Showing papers on "Photometric redshift published in 2002"

The Infrared Spectral Energy Distribution of Normal Star-forming Galaxies: Calibration at Far-Infrared and Submillimeter Wavelengths

Abstract: New far-infrared and submillimeter data are used to solidify and to extend to long wavelengths the empirical calibration of the infrared spectral energy distribution (SED) of normal star-forming galaxies. As was found by Dale and coworkers in 2001, a single parameter family, characterized by fν(60 μm)/fν(100 μm), is adequate to describe the range of normal galaxy SEDs observed by the Infrared Astronomical Satellite and Infrared Space Observatory from 3 to 100 μm. However, predictions based on the first-generation models at longer wavelengths (122-850 μm) are increasingly overluminous compared to the data for smaller fν(60 μm)/fν(100 μm), or alternatively, for weaker global interstellar radiation fields. After slightly modifying the far-infrared/submillimeter dust emissivity in those models as a function of the radiation field intensity to better match the long-wavelength data, a suite of SEDs from 3 μm to 20 cm in wavelength is presented. Results from relevant applications are also discussed, including submillimeter-based photometric redshift indicators, the infrared energy budget and simple formulae for recovering the bolometric infrared luminosity, and dust mass estimates in galaxies. Regarding the latter, since galaxy infrared SEDs are not well described by single blackbody curves, the usual methods of estimating dust masses can be grossly inadequate. The improved model presented herein is used to provide a more accurate relation between infrared luminosity and dust mass.

Photometric Redshifts for the SDSS Early Data Release

Abstract: The Early Data Release from the Sloan Digital Sky survey provides one of the largest multicolor photometric catalogs currently available to the astronomical community. In this paper we present the first application of photometric redshifts to the $\sim 6$ million extended sources within these data (with 1.8 million sources having $r' < 21$). Utilizing a range of photometric redshift techniques, from empirical to template and hybrid techniques, we investigate the statistical and systematic uncertainties present within the redshift estimates for the EDR data. For $r'<21$ we find that the redshift estimates provide realistic redshift histograms with an rms uncertainty in the photometric redshift relation of 0.035 at $r'<18$ and rising to 0.1 at $r'<21$. We conclude by describing how these photometric redshifts and derived quantities, such as spectral type, restframe colors and absolute magnitudes, are stored within the SDSS database. We provide sample queries for searching on photometric redshifts and list the current caveats and issues that should be understood before using these photometric redshifts in statistical analyses of the SDSS galaxies.

Photometric redshifts from evolutionary synthesis with PEGASE: the code ZPEG and the z=0 age constraint

Abstract: Photometric redshifts are estimated on the basis of template scenarios with the help of the code ZPEG, an extension of the galaxy evolution model PEGASE.2 and available on the PEGASE web site. The spectral energy distribution (SED) templates are computed for nine spectral types including starburst, irregular, spiral and elliptical. Dust, extinction and metal effects are coherently taken into account, depending on evolution scenarios. The sensitivity of results to adding near-infrared colors and IGM absorption is analyzed. A comparison with results of other models without evolution measures the evolution factor which systematically increases the estimated photometric redshift values by $\Delta z$ > 0.2 for z > 1.5. Moreover we systematically check that the evolution scenarios match observational standard templates of nearby galaxies, implying an age constraint of the stellar population at z=0 for each type. The respect of this constraint makes it possible to significantly improve the accuracy of photometric redshifts by decreasing the well-known degeneracy problem. The method is applied to the HDF-N sample. From fits on SED templates by a $\chi^2$-minimization procedure, not only is the photometric redshift derived but also the corresponding spectral type and the formation redshift $z_for$ when stars first formed. Early epochs of galaxy formation z > 5 are found from this new method and results are compared to faint galaxy count interpretations. The new tool is available at: this http URL

Photometric redshifts from evolutionary synthesis with PÉGASE: The code Z-PEG and the z=0 age constraint

Abstract: Photometric redshifts are estimated on the basis of template scenarios with the help of the code Z-PEG, an extension of the galaxy evolution model P EGASE.2 and available on the P EGASE web site. The spectral energy distribution (SED) templates are computed for nine spectral types including starburst, irregular, spiral and elliptical. Dust, extinction and metal eects are coherently taken into account, depending on evolution scenarios. The sensitivity of results to adding near-infrared colors and IGM absorption is analyzed. A comparison with results of other models without evolution measures the evolution factor which systematically increases the estimated photometric redshift values byz 0:2 for z> 1:5. Moreover we systematically check that the evolution scenarios match observational standard templates of nearby galaxies, implying an age constraint of the stellar population at z = 0 for each type. The respect of this constraint makes it possible to signicantly improve the accuracy of photometric redshifts by decreasing the well-known degeneracy problem. The method is applied to the HDF-N sample (see in Fern andez-Soto et al. 1999). From ts on SED templates by a 2 -minimization procedure, not only is the photometric redshift derived but also the corresponding spectral type and the formation redshift zfor when stars rst formed. Early epochs of galaxy formationzfor > 5 are found from this new method and results are compared to faint galaxy count interpretations. The new tool is available at: http://www.iap.fr/pegase

