arXiv subjects

Ian Smail

Publications and source records attributed to Ian Smail.

At least 379 records · Page 21Linked to original sources

arXiv2000

A combined HST/CFH12k/XMM survey of X-ray luminous clusters of galaxies at z~0.2

We describe a project to study a sample of X-ray luminous clusters of galaxies at redshift z~0.2 at several scales (with HST/WFPC2 and CFHT/CFH12k) and wavebands (optical and X-ray). The main aims of the project are (i) to determine the mass profiles of the clusters on scales ranging from ~10 kpc/h to >1.5 Mpc/h using weak and strong lensing, thereby testing theoretical predictions of a ``universal mass profile'', and (ii) to calibrate the M_tot - T_X relation in view of future application in the study of the evolution of the cluster mass function at higher redshift.

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arXiv2000

The Identification of the Submillimeter Galaxy SMM J00266+1708

We report the detection of 1.3mm continuum and near-infrared K-band (2.2-micron) emission from the submillimeter galaxy SMM J00266+1708. Although this galaxy is among the brightest sub-mm sources detected in the blank-sky surveys (L~10^{13} L[sun]), SMM J00266+1708 had no reliable optical/near-infrared counter-part. We used sensitive interferometric 1.3mm observations with the Owens Valley Millimeter Array to accurately determine the position of the sub-mm galaxy. Follow-up near-infrared imaging with the Keck I telescope uncovered a new faint red galaxy at K=22.5 mag which is spatially coincident with the 1.3mm emission. This is currently the faintest confirmed counter-part of a sub-mm galaxy. Although the redshift of SMM J00266+1708 is still unknown, its high sub-mm/radio spectral index suggests that the system is at high redshift (z>2). Approximately 50% or more of the sub-mm galaxies are faint/red galaxies similar to that of SMM J00266+1708. These ultraluminous obscured galaxies account for a significant fraction of the total amount of star-formation at high redshift despite being missed by optical/ultraviolet surveys.

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arXiv2000

An excess of submm sources near 4C41.17: a candidate proto-cluster at z=3.8?

Biased galaxy-formation theories predict that massive galaxies at high redshifts should act as signposts to high-density environments in the early Universe, which subsequently evolve into the cores of the richest clusters seen at the present day. These regions are characterised by over-densities of young galaxies, perhaps including a population of dusty, interaction-driven starbursts - the progenitors of massive cluster ellipticals. By searching for this population at submm wavelengths we can therefore test both galaxy- and structure-formation models. We have undertaken such a search in the field of a z=3.8 radio galaxy, 4C41.17, with the SCUBA submm camera. Our extremely deep 450- and 850-micron maps reveal an order-of-magnitude over-density of luminous submm galaxies compared to typical fields (the likelihood of finding such an over-density in a random field is <2E-3). The SCUBA galaxies have bolometric luminosities, >10^13 L(solar), which imply star-formation rates consistent with those required to form a massive galaxy in only a few 10^8 years. We also note that this field exhibits an over-density of extremely red objects (EROs), some of which may be associated with the submm sources, and Lyman-break galaxies. We propose that the over-densities of both submm and ERO sources in this field represent young dusty, starburst galaxies forming within a proto-cluster centered on the radio galaxy at z=3.8, which is also traced by a less-obscured population of Lyman-break galaxies.

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arXiv1999

The diversity of SCUBA-selected galaxies

We present extensive observations of a sample of distant, submm galaxies detected in the field of the massive cluster lens, A1835, using SCUBA. Taken in conjunction with earlier observations of other submm-selected sources (Ivison et al. 1998; Smail et al. 1999; Soucail et al. 1999) we now have detailed, multi-wavelength observations of 7 examples of the submm population, having exploited the combination of achromatic amplification by cluster lenses and lavish archival datasets. These sources, all clearly at z > 1, illustrate the wide range in the radio and optical properties of distant submm-selected galaxies. We include detailed observations of the first candidate `pure' starburst submm galaxy at high redshift, a z=2.56 interacting galaxy which shows no obvious sign of hosting an AGN. The remaining sources have varying degrees of inferred AGN activity (3 from 7 of the most luminous show some evidence of the presence of an AGN) although even when an AGN is obviously present it is still not apparent if reprocessed radiation from this source dominates the submm emission. In contrast with the variation in the spectral properties, we see relatively homogeneous morphologies for the population, with a large fraction of merging or interacting systems. Our study shows that virtually identical SEDs are seen for galaxies which exhibit strikingly different opt/UV spectral-line characteristics. We conclude that standard opt/UV spectral classifications are misleading when applied to distant, highly obscured galaxies and that we must seek other means of determining the various contributions to the overall energy budget of submm galaxies and hence to the far-IR extragalactic background.

