arXiv subjects

David Carter

Publications and source records attributed to David Carter.

At least 37 records · Page 2Linked to original sources

arXiv2010

Design drivers for a wide-field multi-object spectrograph for the William Herschel Telescope

Wide-field multi-object spectroscopy is a high priority for European astronomy over the next decade. Most 8-10m telescopes have a small field of view, making 4-m class telescopes a particularly attractive option for wide-field instruments. We present a science case and design drivers for a wide-field multi-object spectrograph (MOS) with integral field units for the 4.2-m William Herschel Telescope (WHT) on La Palma. The instrument intends to take advantage of a future prime-focus corrector and atmospheric-dispersion corrector that will deliver a field of view 2 deg in diameter, with good throughput from 370 to 1,000 nm. The science programs cluster into three groups needing three different resolving powers R: (1) high-precision radial-velocities for Gaia-related Milky Way dynamics, cosmological redshift surveys, and galaxy evolution studies (R = 5,000), (2) galaxy disk velocity dispersions (R = 10,000) and (3) high-precision stellar element abundances for Milky Way archaeology (R = 20,000). The multiplex requirements of the different science cases range from a few hundred to a few thousand, and a range of fibre-positioner technologies are considered. Several options for the spectrograph are discussed, building in part on published design studies for E-ELT spectrographs. Indeed, a WHT MOS will not only efficiently deliver data for exploitation of important imaging surveys planned for the coming decade, but will also serve as a test-bed to optimize the design of MOS instruments for the future E-ELT.

astro-ph.IM↗

arXiv2010

Ultraviolet tails and trails in cluster galaxies: A sample of candidate gaseous stripping events in Coma

We have used new deep observations of the Coma cluster from GALEX to identify 13 star-forming galaxies with asymmetric morphologies in the ultraviolet. Aided by optical broad-band and H-alpha imaging, we interpret the asymmetric features as being due to star formation within gas stripped from the galaxies by interaction with the cluster environment. The selected objects display a range of structures from broad fan-shaped systems of filaments and knots (`jellyfish') to narrower and smoother tails extending up to 100 kpc in length. Some of the features have been discussed previously in the literature, while others are newly identified here. As an ensemble, the candidate stripping events are located closer to the cluster centre than other star-forming galaxies; their radial distribution is similar to that of all cluster members, dominated by passive galaxies. The fraction of blue galaxies which are undergoing stripping falls from 40% in the central 500 kpc, to less than 5% beyond 1 Mpc. We find that tails pointing away from (i.e. galaxies moving towards) the cluster centre are strongly favoured (11/13 cases). From the small number of `outgoing' galaxies with stripping signatures we conclude that the stripping events occur primarily on first passage towards the cluster centre, and are short-lived compared to the cluster crossing time. Using infall trajectories from simulations, the observed fraction of blue galaxies undergoing stripping can be reproduced if the events are triggered at a threshold radius of ~1 Mpc and detectable for ~500 Myr. HST images are available for two galaxies from our sample and reveal compact blue knots coincident with UV and H-alpha emission, apparently forming stars within the stripped material. Our results confirm that stripping of gas from infalling galaxies, and associated star formation in the stripped material, is a widespread phenomenon in rich clusters.

astro-ph.CO↗

arXiv2010

Barred Galaxies in the Coma Cluster

We use ACS data from the HST Treasury survey of the Coma cluster (z~0.02) to study the properties of barred galaxies in the Coma core, the densest environment in the nearby Universe. This study provides a complementary data point for studies of barred galaxies as a function of redshift and environment. From ~470 cluster members brighter than M_I = -11 mag, we select a sample of 46 disk galaxies (S0--Im) based on visual classification. The sample is dominated by S0s for which we find an optical bar fraction of 47+/-11% through ellipse fitting and visual inspection. Among the bars in the core of the Coma cluster, we do not find any very large (a_bar > 2 kpc) bars. Comparison to other studies reveals that while the optical bar fraction for S0s shows only a modest variation across low-to-intermediate density environments (field to intermediate-density clusters), it can be higher by up to a factor of ~2 in the very high-density environment of the rich Coma cluster core.

astro-ph.CO↗

arXiv2009

Optical and near infra-red colours as a discriminant of the age and metallicity of stellar populations

