub-mm population: predictions for the SCUBA Half-Degree Extragalactic Survey (SHADES)

We present predictions for the angular correlation function and redshift distribution for SHADES, the SCUBA HAlf-Degree Extragalactic Survey, which will yield a sample of around 300 sub-mm sources in the 850 micron waveband in two separate fields. Complete and unbiased photometric redshift information on these sub-mm sources will be derived by combining the SCUBA data with i) deep radio imaging already obtained with the VLA, ii) guaranteed-time Spitzer data at mid-infrared wavelengths, and iii) far-infrared maps to be produced by BLAST, the Balloon-borne Large-Aperture Sub-millimeter Telescope. Predictions for the redshift distribution and clustering properties of the final anticipated SHADES sample have been computed for a wide variety of models, each constrained to fit the observed number counts. Since we are dealing with around 150 sources per field, we use the sky-averaged angular correlation function to produce a more robust fit of a power-law shape w(theta)=(theta/A)^{-delta} to the model data. Comparing the predicted distributions of redshift and of the clustering amplitude A and slope delta, we find that models can be constrained from the combined SHADES data with the expected photometric redshift information.

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arXiv2004

First Insights into the SWIRE Galaxy Populations

We characterize the SWIRE galaxy populations in the SWIRE validation field within the Lockman Hole, based on the 3.6-24$μ$ Spitzer data and deep U,g',r',r' optical imaging within an area ~1/3 sq. deg for ~16,000 Spitzer-SWIRE sources. The entire SWIRE survey will discover over 2.3 million galaxies at 3.6$μ$m and almost 350,000 at 24$μ$m; ~70,000 of these will be 5-band 3.6-24$μ$ detections. The colors cover a broad range, generally well represented by redshifted spectral energy distributions of known galaxy populations, however significant samples of unusually blue objects in the [3.6-4.5]$μ$m color are found, as well as many objects very red in the 3.6-24$μ$m mid-IR. Nine of these are investigated and are interpreted as star-forming systems, starbursts and AGN from z=0.37 to 2.8, with luminosities from L$_{IR}$=10$^{10.3}$ to 10$^{13.7}$ L$_{\odot}$

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arXiv2004

Angular Clustering of Galaxies at 3.6 micron from the SWIRE survey

We present the first analysis of large-scale clustering from the Spitzer Wide-area InfraRed Extragalactic legacy survey: SWIRE. We compute the angular correlation function of galaxies selected to have 3.6 micron fluxes brighter than 32 muJy in three fields totaling two square degrees in area. In each field we detect clustering with a high level of significance. The amplitude and slope of the correlation function is consistent between the three fields and is modeled as $w(theta)=Aθ^{1-γ}$ with $A=(0.6\pm0.3)\times10^{-3}, γ=2.03\pm0.10$. With a fixed slope of $γ=1.8$, we obtain an amplitude of $A=(1.7\pm0.1)\times10^{-3}$. Assuming an equivalent depth of K~18.7 mag we find our errors are smaller but our results are consistent with existing clustering measurements in K-band surveys and with stable clustering models. We estimate our median redshift $z\simeq 0.75$ and this allows us to obtain an estimate of the three-dimensional correlation function $ξ(r)$, for which we find $r_0=4.4\pm0.1 h^{-1}$ Mpc.

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arXiv2003

SWIRE: The SIRTF Wide-area InfraRed Extragalactic Survey

The SIRTF Wide-area InfraRed Extragalactic survey (SWIRE), the largest SIRTF Legacy program, is a wide-area, imaging survey to trace the evolution of dusty, star-forming galaxies, evolved stellar populations, and AGN as a function of environment, from redshifts z~3 to the current epoch. SWIRE will survey 7 high-latitude fields, totaling 60 - 65 sq. deg. in all 7 SIRTF bands: IRAC 3.6, 4.5, 5.6, 8 microns and MIPS 24, 70, 160 microns. The Legacy Extragalactic Catalog may contain in excess of 2 million IR-selected galaxies, dominated by (1) ~150,000 luminous infrared galaxies (LIRGs: L{FIR}>10^11 L_sun), ~7000 of these with z>2; (2) 1 million early-type galaxies, ~10,000 with z>2; and (3) \~20,000 classical AGN, plus significantly more dust-obscured QSO/AGN among the LIRGs. SWIRE will provide an unprecedented view of the evolution of galaxies, structure, and AGN. The key scientific goals of SWIRE are: (1) to determine the evolution of actively star-forming and passively evolving galaxies in order to understand the history of galaxy formation in the context of cosmic structure formation; (2) to determine the evolution of the spatial distribution and clustering of evolved galaxies, starbursts and AGN in the key redshift range, 0.5<z<3, over which much of cosmic evolution has occurred; (3) to determine the evolutionary relationship between ``normal galaxies'' and AGN, and the contribution of AGN accretion energy vs stellar nucleosynthesis to the cosmic backgrounds. SWIRE's large area is important to create statistically significant population samples over enough volume cells that we can resolve the star formation history as a function of epoch and environment The large volume is also optimised for finding rare objects.

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arXiv2001

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 effective area 7.1 sr) has ~100% complete radio detections and is insensitive to dust obscuration and cirrus contamination. After removal of known active galaxies, the best-fit Schechter function has a faint-end slope of -1.27+/-0.07 in agreement with the local Halpha luminosity function, characteristic luminosity L*=(2.6+/-0.7)*10^{22} W/Hz and density phi* = (4.8 +/-1.1)*10^{-4} / Mpc^3. The inferred local radio luminosity density of (1.73+/-0.37+/-0.03)*10^{19} W/Hz/Mpc^3 (Poisson noise, large scale structure fluctuations) implies a volume averaged star formation rate ~2 x larger than the Gallego et al. Halpha estimate, i.e. rho(1.4 GHz} = (2.10+/-0.45+/-0.04) *10^{-2}$ Msun/yr/Mpc^3 for a Salpeter initial mass function from 0.1-125 Msun and Hubble constant of 50 km/s/Mpc. We demonstrate that the Balmer decrement is a highly unreliable extinction estimator, and argue that optical-UV SFRs are easily underestimated, particularly at high redshift.

