arXiv subjects

J. -P. Kneib

Publications and source records attributed to J. -P. Kneib.

At least 217 records · Page 12Linked to original sources

arXiv1999

Hubble Space Telescope Near-infrared and Optical Imaging of Faint Radio Sources in the Distant Cluster Cl0939+4713

We present deep Hubble Space Telescope NICMOS near-infrared and WFPC2 optical imaging of a small region in the core of the distant rich cluster Cl0939+4713 (z=0.41). We compare the optical and near-infrared morphologies of cluster members and find apparent small-scale optical structures within the galaxies which are absent in the near-infrared. We conclude that strong dust obscuration is a common feature in the late-type galaxies in distant clusters. We then concentrate on a sample of ten faint radio galaxies lying within our NICMOS field and selected from a very deep 1.4-GHz VLA map of the cluster with a 1sigma flux limit of 9uJy. Using published data we focus on the spectral properties of the eight radio-selected cluster members and show that these comprise a large fraction of the post-starburst population in the cluster. The simplest interpretation of the radio emission from these galaxies is that they are currently forming massive stars, contradicting their classification as post-starburst systems based on the optical spectra. We suggest that this star formation is hidden from view in the optical by the same obscuring dust which is apparent in our comparison on the optical and near-infrared morphologies of these galaxies. We caution that even in the restframe optical the effects of dust cannot be ignored when comparing samples of distant galaxies to low-redshift systems, particularly if dust is as prevelant in distant galaxies as appears to be the case in our study.

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arXiv1999

Gravitational Lensing in Clusters of Galaxies

Gravitational lensing in clusters of galaxies is an efficient tool to probe the mass distribution of galaxies and clusters, high redshift objects thanks to the gravitational amplification, and the geometry of the universe. We review some important aspects of cluster lensing and related issues in observational cosmology.

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arXiv1999

Two multiple-imaged z=4.05 galaxies in the cluster-lens Abell 2390

We present the first results on the identification and study of very distant field galaxies in the core of cluster-lenses, using a selection criterium based on both lens modelling and photometric redshifts. We concentrate on two multiple-imaged sources at z=4.05 in the cluster A2390. The 2 objects presented in this paper, namely H3 and H5, were identified through lens modelling as multiple images of high-redshift sources at z>3.5. We confirm the excellent agreement between this identification and both their photometric redshifts and morphologies. Our CFHT/WHT program for a systematic redshift survey of arcs in clusters has allowed to obtain a set of spectra on 3 different images at z~4: the brightest image of H3, which redshift was already confirmed by Frye & Broadhurst (1998), and the two brightest images of H5. The later is then confirmed spectroscopically as a multiple image, giving a strong support to the lens model. The main feature in each of these spectra is a strong emission line, identified as Ly-alpha, leading to z=4.05 for both H3 and H5. The spectrophotometric properties of these galaxies are studied, in particular the degeneracy in the parameter-space defined by the SFR type, age, metallicity and reddening. H3 and H5 are intrinsically bright and clumpy sources located ~100 kpc part on the source plane, with mean metallicities compatible with a fraction of solar or even solar values. All these results seem to favour a hierarchical merging scenario, where we are actually seeing a relatively advanced step for these 2 z~4 objects, with stars forming locally and efficiently from a preenriched gas.

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arXiv1999

The history of star formation in dusty galaxies

A population of distant dusty galaxies emitting in the submillimetre waveband has recently been detected using the Submillimetre Common-User Bolometer Array (SCUBA) camera on the James Clerk Maxwell Telescope (JCMT). This population can be used to trace the amount of high-redshift star-formation activity that is obscured from view in the optical waveband by dust, and so is missing from existing inventories of star formation in the distant Universe. By including this population we can construct a complete and consistent picture of the history of star formation. The evolution of obscured star formation at redshifts less than unity is constrained by mid- and far-infrared counts of dusty galaxies. Activity increases with redshift z as (1+z) to a power of about 4, consistent with the form of evolution found in the optical waveband by the Canada-France Redshift Survey (CFRS) to redshifts less than about 1. The form of evolution at higher redshifts is constrained by both our faint SCUBA counts and the intensity of background radiation in the millimetre/submillimetre waveband. We find that the total amount of energy emitted by dusty galaxies is about five times greater than that inferred from restframe ultraviolet observations, and that a larger fraction of this energy is emitted at high redshifts. The simplest explanation for these results is that a large population of luminous strongly-obscured sources at redshifts less than about 5 is missing from optical surveys. We discuss the possible contribution of obscured active galactic nuclei to the submillimetre-wave background and counts. More accurate constraints on the history of star formation will be provided by determinations of the counts in several submillimetre wavebands and crucially, by a reliable redshift distribution of the detected galaxies.