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arXiv1999

Galaxy-galaxy lensing in clusters: new results

A synopsis of the recent results from our study of galaxy-galaxy lensing in clusters is presented. We have applied our analysis techniques to the sample of HST cluster-lenses that span a redshift range from redshift 0.18 to 0.58. We find that there is evidence that the total mass of a typical early-type cluster galaxy increases with redshift. For the lowest redshift bin, a sensible comparison can be made with field galaxies and it is found that cluster galaxies are less massive and less extended than equivalent luminosity field galaxies. This agrees broadly with the theoretical picture of tidal stripping of dark matter halos in high density cluster environments. We also find the following trends -- at a fixed luminosity both the mass-to-light ratio and the fiducial truncation radius increase with redshift from redshift 0.18 to 0.58.

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arXiv1999

Dust-obscured star formation and AGN fuelling in hierarchical models of galaxy evolution

A large fraction of the luminous distant submm-wave galaxies recently detected using SCUBA on the JCMT appear to be associated with interacting optical counterparts. We investigate these systems using a simple hierarchical clustering model of galaxy evolution, in which the large luminosity of the SCUBA galaxies is assumed to be generated at the epoch of galaxy mergers in a burst of either star formation activity or the fuelling of an active galactic nucleus (AGN). The models are well constrained by the observed spectrum of the FIR/submm background radiation and the 60-micron counts of low-z IRAS galaxies. The ratio between the total amount of energy released during mergers and the mass of dark matter involved must increase sharply with z at z<1, and then decrease at higher z, independent of the fraction of the luminosity of mergers that is produced by starbursts and AGN. One additional parameter - the reciprocal of the product of the duration of the enhanced luminosity produced by the merger and the fraction of mergers that induce an enhanced luminosity, which we call the activity parameter - is introduced, to allow the relationship between merging dark matter haloes and the observed counts of distant dusty galaxies to be investigated. The observed counts can only be reproduced if the activity parameter is greater by a factor of about 5 and 100 at redshifts of 1 and 3 respectively, compared with the present epoch. Hence, if mergers account for the SCUBA galaxies, then the merger process must have been much more violent at high redshifts. We discuss the counts of galaxies and the intensity of background radiation in the optical/near-IR wavebands in the context of these hierarchical models, and thus investigate the relationship between the populations of SCUBA and Lyman-break galaxies.

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arXiv1999

Radio Constraints on the Identifications and Redshifts of Submm Galaxies

We present deep radio maps from the Very Large Array (VLA) for 16 sources detected in a sub-millimeter (submm) survey of the distant Universe. Our deep VLA 1.4-GHz maps allow us to identify radio counterparts or place stringent limits (<20uJy in the source plane) on the radio flux of the submm sources. We compare the spectral indices of our sources between 850um and 1.4GHz to empirical and theoretical models for distant starburst galaxies and active galactic nuclei (AGN) as a function of redshift. In this way we can derive redshift limits for the submm sources, even in the absence of an optical or near-infrared counterpart. We conclude that the submm population brighter than \~1mJy has a median redshift of at least ~2, more probably ~2.5-3, with almost all galaxies at z>>1. This estimate is a strong lower limit as both misidentification of the radio counterparts and non-thermal emission from an AGN will bias our redshift estimates to lower values. The high median redshift means that the submm population, if predominately powered by starbursts, contributes a substantial fraction of the total star formation density at high redshifts. A comparison of the spectral index limits with spectroscopic redshifts for proposed optical counterparts to individual submm galaxies suggests that half of the submm sources remain unidentified and thus their counterparts must be fainter than I>24.