Far infrared and radio emission in dusty starburst galaxies

Abstract: We revisit the nature of the far infrared (FIR)/radio correlation by means of the most recent models of star forming galaxies, focusing in particular on the case of obscured starbursts. We model the IR emission with our population synthesis code, GRASIL (Silva et al. 1998). For the radio emission, we revisit the simple model of Condon & Yin (1990). We find that a tight FIR/radio correlation is natural when the synchrotron mechanism dominates over the inverse Compton, and the electron cooling time is shorter than the fading time of the supernova (SN) rate. Observations indicate that both these conditions are met in star forming galaxies, from normal spirals to obscured starbursts. However, since the radio non-thermal (NT) emission is delayed, deviations are expected both in the early phases of a starburst, when the radio thermal component dominates, and in the post-starburst phase, when the bulk of the NT component originates from less massive stars. We show that this delay allows the analysis of obscured starbursts with a time resolution of a few tens of Myrs, unreachable with other star formation (SF) indicators. We suggest a strategy to complement the analysis of the deviations from the FIR/radio correlation with the radio slope (q-radio slope diagram) to obtain characteristic parameters of the burst, e.g. its intensity, age and fading time scale. The analysis of a sample of compact ULIRGs shows that they are intense but transient starbursts, to which one should not apply usual SF indicators devised for constant SF rates. We also discuss the possibility of using the q-radio slope diagram to assess the presence of obscured AGN. A firm prediction of the models is an apparent radio excess during the post-starburst phase, which seems to be typical of a class of star forming galaxies in rich cluster cores. Finally we discuss how deviations from the correlation, due to the evolutionary status of the starburst, aect the technique of photometric redshift determination widely used for high-z sources.

The Star Formation Rate Intensity Distribution Function: Implications for the Cosmic Star Formation Rate History of the Universe

Abstract: We address the effects of cosmological surface brightness dimming on observations of faint galaxies by examining the distribution of unobscured star formation rate intensities versus redshift. We use the star formation rate intensity distribution function to assess the ultraviolet luminosity density versus redshift, based on our photometry and photometric redshift measurements of faint galaxies in the Hubble Deep Field (HDF) and the Hubble Deep Field-South (HDF-S) Wide Field Planetary Camera 2 and Near-Infrared Camera and Multi-Object Spectrometer fields. We find that (1) previous measurements have missed a dominant fraction of the ultraviolet luminosity density of the universe at high redshifts by neglecting cosmological surface brightness dimming effects, which are important at redshifts larger than z ≈ 2; (2) the incidence of the highest intensity star-forming regions increases monotonically with redshift; and (3) the ultraviolet luminosity density plausibly increases monotonically with redshift through the highest redshifts observed. By measuring the spectrum of the luminosity density versus redshift, we also find that (4) previous measurements of the ultraviolet luminosity density at redshifts z < 2 must be reduced by a factor of ≈2 to allow for the spectrum of the luminosity density between rest-frame wavelengths 1500 and 2800 A. And, by comparing with observations of high-redshift damped Lyα absorption systems detected toward background quasi-stellar objects, we further find that (5) the distribution of star formation rate intensities matches the distribution of neutral hydrogen column densities at redshifts z ≈ 2-5, which establishes a quantitative connection between high-redshift galaxies and high column density gas and suggests that high-redshift damped Lyα absorption systems trace lower star formation rate intensity regions of the same galaxies detected in starlight in the HDF and HDF-S. Because our measurements neglect the effects of obscuration by dust, they represent lower limits to the total star formation rate density.

