355 research outputs found
Diffuse neutral hydrogen in the HI Parkes All Sky Survey
Observations of neutral hydrogen can provide a wealth of information about
the distribution and kinematics of galaxies. To detect HI beyond the ionisation
edge of galaxy disks, column density sensitivities have to be achieved that
probe the regime of Lyman limit systems. Typically HI observations are limited
to a brightness sensitivity of NHI~10^19 cm-2 but this has to be improved by at
least an order of magnitude. In this paper, reprocessed data is presented that
was originally observed for the HI Parkes All Sky Survey (HIPASS). HIPASS
provides complete coverage of the region that has been observed for the
Westerbork Virgo Filament HI Survey (WVFS), presented in accompanying papers,
and thus is an excellent product for data comparison. The region of interest
extends from 8 to 17 hours in right ascension and from -1 to 10 degrees in
declination. Although the original HIPASS product already has good flux
sensitivity, the sensitivity and noise characteristics can be significantly
improved with a different processing method. The newly processed data has an
1sigma RMS flux sensitivity of ~10 mJy beam-1 over 26 km s-1, corresponding to
a column density sensitivity of ~3\cdot10^17 cm-2. While the RMS sensitivity is
improved by only a modest 20%, the more substantial benefit is in the reduction
of spectral artefacts near bright sources by more than an order of magnitude.
In the reprocessed region we confirm all previously catalogued HIPASS sources
and have identified 29 additional sources of which 14 are completely new HI
detections. Extended emission or companions were sought in the nearby
environment of each discrete detection. With the improved sensitivity after
reprocessing and its large sky coverage, the HIPASS data is a valuable resource
for detection of faint HI emission.(Abridged)Comment: 22 pages plus appendix, 6 figures, appendix will only appear in
online format. Accepted for publication in A&
Dissecting the IRX - dust attenuation relation: exploring the physical origin of observed variations in galaxies
The use of ultraviolet (UV) emission as a tracer of galaxy star-formation
rate (SFR) is hampered by dust obscuration. The empirical relationship between
UV slope, , and the ratio between far-infrared and UV luminosity, IRX,
is commonly employed to account for obscured UV emission. We present a simple
model that explores the physical origin of variations in the IRX - dust
attenuation relation. A relative increase in FUV attenuation compared to NUV
attenuation and an increasing stellar population age cause variations towards
red UV slopes for a fixed IRX. Dust geometry effects (turbulence, dust screen
with holes, mixing of stars within the dust screen, two-component dust model)
cause variations towards blue UV slopes. Poor photometric sampling of the UV
spectrum causes additional observational variations. We provide an analytic
approximation for the IRX - relation invoking a subset of the explored
physical processes (dust type, stellar population age, turbulence). We discuss
observed variations in the IRX - relation for local (sub-galactic
scales) and high-redshift (normal and dusty star-forming galaxies, galaxies
during the epoch of reionization) galaxies in the context of the physical
processes explored in our model. High spatial resolution imaging of the UV and
sub-mm emission of galaxies can constrain the IRX - dust attenuation
relation for different galaxy types at different epochs, where different
processes causing variations may dominate. These constraints will allow the use
of the IRX - relation to estimate intrinsic SFRs of galaxies, despite
the lack of a universal relation.Comment: Accepted for publication in MNRA
The WSRT Virgo Hi filament survey II; Cross Correlation Data
The extended environment of galaxies contains a wealth of information about
the formation and life cycle of galaxies which are regulated by accretion and
feedback processes. Observations of neutral hydrogen are routinely used to
image the high brightness disks of galaxies and to study their kinematics.
Deeper observations will give more insight into the distribution of diffuse gas
in the extended halo of the galaxies and the IGM, where numerical simulations
predict a cosmic web of extended structures and gaseous filaments. To observe
the extended environment of galaxies, column density sensitivities have to be
achieved that probe the regime of Lyman limit systems. HI observations are
typically limited to a brightness sensitivity of NHI~10^19 cm-2, but this must
be improved upon by ~2 orders of magnitude. In this paper we present the
interferometric data of the Westerbork Virgo HI Filament Survey (WVFS) - the
total power product of this survey has been published in an earlier paper. By
observing at extreme hour angles, a filled aperture is simulated of 300x25
meters in size, that has the typical collecting power and sensitivity of a
single dish telescope, but the well defined bandpass characteristics of an
interferometer. With the very good surface brightness sensitivity of the data,
we hope to make new HI detections of diffuse systems with moderate angular
resolution. The survey maps 135 degrees in Right Ascension between 8 and 17
hours and 11 degrees in Declination between -1 and 10 degrees, including the
galaxy filament connecting the Local Group with the Virgo Cluster. Only
positive declinations could be completely processed and analysed due to
projection effects. A typical flux sensitivity of 6 mJy beam-1 over 16 km s-1
is achieved, that corresponds to a brightness sensitivity of NHI~10^18 cm-2. In
total, 199 objects have been detected, of which 17 are new HI detections.
(Abridged)Comment: 19 pages plus appendix, 19 figures, appendix will only appear in
online format. Accepted for publication in A&
The Standing Wave Phenomenon in Radio Telescopes; Frequency Modulation of the WSRT Primary Beam
Inadequacies in the knowledge of the primary beam response of current
interferometric arrays often form a limitation to the image fidelity. We hope
to overcome these limitations by constructing a frequency-resolved,
full-polarization empirical model for the primary beam of the Westerbork
Synthesis Radio Telescope (WSRT). Holographic observations, sampling angular
scales between about 5 arcmin and 11 degrees, were obtained of a bright compact
source (3C147). These permitted measurement of voltage response patterns for
seven of the fourteen telescopes in the array and allowed calculation of the
mean cross-correlated power beam. Good sampling of the main-lobe, near-in, and
far-side-lobes out to a radius of more than 5 degrees was obtained. A robust
empirical beam model was detemined in all polarization products and at
frequencies between 1322 and 1457 MHz with 1 MHz resolution. Substantial
departures from axi-symmetry are apparent in the main-lobe as well as
systematic differences between the polarization properties. Surprisingly, many
beam properties are modulated at the 5 to 10% level with changing frequency.
