arXiv subjects

Douglas Scott

Publications and source records attributed to Douglas Scott.

At least 289 records · Page 16Linked to original sources

arXiv2005

An upper limit to [C II] emission in a z ~= 5 galaxy

Low-ionization-state far-infrared (FIR) emission lines may be useful diagnostics of star-formation activity in young galaxies, and at high redshift may be detectable from the ground. In practice, however, very little is known concerning how strong such line emission might be in the early Universe. We attempted to detect the 158 micron [C II] line from a lensed galaxy at z = 4.926 using the Caltech Submillimeter Observatory. This source is an ordinary galaxy, in the sense that it shows high but not extreme star formation, but lensing makes it visible. Our analysis includes a careful consideration of the calibrations and weighting of the individual scans. We find only modest improvement over the simpler reduction methods, however, and the final spectrum remains dominated by systematic baseline ripple effects. We obtain a 95 per cent confidence upper limit of 33 mJy for a 200 km/s full width at half maximum line, corresponding to an unlensed luminosity of 1x10^9 L_sun for a standard cosmology. Combining this with a marginal detection of the continuum emission using the James Clerk Maxwell Telescope, we derive an upper limit of 0.4 per cent for the ratio of L_CII/L_FIR in this object.

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arXiv2005

The Hubble Deep Field North SCUBA Super-map III - Optical and near-infrared properties of submillimetre galaxies

We present a new sub-mm Super-map in the HDF-North region (GOODS-North field), containing 40 statistically robust sources at 850 microns. This map contains additional data, and several new sources, including one of the brightest blank-sky extragalactic sub-mm sources ever detected. We have used the ACS HST images and ground-based near-IR observations from GOODS to develop a systematic approach for counterpart identification. 72 per cent of our sources with optical coverage have a unique optical counterpart using our new techniques for counterpart identification, and an additional 18 per cent have more than one possibility that meet our criteria in the ACS images. We have found a much higher ERO rate than other sub-mm surveys, due to the increased depth in the optical images. The median photometric redshift (and quartile range), from optical and near-infrared data, is 1.7 (1.3-2.5) for the radio-detected sub-mm sources, and rises to 2.3 (1.3-2.7) for the radio-undetected sub-sample. We find interesting correlations between the 850 micron flux and both the i magnitude and the photometric redshift, from which there appears to be an absence of high redshift faint counterparts to the lower flux density SCUBA sources. While the quantitative morphologies span a range of values, in general the sub-mm galaxies show larger sizes and a higher degree of asymmetry than other galaxy populations at the same redshifts.

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arXiv2004

The 2-850 micron SED of starforming galaxies

We present preliminary results on a study of the 2--850 micron SEDs of a sample of 30 FIRBACK galaxies selected at 170 micron. These sources are representative of the brightest ~10% of the Cosmic Infrared Background. They are a mixture of mostly local (z<~0.3) starforming galaxies, and a tail of ULIGs that extend up to z~1, and are likely to be a similar population to faint SCUBA sources. We use archival Spitzer IRAC and MIPS data to extend the spectral coverage to the mid-IR regime, resulting in an unprecended (for this redshift range) census of their infrared SEDs. This allows us to study in far greater detail this important population linking the near-IR stellar emission with PAH and thermal dust emission. We do this using a Markov Chain Monte Carlo method, which easily allows for the inclusion of ~6 free parameters, as well as an estimate of parameter uncertainties and correlations.

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arXiv2004

An 850-micron SCUBA map of the Groth Strip and reliable source extraction

We present an 850-micron map and list of candidate sources in a 70 arcmin^2 sub-area of the Groth Strip observed using SCUBA. We initially detect 7 candidate sources with signal-to-noise ratios (SNRs) between 3.0 and 3.5 and 4 candidate sources with SNR > 3.5. Simulations suggest that on average in a map this size one expects 1.6 false positive sources for SNR > 3.5 and 4.5 for 3 < SNR < 3.5. Flux boosting in maps is a well known effect and we have developed a simple Bayesian prescription for estimating the unboosted flux distribution and used this method to determine the best flux estimates of our sources. This method is easily adapted for any other modest signal-to-noise survey in which there is prior knowledge of the source counts. We performed follow-up photometry in an attempt to confirm or reject 5 of our source candidates. We confirm the reality of 2 of the SCUBA sources, although at lower levels than suggested in the map and we find that the photometry results are consistent with and confirm the de-boosted map fluxes. Our final candidate source list contains 3 sources, including the 2 confirmed detections and 1 further candidate source with SNR > 3.5 which has a reasonable chance of being real.

