2,735 research outputs found
Faint star counts in the near-infrared
We discuss near-infrared star counts at the Galactic pole with a view to
guiding the NGST and ground-based NIR cameras. Star counts from deep K-band
images from the CFHT are presented, and compared with results from the 2MASS
survey and some Galaxy models. With appropriate corrections for detector
artifacts and galaxies, the data agree with the models down to K~18, but
indicate a larger population of fainter red stars. There is also a significant
population of compact galaxies that extend to the observational faint limit of
K=20.5. Recent Galaxy models agree well down to K19, but diverge at
fainter magnitudes.Comment: 14 pages and 4 diagrams; to appear in PAS
Near Infrared Imaging of the Proposed z=2 Cluster Behind CL 0939+4713 (Abell 851)
We have obtained deep J and K' images of a 90 arcsec square field containing
the z=2.0 QSO that is viewed through the z=0.4 rich cluster A851. Brightnesses
of individual galaxies are measured from these data, and archival F702W WFPC2
images from HST. The results are used to construct 2-colour diagrams,
colour-magnitude diagrams, and luminosity functions of galaxies in A851 and the
faint objects discovered by Dressler et al (1993) near the QSO. The QSO faint
companions appear to form a separate population from other faint galaxies in
the field. Comparisons with the GISSEL models covering redshifts from 0 to 3
indicate that (1) the A851 galaxies have ages between 3 and 10 Gyr, and (2) if
they are at the QSO redshift, the QSO companion galaxies are within 1Gyr of
cessation of star-formation.Comment: 12 pages plus 1 table, 5 figures. To appear in PASP. Also available
at http://www.hia.nrc.ca/eprints.htm
Low temperature specific heat and possible gap to magnetic excitations in the Heisenberg pyrochlore antiferromagnet Gd2Sn207
The Gd2Sn2O7 pyrochlore Heisenberg antiferromagnet displays a phase
transition to a four sublattice Neel ordered state at a temperature near 1 K.
Despite the seemingly conventional nature of the ordered state, the specific
heat has been found to be described in the temperature range 350-800 mK by an
anomalous T-squared power law. A similar temperature dependence has also been
reported for Gd2Ti2O7, another pyrochlore Heisenberg material. Such anomalous
T-squared behavior in Cv has been argued to be correlated to an unusual
energy-dependence of the density of states which also seemingly manifests
itself in low-temperature spin fluctuations found in muon spin relaxation
experiments. In this paper, we report calculations of Cv that consider spin
wave like excitations out of the Neel order observed in Gd2Sn2O7 and argue that
the parametric T-squared behavior does not reflect the true low-energy
excitations of Gd2Sn2O7. Rather, we find that the low-energy excitations of
this material are antiferromagnetic magnons gapped by single-ion and dipolar
anisotropy effects, and that the lowest temperature of 350 mK considered in
previous specific heat measurements accidentally happens to coincide with a
crossover temperature below which magnons become thermally activated and Cv
takes an exponential form. We argue that further specific heat measurements
that extend down to at least 100 mK are required in order to ascribe an
unconventional description of magnetic excitations out of the ground state of
Gd2Sn2O7 or to invalidate the standard picture of gapped excitations proposed
herein.Comment: 12 pages, 13 figures; shortened introduction and added 1 figur
A comparative HST imaging study of the host galaxies of radio-quiet quasars, radio-loud quasars and radio galaxies: Paper I
We present the first results from a major HST WFPC2 imaging study aimed at
providing the first statistically meaningful comparison of the morphologies,
luminosities, scalelengths and colours of the host galaxies of radio-quiet
quasars, radio-loud quasars, and radio galaxies. We describe the design of this
study and present the images which have been obtained for the first half of our
33-source sample. We find that the hosts of all three classes of luminous AGN
are massive elliptical galaxies, with scalelengths ~=10 kpc, and R-K colours
consistent with mature stellar populations. Most importantly this is the the
first unambiguous evidence that, just like radio-loud quasars, essentially all
radio-quiet quasars brighter than M_R = -24 reside in massive ellipticals. This
result removes the possibility that radio `loudness' is directly linked to host
galaxy morphology, but is however in excellent accord with the
black-hole/spheroid mass correlation recently highlighted by Magorrian et al.
