1,268 research outputs found
A Tunable Echelle Imager
We describe and evaluate a new instrument design called a Tunable Echelle
Imager (TEI). In this instrument, the output from an imaging Fabry-Perot
interferometer is cross-dispersed by a grism in one direction and dispersed by
an echelle grating in the perpendicular direction. This forms a mosaic of
different narrow-band images of the same field on a detector. It offers a
distinct wavelength multiplex advantage over a traditional imaging Fabry-Perot
device.
Potential applications of the TEI include spectrophotometric imaging and
OH-suppressed imaging by rejection.Comment: 11 pages, 12 figures, accepted by PAS
Astrophotonic micro-spectrographs in the era of ELTs
The next generation of Extremely Large Telescopes (ELT), with diameters up to
39 meters, will start opera- tion in the next decade and promises new
challenges in the development of instruments. The growing field of
astrophotonics (the use of photonic technologies in astronomy) can partly solve
this problem by allowing mass production of fully integrated and robust
instruments combining various optical functions, with the potential to reduce
the size, complexity and cost of instruments. In this paper, we focus on
developments in integrated micro-spectrographs and their potential for ELTs. We
take an inventory of the identified technologies currently in development, and
compare the performance of the different concepts. We show that in the current
context of single-mode instruments, integrated spectrographs making use of,
e.g., a photonic lantern can be a solution to reach the desired performance.
However, in the longer term, there is a clear need to develop multimode devices
to improve overall the throughput and sensitivity, while decreasing the
instrument complexity.Comment: 9 pages. 2 figures. Proceeding of SPIE 9147 "Ground-based and
Airborne Instrumentation for Astronomy V
A Technique for Narrowband Time Series Photometry: the X-ray Star V2116 Oph
We have used innovative features of the Taurus Tunable Filter instrument on
the 3.9-m Anglo-Australian Telescope to obtain nearly-continuous,
high-throughput, linear photometry of V2116 Oph in a 7 Angstrom bandpass at the
center of the O I 8446 emission line. This instrumental technique shows promise
for applications requiring precise, rapid, narrowband photometry of faint
objects. The spectrum of V2116 Oph, the counterpart of GX 1+4 (=X1728-247), is
exotic, even among the unusual spectra of other optical counterparts of compact
Galactic X-ray sources. The second strongest emission line is an unusual one,
namely extremely prominent O I 8446, which is likely to result from pumping by
an intense Ly beta radiation field. As the X-radiation from GX 1+4 is steadily
pulsed, with typical pulsed fractions of 0.4, the O I 8446 emission in V2116
Oph may also be strongly modulated with the current 127 s period of the X-ray
source. If so, this may well allow us to obtain high signal-to-noise radial
velocity measurements and thus to determine the system parameters. However, no
such pulsations are detected, and we set an upper limit of ~1% (full-amplitude)
on periodic 8446 oscillations at the X-ray frequency. This value is comparable
to the amplitude of continuum oscillations observed on some nights by other
workers. Thus we rule out an enhancement of the pulsation amplitude in O I
emission, at least at the time of our observations.Comment: 9 pages including 4 figures and no tables. Accepted for publication
in PASP; to appear in Volume 110, August 199
The Escape of Ionizing Photons from the Galaxy
The Magellanic Stream and several high velocity clouds have now been detected
in optical line emission. The observed emission measures and kinematics are
most plausibly explained by photoionization due to hot, young stars in the
Galactic disk. The highly favorable orientation of the Stream allows an
unambiguous determination of the fraction of ionizing photons, F_esc, which
escape the disk. We have modelled the production and transport of ionizing
photons through an opaque interstellar medium. Normalization to the Stream
detections requires F_esc = 6%, in reasonable agreement with the flux required
to ionize the Reynolds layer. Neither shock heating nor emission within a hot
Galactic corona can be important in producing the observed H-alpha emission. If
such a large escape fraction is typical of L_* galaxies, star-forming systems
dominate the extragalactic ionizing background. Within the context of this
model, both the three-dimensional orientation of the Stream and the distances
to high-velocity clouds can be determined by sensitive H-alpha observations.Comment: 4 pages; LaTeX2e, emulateapj.sty, apjfonts.sty; 4 encapsulated PS
figures. For correct labels, may need to print Fig. 3 separately due to psfig
limitation. Astrophysical Journal (Letters), accepte
The Smith Cloud: HI associated with the Sgr dwarf?
