arXiv subjects

Ian Waddington

Publications and source records attributed to Ian Waddington.

10 recordsLinked to original sources

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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arXiv2005

The IRAC galaxy correlation functions from SWIRE

We present an analysis of large-scale structure from the Spitzer Wide-area Infrared Extragalactic legacy survey, SWIRE. The two-point angular correlation functions were computed for galaxies detected in the 3.6-micron IRAC band, on angular scales up to a degree. Significant evolution in the clustering amplitude was detected, as the median redshift of the samples increases from z=0.2 to 0.6. The galaxy clustering in the GALICS semi-analytic models was compared with the observed correlation functions and found to disagree with the data at faint flux limits.

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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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arXiv2002

HST Imaging of a z=1.55 Old Galaxy Group

We present high-resolution imaging in the rest-frame optical of the weak radio source LBDS53W091. Previous optical spectroscopy has shown that this object has an evolved stellar population of age >3Gyr at z=1.55, determined from the amplitude of rest-frame UV spectral breaks. We have obtained deep Hubble Space Telescope imaging over 10 orbits with NICMOS camera 2, using the F160W H-band filter (1.6microns) which is a good approximation to the rest-frame R-band. Our observations reveal a radial light profile which is well fit by a de Vaucouleurs' r^{1/4} law, with a scale length of r_e=0.3" (3kpc/h_{50} for Omega_M=0.3 and Omega_{Lambda}=0). The elliptical morphology of the radio galaxy indicates a dynamically-evolved old system, consistent with the spectroscopic results. Some surrounding objects lie on the color:magnitude relation for a cluster at z=1.55, and are likely to be associated. The group of galaxies are somewhat more luminous than the fundamental plane of ellipticals at z=0 subject to (1+z)^4 surface brightness dimming, but are consistent with estimates of the fundamental plane at high redshift subject to passive luminosity evolution from a formation epoch of z>3.

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arXiv2000

Faint Radio Sources and Star Formation History

The centimeter-wave luminosity of local radio galaxies correlates well with their star formation rate. We extend this correlation to surveys of high-redshift radio sources to estimate the global star formation history. The star formation rate found from radio observations needs no correction for dust obscuration, unlike the values calculated from optical and ultraviolet data. Three deep radio surveys have provided catalogs of sources with nearly complete optical identifications and nearly 60% complete spectroscopic redshifts: the Hubble Deep Field and Flanking Fields at 12h+62d, the SSA13 field at 13h+42d, and the V15 field at 14h+52d. We use the redshift distribution of these radio sources to constrain the evolution of their luminosity function. The epoch dependent luminosity function is then used to estimate the evolving global star formation density. At redshifts less than one, our calculated star formation rates are significantly larger than even the dust-corrected optically-selected star formation rates; however, we confirm the rapid rise from z=0 to z=1 seen in those surveys.

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arXiv1999

Evidence for Large-Scale Structure at z=2.4 From Lyman-alpha Imaging

We present deep wide-field medium-band imaging in redshifted Lyman alpha of fields surrounding HST detections of faint emitters, to characterize their larger-scale environment. The radio galaxy 53W002 was previously found to be part of a rich grouping at z=2.39, including 5 confirmed, compact, star-forming objects. This is now shown to be part of a larger structure traced by bright AGN, all contained within a projected span of 6.8 arcmin (3.2 Mpc). Of the 14 candidate emitters, six have been spectroscopically confirmed as active nuclei in the range z=2.390 +/- 0.008. Various statistical tests give a significance of 95-99% for the reality of this structure on the sky. This grouping is more extended than a relaxed King model, at the 90% confidence level. Either this configuration has yet to decouple fully from the Hubble expansion, or it consists of subgroups which will themselves form a more compact, relaxed structure. The redshift range for measured members is comparable to the Hubble flow across the structure, which may imply that the structure is seen near turnaround. We surveyed two additional fields at z=2.4, and three contiguous fields at z=2.6. Only a single candidate at z=2.4 was found, with six at z=2.6. Comparison with the WFPC2 surveys suggests that the field-to-field contrast is larger for brighter objects. From this survey alone, groupings such as the 53W002 "cluster" must have an area covering fraction <0.04 in this redshift range.

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arXiv1999

Evolution of the 1.4 GHz Radio Luminosity Function

The results of an optical and infrared investigation of a complete sub-sample of the Leiden-Berkeley Deep Survey (flux > 1 mJy at 1.4 GHz) are presented. Optical counterparts have been identified for 69 of the 73 sources in the two Hercules fields, and redshifts obtained for 49 of them. Photometric redshifts are computed from the g,r,i,K data for the remaining 21 sources. This complete sample is compared with the radio luminosity functions (RLFs) of Dunlop and Peacock (1990). The RLF models successfully trace the evolution of the radio sources with redshift, but there is some disagreement between the luminosity-dependence of the models and the data. The observed RLF for the lower luminosity population (log P < 26) shows evidence for a cut-off at lower redshifts (z=0.5-1.5) than for the more powerful objects.

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arXiv1998

Near-Infrared Imaging of a Group or Cluster of Galaxies at a Redshift of 2.39

At z=2.39, the cluster around 53W002 is one of the most distant groups or clusters of galaxies known to date. At this redshift the 4000A-break falls between the J and H bands, thus our infrared observations are designed to identify cluster members by a red J-H colour. Out of the 42 objects we have detected in the field, we find ten galaxies with J-H>1.0 and K>18.8, redder and fainter than the radio galaxy, and consistent with the presence of a 4000A-break at the cluster redshift. Two of these reddest galaxies have been confirmed spectroscopically. The colours, sizes and location of these infrared-selected galaxies suggest a cluster much more similar to those nearby than revealed by Hubble Space Telescope observations alone.

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