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arXiv1999

Redshift Distribution of the Faint Submillimeter Galaxy Population

We present a Keck II LRIS spectroscopic follow-up study of the possible optical counterparts to a flux-limited sample of galaxies selected from an 850-micron survey of massive lensing clusters using the SCUBA bolometer array on the JCMT. These sources represent a population of luminous dusty galaxies responsible for the bulk of the 850-micron background detected by COBE and thus for a substantial fraction of the total far-infrared emission in the Universe. We present reliable redshifts for 20 galaxies and redshift limits for a further 4 galaxies selected from the error-boxes of 14 submm sources. Two other submm detections in the sample have no obvious optical counterparts, and the final submm source was only identified from imaging data after the completion of our spectroscopic observations. The optical identifications for 4 of the submm sources have been confirmed through either their detection in CO at mm-wavelengths (two pairs of galaxies at z=2.55 and z=2.80) or from the characteristics of their spectral energy distributions (two of the central cD galaxies in the lensing clusters). Plausible arguments based on the optical spectral properties (starburst or AGN signatures) of the counterparts allow us to identify a further two likely counterparts at z=1.06 and 1.16. For the remaining 8 cases, it is not always clear which, if any, of the optical sources identified are the true counterparts. Possible counterparts for these have redshifts ranging from z=0.18 to z=2.11. Working with the current identifications, we suggest that the majority of the extragalactic background light in the submm is emitted by sources at z<3 and hence that the peak activity in highly-obscured sources (both AGN and starbursts) lies at relatively modest redshifts. (Abridged)

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arXiv1999

The Detection of Dust in the Central Galaxies of Distant Cooling-Flow Clusters

We present 850-um observations with the SCUBA submillimetre (sub-mm) camera of the central galaxies in seven concentrated clusters of galaxies at redshifts between 0.19 and 0.41. We detect sub-mm emission from the central galaxies in the rich clusters A1835 and A2390, and present upper limits for the central galaxies in the remaining five clusters. The two galaxies which we detect both exhibit unusually blue UV-optical colours and lie in clusters which contain massive cooling flows, >1000 Mo/yr. Moreover, both galaxies host relatively strong radio sources. Focusing on these two systems, we present new and archival radio-optical observations to provide a detailed view of their spectral energy distributions. Our analysis indicates that sub-mm emission from the central galaxy of A1835 can be best understood as arising from dust, heated either by vigorous star formation or an obscured active galactic nucleus. For the central galaxy of A2390, the sub-mm flux is marginally consistent with an extrapolation of the centimetre-millimetre emission from the luminous radio source that lies in its core; although we cannot rule out an excess flux density from dust emission comparable to that seen in A1835. We present details of our multi-wavelength observations and discuss the implications of these data for the interpretation of star formation in cooling-flow galaxies.

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arXiv1999

A Deep and Ultra-deep ISOCAM Cosmological Survey through Gravitationally Lensing Clusters of Galaxies

We present imaging results and source counts from an ISOCAM deep and ultra-deep cosmological survey through gravitationally lensing clusters of galaxies at 7 and 15 microns. A total area of about 53 sq.arcmin was covered in maps of three clusters. The lensing increases the sensitivity of the survey. A large number of luminous mid-infrared (MIR) sources were detected behind the lenses, and most could be unambiguously identified with visible counterparts. Thanks to the gravitational amplification, these results include the faintest MIR detections ever recorded, extending source counts to an unprecedented level. The source counts, corrected for cluster contamination and lensing distortion effects, show an excess by a factor of 10 with respect to the prediction of a no-evolution model, as we reported for A2390 alone in Altieri et al. (1999). These results support the A2390 result that the resolved 7 and 15 microns background radiation intensities are 1.7 (+/- 0.5) x 10^-9 and 3.3 (+/- 1.3) x 10^-9 W/m^2/sr, respectively, integrating from 30 microJy to 50 mJy.

