1,276 research outputs found
Geometrical tests of cosmological models. II. Calibration of rotational widths and disc scaling relations
This series of papers is dedicated to a new technique to select galaxies that
can act as standard rods and standard candles in order to perform geometrical
tests on large samples of high redshift galaxies to constrain different
cosmological parameters. The goals of this paper are (1) to compare different
rotation indicators in order to understand the relation between rotation
velocities extracted from observations of the Halpha line and the [OII]3727
line, and (2) determine the scaling relations between physical size, surface
brightness and magnitude of galaxies and their rotation velocity using the
SFI++, a large catalog of nearby galaxies observed at I-band. A good
correlation is observed between the rotation curve-derived velocities of the
Halpha and [OII] observations, as well as between those calculated from
velocity histograms, justifying the direct comparison of velocities measured
from Halpha rotation curves in nearby galaxies and from [OII] line widths at
higher redshifts. To provide calibration for the geometrical tests, we give
expressions for the different scaling relations between properties of galaxies
(size, surface brightness, magnitude) and their rotation speeds. Apart from the
Tully-Fisher relation, we derive the size-rotation velocity and surface
brightness-rotation velocity relations with unprecedentedly small scatters. We
show how the best size-rotation velocity relation is derived when size is
estimated not from disc scale lengths but from the isophotal diameter r23.5,
once these have been corrected for inclination and extinction effects.Comment: 14 pages and 10 figures. A&A submitte
Are Newly Discovered HI High Velocity Clouds Minihalos in the Local Group?
A set of HI sources extracted from the north Galactic polar region by the
ongoing ALFALFA survey has properties that are consistent with the
interpretation that they are associated with isolated minihalos in the
outskirts of the Local Group (LG). Unlike objects detected by previous surveys,
such as the Compact High Velocity Clouds of Braun & Burton (1999), the HI
clouds found by ALFALFA do not violate any structural requirements or halo
scaling laws of the LambdaCDM structure paradigm, nor would they have been
detected by extant HI surveys of nearby galaxy groups other than the LG. At a
distance of d Mpc, their HI masses range between $5 x 10^4 d^2 and 10^6 d^2
solar and their HI radii between <0.4d and 1.6 d kpc. If they are parts of
gravitationally bound halos, the total masses would be on order of 10^8--10^9
solar, their baryonic content would be signifcantly smaller than the cosmic
fraction of 0.16 and present in a ionized gas phase of mass well exceeding that
of the neutral phase. This study does not however prove that the minihalo
interpretation is unique. Among possible alternatives would be that the clouds
are shreds of the Leading Arm of the Magellanic Stream.Comment: 4 pages, 3 figures, 1 table; to appear Ap.J. Letter
The Arecibo Legacy Fast ALFA Survey: VI. Second HI Source Catalog of the Virgo Cluster Region
We present the third installment of HI sources extracted from the Arecibo
Legacy Fast ALFA extragalactic survey. This dataset continues the work of the
Virgo ALFALFA catalog. The catalogs and spectra published here consist of data
obtained during the 2005 and 2006 observing sessions of the survey. The catalog
consists of 578 HI detections within the range 11h 36m < R.A.(J2000) < 13h 52m
and +08 deg < Dec.(J2000) < +12 deg, and cz_sun < 18000 km/s. The catalog
entries are identified with optical counterparts where possible through the
examination of digitized optical images. The catalog detections can be
classified into three categories: (a) detections of high reliability with S/N >
6.5; (b) high velocity clouds in the Milky Way or its periphery; and (c)
signals of lower S/N which coincide spatially with an optical object and known
redshift. 75% of the sources are newly published HI detections. Of particular
note is a complex of HI clouds projected between M87 and M49 that do not
coincide with any optical counterparts. Candidate objects without optical
counterparts are few. The median redshift for this sample is 6500 km/s and the
cz distribution exhibits the local large scale structure consisting of Virgo
and the background void and the A1367-Coma supercluster regime at cz_sun ~7000
km/s. Position corrections for telescope pointing errors are applied to the
dataset by comparing ALFALFA continuum centroid with those cataloged in the
NRAO VLA Sky Survey. The uncorrected positional accuracy averages
27 arcsec ~(21 arcsec ~median) for all sources with S/N > 6.5 and is of order
~21 arcsec ~(16 arcsec ~median) for signals with S/N > 12. Uncertainties in
distances toward the Virgo cluster can affect the calculated HI mass
distribution.Comment: 25 pages, 1 Table, 8 figures, Accepted by the Astronomical Journa
Lang-Lit from A to BA: student backgrounds and first year content
This report, originally prepared in 2013, investigates the relationship between English ‘langlit’ provision at A level and at BA level (school qualifications outside the A level nations are beyond the direct scope of this report). It builds on an earlier HEA report (Clark and Macrae 2014) which considered general questions about the nature of lang-lit work in both contexts and some issues of transition between the two. This report presents a more detailed comparative analysis of the content and delivery methods of lang-lit work at A level and first year BA level, identifying popular teaching and learning resources, tasks, and assessment methods. This overview is mainly grounded in data gathered from course materials, plus questionnaire responses gathered via a small survey. 20 responses to the survey were received from teachers and lecturers: these responses are not treated as representative of national opinion, but are nonetheless indicative of some widely held opinions in education at school and university. The report also presents limited anecdotal data, some of this gathered from interviews with colleagues delivering courses at both levels
CCAT
Star formation, which drives the evolution of baryonic matter in the universe, occurs in the densest regions of the interstellar medium. As a result much of the emergent short wavelength radiation, UV to near IR, is absorbed by intervening dust and reradiated at longer wavelengths, far IR and subillimeter. Indeed the energy density of post primordial extragalactic light is divided equally between these short and long wavelengths, indicating equal amounts of radiation have passed through dusty and optically transparent environments over cosmic time. Comprehensive understanding of the processes of galaxy, star, and planetary formation requires, therefore, high sensitivity and high angular resolution observations, particularly surveys, in the far IR and submillimeter. A consortium led by Cornell and Caltech with JPL is now jointly planning the construction of a 25 m diameter telescope for submillimeter astronomy on a high mountain in northern Chile. This CCAT will combine high sensitivity, a wide field of view, and a broad wavelength range to provide an unprecedented capability for deep, large area, multi-color submillimeter surveys to complement narrow field, high resolution studies with ALMA. CCAT observations will address fundamental themes in contemporary astronomy, notably the formation and evolution of galaxies, the nature of the dark matter and dark energy that comprise most of the content of the universe, the formation of stars and planets, the conditions in circumstellar disks, and the conditions during the early history of the Solar system. The candidate CCAT site, at 5600m in northern Chile, enjoys superb observing conditions. To accommodate large format bolometer cameras, CCAT is designed with a 20 arcmin field of view. CCAT will incorporate closed loop active control of its segmented primary mirror to maintain a half wavefront error of 10 μm rms or less for the entire telescope. Instrumentation under consideration includes both short (650 μm–200 μm) and long (2 mm–750 μm) wavelength bolometer cameras, direct detection spectrometers, and heterodyne receiver arrays. In addition to Cornell and Caltech with JPL, the University of Colorado, the Universities of British Columbia and of Waterloo, the UK Astronomy Technology Centre on behalf of the UK community, and the Universities of Cologne and of Bonn have joined the CCAT consortium. When complete, CCAT will be the largest and most sensitive facility of its class as well as the highest altitude astronomical facility on Earth
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