92,391 research outputs found
The Kinematic Properties of the Extended Disks of Spiral Galaxies: A Sample of Edge-On Galaxies
We present a kinematic study of the outer regions (R_25<R<2 R_25) of 17
edge-on disk galaxies. Using deep long-slit spectroscopy (flux sensitivity a
few 10^-19 erg s^-1 cm^-2 arcsec^-2), we search for H-alpha emission, which
must be emitted at these flux levels by any accumulation of hydrogen due to the
presence of the extragalactic UV background and any other, local source of UV
flux. We present results from the individual galaxy spectra and a stacked
composite. We detect H-alpha in many cases well beyond R_25 and sometimes as
far as 2 R_25. The combination of sensitivity, spatial resolution, and
kinematic resolution of this technique thus provides a powerful complement to
21-cm observations. Kinematics in the outer disk are generally disk-like (flat
rotation curves, small velocity dispersions) at all radii, and there is no
evidence for a change in the velocity dispersion with radius. We place strong
limits, few percent, on the existence of counter-rotating gas out to 1.5 R_25.
These results suggest that thin disks extend well beyond R_25; however, we also
find a few puzzling anomalies. In ESO 323-G033 we find two emission regions
that have velocities close to the systemic velocity rather than the expected
rotation velocity. These low relative velocities are unlikely to be simply due
to projection effects and so suggest that these regions are not on disk-plane,
circular orbits. In MCG-01-31-002 we find emission from gas with a large
velocity dispersion that is co-rotating with the inner disk.Comment: 18 pages, 14 figures, accepted for publication in Ap
A multi-method approach to radial-velocity measurement for single-object spectra
The derivation of radial velocities from large numbers of spectra that
typically result from survey work, requires automation. However, except for the
classical cases of slowly rotating late-type spectra, existing methods of
measuring Doppler shifts require fine-tuning to avoid a loss of accuracy due to
the idiosyncrasies of individual spectra. The radial velocity spectrometer
(RVS) on the Gaia mission, which will start operating very soon, prompted a new
attempt at creating a measurement pipeline to handle a wide variety of spectral
types.
The present paper describes the theoretical background on which this software
is based. However, apart from the assumption that only synthetic templates are
used, we do not rely on any of the characteristics of this instrument, so our
results should be relevant for most telescope-detector combinations.
We propose an approach based on the simultaneous use of several alternative
measurement methods, each having its own merits and drawbacks, and conveying
the spectral information in a different way, leading to different values for
the measurement. A comparison or a combination of the various results either
leads to a "best estimate" or indicates to the user that the observed spectrum
is problematic and should be analysed manually.
We selected three methods and analysed the relationships and differences
between them from a unified point of view; with each method an appropriate
estimator for the individual random error is chosen. We also develop a
procedure for tackling the problem of template mismatch in a systematic way.
Furthermore, we propose several tests for studying and comparing the
performance of the various methods as a function of the atmospheric parameters
of the observed objects. Finally, we describe a procedure for obtaining a
knowledge-based combination of the various Doppler-shift measurements.Comment: 16 pages, 4 figure
Short-duration lensing events: I. wide-orbit planets? free-floating low-mass objects? or high-velocity stars?