The Smith high velocity cloud (V(LSR) = 98 kms) has been observed at two
locations in the emission lines [OIII]5007, [NII]6548 and H-alpha. Both the
[NII] and H-alpha profiles show bright cores due to the Reynolds layer, and red
wings with emission extending to V(LSR) = 130 kms. This is the first
simultaneous detection of two emission lines towards a high velocity cloud,
allowing us to form the ratio of these line profiles as a function of LSR
velocity. At both cloud positions, we see a clear distinction between emission
at the cloud velocity, and the Reynolds layer emission (V(LSR) = 0). The
[NII]/H-alpha ratio (=0.25) for the Reynolds layer is typical of the warm
ionised medium. At the cloud velocity, this ratio is enhanced by a factor of
3-4 compared to emission at rest with respect to the LSR. A moderately deep
upper limit at [OIII] (0.12R at 3-sigma) was derived from our data. If the
emission arises from dilute photoionisation from hot young stars, the highly
enhanced [NII]/H-alpha ratio, the [OIII] non-detection and weak H-alpha
emission (0.24-0.30R) suggest that the Smith Cloud is 26+/-4 kpc from the Sun,
at a Galactocentric radius of 20+/-4 kpc. This value assumes that the emission
arises from an optically thick slab, with a covering fraction of unity as seen
by the ionizing photons, whose orientation is either (a) parallel to the
Galactic disk, or (b) such as to maximize the received flux from the disk. The
estimated mass and size of the cloud are 4x10^6 Msun and 6 kpc. We discuss a
possible association with the much larger Sgr dwarf, at a galactocentric radius
of 16+/-2 kpc, which lies within 35 degrees (~12 kpc) of the Smith Cloud.Comment: 18 pages, 14 figures, mn.sty. Our first application of a new method
for establishing distances to high velocity clouds. This version matches
paper to appear in MNRAS, 299, 611-624 (Sept. 11 issue
The Taurus Tunable Filter Field Galaxy Survey: Sample Selection and Narrowband Number-Counts
Recent evidence suggests a falling volume-averaged star-formation rate (SFR)
over z ~ 1. It is not clear, however, the extent to which the selection of such
samples influences the measurement of this quantity. Using the Taurus Tunable
Filter (TTF) we have obtained an emission-line sample of faint star-forming
galaxies over comparable lookback times: the TTF Field Galaxy Survey. By
selecting through emission-lines, we are screening galaxies through a quantity
that scales directly with star-formation activity for a given choice of initial
mass function. The scanning narrowband technique furnishes a galaxy sample that
differs from traditional broadband-selected surveys in both its volume-limited
nature and selection of galaxies through emission-line flux. Three discrete
wavelength intervals are covered, centered at H-alpha redshifts z = 0.08, 0.24
and 0.39.
Galaxy characteristics are presented and comparisons made with existing
surveys of both broadband and emission-line selection. When the number-counts
of emission-line objects are compared with those expected on the basis of
existing H-alpha surveys, we find an excess of ~ 3 times at the faintest
limits. While these detections are yet to be independently confirmed,
inspection of the stronger subsample of galaxies detected in both the line and
continuum (line-on-continuum subsample; 13 %) is sufficient to support an
excess population. This increase in the emission-line field population implies
higher star-formation densities over z ~ 0.4. However, further study in the
form of multi-object spectroscopic follow-up is necessary to quantify this and
confirm the faintest detections in the sample.Comment: 48 pages, 12 figures. To appear in the Astrophysical Journal. An
abridged version of the Abstract is shown her
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