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arXiv2001

Galaxy populations from Deep ISO Surveys

I discuss some of the main extra-galactic field surveys which have been undertaken by the Infrared Space Observatory (ISO). I review the findings from the source counts analysies and then examine some of the more recent detailed investigations into the explicit nature of the populations that make up these source counts.

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arXiv2001

FIRST Extra-Galactic Surveys: Practical Considerations

We discuss some of the practical considerations that need to be made in the design of extra-galactic field surveys using FIRST. We investigate the various limitations that confusion noise imposes on possible FIRST surveys and the benefits of super resolution or decconvolution techniques. We consider the possible sizes and depths of survey fields in order to meet many of the scientific objectives of the FIRST mission. In particular we discuss the factors that need to be taken into account in selecting the survey fields. The final choice of fields will be influenced by the location of other surveys that are currently being planned; so it is vital that the FIRST community begin debating this issue now. We conclude that a substantial survey of order of 100 square degrees is likely to be sited in the region of the SIRTF Legacy survey, SWIRE.

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arXiv2000

HST imaging survey of sub-mJy star-forming galaxies I: morphologies at z ~ 0.2

We present the first results of our HST WFPC2 F814W snapshot imaging survey, targeting virtually all sub-mJy decimetric radio-selected star-forming galaxies. The radio selection at ~1 GHz is free from extinction effects and the radio luminosities are largely unaffected by AGN contamination, making these galaxies ideal tracers of the cosmic star formation history. A sub-sample of 4 targets is presented here, selected at 1.4 GHz from the spectroscopically homogenous and complete samples of Benn et al. (1993) and Hopkins et al. (1999). The redshifts are confined to a narrow range around z~0.2, to avoid differential evolution, with a radio luminosity close to L* where the galaxies dominate the comoving volume-averaged star formation rate. We find clearly disturbed morphologies resembling those of ultraluminous infrared galaxies, indicating that galaxy interactions may be the dominant mechanism for triggering star formation at these epochs. The morphologies are also clearly different from coeval quasars and radiogalaxies, as found in star-forming galaxies selected at other wavelengths. This may prove challenging for models which propose direct causal links between AGN evolution and the cosmic star formation history at these epochs. The asymmetries are typically much larger than seen in the CFRS at similar redshifts, optical luminosities and Halpha-derived star formation rates, indicating the possible existence of an obscuration-related morphological bias in such samples.

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arXiv2000

The European Large Area ISO Survey: Latest Result

We present some recent results from the European Large Area ISO Survey (ELAIS). This survey was the largest non-serendipitous ISO field survey. A preliminary reduction has recently been completed and catalogues of sources released to the community. Early results show strongly evolving source counts. A comprehensive identification programme is underway and a number of extremely luminous objects have already been discovered. This survey provides an exciting legacy from the ISO mission and (amongst many goals) will allow us to provide important constraints on the obscured star-formation history of the Universe.

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arXiv2000

The European Large Area ISO Survey I: Goals, Definition and Observation

We describe the European Large Area ISO Survey (ELAIS). ELAIS was the largest single Open Time project conducted by ISO, mapping an area of 12 square degrees at 15mu with ISO-CAM and at 90mu with ISO-PHOT. Secondary surveys in other ISO bands were undertaken by the ELAIS team within the fields of the primary survey, with 6 square degrees being covered at 6.7mu and 1 square degree at 175mu. This paper discusses the goals of the project and the techniques employed in its construction, as well as presenting details of the observations carried out, the data from which are now in the public domain. We outline the ELAIS ``Preliminary Analysis'' which led to the detection of over 1000 sources from the 15 and 90 mu surveys (the majority selected at 15mu with a flux limit of ~3 mJy), to be fed into a ground-based follow-up campaign, as well as a programme of photometric observations of detected sources using both ISO-CAM and ISO-PHOT. We detail how the ELAIS survey complements other ISO surveys in terms of depth and areal coverage, and show that the extensive multi-wavelength coverage of the ELAIS fields resulting from our concerted and on-going follow-up programme has made these regions amongst the best studied areas of their size in the entire sky, and, therefore, natural targets for future surveys. This paper accompanies the release of extremely reliable sub-sets of the ``Preliminary Analysis'' products. Subsequent papers in this series will give further details of our data reduction techniques, reliability & completeness estimates and present the 15 and 90 nu number counts from the ``Preliminary Analysis'', while a further series of papers will discuss in detail the results from the ELAIS ``Final Analysis'', as well as from the follow-up programme.

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arXiv1999

Density and Velocity Fields from the PSCz Survey

We present the results for the predicted density and peculiar velocity fields and the dipole from the PSCz survey of 15,000 IRAS galaxies over 84% of the sky. We find a significant component to the dipole arising between 6000 and 15,000 km/s, but no significant component from greater distances. The misalignment with the CMB is 20 degrees. The most remarkable feature of the PSCz model velocity field is a coherent large-scale flow along the baseline connecting Perseus-Pisces, the Local Supercluster, Great Attractor and the Shapley Concentration. We have measured the parameter beta using the amplitude of the dipole, bulk flow and point by point comparisons between the individual velocities of galaxies in the MarkIII and SFI datasets, and the large-scale clustering distortion in redshift space.All our results are consistent with beta = 0.6 +- 0.1.

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