These include: (1) the main beam area, (2) the side-lobe to main-lobe power
ratio, and (3) the effective telescope aperture. These semi-sinusoidsal
modulations have a basic period of about 17 MHz, consistent with the natural
'standing wave' period of a 8.75 m focal distance. The deduced frequency
modulations of the beam pattern were verified in an independent long duration
observation using compact continuum sources at very large off-axis distances.
Application of our frequency-resolved beam model should enable higher dynamic
range and improved image fidelity for interferometric observations in complex
fields. (abridged)Comment: 12 pages, 11 figures, Accepted for publication in A&A, figures
compressed to low resolution; high-resolution version available at:
http://www.astro.rug.nl/~popping/wsrtbeam.pd
The Molecular Gas Reservoirs of Galaxies: A comparison of CO(1-0) and dust-based molecular gas masses
We test the use of long-wavelength dust continuum emission as a molecular gas
tracer at high redshift, via a unique sample of 12, z~2 galaxies with
observations of both the dust continuum and CO(1-0) line emission (obtained
with the Atacama Large Millimeter Array and Karl G. Jansky Very Large Array,
respectively). Our work is motivated by recent, high redshift studies that
measure molecular gas masses (\ensuremath{\rm{M}_{\rm{mol}}}) via a calibration
of the rest-frame m luminosity () against the
CO(1-0)-derived \ensuremath{\rm{M}_{\rm{mol}}}\ of star-forming galaxies. We
hereby test whether this method is valid for the types of high-redshift,
star-forming galaxies to which it has been applied. We recover a clear
correlation between the rest-frame m luminosity, inferred from the
single-band, long-wavelength flux, and the CO(1-0) line luminosity, consistent
with the samples used to perform the m calibration. The molecular gas
masses, derived from , agree to within a factor of
two with those derived from CO(1-0). We show that this factor of two
uncertainty can arise from the values of the dust emissivity index and
temperature that need to be assumed in order to extrapolate from the observed
frequency to the rest-frame at 850. The extrapolation to
850 therefore has a smaller effect on the accuracy of \Mmol\
derived via single-band dust-continuum observations than the assumed
CO(1-0)-to-\ensuremath{\rm{M}_{\rm{mol}}}\ conversion factor. We therefore
conclude that single-band observations of long-wavelength dust emission can be
used to reliably constrain the molecular gas masses of massive, star-forming
galaxies at
The ESO UVES Advanced Data Products Quasar Sample - VI. Sub-Damped Lyman- Metallicity Measurements and the Circum-Galactic Medium
The Circum-Galactic Medium (CGM) can be probed through the analysis of
absorbing systems in the line-of-sight to bright background quasars. We present
measurements of the metallicity of a new sample of 15 sub-damped Lyman-
absorbers (sub-DLAs, defined as absorbers with 19.0 < log N(H I) < 20.3) with
redshift 0.584 < < 3.104 from the ESO Ultra-Violet Echelle
Spectrograph (UVES) Advanced Data Products Quasar Sample (EUADP). We combine
these results with other measurements from the literature to produce a
compilation of metallicity measurements for 92 sub-DLAs as well as a sample of
362 DLAs. We apply a multi-element analysis to quantify the amount of dust in
these two classes of systems. We find that either the element depletion
patterns in these systems differ from the Galactic depletion patterns or they
have a different nucleosynthetic history than our own Galaxy. We propose a new
method to derive the velocity width of absorption profiles, using the modeled
Voigt profile features. The correlation between the velocity width delta_V90 of
the absorption profile and the metallicity is found to be tighter for DLAs than
for sub-DLAs. We report hints of a bimodal distribution in the [Fe/H]
metallicity of low redshift (z < 1.25) sub-DLAs, which is unseen at higher
redshifts. This feature can be interpreted as a signature from the metal-poor,
accreting gas and the metal-rich, outflowing gas, both being traced by sub-DLAs
at low redshifts.Comment: 64 pages, 31 figures, 27 tables. Submitted to MNRA
Discussion on American Mutual Fund Governance from the Perspective of Conflicts of Interest and Its Experiences for China
美国的共同基金是通过以基金投资顾问为主的外部机构进行基金的管理、运作的,这种独特的外部管理结构必然带来基金投资者与基金管理者及其他相关主体之间的利益冲突。我国的证券投资基金同样也存在主体利益冲突。美国是世界上基金业最发达的国家,其解决共同基金主体利益冲突的相对规范和完善的基金治理和监管制度的安排,值得我国证券投资基金借鉴。除引言和结论外,本文分为四章:第一章,美国共同基金的基金费用引起的主体冲突分析。首先,分析了美国共同基金的12b-1费引起的主体利益冲突;其次,分析了软美元安排引起的主体利益冲突。第二章,美国共同基金运作、交易中的主体利益冲突分析。首先,分析了延时交易和择时交易引起的主体利益...American mutual fund is an investment pool organized in the form of a corporation or trust to facilitate collective investments and it managed by a professional investment manager outside the mutual fund itself, and thus causes the inherent conflicts of interest between the fund shareholders and manager. There are conflicts of interest between the fund investors and manager in chinese securities i...学位:法学硕士院系专业:法学院法律系_民商法学(含劳动法学、社会保障法学)学号:2005130040
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