We present a comprehensive analysis of the ability of current stellar population models to reproduce the optical (ugriz) and near infra-red (JHK) colours of a small sample of well-studied nearby elliptical and S0 galaxies. We find broad agreement between the ages and metallicities derived using different population models, although different models show different systematic deviations from the measured broad-band fluxes. Although it is possible to constrain Simple Stellar Population models to a well defined area in age-metallicity space, there is a clear degeneracy between these parameters even with such a full range of precise colours. The precision to which age and metallicity can be determined independently, using only broad band photometry with realistic errors, is Delta{[Fe/H]} ~ 0.18 and Delta{log(Age)} ~ 0.25. To constrain the populations and therefore the star formation history further it will be necessary to combine broad-band optical-IR photometry with either spectral line indices, or else photometry at wavelengths outside of this range.

astro-ph.CO↗

arXiv2009

Optical flashes, reverse shocks and magnetization

Despite the pre-Swift expectation that bright optical flashes from reverse shocks would be prevalent in early-time afterglow emission, rapid response observations show this not to be the case. Although very bright at early times, some GRBs such as GRB 061007 and GRB 060418, lack the short-lived optical flash from the reverse shock within minutes after the GRB. In contrast, other optical afterglows, such as those of GRB 990123, GRB 021211, GRB 060111B, GRB 060117, GRB 061126, and recently GRB 080319B, show a steep-to-flat transition within first 10^3 s typical of a rapidly evolving reverse + forward shock combination. We review the presence and absence of the reverse shock components in optical afterglows and discuss the implications for the standard model and the magnetization of the fireball. We show that the previously predicted optical flashes are likely to occur at lower wavelengths, perhaps as low as radio wavelengths and, by using the case of GRB 061126 we show that the magnetic energy density in the ejecta, expressed as a fraction of the equipartion value, is a key physical parameter.

astro-ph.HE↗

arXiv2008

Globular Cluster Populations in Four Early-Type Poststarburst Galaxies

We present a study of the globular cluster systems of four early-type poststarburst galaxies using deep g and I-band images from the ACS camera aboard the Hubble Space Telescope (HST). All the galaxies feature shells distributed around their main bodies and are thus likely merger remnants. The color distribution of the globular clusters in all four galaxies shows a broad peak centered on g-I ~ 1.4, while PGC 6240 and PGC 42871 show a significant number of globular clusters with g-I ~ 1.0. The latter globular clusters are interpreted as being of age ~ 500 Myr and likely having been formed in the merger. The color of the redder peak is consistent with that expected for an old metal-poor population that is very commonly found around normal galaxies. However, all galaxies except PGC 10922 contain several globular clusters that are significantly brighter than the maximum luminosity expected of a single old metal-poor population. To test for multiple-age populations of overlapping g-I color, we model the luminosity functions of the globular clusters as composites of an old metal-poor subpopulation with a range of plausible specific frequencies and an intermediate-age subpopulation of solar metallicity. We find that three of the four sample galaxies show evidence for the presence of an intermediate-age (~ 1 Gyr) globular cluster population, in addition to the old metal-poor GC population seen in normal early-type galaxies. None of the galaxies show a significant population of clusters consistent with an old, metal-rich red cluster population that is typically seen in early-type galaxies.

astro-ph↗

arXiv2008

A large population of recently-quenched red-sequence dwarf galaxies in the outskirts of the Coma Cluster

We analyse the stellar populations of 75 red-sequence dwarf galaxies in the Coma cluster, based on high signal-to-noise spectroscopy from the 6.5m MMT. The sample covers a luminosity range 3-4 magnitudes below M*, in the cluster core and in a field centred 1 deg to the south-west. We find a strong dependence of the absorption line strengths with location in the cluster. Galaxies further from the cluster centre have stronger Balmer lines than inner-field galaxies of the same luminosity. The magnesium lines are weaker at large radius, while the iron lines are not correlated with radius. Converting the line strengths into estimates of stellar age, metallicity and abundance ratios, we find the gradients are driven by variations in age (>6 sigma significance) and in the iron abundance Fe/H (~2.7 sigma significance). The light element (Mg, C, N, Ca) abundances are almost independent of radius. At radius of 0.4-1.3 degree (~0.3-1.0x the virial radius), dwarf galaxies have ages ~3.8 Gyr on average, compared to ~6 Gyr near the cluster centre. The outer dwarfs are also ~50% more iron-enriched, at given luminosity. Our results confirm earlier indications that the ages of red-sequence galaxies depend on location within clusters, and in Coma in particular. The exceptionally strong trends found here suggest that dwarf galaxies are especially susceptible to environmental "quenching", and/or that the south-west part of Coma is a particularly clear example of recent quenching in an infalling subcluster.