Likelihood analysis of cosmic shear on simulated and VIRMOS-DESCART data

Abstract: We present a maximum likelihood analysis of cosmological parameters from measurements of the aperture mass up to 35 arcmin using simulated and real cosmic shear data. A four-dimensional parameter space is explored which examines the mean densityM, the mass power spectrum normalisation8, the shape parameter and the redshift of the sources zs. Constraints onM and8 (resp. and zs) are provided by marginalising over and zs (resp.M and8). For a flatCDM cosmologies, using a photometric redshift prior for the sources and 2 (0:1; 0:4), we find8= (0:57 0:04) (0:240:18) M 0:49 M at the 68% confidence level (the error budget includes statistical noise, full cosmic variance and residual systematics). The estimate of , marginalised overM 2 (0:1; 0:4),8 2 (0:7; 1:3) and zs constrained by photometric redshifts, gives= 0:25 0:13 at 68% confidence. Adopting h= 0:7, a flat universe,= 0: 2a ndm = 0: 3w e fi nd8 = 0:98 0:06. Combined with CMB measurements, our results suggest a non-zero cosmological constant and provide tight constraints onM and8. Finally, we compare our results to the cluster abundance ones, and discuss the possible discrepancy with the latest determinations of the cluster method. In particular we point out the actual limitations of the mass power spectrum prediction in the non-linear regime, and the importance in improving this.

The 1.6 Micron Bump as a Photometric Redshift Indicator

Abstract: I describe the principle of using the 1.6 μm H- spectral feature as a photometric redshift indicator and demonstrate that the technique holds promise by successfully recovering the redshifts of a small sample of z = 0–1 galaxies by using only their infrared (JHKL) photometry. I then consider the applicability of the technique to the 3.6–8 μm Space Infrared Telescope Facility (SIRTF) filter set and investigate the systematic errors that could arise in photometric redshifts from random photometric errors or from a mismatch between target galaxies and fitting templates in metallicity, star formation history, and amount of interstellar dust. It appears that SIRTF near-IR data alone should be sufficient to estimate redshift of most galaxies that are at z 1.5 and are dominated by stellar populations older than 20 Myr. Galaxies whose photometric fits indicate them to be at lower redshifts, zfit 1.5, or dominated by very young stellar populations, agefit 20 Myr, suffer from severe degeneracies in photometric redshift, and a reliable photometric determination of their redshifts will have to include either IR observations at shorter wavelengths (H and K) or optical data. Overall, it appears that with care and caveats the 1.6 μm bump can provide a powerful way of estimating redshifts of distant galaxies in deep infrared imaging surveys that will soon be provided by SIRTF and eventually by Next Generation Space Telescope.

Suppressing the contribution of intrinsic galaxy alignments to the shear two-point correlation function

Abstract: Cosmological weak lensing gives rise to correlations in the ellipticities of faint galaxies. This cosmic shear signal depends upon the matter power spectrum, thus providing a means to constrain cosmological parameters. It has recently been proposed that intrinsic alignments arising at the epoch of galaxy formation can also contribute significantly to the observed correlations, the amplitude increasing with decreasing survey depth. Here we consider the two-point shear correlation function, and demonstrate that photometric redshift information can be used to suppress the intrinsic signal; at the same time Poisson noise is increased, due to a decrease in the effective number of galaxy pairs. The choice to apply such a redshift-depending weighting will depend on the characteristics of the survey in question. In surveys with $\left\langle z \right\rangle \sim 1$, although the lensing signal dominates, the measurement error bars may soon become smaller than the intrinsic alignment signal; hence, in order not to be dominated by systematics, redshift information in cosmic shear statistics will become a necessity. We discuss various aspects of this.

The Las Campanas Infrared Survey – II. Photometric redshifts, comparison with models and clustering evolution

Abstract: The Las Campanas Infrared (LCIR) Survey, using the Cambridge Infra-Red Survey Instrument (CIRSI), reaches H∼21 over nearly 1 deg^2. In this paper we present results from 744 arcmin^2 centred on the Hubble Deep Field South for which UBVRI optical data are publicly available. Making conservative magnitude cuts to ensure spatial uniformity, we detect 3177 galaxies to H=20.0 in 744 arcmin^2 and a further 842 to H=20.5 in a deeper subregion of 407 arcmin^2. We compare the observed optical–infrared (IR) colour distributions with the predictions of semi-analytic hierarchical models and find reasonable agreement. We also determine photometric redshifts, finding a median redshift of ∼0.55. We compare the redshift distributions N(z) of E, Sbc, Scd and Im spectral types with models, showing that the observations are inconsistent with simple passive-evolution models while semi-analytic models provide a reasonable fit to the total N(z) but underestimate the number of z∼1 red spectral types relative to bluer spectral types. We also present N(z) for samples of extremely red objects (EROs) defined by optical–IR colours. We find that EROs with R-H>4 and H 4 comprise ∼18 per cent of the observed galaxy population, while in semi-analytic models they contribute only ∼4 per cent. We also determine the angular correlation function w(θ) for magnitude, colour, spectral type and photometric redshift-selected subsamples of the data and use the photometric redshift distributions to derive the spatial clustering statistic ξ(r) as a function of spectral type and redshift out to z∼1.2. Parametrizing ξ(r) by ξ(rc,z)=[rc/r∗(z)]^(-1.8), where r_c is in comoving coordinates, we find that r∗(z) increases by a factor of 1.5–2 from z=0 to z∼1.2. We interpret this as a selection effect – the galaxies selected at z∼1.2 are intrinsically very luminous, about 1–1.5 mag brighter than L∗. When galaxies are selected by absolute magnitude, we find no evidence for evolution in r∗ over this redshift range. Extrapolated to z=0, we find r∗(z=0)∼6.5 h^(-1) Mpc for red galaxies and r∗(z=0)∼2–4 h^(-1) Mpc for blue galaxies. We also find that, while the angular clustering amplitude of EROs with R-H>4 or I-H>3 is up to four times that of the whole galaxy population, the spatial clustering length r∗(z=1) is ∼7.5–10.5 h^(-1) Mpc, which is only a factor of ∼1.7 times r∗(z=1) for R-H<4 and I-H<3 galaxies lying in a similar redshift and luminosity range. This difference is similar to that observed between red and blue galaxies at low redshifts.