Short duration lensing events tend to be generated by low-mass lenses or by
lenses with high transverse velocities. Furthermore, for any given lens mass
and speed, events of short duration are preferentially caused by nearby lenses
(mesolenses) that can be studied in detail, or else by lenses so close to the
source star that finite-source-size effects may be detected, yielding
information about both the Einstein ring radius and the surface of the lensed
star. Planets causing short-duration events may be in orbits with any
orientation, and may have semimajor axes smaller than an AU, or they may reach
the outer limits of their planetary systems, in the region corresponding to the
Solar System's Oort Cloud. They can have masses larger than Jupiter's or
smaller than Pluto's. Lensing therefore has a unique potential to expand our
understanding of planetary systems. A particular advantage of lensing is that
it can provide precision measurements of system parameters, including the
masses of and projected separation between star and planet. We demonstrate how
the parameters can be extracted and show that a great deal can be learned. For
example, it is remarkable that the gravitational mass of nearby free-floating
planet-mass lenses can be measured by complementing observations of a
photometric event with deep images that detect the planet itself. A fraction of
short events may be caused by high-velocity stars located within a kpc. Many
high-velocity lenses are likely to be neutron stars that received large natal
kicks. Other high-speed stars may be members of the halo population. Still
others may be hypervelocity stars that have been ejected from the Galactic
Center, or runaway stars escaped from close binaries, possibly including the
progenitor binaries of Type Ia supernovae.Comment: 17 pages; 2 figures; submitted to ApJ 3 July 200
Deficient Reasoning for Dark Matter in Galaxies
Astronomers have been using the measured luminosity to estimate the {\em
luminous mass} of stars, based on empirically established mass-to-light ratio
which seems to be only applicable to a special class of stars---the
main-sequence stars---with still considerable uncertainties. Another basic tool
to determine the mass of a system of stars or galaxies comes from the study of
their motion, as Newton demonstrated with his law of gravitation, which yields
the {\em gravitational mass}. Because the luminous mass can at best only
represent a portion of the gravitational mass, finding the luminous mass to be
different or less than the gravitational mass should not be surprising. Using
such an apparent discrepancy as a compelling evidence for the so-called dark
matter, which has been believed to possess mysterious nonbaryonic properties
and present a dominant amount in galaxies and the universe, seems to be too far
a stretch when seriously examining the facts and uncertainties in the
measurement techniques. In our opinion, a galaxy with star type distribution
varying from its center to edge may have a mass-to-light ratio varying
accordingly. With the thin-disk model computations based on measured rotation
curves, we found that most galaxies have a typical mass density profile that
peaks at the galactic center and decreases rapidly within of the
cut-off radius, and then declines nearly exponentially toward the edge. The
predicted mass density in the Galactic disk is reasonably within the reported
range of that observed in interstellar medium. This leads us to believe that
ordinary baryonic matter can be sufficient for supporting the observed galactic
rotation curves; speculation of large amount of non-baryonic matter may be
based on an ill-conceived discrepancy between gravitational mass and luminous
mass which appears to be unjustified
Effects of Lens Motion and Uneven Magnification on Image Spectra
Counter to intuition, the images of an extended galaxy lensed by a moving
galaxy cluster should have slightly different spectra in any metric gravity
theory. This is mainly for two reasons. One relies on the gravitational
potential of a moving lens being time-dependent (the
, ). The other is due to uneven
magnification across the extended, rotating source (the
, ). The time delay between
the images can also cause their redshifts to differ because of cosmological
expansion. This Differential Expansion Effect is likely to be small. Using a
simple model, we derive these effects from first principles.
One application would be to the Bullet Cluster, whose large tangential
velocity may be inconsistent with the paradigm. This velocity can
be estimated with complicated hydrodynamic models. Uncertainties with such
models can be avoided using the MCE. We argue that the MCE should be observable
with ALMA.
However, such measurements can be corrupted by the DME if typical spiral
galaxies are used as sources. Fortunately, we find that if detailed spectral
line profiles were available, then the DME and MCE could be distinguished. It
might also be feasible to calculate how much the DME should affect the mean
redshift of each image. Resolved observations of the source would be required
to do this accurately.