astro-ph↗

arXiv2008

The HST/ACS Coma Cluster Survey: I - Survey Objectives and Design

We describe the HST ACS Coma cluster Treasury survey, a deep two-passband imaging survey of one of the nearest rich clusters of galaxies, the Coma cluster (Abell 1656). The survey was designed to cover an area of 740 square arcmin in regions of different density of both galaxies and intergalactic medium within the cluster. The ACS failure of January 27th 2007 leaves the survey 28% complete, with 21 ACS pointings (230 square arcmin) complete, and partial data for a further 4 pointings (44 square arcmin). Predicted survey depth for 10 sigma detections for optimal photometry of point sources is g' = 27.6 in the F475W filter, and IC=26.8 mag in F814 (AB magnitudes). Initial simulations with artificially injected point sources show 90% recovered at magnitude limits of g' = 27.55 and IC = 26.65. For extended sources, the predicted 10 sigma limits for a 1 square arcsecond region are g' = 25.8 mag/sq. arcsec and IC = 25.0 mag/sq. arcsec. We highlight several motivating science goals of the survey, including study of the faint end of the cluster galaxy luminosity function, structural parameters of dwarf galaxies, stellar populations and their effect on colors and color gradients, evolution of morphological components in a dense environment, the nature of ultra compact dwarf galaxies, and globular cluster populations of cluster galaxies of a range of luminosities and types. This survey will also provide a local rich cluster benchmark for various well known global scaling relations and explore new relations pertaining to the nuclear properties of galaxies.

astro-ph↗

arXiv2007

Early Optical Polarization of a Gamma Ray Burst Afterglow

We report the optical polarization of a gamma ray burst (GRB) afterglow, obtained 203 seconds after the initial burst of gamma rays from GRB 060418, using a ring polarimeter on the robotic Liverpool Telescope. Our robust (2-sigma) upper limit on the percentage of polarization, less than 8%, coincides with the fireball deceleration time at the onset of the afterglow. The combination of the rate of decay of the optical brightness and the low polarization at this critical time constrains standard models of GRB ejecta, ruling out the presence of a large-scale ordered magnetic field in the emitting region.

astro-ph↗

arXiv2003

A Photometric and Spectroscopic Study of Dwarf and Giant Galaxies in the Coma Cluster - IV. The Luminosity Function

A large spectroscopic survey is constructed of galaxies in the Coma cluster. The survey covers a wide area (1 deg$^2$) to deep magnitudes (R ~ 19.5), covering both the core (high density) and outskirts (intermediate to low density) of the cluster. The spectroscopic sample consists of a total of 1191 galaxies, of which, 760 galaxies are confirmed members of the Coma cluster. A statistical technique is developed to correct the spectroscopic sample for incompletness. The corrected sample is then used to construct R-band luminosity function (LF) spanning a range of 7 magnitudes (-23 < M_R < -16) both at the core and outskirts of the cluster. Dependence of the LF on local environment in Coma is explored. The LFs are found to be the same, within the errors, between the inner and outer regions and close to those from recent measurements for field galaxies. The total B-band LF for the Coma cluster, fitted to a Schecter form is also measured and shows a dip at M(B) = -18 mag., in agreement with previous studies. The implications of this feature are discussed. The LF is studied in B-R color intervals and shows a steep faint-end slope for red (B-R > 1.35) galaxies, both at the core and outskirts of the cluster. This population of low luminosity red galaxies has a higher surface density than the blue (B-R < 1.35) star-forming population, dominating the faint-end of the Coma cluster LF. It is found that relative number of high surface brightness galaxies is larger at the cluster core, implying destruction of low surface brightness galaxies in dense core environment.

astro-ph↗

arXiv1997

The TreeBanker: a Tool for Supervised Training of Parsed Corpora

I describe the TreeBanker, a graphical tool for the supervised training involved in domain customization of the disambiguation component of a speech- or language-understanding system. The TreeBanker presents a user, who need not be a system expert, with a range of properties that distinguish competing analyses for an utterance and that are relatively easy to judge. This allows training on a corpus to be completed in far less time, and with far less expertise, than would be needed if analyses were inspected directly: it becomes possible for a corpus of about 20,000 sentences of the complexity of those in the ATIS corpus to be judged in around three weeks of work by a linguistically aware non-expert.

