438 research outputs found
New, nearby bright southern ultracool dwarfs
We report the discovery of twenty-one hitherto unknown bright southern
ultracool dwarfs with spectral types in the range M7 to L5.5, together with new
observations of a further three late M dwarfs previously confirmed. Three more
objects are already identified in the literature as high proper motion stars;we
derive their spectral types for the first time. All objects were selected from
the 2MASS All Sky and SuperCOSMOS point source databases on the basis of their
optical/near-infrared colours, -band magnitudes and proper motions. Low
resolution (R 1000) spectroscopy with the ESO/NTT SOFI spectrograph
has confirmed the ultracool nature of 24 targets, out of a total of 25
candidates observed. Spectral types are derived by direct comparison with
template objects and compared to results from HO and FeH indices. We also
report the discovery of one binary, as revealed by SOFI acquisition imaging;
spectra were taken for both components. The spectral types of the two
components are L2 and L4 and the distance 19 pc. Spectroscopic distances
and transverse velocities are derived for the sample. Two L5 objects lie
only 10 pc distant. Such nearby objects are excellent targets for
further study to derive their parallaxes and to search for fainter, later
companions with AO and/or methane imaging.Comment: 11 pages, 10 figures, accepted to MNRA
A composite HII region luminosity function in H alpha of unprecedented statistical weight
Context. Statistical properties of HII region populations in disk galaxies
yield important clues to the physics of massive star formation.
Aims. We present a set of HII region catalogues and luminosity functions for
a sample of 56 spiral galaxies in order to derive the most general form of
their luminosity function.
Methods. HII region luminosity functions are derived for individual galaxies
which, after photometric calibration, are summed to form a total luminosity
function comprising 17,797 HII regions from 53 galaxies.
Results. The total luminosity function, above its lower limit of
completeness, is clearly best fitted by a double power law with a significantly
steeper slope for the high luminosity portion of the function. This change of
slope has been reported in the literature for individual galaxies, and occurs
at a luminosity of log L = 38.6\pm0.1 (L in erg/s) which has been termed the
Stromgren luminosity. A steep fall off in the luminosity function above log L =
40 is also noted, and is related to an upper limit to the luminosities of
underlying massive stellar clusters. Detailed data are presented for the
individual sample galaxies.
Conclusions. The luminosity functions of HII regions in spiral galaxies show
a two slope power law behaviour, with a significantly steeper slope for the
high luminosity branch. This can be modelled by assuming that the high
luminosity regions are density bounded, though the scenario is complicated by
the inhomogeneity of the ionized interstellar medium. The break, irrespective
of its origin, is of potential use as a distance indicator for disc galaxies.Comment: Accepted for publication as a Letter in Astronomy & Astrophysics.
Latex with postscript figures. Online-only tables and figures are included in
this preprint. The HII region catalogues for 56 galaxies will be published
electronically on the CDS but are available also on request from the author
The sub-stellar birth rate from UKIDSS
We present a new sample of mid-L to mid-T dwarfs with effective temperatures of 11001700 K selected from the UKIDSS Large Area Survey (LAS) and confirmed with infrared spectra from X-shooter/Very Large Telescope. This effective temperature range is especially sensitive to the formation history of Galactic brown dwarfs and allows us to constrain the form of the sub-stellar birth rate, with sensitivity to differentiate between a flat (stellar like) birth rate and an exponentially declining form. We present the discovery of 63 new L and T dwarfs from the UKIDSS LAS DR7, including the identification of 12 likely unresolved binaries, which form the first complete sub-set from our programme, covering 495 square degrees of sky, complete to J = 18.1. We compare our results for this sub-sample with simulations of differing birth rates for objects of masses 0.10-0.03 M-circle dot and ages 1-10 Gyr. We find that the more extreme birth rates (e. g. a halo type form) can likely be excluded as the true form of the birth rate. In addition, we find that although there is substantial scatter we find a preference for a mass function, with a power-law index a in the range -1 <alpha <0 that is consistent (within the errors) with the studies of late T dwarfs.Peer reviewe
A composite H region luminosity function in Hα of unprecedented statistical weight
ABSTRACT Context. Statistical properties of H region populations in disk galaxies yield important clues to the physics of massive star formation. Aims. We present a set of H region catalogues and luminosity functions for a sample of 56 spiral galaxies in order to derive the most general form of their luminosity function. Methods. H region luminosity functions are derived for individual galaxies which, after photometric calibration, are summed to form a total luminosity function comprising 17,797 H regions from 53 galaxies. Results. The total luminosity function, above its lower limit of completeness, is clearly best fitted by a double power law with a significantly steeper slope for the high luminosity portion of the function. This change of slope has been reported in the literature for individual galaxies, and occurs at a luminosity of log L = 38.6 ± 0.1 (L in erg s −1 ) which has been termed the Strömgren luminosity. A steep fall off in the luminosity function above log L = 40 is also noted, and is related to an upper limit to the luminosities of underlying massive stellar clusters. Detailed data are presented for the individual sample galaxies. Conclusions. The luminosity functions of H regions in spiral galaxies show a two slope power law behaviour, with a significantly steeper slope for the high luminosity branch. This can be modelled by assuming that the high luminosity regions are density bounded, though the scenario is complicated by the inhomogeneity of the ionized interstellar medium. The break, irrespective of its origin, is of potential use as a distance indicator for disc galaxies