The 1.6um Bump as a Photometric Redshift Indicator

Abstract: I describe the principle of using the 1.6um H- spectral feature as a photometric redshift indicator and demonstrate that the technique holds promise by successfully recovering the redshifts of a small sample of z=0-1 galaxies using only their infrared (JHKL) photometry. I then consider the applicability of the technique to the 3.6--8um SIRTF filter set and investigate the systematic errors that could arise in photometric redshifts from random photometric errors or from a mismatch between target galaxies and fitting templates in metallicity, star formation history, and amount of interstellar dust. It appears that SIRTF near-IR data alone should be sufficient to estimate redshift of most galaxies that are at z>~1.5 and are dominated by stellar populations older than >~20Myr. Galaxies whose photometric fits indicate them to be at lower redshifts, z_fit<~1.5, or dominated by very young stellar populations, age_fit<~20Myr, suffer from severe degeneracies in photometric redshift, and a reliable photometric determination of their redshifts will have to include either IR observations at shorter wavelengths (H and K) or optical data. Overall, it appears that - with care and caveats - the 1.6um bump can provide a powerful way of estimating redshifts of distant galaxies in deep infrared imaging surveys that will soon be provided by SIRTF, and, eventually, by NGST.

Warm-hot intergalactic baryons revealed

Abstract: Several popular cosmological models predict that most of the baryonic mass in the local universe is located in filamentary and sheet-like structures associated with galaxy overdensities. This gas is expected to be gravitationally heated to ∼10 6 K and therefore emitting in the soft X-rays. We have detected diffuse soft X-ray structures in a high Galactic latitude ROSAT field after point source subtraction and correction for Galactic absorption. These diffuse structures have an X-ray energy distribution that is much softer than expected from clusters, groups or unresolved emission from AGNs, but are consistent with that expected from a diffuse warm intergalactic medium. To discriminate between a Galactic or extragalactic nature of the diffuse gas we have correlated the soft X-map with multiband optical images in this field. We have found a significant overdensity of galaxies in correspondence with the strongest diffuse X-ray structure. The photometric redshift distribution of the galaxies over the X-ray peak has an excess over field galaxies at z ∼ 0.45. This result strongly suggests that the diffuse X-ray flux is due to extragalactic emission by warm gas associated with an overdense galaxy region at z ∼ 0.45.

Error analysis of the photometric redshift technique

Abstract: ABSTRA C T We present a calculation of the systematic component of the error budget in the photometric redshift technique. We make use of it to describe a simple technique that allows the assignment of confidence limits to redshift measurements obtained through photometric methods. We show that our technique, through the calculation of a redshift probability function, gives complete information on the probable redshift of an object and its associated confidence intervals. This information can and must be used in the calculation of any observable quantity that makes use of the redshift.