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arXiv1999

Submillimeter-selected galaxies

The first generation of submillimeter(submm)-wave surveys are being carried out using the 450/850-micron SCUBA camera at the JCMT on Mauna Kea. These surveys are potentially sensitive to galaxies at very high redshift, and the galaxies that have been detected so far appear to contribute the greater fraction of the mm/submm-wave background radiation intensity measured by COBE. In order to understand this new population of galaxies, individual examples must be studied in detail across many wavebands; in particular their redshifts must be determined. We discuss the potential selection effects at work in submm-wave surveys and describe the spectral energy distributions of galaxies selected or luminous in the submm waveband. We also describe the general procedure for, and emphasize the difficulty of, identifying optical counterparts to submm-selected galaxies. Finally, we summarize what is known about the redshifts of these galaxies and the source of their luminosity.

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arXiv1999

Galaxy counts at 450 and 850 microns

Surveys of the distant Universe have been made using the SCUBA submillimeter(submm)-wave camera at the JCMT. 450- and 850-micron data is taken simultaneously by SCUBA in the same 5-square-arcmin field. Deep 850-micron counts of high-redshift dusty galaxies have been published; however, at 450 microns both the atmospheric transmission and antenna efficiency are lower, and the atmospheric noise is higher, and so only upper limits to the 450-micron counts have been reported so far. Here we apply the methods used by Blain et al. (1999) to derive deep 850-micron counts from SCUBA images of lensing clusters to the 450-micron images that were obtained in parallel, in which four sources were detected. We present the first 450-micron galaxy count. This analysis has only just become possible because the volume of data and the difficulty of calibration are both greater for the 450-micron array. In light of recent work, in which the identification of two of the galaxies in our sample was clarified, we also update our deep 850-micron counts.

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arXiv1999

Constraints on the evolution of S0 galaxies in rich clusters at moderate redshifts

We combine morphological classifications from deep HST imaging of a sample of 3 clusters at z=0.31 and a further 9 clusters at z=0.37-0.56 with existing spectroscopic observations of their E and S0 populations, to study the relative spectral properties of these two galaxy types. We have also used similar data for the Coma cluster as a present day bench-mark with which to compare our data at higher redshift. The data span the range where strong evolution is claimed in the proportion of S0 galaxies within rich clusters. Techniques have been recently developed to analyse the strengths of absorption lines in the spectra of local, passive galaxies to separate the effects of age & metallicity and hence date the ages of the most recent, substantial star formation episode in these galaxies. We have applied these techniques to the co-added spectra of the distant cluster galaxies to determine the relative ages of the E and S0 populations. We find no statistically significant difference between the luminosity-weighted ages of the E and S0 galaxies in these clusters. The precision of this measurement would indicate that any recent star formation in the cluster S0s at z=0.31 could have involved bursts of no more than ~11% (by mass) in strength in no more than half of these galaxies. Our results, in conjunction with other work, suggest that the progenitors of S0 galaxies in rich clusters are mostly early-type spirals who, through interactions with the cluster environment, have had their star formation truncated. This indicates a relatively unspectacular origin for the missing S0 population at high redshift.

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arXiv1999

The Discovery of ERO Counterparts to Faint Submm Galaxies

We use deep ground-based imaging in the near-IR to search for counterparts to the luminous submm sources in the catalog of Smail et al (1998). For the majority of the submm sources the near-IR imaging supports the counterparts originally selected from deep optical images. However, in two cases (10% of the sample) we find a relatively bright near-IR source close to the submm position, sources that were unidentified in the deep HST and ground-based R-band images used in Smail et al (1998). We place limits on colours of these sources from deep high-resolution Keck II imaging and find they have 2-sigma limits of (I-K)>6.8 and (I-K)>6.0 respectively. Both sources thus class as EROs. Using the spectral properties of the submm source in the radio and submm we argue that these EROs are probably the source of the submm emission, rather than the bright spiral galaxies previously identified by Smail et al. (1998). From the surface density of these submm-bright EROs we suggest that this class accounts for the majority of the reddest members of the ERO population, in good agreement with the preliminary conclusions of pointed submm observations of individual EROs. We conclude that the most extreme EROs represent a population of dusty, ultraluminous galaxies at high redshifts; further study of these will provide insights into the nature of star formation in obscured galaxies in the early Universe. The identification of similar counterparts in blank field submm surveys will be extremely difficult owing to their faintness (K~20.5, I>26.5). Finally, we discuss the radio and submm properties of the two submm-bright EROs discovered here and suggest that both galaxies lie at z>2.