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arXiv1999

Molecular Gas in the z=2.565 Submillimeter Galaxy SMM J14011+0252

We report the detection of CO(3-2) emission from the submillimeter-selected luminous galaxy SMM J14011+0252. The optical counterpart of the submillimeter source has been identified as a merger system with spectral characteristics consistent with a starburst at z=2.565. The CO emission confirms the optical identification of the submillimeter source and implies a molecular gas mass of 5x10^{10}h_{75}^{-2} M(sun), after correcting for a lensing amplification factor of 2.75. The large molecular gas mass and the radio emission are consistent with the starburst interpretation of the source. These results are similar to those found for SMM J02399-0136, which was the first submillimeter selected CO source found at high redshift. The CO detections of these two high-redshift submillimeter galaxies suggest the presence of massive reservoirs of molecular gas which is consistent with the inferred high rates of star-formation (1000 M(sun)/yr). These two systems appear to be associated with merger events which may evolve into present day luminous elliptical galaxies.

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arXiv1998

Molecular Gas in the z=2.8 Submillimeter Galaxy SMM 02399-0136

We report the detection of CO(3-2) emission from the submillimeter-selected hyperluminous galaxy SMM 02399-0136. This galaxy is the brightest source detected in the recent SCUBA surveys of the submillimeter sky. The optical counterpart of the submillimeter source has been identified as a narrow-line AGN/starburst galaxy at z=2.8. The CO emission is unresolved, theta < 5 arcsec, and is coincident in redshift and position with the optical counterpart. The molecular gas mass derived from the CO observations is 8x10^{10} h_{75}^{-2} M_{sun}, after correcting for a lensing amplification factor of 2.5. The large CO luminosity suggests that a significant fraction of the infrared luminosity of SMM 02399-0136 arises from star formation. The high inferred star-formation rate of 10^{3} M_{sun}/yr and the large gaseous reservoir may suggest that we are seeing the formation phase of a massive galaxy. Future CO observations of other galaxies detected in deep submillimeter surveys will test the generality of these conclusions for the bulk of the faint submillimeter population.

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arXiv1998

Confusion noise in millimetre/submillimetre surveys

The flux density from undetected sources in the observing beam of a telescope produces source confusion noise in the resulting maps of the sky, and thus limits sensitivity. In a recent paper we discussed this effect in the millimetre/submillimetre waveband, using a simple model of galaxy evolution that could account for the gravitationally lensed images of distant dusty galaxies newly discovered in the fields of clusters of galaxies. New models explain all the available mm, submm and far-infrared data. The associated predictions of source confusion noise are presented here. The new and old estimates agree to within a factor of two on angular scales between about 0.3 and 50 arcmin, and are most similar on angular scales of about 1 arcmin. The new estimates are greater/less than the old ones on larger/smaller angular scales.

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arXiv1998

The Faint Sub-mm Galaxy Population: HST Morphologies and Colors

We present optical morphologies obtained from deep HST and ground-based images for galaxies selected from the first sub-millimeter survey of the distant Universe. Our sample comprises galaxies detected in deep 850-micron continuum maps of seven massive clusters, obtained using SCUBA, the new bolometer camera on the JCMT. The survey covers a total area of 0.01 square degrees to 1-sigma noise levels of about 2 mJy/beam. We detect a total of 25 sources at 850 microns, of which 17 and 10 are brighter than the respective 50% and 80% completeness limits. Optical counterparts are identified for 14 of the 16 sources in the f(50%) sample and for 9 of the 10 sources in the f(80%) sample that lie within our optical fields. The morphologies of those galaxies for which we have HST imaging fall into three broad categories: faint disturbed galaxies and interactions; faint galaxies too compact to classify reliably; and dusty, star-forming galaxies at intermediate redshifts. The disturbed and interacting galaxies constitute the largest class, which suggests that interactions remain an important mechanism for triggering star formation and the formation of ultraluminous galaxies in the distant Universe. The faint, compact galaxies may represent a later evolutionary stage in these mergers, or more centrally-concentrated starbursts. It is likely that some of these will host AGN. Analysis of the colors of our sample allow us to estimate a crude redshift distribution: >75% have z<5.5 whilst >50% lie at z<4.5, suggesting that the luminous sub-mm population is coeval with the more modestly star-forming galaxies selected by UV/optical surveys of the distant Universe. This imposes important constraints on models of galaxy formation and evolution.

