2,319 research outputs found
Stellar populations -- the next ten years
The study of stellar populations is a discipline that is highly dependent on
both imaging and spectroscopy. I discuss techniques in different regimes of
resolving power: broadband imaging (R~4), intermediate band imaging (R~16, 64),
narrowband spectral imaging (R~256, 1024, 4096). In recent years, we have seen
major advances in broadband all-sky surveys that are set to continue across
optical and IR bands, with the added benefit of the time domain, higher
sensitivity, and improved photometric accuracy. Tunable filters and integral
field spectrographs are poised to make further inroads into intermediate and
narrowband imaging studies of stellar populations. Further advances will come
from AO-assisted imaging and imaging spectroscopy, although photometric
accuracy will be challenging. Integral field spectroscopy will continue to have
a major impact on future stellar population studies, extending into the near
infrared once the OH suppression problem is finally resolved. A sky rendered
dark will allow a host of new ideas to be explored, and old ideas to be
revisited.Comment: Invited review, IAUS 241, "Stellar Populations as Building Blocks of
Galaxies," eds. Vazdekis, Peletier. 12 pages, 1 table. (The sideways table
should print ok; there are 10 columns.
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
Episodic starbursts in dwarf spheroidal galaxies: a simple model
Dwarf galaxies in the Local Group appear to be stripped of their gas within
270 kpc of the host galaxy. Color-magnitude diagrams of these dwarfs, however,
show clear evidence of episodic star formation (\Delta{}t ~ a few Gyr) over
cosmic time. We present a simple model to account for this behaviour. Residual
gas within the weak gravity field of the dwarf experiences dramatic variations
in the gas cooling time around the eccentric orbit. This variation is due to
two main effects. The azimuthal compression along the orbit leads to an
increase in the gas cooling rate of ~([1+\epsilon]/[1-\epsilon])^2. The
Galaxy's ionizing field declines as 1/R^2 for R>R_disk although this reaches a
floor at R~150 kpc due to the extragalactic UV field ionizing intensity. We
predict that episodic SF is mostly characteristic of dwarfs on moderately
eccentric orbits (\epsilon>0.2) that do not come too close to the centre
(R>R_disk) and do not spend their entire orbit far away from the centre (R>200
kpc). Up to 40% of early infall dwarf spheroidals can be expected to have
already had at least one burst since the initial epoch of star formation, and
10% of these dwarf spheriodals experiencing a second burst. Such a model can
explain the timing of bursts in the Carina dwarf spheroidal and restrict the
orbit of the Fornax dwarf spheroidal. However, this model fails to explain why
some dwarfs, such as Ursa Minor, experience no burst post-infall.Comment: 8 pages, 8 figures. ApJ accepte
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
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