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arXiv2004

Optical and near-infrared properties of submillimetre galaxies in the GOODS-North field

We investigate the photometric redshift distribution, colours and morphologies of a large sample of submillimetre sources in the GOODS-North region. With the depth achieved by the deep ACS images, optical counterparts have been found for all the radio-detected sub-mm sources, and we have used several techniques to identify counterparts to the radio-undetected sources. The sub-mm counterparts have a median photometric redshift, from optical and near-IR photometry, of 1.9. Our sample contains a much higher fraction of EROs than other sub-mm surveys, and larger angular sizes than the average galaxy at all redshifts. The sub-mm galaxy counterparts are consistent with massive star-forming galaxies at high redshift.

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arXiv2004

Submillimetre Constraints on Hyper-Extremely Red Objects in the Subaru Deep Field

We have mapped the submillimetre wavelength continuum emission from the Subaru Deep Field (SDF) at 450 and 850 microns with the Submillimetre Common-User Bolometer Array (SCUBA) detector on the James Clerk Maxwell Telescope (JCMT). The near-IR image of the SDF is one of the deepest near-IR images available and contains four `hyper extremely red objects' (HEROs). These data allow us to test the connection between `extremely red objects' (EROs) found in IR surveys and the population of bright submillimetre sources found with SCUBA. We present a weak measurement of the average flux of the four K-band selected HEROs of 1.15 (+/-0.46) mJy, which fails to support the hypothesis that HEROs should be bright SCUBA sources. Our data are consistent with the HEROs being objects with SEDs like that of Arp220 out to z~1.7, however, the extinction in the HEROs must be about 1 magnitude greater in the J-band than is the case for Arp220 and they would need to be 1.7 times as luminous as Arp220. On the other hand, an evolutionary model of elliptical galaxies at z~2-3 in a dusty starburst phase is also in agreement with the submillimetre data, as was originally proposed for the HEROs.

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arXiv2004

Cosmic Background Radiation Mini-Review

This is a complete re-write of the mini-review for the Review of Particle Physics (a.k.a the Partcicle Data Book), which includes an assessment of the CMB anisotropy results and their interpretation up until the end of 2003. It forms a compact overview of the field at this time. Sections headings are: Introduction; Description of CMB Anisotropies; Cosmological Parameters; Physics of Anisotropies; Current Anisotropy Data; CMB Polarization; Complications; Constraints on Cosmologies; Particle Physics Constraints; Fundamental Lessons; and Future Directions.

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arXiv2003

Measurement of the Sunyaev-Zel'dovich Increment in Massive Galaxy Clusters

We have detected the Sunyaev-Zel'dovich (SZ) increment at 850 microns in two galaxy clusters (Cl 0016+16 and MS 1054.4-0321) using SCUBA (Sub-millimetre Common User Bolometer Array) on the James Clerk Maxwell Telescope. Fits to the isothermal βmodel yield a central Compton y parameter of (2.2 \pm 0.7) x 10^{-4} and a central 850 micron flux of ΔI_{0} = 2.2 \pm 0.7 mJy/beam in Cl 0016. This can be combined with decrement measurements to infer y = (2.38 \pm_{0.34}^{0.36}) x 10^{-4} and v_{pec} = 400 \pm_{1400}^{1900} km/s. In MS 1054 we find a peak 850 micron flux of ΔI_{0} = 2.0 \pm 1.0 mJy/beam and y = (2.0 \pm 1.0) x 10^{-4}. To be successful such measurements require large chop throws and non-standard data analysis techniques. In particular, the 450 micron data are used to remove atmospheric variations in the 850 micron data. An explicit annular model is fit to the SCUBA difference data in order to extract the radial profile, and separately fit to the model differences to minimize the effect of correlations induced by our scanning strategy. We have demonstrated that with sufficient care, SCUBA can be used to measure the SZ increment in massive, compact galaxy clusters.