(1998). We apply the relations given by Magorrian et al. to infer the expected
Eddington luminosity of the putative black hole at the centre of each of the
spheroidal host galaxies we have uncovered. Comparison with the actual nuclear
R-band luminosities suggests that the black holes in most of these galaxies are
radiating at a few percent of the Eddington luminosity; the brightest host
galaxies in our low-z sample are capable of hosting quasars with M_R = -28,
comparable to the most luminous quasars at z = 3. Finally we discuss our
host-derived black-hole masses in the context of the
radio-luminosity:black-hole mass correlation recently uncovered for nearby
galaxies by Franceschini et al. (1998), and the resulting implications for the
physical origin of radio loudness.Comment: Submitted for publication in the Astrophysical Journal, 55 pages of
latex, plus 12 postscript figures (Figures 1a-1s (greyscales of images and
model fits, and Figures 2a-2g (luminosity profiles and model fits) can be
downloaded from http://www.roe.ac.uk/astronomy/html/rjm1.shtml
Magellanic Cloud X-ray Sources: III. Completion of a ROSAT Survey
This paper concludes a series of three papers presenting ROSAT
High-Resolution Imager (HRI) observations of unidentified Einstein and
serendipitous ROSAT X-ray sources in the direction of the Magellanic Clouds.
Accurate positions and fluxes have been measured for these sources. Optical
photometry and spectroscopy were obtained to search for identifications in
order to determine the physical nature of these sources. The present paper
includes new data for 24 objects; identifications are given or confirmed for 30
sources. For six sources optical finding charts showing the X-ray positions are
provided. The results from this program are summarized, showing the populations
of luminous X-ray sources in the Magellanic Clouds are quite different from
those in the Galaxy.Comment: 28 pages, 2 figures; to appear in Astronomical Journa
A comparison of cultivation techniques for successful tree establishment on compacted soil
Soil compaction is often responsible for the poor establishment of trees on restored brownfield sites. This paper examines the root development, survival and growth of Alnus cordata, Larix kaempferi, Pinus nigra and Betula pendula after cultivation with complete cultivation, a standard industrial ripper and a prototype ripper. The industrial ripper was used in one pass across the experimental plots and the prototype ripper in both two and four passes. While the maximum root depths, after five growing seasons, attained by trees were similar to the target soil loosening depths for the cultivation techniques, the total number of roots suggests that root development was not uniform across the soil profile. All treatments significantly increased both the maximum root depth and total number of roots compared with the untreated control; the complete cultivation had approximately double the number of roots compared with the other treatments. Larger average root diameters and a higher percentage of coarse roots also suggest that roots experienced physical restriction in the control, two-pass prototype and industrial ripper plots. Similarly, while all species had attained significantly greater height growth on the treated soils compared with the control, the height of A. cordata, L. kaempferi and B. pendula was greatest after complete cultivation. The results demonstrate that complete cultivation is the most effective method of alleviating soil compaction for tree establishment. © Institute of Chartered Foresters, 2008. All rights reserved
Simultaneous Ultraviolet and X-ray Observations of the Seyfert Galaxy NGC 4151. I. Physical Conditions in the X-ray Absorbers
We present a detailed analysis of the intrinsic X-ray absorption in the
Seyfert 1 galaxy NGC 4151 using Chandra/HETGS data obtained 2002 May, as part
of a program which included simultaneous UV spectra using HST/STIS and FUSE.
NGC 4151 was in a relatively low flux state during the observations reported
here, although roughly 2.5 times as bright in the 2 --10 keV band as during a
Chandra observation in 2000. The soft X-ray band was dominated by emission
lines, which show no discernible variation in flux between the two
observations. The 2002 data show the presence of a very highly ionized
absorber, in the form of H-like and He-like Mg, Si, and S lines, as well as
lower ionization gas via the presence of inner-shell absorption lines from
lower-ionization species of these elements. The former is too highly ionized to
be radiatively accelerated in a sub-Eddington source such as NGC 4151. We find
that the lower ionization gas had a column density a factor of ~ 3 higher
during the 2000 observation. If due to bulk motion, we estimate that this
component must have a velocity of more than 1250 km/sec transverse to our
line-of-sight. We suggest that these results are consistent with a
magneto-hydrodynamic flow.Comment: 42 pages, 14 figures. Accepted for publication in The Astrophysical
Journa
Magnetic Behavior in RRhX (R = rare earths; X=B, C) Compounds
We report on the magnetic behavior of RRhB (R = La, Ce, Pr, Nd, Gd, Tb and
Tm) and RRhC (R = La, Ce, Pr and Gd) compounds crystallizing in the cubic
perovskite type structure with space group Pm3m. The heat capacity data on
Pauli-paramagnetic LaRhB and LaRhC indicate a high frequency vibrating motion
of boron and carbon atoms in the unit cell. Ce is in -like nonmagnetic state in
both the compounds. Pr compounds show a dominant crystal field effect with a
nonmagnetic singlet ground state in PrRhB and a nonmagnetic quadrupolar doublet
in PrRhC. Compounds with other rare earths order ferromagnetically at low
temperatures except TmRhB in which the zero field evolution of magnetic
interactions is relatively more complicated. The electrical resistivity of
GdRhB decreases with increasing temperature in the paramagnetic state in the
vicinity of T, which is rarely seen in ferromagnets. The behavior is discussed
to be arising due to the short range spin fluctuation and a possible
contribution from Fermi surface geometry.Comment: 14 Figs and a text fil
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