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arXiv1998

Unveiling the nature of the Cloverleaf lens-system: HST/NICMOS-2 observations

We present new elements in the identification of the lens-system producing the 4 images of the BAL quasar H1413+117, based on the recent HST/NICMOS-2/F160W observations. After a careful PSF subtraction of the 4 images of the quasar, the residual H image reveals the presence of a faint object (H~20.5) within the region enclosed by the 4 quasar images. This object corresponds to a single galaxy: the primary lens of the lens-system. We also identify the galaxies around the Cloverleaf which had been proposed to belong to a galaxy cluster/group at high redshift (Kneib et al 1998): the other component in the lens-system that provides the additional "external" shear. For these galaxies, we have derived a likely redshift based upon their R, I and H magnitudes. We find that most of them are consistent with belonging to a galaxy cluster/group with mean redshift =0.9 +/- 0.1. Furthermore we detect 2 very red objects (I-H~4): the faintest one has no observed optical (R and I) counterpart, while the brightest has a predicted redshift around z~2, and may be identified with one of the Cloverleaf absorbers. This gravitational-lens system constitutes an excellent target for IR imaging/spectroscopy with the new generation of 8m ground-based telescopes.

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arXiv1998

A hyperluminous galaxy at z = 2.8 found in a deep submillimetre survey

We present a detailed study of SMM 02399-0136, a hyperluminous, active galaxy selected from a sub-mm survey of the distant Universe. This galaxy is the brightest source in the fields of seven rich, lensing clusters - total area 0.01 deg^2 - that we have mapped with a sensitivity of ~2 mJy/bm at 850um. We identify a compact optical counterpart with B ~ 23 and a LSB companion 3" away. Our spectroscopy shows that components have the same redshift; z = 2.803 +/- 0.003. The emission lines widths, FWHM ~ 1000-1500 km/s, and line ratios, along with the compact morphology and high luminosity (M_B ~ -24.0) of the galaxy indicate that SMM 02399-0136 contains a rare dust-embedded, narrow-line or type-2 AGN. The source is lensed by the foreground cluster, amplifying its apparent luminosity by a factor of 2.5. Taking this into account, we estimate that SMM 02399-0136 is five times more luminous than F10214+4724. Its far-IR and H-alpha luminosities and LSB radio emission are indicative of an extremely high SFR - several thousand Mo/yr. This assumes that a starburst is the dominant source of energy, but we cannot yet determine the relative contributions of the starburst and the buried AGN. A dust mass of 5-7 x 10^8 Mo is indicated by our data for T(dust) ~ 40-50K, independent of whether the dominant energy source is an AGN or a starburst. We estimate the possible space density of such luminous sub-mm sources and find that while a very large population of these obscured sources could be detected in future wide-field sub-mm surveys, they are unlikely to dominate the faint counts in this waveband. Galaxies such as SMM 02399-0136 and F10214+4724 cannot be easily detected in conventional AGN/QSO surveys, and so estimates of the prevalence of AGN in the early Universe may require significant revision.

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arXiv1998

Discovery of X-ray emission rom the distant lensing cluster of galaxies CL2236-04 at z = 0.552

X-ray emission from the distant lensing cluster CL2236-04 at $z$ = 0.552 was discovered by ASCA and ROSAT/HRI observations. If the spherical symmetric mass distribution model of the cluster is assumed, the lensing estimate of the cluster mass is a factor of two higher than that obtained from X-ray observations as reported for many distant clusters. However, the elliptical and clumpy lens model proposed by Kneib et al.(1993) is surprisingly consistent with the X-ray observations assuming that the X-ray emitting hot gas is isothermal and in a hydrostatic equilibrium state. The existence of the cooling flow in the central region of the cluster is indicated by the short central cooling time and the excess flux detected by ROSAT/HRI compared to the ASCA flux. However, it is shown that even if the AXJ2239-0429 has a cooling flow in the central region, the temperature measured by ASCA which is the mean emission-weighted cluster temperature in this case, should not be cooler than and different from the virial temperature of the cluster. Therefore, we conclude that the effect of the clumpiness and non-zero ellipticity in the mass distribution of the cluster are essential to explain the observed feature of the giant luminous arc, and there is no discrepancy between strong lensing and X-ray estimation of the mass of the cluster in this cluster.

