arXiv subjects

Ian Smail

Publications and source records attributed to Ian Smail.

At least 397 records · Page 22Linked to original sources

arXiv1999

Deep counts of submillimeter galaxies

We present the counts of luminous submillimeter (sub-mm) galaxies from an analysis of our survey of the distant Universe through lensing clusters, which uses massive cluster lenses with well-constrained mass models to magnify the background sky. This both increases the sensitivity and reduces the effects of source confusion. Accurate lens models are used to correct the observed sub-mm source counts. The associated uncertainties do not dominate the final errors. We derive counts by two independent methods: a direct inversion of the observed sources, corrected individually for lens amplification; and a Monte-Carlo simulation of our observations using a parametric model for the background counts, which is folded through the lens models and incompleteness estimates to determine best-fitting values of the count parameters. Both methods agree well and confirm the robustness of our analysis. Detections that are optically identified with galaxies in the lensing clusters are removed prior to our analysis, and the results are insensitive both to the details of the correction and to the redshift distribution of the detections. We present 850-micron counts between 0.5 and 8 mJy. The most accurate count is determined at 1 mJy: 7900 +- 3000 deg^-2, including both Poisson and systematic errors. These are the deepest sub-mm counts published, and are not subject to source confusion because the detected galaxies are separated and magnified by the lens. Down to the 0.5-mJy limit, the resolved 850-micron background radiation intensity is (0.5 +- 0.2) nW m^{-2} sr^{-1}, comparable to the current COBE estimate of the background, and so the bulk of the 850-micron background radiation originated in distant ultraluminous galaxies.

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arXiv1998

Deep sub-mm surveys with SCUBA

We review published deep surveys in the submillimeter (sub-mm) regime from the new Sub-millimetre Common User Bolometer Array (SCUBA) on the 15-m James Clerk Maxwell Telescope (JCMT), Mauna Kea, Hawaii. Summarising the number counts of faint sub-mm sources determined from the different surveys we show that the deepest counts from our completed SCUBA Lens Survey, down to 0.5mJy at 850um fully account for the far-infrared background (FIRB) detected by COBE. We conclude that a population of distant, dust-enshrouded ultraluminous infrared galaxies dominate the FIRB emission around 1mm. We go on to discuss the nature of this population, starting with the identification of their optical counterparts, where we highlight the important role of deep VLA radio observations in this process. Taking advantage of the extensive archival Hubble Space Telescope (HST) observations of our fields, we then investigate the morphological nature of the sub-mm galaxy population and show that a large fraction exhibit disturbed or interacting morphologies. By employing existing broadband photometry, we derive crude redshift limits for a complete sample of faint sub-mm galaxies indicating that the majority lie at z<5, with at most 20% at higher redshifts. We compare these limits to the initial spectroscopic results from various sub-mm samples. Finally we discuss the nature of the sub-mm population, its relationship to other classes of high-redshift galaxies and its future role in our understanding of the formation of massive 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

Observational limits to source confusion in the millimetre/submillimetre waveband

The first observations to detect a population of distant galaxies directly in the submillimetre waveband have recently been made using the new Submillimetre Common-User Bolometer Array (SCUBA) on the James Clerk Maxwell Telescope (JCMT). The results indicate that a large number of distant galaxies are radiating strongly in this waveband. Here we discuss their significance for source confusion in future millimetre/submillimetre-wave observations of both distant galaxies and cosmic microwave background radiation (CMBR) anisotropies. Earlier estimates of such confusion involved significant extrapolation of the results of observations of galaxies at small redshifts; our new estimates do not, as they are derived from direct observations of distant galaxies in the submillimetre waveband. The results have important consequences for the design and operation of existing and proposed millimetre/submillimetre-wave telescopes: the Planck Surveyor mission will be confusion-limited at frequencies greater than 350 GHz, even in the absence of galactic dust emission; 1-sigma confusion noise of 0.44 mJy/beam is expected for the JCMT/SCUBA at a wavelength of 850 microns; and the sub-arcsecond resolution of large millimetre/submillimetre-wave interferometer arrays will be required in order to execute very deep galaxy surveys. For the latest information see astro-ph/9806063.