A New Automatic Method to Identify Galaxy Mergers I. Description and Application to the STAGES Survey
We present an automatic method to identify galaxy mergers using the
morphological information contained in the residual images of galaxies after
the subtraction of a Sersic model. The removal of the bulk signal from the host
galaxy light is done with the aim of detecting the fainter minor mergers. The
specific morphological parameters that are used in the merger diagnostic
suggested here are the Residual Flux Fraction and the asymmetry of the
residuals. The new diagnostic has been calibrated and optimized so that the
resulting merger sample is very complete. However, the contamination by
non-mergers is also high. If the same optimization method is adopted for
combinations of other structural parameters such as the CAS system, the merger
indicator we introduce yields merger samples of equal or higher statistical
quality than the samples obtained through the use of other structural
parameters. We explore the ability of the method presented here to select minor
mergers by identifying a sample of visually classified mergers that would not
have been picked up by the use of the CAS system, when using its usual limits.
Given the low prevalence of mergers among the general population of galaxies
and the optimization used here, we find that the merger diagnostic introduced
in this work is best used as a negative merger test, i.e., it is very effective
at selecting non-merging galaxies. As with all the currently available
automatic methods, the sample of merger candidates selected is contaminated by
non-mergers, and further steps are needed to produce a clean sample. This
merger diagnostic has been developed using the HST/ACS F606W images of the
A901/02 cluster (z=0.165) obtained by the STAGES team. In particular, we have
focused on a mass and magnitude limited sample (log M/M_{O}>9.0,
R_{Vega}<23.5mag)) which includes 905 cluster galaxies and 655 field galaxies
of all morphological types.Comment: 25 pages, 14 figures, 4 tables. To appear in MNRA
The discovery of an M4+T8.5 binary system
The original article can be found at: http://www3.interscience.wiley.com Copyright Blackwell Publishing / Royal Astronomical Society. DOI: 10.1111/j.1365-2966.2009.14620.xWe report the discovery of a T8.5 dwarf, which is a companion to the M4 dwarf Wolf 940. [Please see original online abstract for complete version with correct notation]Peer reviewe
Finding benchmark brown dwarfs to probe the IMF as a function of time
Using a simulated disk brown dwarf (BD) population, we find that new large
area infrared surveys are expected to identify enough BDs covering wide enough
mass--age ranges to potentially measure the mass function down to ~0.03Mo, and
the BD formation history out to 10 Gyr, at a level capable of establishing if
BD formation follows star formation. We suggest these capabilities are best
realised by spectroscopic calibration of BD properties (Teff, g and [M/H])
which, when combined with a measured luminosity and an evolutionary model can
give BD mass and age relatively independent of BD atmosphere models. Such
calibration requires an empirical understanding of how BD spectra are affected
by variations in these properties, and thus the identification and study of
"benchmark BDs" whose age and composition can be established independently. We
identify the best sources of benchmark BDs as young open cluster members,
moving group members, and wide (>1000AU) BD companions to both subgiant stars
and high mass white dwarfs (WDs). We have used 2MASS to measure a wide L dwarf
companion fraction of 2.7(+0.7/-0.5)%, which equates to a BD companion fraction
of 34(+9/-6)% for an alpha~1 companion mass function. Using this value we
simulate populations of wide BD binaries, and estimate that 80(+21/-14)
subgiant--BD binaries, and 50(+13/-10) benchmark WD--BD binaries could be
identified using current and new facilities. The WD--BD binaries should all be
identifiable using the Large Area Survey component of UKIDSS combined with
Sloan. Discovery of the subgiant--BD binaries will require a NIR imaging
campaign around a large (~900) sample of Hipparcos subgiants. If identified,
spectral studies of these benchmark brown dwarfs could reveal the spectral
sensitivities across the Teff, g and [M/H] space probed by new surveys.Comment: 18 pages, 12 figures, accepted for publication in MNRA
Distributions of Galaxy Spectral Types in the Sloan Digital Sky Survey
We perform an objective classification of 170,000 galaxy spectra in the Sloan
Digital Sky Survey (SDSS) using the Karhunen-Lo\`eve (KL) transform. With about
one-sixth of the total set of galaxy spectra which will be obtained by the
survey, we are able to carry out the most extensive analysis of its kind to
date. The formalism proposed by Connolly and Szalay is adopted to correct for
gappy regions in the spectra, and to derive eigenspectra and eigencoefficients.