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arXiv1999

Submm Continuum Surveys for Obscured Galaxies

We discuss deep surveys in the submm regime using the SCUBA bolometer array on the JCMT. At 850um SCUBA has resolved the bulk of the submm background (SMBR) detected by COBE into discrete sources brighter than 0.5mJy. The on-going identification and characterisation of this population at other wavelengths suggests that the bulk of the submm sources brighter than ~1mJy lie at z>1, with a median redshift for the population of ~2-3. The optical/near-infrared properties of the counterparts to the submm sources breakdown as follows: roughly two-thirds have reliable identifications with the others being more ambiguous. Of those with identifications about half are optically bright (I<23) mergers or interactions with a high proportion showing signatures of AGN activity and the other half are optically faint (I>25) including both blank fields and Extremely Red Objects (EROs). We conclude that a population of distant, highly obscured ultraluminous infrared galaxies (ULIRGs) dominates the SMBR. The wide range in the characteristics of the optical counterparts is consistent with the dispersion in the restframe UV properties of local ULIRGs. We suggest that the faint submm population comprises a class of high redshift dusty, mergers associated with the formation of present day luminous elliptical galaxies.

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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

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

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

SCUBA's first-born: SMM J02399-0136

We discuss observations of the submm-selected galaxy, SMM J02399-0136, and what has been learnt about it during the year following its discovery. SMM J02399-0136 was the first distant galaxy detected in submm surveys with SCUBA. Its association with a massive, gas-rich starburst/AGN at z=2.8 has lead to suggestions that the prevalence of AGN in the early Universe may be high and that these AGN may account for a significant fraction of the far-IR background.

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arXiv1999

The Star Formation Histories of Galaxies in Distant Clusters

We present a detailed analysis of the spectroscopic catalog of galaxies in 10 distant clusters from Dressler et al. (1999, D99). We investigate the nature of the different spectral classes defined by D99 including star forming, post-starburst and passive galaxy populations, and reproduce their basic properties using our spectral synthesis model. We attempt to identify the evolutionary pathways between the various spectral classes in order to search for the progenitors of the numerous post-starburst galaxies. The comparison of the spectra of the distant galaxy populations with samples drawn from the local Universe leads us to identify a significant population of dust-enshrouded starburst galaxies, showing both strong Balmer absorption and relatively modest [OII] emission, that we believe are the most likely progenitors of the post-starburst population. We present the differences between the field and cluster galaxies at z=0.4-0.5. We then compare the spectral and the morphological properties of the distant cluster galaxies, exploring the connection between the quenching of star formation inferred from the spectra and the strong evolution of the S0 population discussed by Dressler et al. (1997). We conclude that either two different timescales and/or two different physical processes are responsible for the spectral and the morphological transformation.

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arXiv1999

A Spectroscopic Catalog of 10 Distant Rich Clusters of Galaxies

We present spectroscopic observations of galaxies in the fields of 10 distant clusters for which we have previously presented deep imaging with WFPC2 on board the HST. 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, redshifts, spectral line strengths and classifications for 657 galaxies in the fields of the 10 clusters. The catalog comprises 424 cluster members across the 10 clusters and 233 field galaxies, with detailed morphological information from our WFPC2 images for 204 of the cluster galaxies and 71 in the field. We illustrate some basic properties of the catalog, including correlations between the morphological and spectral properties of our large sample of cluster galaxies. A direct comparison of the spectral properties of the high redshift cluster and field populations suggest that the phenomenon of strong Balmer lines in otherwise passive galaxies (commonly called E+A, but renamed here as the k+a class) shows an order-of-magnitude increase in the rich cluster environment, compared to a more modest increase in the field population. This suggests that the process or processes involved in producing k+a galaxies are either substantially more effective in the cluster environment or that this environment prolongs the visibility of this phase. A more detailed analysis and modeling of these data will be presented in Poggianti et al. (1998).

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