Weak lensing analysis of MS 1008–1224 with the VLT

Abstract: We present a gravitational lensing analysis of the cluster of galaxies MS 1008–1224 ($z=0.31$), based on very deep observations obtained using the VLT with FORS1 and ISAAC during the science verification phase. Two different mass reconstruction algorithms were applied to the B -, V -, R - and I -band data to obtain similar projected mass distributions in all the bands. The FORS1 (BVRI) and ISAAC (JK) data were combined to determine the photometric redshift distribution of galaxies within the ISAAC field and to estimate the mass. We inferred from weak shear a minimum mass of $2.3 \times 10^{14}$ $h^{-1}~M_{\odot}$ on large scales (within ~$ 700$ h -1 kpc, diameter) which agrees well with the X-ray mass mass estimate. The Mass-to-light ratios are also in excellent agreement. The observed mass profile is consistent with Pseudo-Isothermal Sphere models as well as a Navarro, Frenk and White model. In the inner regions the lensing mass is about twice as high as the X-ray mass which supports the long-held view that complex physical processes occuring in the innermost parts of lensing-clusters are mainly responsible for the X-ray-lensing mass discrepancy. We found that the central part of the cluster comprises two mass peaks whose center of mass is located 10–20 arcsec north of the cD galaxy. A similar offset between the cD and the peak of the X-ray distribution has been reported before. The optical, X-ray and the mass distributions show that MS 1008–1224 is composed of several subsystems which are probably undergoing a merger. It is likely that the gas is not in equilibrium in the innermost regions which vitiates the X-ray mass estimate there. We discovered that MS 1008–1224 shows a remarkable case of cluster-cluster lensing. The photometric redshifts show an excess of galaxies located 30 arcsec south-west of the cD galaxy at a redshift of ~0.9. This distant cluster is therefore also lensed by MS 1008–1224, which, if confirmed with spectroscopic data, would make this the first known case of magnification of a distant cluster by another one.

Neural Networks and Photometric Redshifts

Abstract: We present a neural network based approach to the determination of photometric redshift. The method was tested on the Sloan Digital Sky Survey Early Data Release (SDSS-EDR) reaching an accuracy comparable and, in some cases, better than SED template fitting techniques. Different neural networks architecture have been tested and the combination of a Multi Layer Perceptron with 1 hidden layer (22 neurons) operated in a Bayesian framework, with a Self Organizing Map used to estimate the accuracy of the results, turned out to be the most effective. In the best experiment, the implemented network reached an accuracy of 0.020 (interquartile error) in the range 0

Mapping the 3-D Dark Matter potential with weak shear

Abstract: We investigate the practical implementation of Taylor's (2002) 3-dimensional gravitational potential reconstruction method using weak gravitational lensing, together with the requisite reconstruction of the lensing potential. This methodology calculates the 3-D gravitational potential given a knowledge of shear estimates and redshifts for a set of galaxies. We analytically estimate the noise expected in the reconstructed gravitational field, taking into account the uncertainties associated with a finite survey, photometric redshift uncertainty, redshift-space distortions, and multiple scattering events. In order to implement this approach for future data analysis, we simulate the lensing distortion fields due to various mass distributions. We create catalogues of galaxies sampling this distortion in three dimensions, with realistic spatial distribution and intrinsic ellipticity for both ground-based and space-based surveys. Using the resulting catalogues of galaxy position and shear, we demonstrate that it is possible to reconstruct the lensing and gravitational potentials with our method. For example, we demonstrate that a typical ground-based shear survey with redshift limit z=1 and photometric redshifts with error Delta z=0.05 is directly able to measure the 3-D gravitational potential for mass concentrations >10^14 M_\odot between 0.1

Completeness in Photometric and Spectroscopic Searches for Clusters

Abstract: We investigate, using simulated galaxy catalogs, the completeness of searches for massive clusters of galaxies in redshift surveys or imaging surveys with photometric redshift estimates, i.e., what fraction of clusters (M > 1014 h-1 M☉) are found in such surveys. We demonstrate that the matched filter method provides an efficient and reliable means of identifying massive clusters even when the redshift estimates are crude. In true redshift surveys the method works extremely well. We demonstrate that it is possible to construct catalogs with high completeness and low contamination, where both vary little with redshift.

The 1.6um Bump as a Photometric Redshift Indicator

Abstract: I describe the principle of using the 1.6um H- spectral feature as a photometric redshift indicator and demonstrate that the technique holds promise by successfully recovering the redshifts of a small sample of z=0-1 galaxies using only their infrared (JHKL) photometry. I then consider the applicability of the technique to the 3.6--8um SIRTF filter set and investigate the systematic errors that could arise in photometric redshifts from random photometric errors or from a mismatch between target galaxies and fitting templates in metallicity, star formation history, and amount of interstellar dust. It appears that SIRTF near-IR data alone should be sufficient to estimate redshift of most galaxies that are at z>~1.5 and are dominated by stellar populations older than >~20Myr. Galaxies whose photometric fits indicate them to be at lower redshifts, z_fit<~1.5, or dominated by very young stellar populations, age_fit<~20Myr, suffer from severe degeneracies in photometric redshift, and a reliable photometric determination of their redshifts will have to include either IR observations at shorter wavelengths (H and K) or optical data. Overall, it appears that - with care and caveats - the 1.6um bump can provide a powerful way of estimating redshifts of distant galaxies in deep infrared imaging surveys that will soon be provided by SIRTF, and, eventually, by NGST.

