674 research outputs found
Measuring Outer Disk Warps with Optical Spectroscopy
Warps in the outer gaseous disks of galaxies are a ubiquitous phenomenon, but
it is unclear what generates them. One theory is that warps are generated
internally through spontaneous bending instabilities. Other theories suggest
that they result from the interaction of the outer disk with accreting
extragalactic material. In this case, we expect to find cases where the
circular velocity of the warp gas is poorly correlated with the rotational
velocity of the galaxy disk at the same radius. Optical spectroscopy presents
itself as an interesting alternative to 21-cm observations for testing this
prediction, because (i) separating the kinematics of the warp from those of the
disk requires a spatial resolution that is higher than what is achieved at 21
cm at low HI column density; (ii) optical spectroscopy also provides important
information on star formation rates, gas excitation, and chemical abundances,
which provide clues to the origin of the gas in warps. We present here
preliminary results of a study of the kinematics of gas in the outer-disk warps
of seven edge-on galaxies, using multi-hour VLT/FORS2 spectroscopy.Comment: 7 pages, 7 figures; to appear in the proceedings of IAU Symposium 254
"The Galaxy disk in a cosmological context", Copenhagen, June 200
The Galaxy in Context: Structural, Kinematic and Integrated Properties
Our Galaxy, the Milky Way, is a benchmark for understanding disk galaxies. It
is the only galaxy whose formation history can be studied using the full
distribution of stars from white dwarfs to supergiants. The oldest components
provide us with unique insight into how galaxies form and evolve over billions
of years. The Galaxy is a luminous (L-star) barred spiral with a central
box/peanut bulge, a dominant disk, and a diffuse stellar halo. Based on global
properties, it falls in the sparsely populated "green valley" region of the
galaxy colour-magnitude diagram. Here we review the key integrated, structural
and kinematic parameters of the Galaxy, and point to uncertainties as well as
directions for future progress. Galactic studies will continue to play a
fundamental role far into the future because there are measurements that can
only be made in the near field and much of contemporary astrophysics depends on
such observations.Comment: 69 pages, 18 figures, LaTeX. See
http://www.physics.usyd.edu.au/~jbh/S/ARAA_2016.pdf for published versio
Microslit Nod-shuffle Spectroscopy - a technique for achieving very high densities of spectra
We describe a new approach to obtaining very high surface densities of
optical spectra in astronomical observations with extremely accurate
subtraction of night sky emission. The observing technique requires that the
telescope is nodded rapidly between targets and adjacent sky positions; object
and sky spectra are recorded on adjacent regions of a low-noise CCD through
charge shuffling. This permits the use of extremely high densities of small
slit apertures (`microslits') since an extended slit is not required for sky
interpolation. The overall multi-object advantage of this technique is as large
as 2.9x that of conventional multi-slit observing for an instrument
configuration which has an underfilled CCD detector and is always >1.5 for high
target densities. The `nod-shuffle' technique has been practically implemented
at the Anglo-Australian Telescope as the `LDSS++ project' and achieves
sky-subtraction accuracies as good as 0.04%, with even better performance
possible. This is a factor of ten better than is routinely achieved with
long-slits. LDSS++ has been used in various observational modes, which we
describe, and for a wide variety of astronomical projects. The nod-shuffle
approach should be of great benefit to most spectroscopic (e.g. long-slit,
fiber, integral field) methods and would allow much deeper spectroscopy on very
large telescopes (10m or greater) than is currently possible. Finally we
discuss the prospects of using nod-shuffle to pursue extremely long
spectroscopic exposures (many days) and of mimicking nod-shuffle observations
with infrared arrays.Comment: Accepted for publication in PASP; 25 pages, 12 figures. A
higher-quality compressed Postscript file (2.2Mb) is available from
http://www.pha.jhu.edu/~kgb/papers/nodshuffle2000hq.ps.g
Astrophysical signatures of leptonium
More than 10^43 positrons annihilate every second in the centre of our Galaxy
yet, despite four decades of observations, their origin is still unknown. Many
candidates have been proposed, such as supernovae and low mass X-ray binaries.
However, these models are difficult to reconcile with the distribution of
positrons, which are highly concentrated in the Galactic bulge, and therefore
require specific propagation of the positrons through the interstellar medium.
Alternative sources include dark matter decay, or the supermassive black hole,
both of which would have a naturally high bulge-to-disc ratio.
The chief difficulty in reconciling models with the observations is the
intrinsically poor angular resolution of gamma-ray observations, which cannot
resolve point sources. Essentially all of the positrons annihilate via the
formation of positronium. This gives rise to the possibility of observing
recombination lines of positronium emitted before the atom annihilates. These
emission lines would be in the UV and the NIR, giving an increase in angular
resolution of a factor of 10^4 compared to gamma ray observations, and allowing
the discrimination between point sources and truly diffuse emission.
Analogously to the formation of positronium, it is possible to form atoms of
true muonium and true tauonium. Since muons and tauons are intrinsically
unstable, the formation of such leptonium atoms will be localised to their
places of origin. Thus observations of true muonium or true tauonium can
provide another way to distinguish between truly diffuse sources such as dark
matter decay, and an unresolved distribution of point sources.Comment: Accepted for publication in EPJ-D, 9 pages, 4 figure
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