cmp-lg↗

arXiv1997

Translation Methodology in the Spoken Language Translator: An Evaluation

In this paper we describe how the translation methodology adopted for the Spoken Language Translator (SLT) addresses the characteristics of the speech translation task in a context where it is essential to achieve easy customization to new languages and new domains. We then discuss the issues that arise in any attempt to evaluate a speech translator, and present the results of such an evaluation carried out on SLT for several language pairs.

cmp-lg↗

arXiv1997

Recycling Lingware in a Multilingual MT System

We describe two methods relevant to multi-lingual machine translation systems, which can be used to port linguistic data (grammars, lexicons and transfer rules) between systems used for processing related languages. The methods are fully implemented within the Spoken Language Translator system, and were used to create versions of the system for two new language pairs using only a month of expert effort.

cmp-lg↗

arXiv1997

Hybrid language processing in the Spoken Language Translator

The paper presents an overview of the Spoken Language Translator (SLT) system's hybrid language-processing architecture, focussing on the way in which rule-based and statistical methods are combined to achieve robust and efficient performance within a linguistically motivated framework. In general, we argue that rules are desirable in order to encode domain-independent linguistic constraints and achieve high-quality grammatical output, while corpus-derived statistics are needed if systems are to be efficient and robust; further, that hybrid architectures are superior from the point of view of portability to architectures which only make use of one type of information. We address the topics of ``multi-engine'' strategies for robust translation; robust bottom-up parsing using pruning and grammar specialization; rational development of linguistic rule-sets using balanced domain corpora; and efficient supervised training by interactive disambiguation. All work described is fully implemented in the current version of the SLT-2 system.

cmp-lg↗

arXiv1996

Adapting the Core Language Engine to French and Spanish

We describe how substantial domain-independent language-processing systems for French and Spanish were quickly developed by manually adapting an existing English-language system, the SRI Core Language Engine. We explain the adaptation process in detail, and argue that it provides a fairly general recipe for converting a grammar-based system for English into a corresponding one for a Romance language.

cmp-lg↗

arXiv1996

Fast Parsing using Pruning and Grammar Specialization

We show how a general grammar may be automatically adapted for fast parsing of utterances from a specific domain by means of constituent pruning and grammar specialization based on explanation-based learning. These methods together give an order of magnitude increase in speed, and the coverage loss entailed by grammar specialization is reduced to approximately half that reported in previous work. Experiments described here suggest that the loss of coverage has been reduced to the point where it no longer causes significant performance degradation in the context of a real application.

cmp-lg↗

arXiv1995

Rapid Development of Morphological Descriptions for Full Language Processing Systems

I describe a compiler and development environment for feature-augmented two-level morphology rules integrated into a full NLP system. The compiler is optimized for a class of languages including many or most European ones, and for rapid development and debugging of descriptions of new languages. The key design decision is to compose morphophonological and morphosyntactic information, but not the lexicon, when compiling the description. This results in typical compilation times of about a minute, and has allowed a reasonably full, feature-based description of French inflectional morphology to be developed in about a month by a linguist new to the system.

cmp-lg↗

arXiv1994

The Speech-Language Interface in the Spoken Language Translator

The Spoken Language Translator is a prototype for practically useful systems capable of translating continuous spoken language within restricted domains. The prototype system translates air travel (ATIS) queries from spoken English to spoken Swedish and to French. It is constructed, with as few modifications as possible, from existing pieces of speech and language processing software. The speech recognizer and language understander are connected by a fairly conventional pipelined N-best interface. This paper focuses on the ways in which the language processor makes intelligent use of the sentence hypotheses delivered by the recognizer. These ways include (1) producing modified hypotheses to reflect the possible presence of repairs in the uttered word sequence; (2) fast parsing with a version of the grammar automatically specialized to the more frequent constructions in the training corpus; and (3) allowing syntactic and semantic factors to interact with acoustic ones in the choice of a meaning structure for translation, so that the acoustically preferred hypothesis is not always selected even if it is within linguistic coverage.

cmp-lg↗