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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

The Mass/Light Ratio of Early-Type Galaxies: Constraints from Gravitational Lensing in the Rich Cluster AC114

We present a new wide field image of the distant cluster AC114 ($z=0.31$) obtained with Wide Field and Planetary Camera II onboard the Hubble Space Telescope. This image considerably extends our knowledge of the lensing properties of the cluster beyond those derived by Smail \et (1995a) from a single WF/PC-1 pointing. In conjunction with published ground-based spectroscopy, we utilise several newly-discovered multiple images to construct an improved mass model for the central regions of the cluster. Using this model, we apply the methodology introduced by Natarajan & Kneib (1997) to interpret local perturbations to the cluster shear field on small scales resulting from mass associated with individual cluster galaxies. We use the lensing signal to place new constraints on the average mass-to-light ratio and spatial extents of the dark matter halos associated with morphologically-classified early-type cluster members. We find that the total mass of a fiducial $L^*$ cluster spheroidal galaxy is largely contained within $\sim$ 15 kpc radius halo ($\sim$~8--10 $R_e$) with a mass-to-light ratio ${M/L_V} \sim {15^{+10}_{-4}} $ (90% c.l., $h=0.5$) in solar units within this radius. Comparisons with similar estimates for fieldgalaxies indicates that the cluster galaxies in AC114 may possess less extensive and less massive halos. We discuss the consequences of our result in the context of models for the dynamical evolution of cluster galaxies and the observational prospects for extending this analysis.

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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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arXiv1997

A Constant Clustering Amplitude for Faint Galaxies?

The angular clustering of faint field galaxies is investigated using deep imaging (I~25) obtained with the 10-m Keck-I telescope. The autocorrelation function is consistent with w(theta) ~ theta^-0.8 and, although less steep correlation functions cannot be ruled out with high confidence, we find no compelling evidence for a systematic decrease in the power law index at the faintest magnitude limits. Results from a number of independent observational studies are combined in order to investigate the variation of the correlation amplitude with median I-magnitude. At Imed~23 the results obtained by different studies are all in rough agreement and indicate that for Imed > 22 the correlation amplitude declines far less steeply than would be expected from an extrapolation of the trend in the brighter samples. In particular, at Imed~24 our data indicate w(theta) to be a factor ~7 higher than the extrapolation. A near-independence of magnitude is a general feature of the correlation amplitude in models in which the redshift distribution of the faint field population contains a substantial fraction of galaxies with z > 1. In order to reproduce the apparent abrupt flattening of the amplitude of w(theta) observed at faint limits, approximately 50% of the galaxies in a sample with a depth of I~25 must be at z > 1.

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arXiv1997

A Deep Sub-millimeter Survey of Lensing Clusters: A New Window on Galaxy Formation and Evolution

We present the first results of a sub-millimeter survey of distant clusters using the new Sub-mm Common-User Bolometer Array (SCUBA) on the James Clerk Maxwell Telescope. We have mapped fields in two massive, concentrated clusters, A370 at z=0.37 and Cl2244-02 at z=0.33, at wavelengths of 450 and 850-um. The resulting continuum maps cover a total area of about 10 sq. arcmin to 1 sigma noise levels less than 14 and 2 mJy per beam at the two wavelengths, 2-3 orders of magnitude deeper than was previously possible. We have concentrated on lensing clusters to exploit the amplification of all background sources by the cluster, improving the sensitivity by a factor of 1.3--2 as compared with a blank-field survey. A cumulative source surface density of (2.4+/-1.0) x 10^3 per sq. degree is found to a 50% completeness limit of ~4 mJy at 850-um. The sub-mm spectral properties of these sources indicate that the majority lie at high redshift, z>1. Without correcting for lens amplification, our observations limit the blank-field counts at this depth. The surface density is 3 orders of magnitude greater than the expectation of a non-evolving ber density of strongly star-forming galaxies in the high-redshift Universe and suggest that optical surveys may have substantial underestimated the star formation density in the distant Universe. Deeper sub-mm surveys with SCUBA should detect large numbers of star-forming galaxies at high redshift, and so provide strong constraints on the formation of normal galaxies.

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arXiv1997

A Statistical Analysis of the Galaxy Populations of Distant Luminous X-ray Clusters