Photometric redshift determination with the BATC multicolor system

Abstract: In this paper, we present the methodology of photometric redshift determination with the BATC 15 color system by using the hyperz program. Both simulated galaxies and real galaxies with known redshifts were used to estimate the accuracy of redshifts inferred from multicolor photometry. From the test with simulated galaxies, the uncertainty in the inferred redshifts is about 0.02-0.03 for a given range of photometric uncertainty of 0.05-0.10 mag. The results with the 27 real galaxies are in good agreement with the simulated ones. The advantage of using the BATC intermediate-band system to derive redshift is clear after comparison with the UBVRI broadband system. The accuracy in redshift determination with the BATC system is mainly affected by the selection of filters and the photometric uncertainties in the observation. When we take the limiting magnitudes of the 15 filters into account, we find that redshift can be determined with good accuracy for galaxies with redshifts less than 0.5, using only filters with central wavelengths shorter than 6000 A.

The lensing system towards the doubly imaged quasar SBS 1520+530

Abstract: The gravitational potential responsible for the lensing effect in SBS 1520+530 is studied over length scales from a few arc-seconds to a few arc-minutes. For this purpose, we use sharply deconvolved Hubble Space Telescopeimages in the optical and near-IR, in combination with ground based optical data obtained over a wider field-of-view. In particular, we have carried out a multi-color analysis in order to identify groups or clusters of galaxies along the line of sight. Photometric redshifts are measured for 139 galaxies unveiling significant excesses of galaxies 1.0' NW and 1.7' SW of the main lensing galaxy. The photometric redshift inferred both for the main lensing galaxy and for the galaxy concentrations is z = 0.9 + 0 . 1 0 - 0 . 2 5 . This is in rough agreement with the measured spectroscopic redshift of the main lensing galaxy, z = 0.71 (Burud et al. 2002), suggesting that it is part of a larger group or cluster. We investigate the impact of including the galaxy cluster, first on the modelling of the lensing system, and second on the expected time-delay between the two quasar images.

The lensing system towards the doubly imaged quasar SBS 1520+530

Abstract: The gravitational potential responsible for the lensing effect in SBS 1520+530 is studied over length scales from a few arc-seconds to a few arc-minutes. For this purpose, we use sharply deconvolved Hubble Space Telescope images in the optical and near-IR, in combination with ground based optical data obtained over a wider field-of-view. In particular, we have carried out a multi-color analysis in order to identify groups or clusters of galaxies along the line of sight. Photometric redshifts are measured for 139 galaxies unveiling significant excesses of galaxies 1.0 arcmin, NW and 1.7 arcmin, SW of the main lensing galaxy. The photometric redshift inferred both for the main lensing galaxy and for the galaxy concentrations is z=0.9^{+0.10}_{-0.25}. This is in rough agreement with the measured spectroscopic redshift of the main lensing galaxy, z=0.71 (Burud et al. 2002), suggesting that it is part of a larger group or cluster. We investigate the impact of including the galaxy cluster, first on the modelling of the lensing system, and second on the expected time--delay between the two quasar images.

ASTRO-F - Super IRAS - The All Sky Infra-Red Survey

Abstract: We review the next generation Japanese infrared space mission, ASTRO-F. ASTRO-F will be the first survey of the entire sky at infrared wavelengths since the IRAS mission almost 20 years ago. ASTRO-F will survey the entire sky in 4 far-infrared bands from 50-200microns and 2 mid-infrared bands at 9 and 20microns to sensitivities of 10-1000 times deeper than the IRAS satellite at angular resolutions of 25-45arcsec (c.f. IRAS 2-5arcmins). ASTRO-F can be considered a SUPER-IRAS. Using the galaxy evolution model of Pearson (2001) we produce expected numbers of sources under 3 different cosmological world models. We predict that ASTRO-F will detect of the order of 10's millions of sources in the far-infrared wavelength bands, most of which will be dusty LIG/ULIGs of which as many as half will lie at redshifts greater than unity. We produce number-redshift distributions, flux-redshift and colour-colour diagrams for the survey and discuss various segregation and photometric redshift techniques. Furthermore, we investigate the large scale structure scales that will be accessed by ASTRO-F, discovering that ASTRO-F and SIRTF-SWIRE probe both different scales and redshift domains and concluding that the 2 missions will supplement rather than supplant one another.