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arXiv2002

The clustering of halo mergers

We analyse the spatial distribution of halo merger sites using four numerical simulations with high time resolution of structure growth in a Lambda-CDM cosmology. We find no evidence for any large-scale relative bias between mergers and randomly selected haloes of the same mass at high redshift. Given a sample of galaxies that form a Poisson sampling of halo mergers, then the amplitude of the measured clustering ought to lead to a robust estimate of the median host halo mass. In the hierarchical picture of structure growth, mergers between galaxies at high redshifts are thought to create dust enshrouded starbursts leading to increased emission in the sub-mm wavebands. Hence, a measurement of the clustering strength of SCUBA galaxies, for example, can determine whether or not they are giant elliptical galaxies in the process of formation.

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arXiv2002

Cosmological Hydrogen Reionization with Three Dimensional Radiative Transfer

We present new calculations of the inhomogeneous process of cosmological reionization by carefully following the radiative transfer in pre-computed hydrodynamical simulations of galaxy formation. These new computations represent an important step on the way towards fully self-consistent and adaptive calculations which will eventually cover the enormous range of scales from the sizes of individual mini-halos to the mean free path of ionizing photons in the post-overlap Universe. The goal of such simulations is to include enough realistic physics to accurately model the formation of early structures and the end of the `dark ages'. Our new calculations demonstrate that the process by which the ionized regions percolate the Universe is complex, and that the idea of voids being ionized before overdense regions is perhaps too simplistic. It seems that observational information pertaining to the reionization epoch may now be in our grasp, through the detection of Gunn-Peterson troughs at z~6. If so, then the comparison of information from many lines of sight with simulations such as ours may allow us to disentangle details of the ionization history and trace the early formation of structure.

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arXiv2002

A SCUBA Scanmap of the HDF: Measuring the bright end of the sub-mm source counts

Using the 850 micron SCUBA camera on the JCMT and a scanning technique different from other sub-mm surveys, we have obtained a 125 square arcminute map centered on the Hubble Deep Field. The one-sigma sensitivity to point sources is roughly 3 mJy and thus our map probes the brighter end of the sub-mm source counts. We find 6 sources with a flux greater than about 12 mJy (>4 sigma) and, after a careful accounting of incompleteness and flux bias, estimate the integrated density of bright sources N(>12 mJy)= 164 (+77/-58) per square degree (68 per cent confidence bounds).

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arXiv2001

Understanding the nature of the optically faint radio sources and their connection to the submillimeter population

We present a sample of 43 submillimeter sources detected (at >3 sigma), drawn from our program to follow-up optically faint radio sources with SCUBA. These sources already have associated radio and in many cases optical identifications, and many are also detected at 450 microns. We compare these with 12 submillimeter sources drawn from the literature, which were discovered in blank field mapping campaigns, but also have radio detections. We then use this total sample (55 sources) to study and model the evolution of dusty galaxies. A correlation is observed in the sub-mm/radio color-magnitude diagram, which can be modeled by strong luminosity evolution. The selection effects of the radio/optical pre-selection technique are determined from the models, and a corrected redshift distribution is constrained using a range of model assumptions. The temperature/redshift effects on the 450 microns detected subset of our sample are studied in relation to the models, and prospects for improved measurements in the shorter sub-mm wavelength windows (450 microns and 350 microns) are explored.

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arXiv2001

Cosmological Difficulties with Modified Newtonian Dynamics (or: La Fin du MOND?)

