last modified by NGD September 21, 2000
Planetary Nebulae (PNe) are excellent probes of a distant galaxy's rotational dynamics. Through their strong [OIII] emission line at 5007Å rest wavelength they can be easily detected and their radial velocity determined. In addition, it is now widely accepted that their luminosity in the [OIII] line (for an ensemble of PNe this is called the PNLF - Planetary Nebula Luminosity Function) has a fairly sharp bright-end cutoff, which means that they can be used as `standard candles' in determining distances. The PNS project aims to build a dedicated Planetary Nebula Spectrograph in order to push this technique to its limit. We have proven that two counter-dispersed images contain the information needed to obtain distances and dynamics in a single observation -- to our knowledge the only way of doing so!
PARTICIPANTS:
Image of the 600 g/mm gratings from Richardson Labs (Gabe Bloxham, Sep 21 2000, a t MSO).
Upon unpacking (Sep 14) Gabe found cosmetic defects, best described as several visual lines, in same direction as the rulings. One extends to about 1/2 length of the grating.
Apart from a few small spots, the cosmetic lines are visible, near the left edge on left grating, and near center on the right grating.
ON THIS PAGE:
LINKS TO FURTHER INFORMATION:
To use PNe to study galaxies one needs to detect them and
then to measure their line-of-sight velocity. Normally
these two steps are distinct (see using PNe.
We have discovered an elegant alternative in which the
detection phase can be skipped entirely. The
galaxy field of interest is simply imaged through a
slitless spectrograph tuned
to the O[III] line as shown in the lower half of the figure.
When this image is studied, the background light of the galaxy and the
Before the night is out, a second image is made but with the spectrograph
rotated by 180o (upper half of figure). The same
PNe will be readily identified in this second image, but with
the sense of their displacement reversed. Obviously, these two images
taken together yield (1) position and (2) velocity, while the
(3) brightness can be obtained from either image. So in one night
we can do as well as, or better than, the traditional
procedure does in two or three.
DEMONSTRATION:
The following pair of images illustrate the idea (these
data were taken at the WHT in April 1997). Click on them to
`blink' them. It will be obvious that the displacement of
various PNe differs (notice especially how the shape of
groups of objects changes). These differences arise because the PNe
have different velocities.
What are the merits of the PNS idea over other techniques?
OBJECTS:
How many early-type galaxies out to 15 Mpc, or 25 Mpc for
dynamical studies, are out there?
Just how reliable is the PNLF as a distance indicator? Don't
take our word for it, but look at what Robin Ciardullo, one of
the best-known workers in the field, has to say
(click here).
Note that [OIII] observations alone do not provide
sufficient discrimination against HII regions to prevent
contamination of the PNLF. The PNLF method can be
applied to spiral galaxies (read this Ap.J.
abstract)
but only if auxiliary measurements are made. Distance work
with the basic PNS is
therefore limited to early-type galaxies.
Where do PNe distances fit in with other distance indicators?
Ciardullo provides the following
schematic.
To what distance will reliable results be obtained?
The bright end of the
PNLF is unambiguously measured when the `flat' portion of the
luminosity function can be fitted. This requires completeness down
to approximately one magnitude below the brightest PNe (`completeness'
here means freedom from flux-dependent selection). According to
the simulations done this occurs for D=15 Mpc in 7h observing
with a 4m-class telescope.
Observations to date suggest that velocities in the range
± 500 km/s will be encountered so a filter bandpass of at least
17Å is indicated. In the PERFORMANCE table given above the PNe
images were assumed to be located to an accuracy of 0.5 pixel.
Simulations show that 0.2 pixel can be expected. aking these
values as limits,
radial velocities will be obtained with a precision of between
15 and 40 km/s.
To what distance, and how far from the
galaxy centre, will reliable results be obtained?
For dynamical information completeness of the PNe sample
(and for that matter contamination by possible HII regions
or SNR) is less of an issue than for PNLF-distance determination.
Simulations show that good results will be obtained to
D=25Mpc in a single night of observing.
The Planetary Nebulae Spectrograph
M. Arnaboldi, M. Capaccioli, N. Douglas, K. Kuijken, K. Freeman, T. Axelrod, K. Taylor,
R. Gilmozzi and R. Kudritzki
PN.S Version 3 (for TNG and WHT) is being built under various
contracts. The optics have been delivered but not tested or coated.
The housing, and some of the mechanical components, have been completed
at ASTRON in Dwingeloo and will be shipped to Mt Stromlo, where assembly
will take place. Also in Mt Stromlo, other mechanical components have
been made. The gratings have been ordered, filters are still under
study. The carrying out of electrical work, interfacing, and the making
of the telescope flanges, are all to be decided.
CONTACTING US FOR FURTHER INFORMATION:
email:
kt@astro.caltech.edu
Prime Optics is based in Eumundi, Queensland,
Australia: 
click to enlarg$
images of foreground stars will be found to be blurred, but the PNe are
instantly recognisable as bright point-like images. This is because
they, and perhaps an occasional giant cloud of ionised gas, are the only
objects with a powerful emission line at this wavelength. The PNe
will be slightly displaced from their true position
on the sky by an amount determined by their exact wavelength, and hence by
their velocity.
Here is a glance at the
RSAA optical and mechanical workshop team
which is currently working on detailed aspects
of the instrument design and construction.
From left to right: Julia Hu, John Hart, Gabe Bloxham
Standing on the balcony of the
Kapteyn Institute in Groningen
are Nigel Douglas (left) and Koen Kuijken.
Here is a picture of part of the spectrograph
housing, work in progress at NFRA (Dwingeloo).
From left to right: Jos de Haas, Jan Idserda, Jaap Bakker
magda@cerere.na.astro.it
ndouglas@astro.rug.nl,
home page
kuijken@astro.rug.nl
kcf@mso.anu.edu.au
michael.merrifield@nottingham.ac.uk
tsa@merlin.anu.edu.au
rgilmozz@eso.org
whole team
tel +61(07)5442 8831, fax +61(07)5442 8804,
djajones@ozemail.com.au