Galaxies Associated with z~4 Damped Lya Systems: I. Imaging and Photometric Selection

Abstract: This paper describes the acquisition and analysis of imaging data for the identification of galaxies associated with z~4 damped Lya systems. We present deep BRI images of three fields known to contain four z~4 damped systems. We discuss the reduction and calibration of the data, detail the color criteria used to identify z~4 galaxies, and present a photometric redshift analysis to complement the color selection. We have found no galaxy candidates closer to the QSO than 7'' which could be responsible for the damped Lya systems. Assuming that at least one of the galaxies is not directly beneath the QSO, we set an upper limit on this damped Lya system of L < L*/4. Finally, we have established a web site to release these imaging data to the public.

Galaxies Associated with z ∼ 4 Damped Lyα Systems. I. Imaging and Photometric Selection

Abstract: This paper describes the acquisition and analysis of imaging data for the identification of galaxies associated with z ~ 4 damped Lyα systems. We present deep BRI images of three fields known to contain four z ~ 4 damped systems. We discuss the reduction and calibration of the data, detail the color criteria used to identify z ~ 4 galaxies, and present a photometric redshift analysis to complement the color selection. We have found no galaxy candidates closer to the QSO than 7'' that could be responsible for the damped Lyα systems. Assuming that at least one of the galaxies is not directly beneath the QSO, we set an upper limit on this damped Lyα system of L < L/4. Finally, we have established a Web site to release these imaging data to the public.

Very Large Telescope near-infrared spectra of hard serendipitous Chandra sources

Abstract: We present near-infrared long-slit spectra of eight optically dim X-ray sources obtained with ISAAC on the Very Large Telescope. Six of the sources have hard X-ray emission with a significant fraction of the counts emerging above 2 keV. All were discovered serendipitously in the fields of three nearby galaxy clusters observed with Chandra, and identified through near-infrared imaging. The X-ray fluxes lie close to the break in the source counts. Two of the sources show narrow emission lines and a third has a broad line. One of the narrow line-emitting sources has a clear redshift identification at z = 2.18, while the other has a tentative determination based on the highest-redshift detection of He I λ 10830 at z = 1.26. The remainder have featureless spectra to deep limiting equivalent widths of ∼20-60 A and line flux ∼5 × 10 - 1 7 erg s - 1 cm - 2 in the K band. High-quality J-, H- and K-band images of the sources were combined with archival optical detections or limits to estimate a photometric redshift for six of them. Two sources show a complex double morphology. The hard sources have spectral count ratios consistent with heavily obscured active galactic nuclei, while the host galaxy emits much of the optical and near-infrared flux. The most likely explanation for the featureless continua is that the line photons are being scattered or destroyed by optically thick gas and associated dust with large covering fractions.

VLT near-infrared spectra of hard serendipitous Chandra sources

Abstract: We present near-infrared long-slit spectra of eight optically-dim X-ray sources obtained with ISAAC on the Very Large Telescope. Six of the sources have hard X-ray emission with a signican t fraction of the counts emerging above 2 keV. All were discovered serendipitously in the elds of three nearby galaxy clusters observed with Chandra, and identied through near-infrared imaging. The X-ray uxes lie close to the break in the source counts. Two of the sources show narrow emission lines, and a third has a broad line. One of the narrow line-emitting sources has a clear redshift identication at z = 2:18, while the other has a tentative determination based on the highest redshift detection of He I 10830 at z = 1:26. The remainder have featureless spectra to deep limiting equivalent widths of 20{60 A and line ux 5 10 17 erg s 1 cm 2 in the K-band. High-quality J, H and Ks{band images of the sources were combined with archival optical detections or limits to estimate a photometric redshift for six. Two sources show complex double morphology. The hard sources have spectral count ratios consistent with heavily obscured AGN, while the host galaxy emits much of the optical and near-infrared ux. The most likely explanation for the featureless continua is that the line photons are being scattered or destroyed by optically-thick gas and associated dust with large covering fractions.

ERO R1 in CL0939+4713 field - Evidence for an S0-like galaxy at z \sim 1.5

Abstract: We present further observations of the extremely red object ERO J094258+4659.2, identified by \citet{iye00} as ERO R1 in their deep images of the cluster A851. We estimate its redshift independently by eight-band photometric redshift determination and cross-correlation of a new H-band spectrum with the optical spectra of local E/S0 galaxies, and conclude that it lies at $z \sim 1.5$. Although its colors are consistent both with an elliptical galaxy and an S0 galaxy at that redshift, its elongated shape and exponential luminosity profile suggest the presence of an evolved stellar disk component. We rule out the possibility that these properties are strongly influenced by gravitational lensing by the foreground cluster, and therefore conclude that this object is more likely to be an S0-like galaxy, rather than a lensed elliptical. The H-band spectrum does not show strong H$\alpha$ emission and the star formation rate therefore appears to be very modest. The presence of such a galaxy with an apparently relaxed disk of stars at this high redshift provides a new and strong constraint on theoretical models which aim to explain the formation and evolution of galaxies.