The cold dark matter paradigm has been extremely successful for explaining a wide range of cosmological phenomena. Nevertheless, since evidence for non-baryonic dark matter remains indirect, all reasonable alternatives should be explored. One proposed idea, involving a fundamental acceleration scale a_0 = 1-2e-10 m/s^2, is called MOdified Newtonian Dynamics or MOND. MOND was suggested to explain the flat rotation curves of galaxies without the need for dark matter. Whether or not it can adequately fit the available data has been debated for almost 20 years (and we summarise many of these studies), but only recently have there been attempts to extend MOND to larger scale regimes. We discuss how the basic properties of MOND make it at best ambiguous to apply these ideas to cosmological scales. We emphasize the difficulties inherent in developing a full theory in which to embed MOND. Without such a theory there is no obviously consistent way to discuss the early Universe and the growth of perturbations. Recent claims that MONDian cosmology works very well are therefore not supportable. We also provide an argument for why a_0~cH_0 naturally, a coincidence which is often suggested as a motivation for taking MOND seriously. We discuss other alternative theories of gravity concluding, as others have, that no metric theory extensions appear workable for explaining rotation curves as well as other observed phenomena. The whole premise of many of these attempts is fatally flawed - galaxies are not pre-selected, discrete, isolated regions which formed monolithically and around which one can construct an axially-symmetric dynamical model in order to remove the need for dark matter. In the modern view, galaxies are part of a dynamic continuum of objects which collectively make up the evolving large-scale structure of the universe.

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arXiv2001

The Balloon-borne Large Aperture Sub-millimetre Telescope

The Balloon-borne Large-Aperture Sub-millimetre Telescope (BLAST) will operate on a Long Duration Balloon platform with large format bolometer arrays at 250, 350 and 500 microns, initially using a 2m mirror, with plans to increase to 2.5m. BLAST is a collaboration between scientists in the USA, Canada, UK, Italy and Mexico. Funding has been approved and it is now in its building phase. The test flight is scheduled for 2002, with the first long duration flight the following year. The scientific goals are to learn about the nature of distant extragalactic star forming galaxies and cold pre-stellar sources by making deep maps both at high and low galactic latitudes. BLAST will be useful for planning Herschel key projects which use SPIRE.

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arXiv2000

Known and unknown SCUBA sources

Summary and discussion of some projects to use SCUBA to target sources selected at other wavebands, as well as to find new sub-mm galaxies in `blank fields': FIRBACK galaxies; Lyman break galaxies and `the Blob'; HDF flanking fields and the Groth Strip; survey of lensing cluster fields.

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arXiv2000

The brighter side of sub-mm source counts: a SCUBA scan-map of the Hubble Deep Field

We present an 11 X 11 arcminute map centred on the Hubble Deep Field taken at 850 microns with the SCUBA camera on the JCMT. The map has an average one-sigma sensitivity to point sources of about 2.3 mJy and thus probes the brighter end of the sub-mm source counts. We find 7 sources with a flux greater than 9 mJy (roughly 4 sigma), and therefore estimate N(>9 mJy)= 208 (+90/-72) per degree. This result is consistent with work from other groups, but improves the statistics at the bright end, and is suggestive of a steepening of the counts.

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arXiv2000

Submillimetre sources in rich cluster fields - source counts, redshift estimates, and cooling flow limits

Recent submillimetre surveys have revealed a population of dusty, high redshift sources of great cosmological significance for understanding dust-enshrouded star formation in distant galaxies, and for determining the origin of the far-IR background. In this paper, we analyze nine rich cluster fields mapped at 850 and 450 microns with the SCUBA array on the James Clerk Maxwell telescope. Lensing models of the clusters are developed in order to derive accurate source counts for our sample. VLA maps of the same clusters are used to help constrain the redshift distribution of our SCUBA detections. Implications for high redshift galaxies and for the far-IR background are discussed. We also provide limits on distributed dust produced by cooling flows in these clusters.

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arXiv2000

Boomerang returns unexpectedly

Experimental study of the anisotropy in the cosmic microwave background (CMB) is gathering momentum. The eagerly awaited Boomerang results have lived up to expectations. They provide convincing evidence in favor of the standard paradigm: the Universe is close to flat and with primordial fluctuations which are redolent of inflation. Further scrutiny reveals something even more exciting however -- two hints that there may be some unforeseen physical effects. Firstly the primary acoustic peak appears at slightly larger scales than expected. Although this may be explicable through a combination of mundane effects, we suggest it is also prudent to consider the possibility that the Universe might be marginally closed. The other hint is provided by a second peak which appears less prominent than expected. This may indicate one of a number of possibilities, including increased damping length or tilted initial conditions, but also breaking of coherence or features in the initial power spectrum. Further data should test whether the current concordance model needs only to be tweaked, or to be enhanced in some fundamental way.

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