We present a deep UBI CCD survey using the Palomar 5-m telescope of a sample of high X-ray luminosity, distant clusters selected from the ROSAT All-Sky Survey. The 10 clusters lie at z=0.22-0.28, an era where evolutionary effects have been reported in the properties of cluster galaxy populations. Our clusters thus provide a well-defined sample of the most massive systems at these redshifts to quantify the extent and variability of these evolutionary effects. Moreover, by concentrating on a narrow redshift range, we can take advantage of the homogeneity of our sample to combine the catalogues from all the clusters to analyse the bulk properties of their populations. The core regions of these clusters ontain only a small proportion of star-forming galaxies, and they therefore do not exhibit a `Butcher-Oemler' effect. Focusing on the redder cluster galaxies we find that their integrated luminosity is well correlated with the cluster X-ray temperatures, and hence with cluster mass. Furthermore, the typical UV-optical colours of the elliptical sequences in the clusters exhibit a small cluster-to-cluster scatter, <=2%, indicating that these galaxies are highly homogeneous between clusters. However, at fainter magnitudes we observe an increase in the range of mid-UV colours of galaxies possessing strong 4000A breaks. In the light of the apparent decline in the S0 populations of z>=0.4 clusters (Dressler et al. 1997), and in view of the luminosities and colours of this population, we propose that they may be the progenitors of the S0 population of local rich clusters, caught in the final stage before they become completely quiescent. [abridged]

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arXiv1997

Evolution since z = 0.5 of the Morphology-Density relation for Clusters of Galaxies

Using traditional morphological classifications of galaxies in 10 intermediate-redshift (z~0.5) clusters observed with WFPC-2 on the Hubble Space Telescope, we derive relations between morphology and local galaxy density similar to that found by Dressler for low-redshift clusters. Taken collectively, the `morphology-density' relationship, M-D, for these more distant, presumably younger clusters is qualitatively similar to that found for the local sample, but a detailed comparison shows two substantial differences: (1) For the clusters in our sample, the M-D relation is strong in centrally concentrated ``regular'' clusters, those with a strong correlation of radius and surface density, but nearly absent for clusters that are less concentrated and irregular, in contrast to the situation for low redshift clusters where a strong relation has been found for both. (2) In every cluster the fraction of elliptical galaxies is as large or larger than in low-redshift clusters, but the S0 fraction is 2-3 times smaller, with a proportional increase of the spiral fraction. Straightforward, though probably not unique, interpretations of these observations are (1) morphological segregation proceeds hierarchically, affecting richer, denser groups of galaxies earlier, and (2) the formation of elliptical galaxies predates the formation of rich clusters, and occurs instead in the loose-group phase or even earlier, but S0's are generated in large numbers only after cluster virialization.

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arXiv1997

Spectroscopic Confirmation of Redshifts Predicted by Gravitational Lensing

We present deep spectroscopic measurements of 18 distant field galaxies identified as gravitationally-lensed arcs in a Hubble Space Telescope image of the cluster Abell 2218. Redshifts of these objects were predicted by Kneib et al. (1996) using a lensing analysis constrained by the properties of two bright arcs of known redshift and other multiply-imaged sources. The new spectroscopic identifications were obtained using long exposures with the LDSS-2 spectrograph on the William Herschel Telescope and demonstrate the capability of that instrument to new limits, $R\simeq$24; the lensing magnification implies true source magnitudes as faint as $R\simeq$25. Statistically, our measured redshifts are in excellent agreement with those predicted from Kneib et al.'s lensing analysis which gives considerable support to the redshift distribution derived by the lensing inversion method for the more numerous and fainter arclets extending to $R\simeq 25.5$. We explore the remaining uncertainties arising from both the mass distribution in the central regions of Abell 2218 and the inversion method itself, and conclude that the mean redshift of the faint field population at $R\simeq$25.5 ($B\sim 26$-27) is low, $ $=0.8-1. We discuss this result in the context of redshift distributions estimated from multi-colour photometry. Although such comparisons are not straightforward, we suggest that photometric techniques may achieve a reasonable level of agreement particularly when they include near-infrared photometry with discriminatory capabilities in the $1<z<$2 range.

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arXiv1997

Counts and Colors of Faint Galaxies in the U and R Bands

Ground-based counts and colors of faint galaxies in the U and R bands in one field at high Galactic latitude are presented. Integrated over flux, a total of 1.2x10^5 sources per square degree are found to U=25.5 mag and 6.3x10^5 sources per square degree to R=27 mag, with d log N/dm ~ 0.5 in the U band and d log N/dm ~ 0.3 in the R band. Consistent with these number-magnitude curves, sources become bluer with increasing magnitude to median U-R=0.6 mag at 24<U<25 mag and U-R=1.2 mag at 25 < R < 26 mag. Because the Lyman break redshifts into the U band at z~3, at least 1.2x10^5 sources per square degree must be at redshifts z<3. Measurable U-band fluxes of 73 percent of the 6.3x10^5 sources per square degree suggest that the majority of these also lie at z < 3. These results require an enormous space density of objects in any cosmological model.