Deep optical imaging of the field of PC 1643+4631A&B – II. Estimating the colours and redshifts of faint galaxies

Abstract: ABSTRA C T In an investigation of the cause of the cosmic microwave background decrement in the field of the z o 3:8 quasar pair PC 1643˛4631, we have developed techniques for making photometric estimates of galaxy redshifts in deep multicolour optical images of the field taken with the William Herschel Telescope (WHT). Using a combination of low-redshift observed spectra and high-redshift model spectra, we find that we can assign well-constrained redshift estimates for z , 1 and z . 2. The lack of strong spectral features at intermediate redshifts means that the estimates are poorly constrained, and galaxies are typically misassigned to lower or higher redshifts. We find that such ambiguous redshifts can be reliably flagged by calculating both the mode and mean of the photometric redshift likelihood distribution. To examine the possibility that a massive cluster of galaxies lies in the field, we have attempted to recover simulated galaxies with intrinsic colours matching those of the model galaxies used in the photometric redshift estimation. We find that a model cluster at z . 1 can easily remain undetected in the images. We further find that when model galaxies are recovered from the images, there is considerable scatter of the recovered galaxy redshifts away from the model value; this scatter is larger than that expected from photometric errors and is the result of confusion, simply due to ground-based seeing, between objects in the field. Lastly we have carried out a simulation to test the efficiency of the selection of z * 3 Lyman-break galaxies (LBGs). We find that the number of LBGs recovered from our images by the standard UGR technique is several times smaller than the number of test galaxies in our simulations. A significant contribution to this inefficiency is due to confusion between the high-redshift galaxies and other objects near the line of sight. We argue that the actual surface density of z < 3 galaxies may be several times greater than that recovered from typical 4-m class ground-based imaging, and note that this conclusion is consistent with the surface density of high-redshift objects found in the Hubble Deep Field (HDF).

Morphological number counts and redshift distributions to I = 25 from the Hubble Deep Fields: constraints on cosmological models from early‐type galaxies

Abstract: ABSTRA C T We combine magnitude and photometric redshift data on galaxies in the Hubble Deep Fields with morphological classifications in order to separate out the distributions for early-type galaxies. The updated morphological galaxy number counts down to Ia 25 and the corresponding redshift distributions are used as joint constraints on cosmological models; in particular, on the values of the density parameter V0 and the normalized cosmological constant l 0. We find that an Einstein‐de Sitter universe with simple passive evolution gives an excellent fit to the counts and redshift data at all magnitudes. An open, low-V0 model with no net evolution (and conservation of the number of ellipticals), which fits the counts equally well, is somewhat less successful, predicting slightly lower mean redshifts and, more significantly, the lack of a high-z tail. A number-conserving model with a dominant contribution from l 0, on the other hand, is far less successful, predicting a much narrower distribution than is seen. More complex models are obviously possible, but we conclude that if large-scale transmutation between types does not occur, then the lambda-dominated models provide a very poor fit to the current data.

A Test of Photometric Redshifts for X-Ray-selected Sources

Abstract: We test the effectiveness of photometric redshifts based on galaxy spectral template fitting for X-ray-luminous objects, using a sample of 65 sources detected by Chandra in the field of the Caltech Faint Galaxy Redshift Survey (CFGRS). We find that sources with quasar-dominated spectra (for which galaxy spectral templates are not appropriate) are easily identified and that photometric redshifts are robust for the rest of the sources in our sample. Specifically, for the 59 sources that are not quasar-dominated at optical wavelengths, we find that the photometric redshift estimates have scatter comparable to the field galaxy population in this region. There is no evidence for a trend of increasing dispersion with X-ray luminosity over the range LX = 1039-5 × 1043 ergs s-1, nor is there a trend with the ratio of X-ray to optical flux, fX/fR. The practical implication of this work is that photometric redshifts should be robust for the majority (~90%) of the X-ray sources down to fX ≈ 10-16 ergs s-1 cm-2 that have optical counterparts brighter than R ≈ 24. Furthermore, the same photometry can easily be used to identify the sources for which the photometric redshifts are likely to fail. Photometric redshift estimation can thus be utilized as an efficient tool in analyzing the statistical properties of upcoming large Chandra and XMM-Newton data sets and identifying interesting subsamples for further study.