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arXiv1998

High Resolution Data on the Cloverleaf in the UV and in CO(7-6)

We present a new investigation of the Cloverleaf (z$=$2.56) based on the HST/WFPC2, IRAM and CFHT/FOCAM images data. The WFPC2 data shows a significant overdensity of galaxies around the Cloverleaf that we interpret as the presence of a distant cluster of galaxies along the line of sight. The Cloverleaf is probably the result of the lensing effects which include, in addition to a single galaxy, a distant clusters of galaxies. We have modelled the lens accordingly. The high-resolution IRAM/CO map provides the orientation and the ellipticity of the CO spots induced by the shear component. Velocity-positional effects are detected at the 8$σ$ level in the CO map. A strong limit can then be put on the size, shape and location of the CO source around the quasar. The CO source is found to form a disk- or ring- structure orbiting the central engine at $\sim$ 100km/s at a radial distance of $\sim$ 100pc, leading to a central mass of $\sim$ 10$^{9}$ M$_\odot$ possibly in the form of a black hole.

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arXiv1997

Gravitational Lensing as a Cosmological Probe

Gravitational Lensing is an efficient tool to probe: the mass distribution of collapsed systems: galaxies and clusters; high redshift objects thanks to the gravitational amplification; and the geometry of the Universe. I will review here some important aspects of lensing and related issues in observational cosmology.

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arXiv1997

Modelling the Cloverleaf: Contribution of a Galaxy Cluster at z~1.7

We present a new investigation of the Cloverleaf (z=2.558) based on the combination of archival HST/WFPC2 data, recent IRAM CO(7-6) maps and wide field CFHT/FOCAM images. The deepest WFPC2 observation shows a significant overdensity of I ~ 23--25 galaxies around the Cloverleaf that we interpret as the presence of a cluster of galaxies along the line of sight. The typical magnitude, red color (R-I ~ 0.9) and small angular size of these galaxies suggest that the cluster is very distant and could be associated with the absorption systems observed in the spectra of the quasar spots. The Cloverleaf is probably the result of the lensing effects of a system which includes a single-galaxy and one of the most distant cluster of galaxies. We have modelled the lens using altogether the WFPC2 data and the IRAM/CO(7-6) map, considereing two cases: one in which the mass model is a galaxy and a dark halo at z=1.7, and a second one in which the mass model is the combination of a cluster and an individual galaxy located amid the Cloverleaf, both at z=1.7. The CO source is found to form a ring-like structure orbiting the central engine at ~ 100km/s at a radial distance of ~ 100pc, leading to a black-hole mass of ~ 10^9 M_solar. The cluster component increases significantly the convergence of the lens and this pulls down the requirement on the mass of the lensing galaxy by a factor 2. This may explain the mystery of the lensing galaxy not detected yet.

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arXiv1996

Identification of a Gravitationally Lensed z=2.515 Star-Forming Galaxy

We discuss the optical spectrum of a multiply-imaged arc resolved by HST in the $z$=0.175 cluster A2218. The spectrum, obtained with LDSS-2 on the 4.2m William Herschel telescope, reveals the source to be a galaxy at a redshift $z$=2.515 in excellent agreement with the value predicted by Kneib et al. (1996) on the basis of their inversion of a highly-constrained mass model for the lensing cluster. The source is extremely blue in its optical-infrared colours, consistent with active star formation, and the spectrum reveals absorption lines characteristic of a young stellar population. Of particular significance is the absence of Lyman-$α$ emission but the presence of a broad Lyman-$α$ absorption. The spectrum is similar to that of other, much fainter, galaxies found at high redshift by various techniques and illustrates the important role that lensing can play in detailed studies of the properties of distant galaxies.

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