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arXiv1997

The Homogeneity of Spheroidal Populations in Distant Clusters

The small scatter observed for the U-V colors of spheroidal galaxies in nearby clusters of galaxies provides a powerful constraint on the history of star formation in dense environments. However, with local data alone, it is not possible to separate models where galaxies assembled synchronously over redshifts 0<z<1 from ones where galaxies formed stochastically at much earlier times. Here we attempt to resolve this ambiguity via high precision rest-frame UV-optical photometry of a large sample of morphologically-selected spheroidal galaxies in three z~0.54 clusters which have been observed with HST. We demonstrate the robustness of using HST to conduct the morphological separation of spheroidal and disk galaxies at this redshift and use our new data to repeat the analysis conducted locally at a significant look-back time. We find a small scatter (< 0.1 mag rms) for galaxies classed as Es and E/S0s, both internally within each of the three clusters and externally from cluster to cluster. We do not find any trend for the scatter to increase with decreasing luminosity down to L = L*+3, other than can be accounted for by observational error. Neither is there evidence for a distinction between the scatter observed for galaxies classified as ellipticals and S0. Our result provides a new constraint on the star formation history of cluster spheroidals prior to z=0.5 confirming and considerably strengthening the earlier conclusions. Most of the star formation in the elliptical galaxies in dense clusters was completed before z~3 in conventional cosmologies. Although we cannot rule out the continued production of some ellipticals, our results do indicate an era of initial star formation consistent with the population of star-forming galaxies recently detected beyond z~3.

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arXiv1996

A Weak Lensing Survey in the Fields of z~1 Luminous Radio Sources

In this paper we present weak lensing observations of the fields around 8 z~1 luminous radio sources. These data are searched for the lensing signatures of clusters that are either physically associated with the radio objects, or are foreground systems projected along the line of sight. The radio sources were all imaged with WFPC-2/HST providing high quality shape information on large numbers of faint galaxies around them. Statistical analysis of the coherent shear field visible in the shapes of the faint galaxies indicates that we have detected a weak lensing signal close to one of the targets, 3C336 at z=0.927, with a high level of confidence. A second, independent WFPC-2 observation of this target reinforces this detection. Our results support the earlier suggestion of weak lensing in this field by Fort et al (1996) using ground-based data. We also combined the shear distributions in the remaining 7 field to improve our sensitivity to weak shear signals from any structure typically associated with these sources. We find no detectable signal and estimate an upper limit on the maximum shear allowed by our observations. Using an N(z) estimated from lensing analyses we convert our observed lensing signal and limits into estimates of the masses of the various structures. We suggest that further lensing observations of distant radio sources and their host environments may allow the cluster L_X-mass relationship to be mapped at high-z. This is crucial for interpreting the results of the next generation of deep X-ray surveys, and thus constraining the redshift evolution of the cluster mass function out to z=1.

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arXiv1996

A Catalog of Morphological Types in 10 Distant Rich Clusters of Galaxies

We present catalogs of objects detected in deep images of 11 fields in 10 distant clusters obtained using WFPC-2 on board the Hubble Space Telescope. The clusters span the redshift range z=0.37-0.56 and are the subject of a detailed ground- and space-based study to investigate the evolution of galaxies as a function of environment and epoch. The data presented here include positions, photometry and basic morphological information on ~9000 objects in the fields of the 10 clusters. For a brighter subset of 1857 objects in these areas we provide more detailed morphological information.

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arXiv1996

HST Observations of Distant Clusters: Implications for Galaxy Evolution

The ``MORPHS'' group has completed the cataloging, parameterization, and morphological classification of ~2000 galaxies in 10 rich clusters from 0.36 < z < 0.56. From a weak lensing analysis using these data, which compares the X-ray properties (L_X) of the clusters with virial temperature estimates (T_v) from the lensing shear strength, we find little evidence for evolution in the L_X-T_v relation from that observed for local clusters. We discuss how this observation constrains models for the X-ray evolution of clusters. The data have also been used to study the color dispersion of bona-fide ellipticals in high-z clusters: we find the spread to be very small, suggesting an early formation epoch for the stellar populations of cluster ellipticals. This is consistent with the evolution of the morphology-density relationship, in which we find ellipticals to be as abundant at z=0.5 as in clusters today, and already well ensconced in the dense regions. In contrast, S0's are less plentiful and less well-concentrated compared to the present epoch, and spiral galaxies everywhere more abundant. Combined with other spectroscopic and morphological data, these observations suggest that most of these rapidly evolving systems are not likely to become bright ellipticals, which were more likely formed at early epochs. Cluster S0 galaxies, on the other hand, are likely to have been produced in large numbers in the recent past.

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