From this analysis, we show that this gap-correction formalism leads to a
converging set of eigenspectra and KL-repaired spectra. Furthermore, KL
eigenspectra of galaxies are found to be convergent not only as a function of
iteration, but also as a function of the number of randomly selected galaxy
spectra used in the analysis. From these data a set of ten eigenspectra of
galaxy spectra are constructed, with rest-wavelength coverage 3450-8350
angstrom. The eigencoefficients describing these galaxies naturally place the
spectra into several classes defined by the plane formed by the first three
eigencoefficients of each spectrum. Spectral types, corresponding to different
Hubble-types and galaxies with extreme emission lines, are identified for the
170,000 spectra and are shown to be complementary to existing spectral
classifications. From a non-parametric classification technique, we find that
the population of galaxies can be divided into three classes which correspond
to early late- through to intermediate late-types galaxies. (Abridged)Comment: AJ in press (August 2004 issue) 25 pages, 24 figures (lowered
resolutions). Eigenspectra and mean spectra are available in:
http://www.sdss.org/dr2/products/value_added
Tree structure of the percolating Universe
We present a numerical study of topological descriptors of initially Gaussian
and scale-free density perturbations evolving via gravitational instability in
an expanding universe. We carefully evaluate and avoid numerical contamination
in making accurate measurements on simulated fields on a grid in a finite box.
Independent of extent of non linearity, the measured Euler number of the
excursion set at the percolation threshold, , is positive and nearly
equal to the number of isolated components, suggesting that these structures
are trees. Our study of critical point counts reconciles the clumpy appearance
of the density field at with measured filamentary local curvature.
In the Gaussian limit, we measure in contrast to widely
held belief that , where is the variance of
the density field.Comment: 4 pages, 2 figures, Accepted for publication in Phys. Rev. Let
A review of reporting of participant recruitment and retention in RCTs in six major journals
<p>Abstract</p> <p>Background</p> <p>Poor recruitment and retention of participants in randomised controlled trials (RCTs) is problematic but common. Clear and detailed reporting of participant flow is essential to assess the generalisability and comparability of RCTs. Despite improved reporting since the implementation of the CONSORT statement, important problems remain. This paper aims: (i) to update and extend previous reviews evaluating reporting of participant recruitment and retention in RCTs; (ii) to quantify the level of participation throughout RCTs.</p> <p>Methods</p> <p>We reviewed all reports of RCTs of health care interventions and/or processes with individual randomisation, published July–December 2004 in six major journals. Short, secondary or interim reports, and Phase I/II trials were excluded. Data recorded were: general RCT details; inclusion of flow diagram; participant flow throughout trial; reasons for non-participation/withdrawal; target sample sizes.</p> <p>Results</p> <p>133 reports were reviewed. Overall, 79% included a flow diagram, but over a third were incomplete. The majority reported the flow of participants at each stage of the trial after randomisation. However, 40% failed to report the numbers assessed for eligibility. Percentages of participants retained at each stage were high: for example, 90% of eligible individuals were randomised, and 93% of those randomised were outcome assessed. On average, trials met their sample size targets. However, there were some substantial shortfalls: for example 21% of trials reporting a sample size calculation failed to achieve adequate numbers at randomisation, and 48% at outcome assessment. Reporting of losses to follow up was variable and difficult to interpret.</p> <p>Conclusion</p> <p>The majority of RCTs reported the flow of participants well after randomisation, although only two-thirds included a complete flow chart and there was great variability over the definition of "lost to follow up". Reporting of participant eligibility was poor, making assessments of recruitment practice and external validity difficult. Reporting of participant flow throughout RCTs could be improved by small changes to the CONSORT chart.</p
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