The DME is of order the source angular size divided by the Einstein radius
times the redshift variation across the source. Thus, it mostly affects nearly
edge-on spiral galaxies in certain orientations. This suggests that observers
should reduce the DME by careful choice of target, a possibility we discuss in
some detail.Comment: 15 pages, 8 figures, 2 tables. This is the peer-reviewed version
which has been accepted for publication in Monthly Notices of the Royal
Astronomical Societ
The peculiar motions of early-type galaxies in two distant regions - II. The spectroscopic data
We present the spectroscopic data for the galaxies studied in the EFAR
project, which is designed to measure the properties and peculiar motions of
early-type galaxies in two distant regions. We have obtained 1319 spectra of
714 early-type galaxies over 33 observing runs on 10 different telescopes. We
describe the observations and data reductions used to measure redshifts,
velocity dispersions and the Mgb and Mg2 Lick linestrength indices. Detailed
simulations and intercomparison of the large number of repeat observations lead
to reliable error estimates for all quantities. The measurements from different
observing runs are calibrated to a common zeropoint or scale before being
combined, yielding a total of 706 redshifts, 676 velocity dispersions, 676 Mgb
linestrengths and 582 Mg2 linestrengths. The median estimated errors in the
combined measurements are dcz=20 km/s, dsigma/sigma=9.1%, dMgb/Mgb=7.2% and
dMg2=0.015 mag. Comparison of our measurements with published datasets shows no
systematic errors in the redshifts or velocity dispersions and only small
zeropoint corrections to bring our linestrengths onto the standard Lick system.
We have assigned galaxies to physical clusters by examining the line-of-sight
velocity distributions based on EFAR and ZCAT redshifts, together with the
projected distributions on the sky. We derive mean redshifts and velocity
dispersions for these clusters, which will be used in estimating distances and
peculiar velocities and to test for trends in the galaxy population with
cluster mass. The spectroscopic parameters presented here for 706 galaxies
combine high quality data, uniform reduction and measurement procedures, and
detailed error analysis. They form the largest single set of velocity
dispersions and linestrengths for early-type galaxies published to date.Comment: 27 pages, 18 figures, accepted by MNRA
Differential Proper-Motion Study of the Circumstellar Dust Shell of the Enigmatic Object, HD 179821
HD179821 is an enigmatic evolved star that possesses characteristics of both
a post-asymptotic giant branch star and a yellow hyper-giant, and there has
been no evidence that unambiguously defines its nature. These two hypotheses
are products of an indeterminate distance, presumed to be 1 kpc or 6 kpc. We
have obtained the two-epoch Hubble Space Telescope WFPC2 data of its
circumstellar shell, which shows multiple concentric arcs extending out to
about 8 arcsec. We have performed differential proper-motion measurements on
distinct structures within the circumstellar shell of this mysterious star in
hopes of determining the distance to the object, and thereby distinguishing the
nature of this enigmatic stellar source. Upon investigation, rather than
azimuthal radially symmetric expansion, we discovered a bulk motion of the
circumstellar shell of (2.41+-0.43, 2.97+-0.32) mas/yr. This corresponded to a
translational ISM flow of (1.28+-0.95, 7.27+-0.75) mas/yr local to the star.
This finding implies that the distance to HD 179821 should be rather small in
order for its circumstellar shell to preserve its highly intact spherical
structure in the presence of the distorting ISM flow, therefore favoring the
proposition that HD 179821 is a post-AGB object.Comment: Accepted for publication in ApJ
Treatment of biodiesel wastewater using ferric chloride and ferric sulfate
The production of biodiesel through the transesterification method produces a large amount of wastewater that contains high level of chemical oxygen demand (COD) and oil and grease. In this study, coagulation was adopted to treat the biodiesel wastewater. Two types of coagulants were examined using standard jar test apparatus, i.e. ferric chloride and ferric sulfate. The effects of pH and coagulant dosage were examined at 150 rpm of rapid mixing and 20 rpm slow mixing and 30 min settling time, higher removal of SS (over 80%), colour (over 80%), COD (over 50%) and Oil and Grease (over 90%) were achieved at pH 6. Ferric Chloride was found to be superior was observed at reasonable lower amount of coagulant i.e. 300 mg/L. The result indicated that coagulation and flocculation process had contributed bigger roles in integrated treatment system
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