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The new AAO/UKST H-alpha survey
Test H-alpha image of the Vela Supernova remnant
The negative image shown was taken from a 2 hour test H-alpha exposure of the Vela Supernova remnant using the new H-alpha interference filter and Tech-Pan emulsion as the detector. The scanned image is 3.5 x 4.7 degrees with the top left hand corner being in the North-East and viewed `emulsion up'.
A UKST H-alpha
survey of the Galactic plane & Magellanic Clouds
The need
for the survey
The advantages
of Tech-Pan film & the UKST
Tie-ins with
other surveys
The scientific
aims
Survey planning
and logistics
Survey Availability
& Announcement of Opportunity
Map
of AAO/UKST H-alpha survey area
Survey availability
& Announcement of opportunity
International
Workshop, Sydney (held April 16-18th 1997)
The high quality
H-alpha Interference filter
Preliminary
H-alpha image comparisons
First
DISCOVERIES from the survey
The Consortium
UKST H-alpha survey of the Galactic plane & Magellanic Clouds
The UK Schmidt Telescope (UKST) of the Anglo-Australian Observatory (AAO) has embarked on a new H-alpha survey of the Southern Galactic Plane, Magellanic Clouds and selected regions using a specially designed high specification, monolithic interference filter which is probably the largest of its kind in use for astronomy. It is being used in combination with Kodak Tech-Pan film-based emulsion which not only has a useful sensitivity peak at H-alpha but also posseses extremely fine grain and an exceptionally high DQE for the hypersentised product of ~10%. This leads to excellent imaging, sensitivity and low noise. It is clear that CCDs cannot yet match the wide-area coverage, uniformity and resolution of the UKST/Tech Pan combination for undertaking a Galactic Plane H-alpha survey. The survey will initially include 233 4-degree field centres and will take about 3 years to complete. Some preliminary images from the new survey are presented and compared with the best previously available from the UKST. A survey of unprecedented area coverage, depth and resolution should result, superior to any previous optical survey of ionized gas in the galaxy. Discoveries are already being made and new research avenues are expected. The survey commenced in earnest in July 1997 and is expected to take 3 years.
The need for the survey
Despite the importance of star formation in our own galaxy, its variation between galaxies and the need to understand the resultant gaseous emission line structures visible on a wide range of angular scales, little survey work has been undertaken in a form that combines both large area coverage, high sensitivity and good resolution. The nearest star forming complexes may lie as close as 100pc with sizes of tens of parsecs. Such structures often subtend angular sizes of a degree or more yet exhibit fine detail at arc-second level. To study the interaction of these ionized structures with their large scale environment we need surveys of considerable extent at good angular resolution. Most work to date has concentrated on relatively small regions for specific study at high spatial or velocity resolution or very large areas at low resolution. It is only now that surveys are beginning to simultaneously tackle issues of coverage, sensitivity and resolution in either a spatial sense (our survey) or in velocity space (e.g. the Reynolds et al `WHAM' survey) There is a clear need for a high angular resolution optical survey to complement studies at other wavebands. The only existing UK Schmidt Telescope (UKST) wide area H-alpha survey work dates to the late 1970's (Davis, Elliot & Meaburn, 1976) using mainly coarse grained (though fast) 098 emulsion and a far from optimum filter. Many parts of the Galactic plane are unsurveyed at decent resolutions, particularly the outer extensions beyond a few degrees from the Galactic equator, whilst the Northern Milky-Way above Dec -20degrees has not been covered at all. Progress in other wavebands highlights the paucity of the optical counterpart for the detailed study of Galactic gas.Table 1 gives a list of the various H-alpha surveys currently underway and known to the authors together with the KYOTO survey (Kogure et al. 1982) as an example from the older photographic work.
Our new survey will clearly contribute much to the detailed investigation of star formation and the general ISM in terms of its coverage and resolution.
The advantages of Tech Pan film and the UKST
The successful implementation of high resolution, panchromatic Tech-Pan film on the UKST, coupled with its peak sensitivity at H-alpha was a prime motivation behind the new Galactic Plane survey. Generally speaking we have found that in good seeing Tech-Pan UKST `R' band exposures go about 1 magnitude deeper than the equivalent standard IIIaF `R' band images with improved imaging, resolution and lower noise characteristics. It is an ideal wide-field photographic detector for use with an H-alpha filter.Figure 1 gives the IIIaF (left) and Tech-Pan (right) emulsion sensitivities as a function of wavelength through a 100Angstrom H-alpha filter as obtained with a calibration spectrograph. Identical aperture settings and exposure times (180mins) were used for both emulsion samples. The figures basically represent flux trasmitted through the narrow-band filter and recorded by each emulsion. These results convinced us there was no speed, sensitivity or reciprocity penalty in adopting Tech-Pan for use with a narrow-band H-alpha filter c.f. IIIaF. the tick mark is at a wavelength of 5460Angstroms whilst the right hand edge of both sensitivity curves is close to 7000Angstroms.
Figure 1. IIIaF versus Tech-Pan H-alpha sensitivity curves
The nearest star forming complexes may lie as close as 100pc with sizes of tens of parsecs. Such structures often present large angular sizes (a degree or more) yet exhibit fine detail at arc-second level. To study the interaction of ionized structures with their large scale environment we need surveys of considerable extent at good resolution. CCDs cannot yet match the wide-area coverage, uniformity and resolution of the UKST/Tech Pan combination. A wide angle, yet deep H-alpha survey of the Galactic plane is particularly suited to the UKST and Tech Pan film.
Tech-Pan's superiority can also be seen in Figure 2a-d, a set of 4.4x3arcmin indentical sky regions taken from near the field centre of 4 UKST exposures of standard galactic plane survey field 213. The top left image (figure 2a) is from the A-grade survey R plate taken with IIIaF emulsion and the OG590 R-band filter with 63mins exposure and a visual point source detection limit of R~21.5. The top right is from the equivalent exposure taken with Tech-Pan and the new H-alpha filter with 120mins exposure (figure 2b). The bottom left (figure 2c) is the short red A-grade survey plate taken with 098-04 fast but coarser grained emulsion with a narrower 630 red filter (600A) in 10minutes exposure whilst the bottom left (figure 2d) gives the A-grade `I' survey plate taken with IV-N emulsion with a 715 filter and 90minutes exposure.
Figure 2a-d. Emulsion/filter combination imaging comparisons
Prior to the availability of quantitative H-alpha data from SuperCOSMOS measuring machine scans of the new films, simple visual examination of these exposures can be made. It is estimated that for point source detection the H-alpha 120 minute Tech-Pan exposures goes at least 0.5 magnitude deeper than the short red and is about 1 magnitude less deep than the R survey but with considerably superior resolution and is also about 0.5 magnitude deeper than the I survey. We are not yet sky-limited with the 2 hour H-alpha exposure.
The survey exposures are of 3 hours duration
The high resolution of Tech Pan H-alpha imaging should represent a significant advance in the ability to:
Resolve out point sources from
more extended emission
Enable detection of more distant
planetary nebulae in the Magellanic clouds.
Determine accurately surface
brightness and its variations in extended regions.
Provide better definition of
the sharp shock fronts seen around ionized gas clouds.
Investigate in more detail the
morphology and environment of Herbig-Haro objects and find more distant
or less extended examples.
Tie-ins with other surveys
Of particular interest on the large scale will be comparisons between H-alpha emission and other indicators of interstellar gas and/or star formation activity. These include giant molecular cloud complexes and the general molecular ISM traced by CO observations, radio continuum emission, gamma-rays, HI, dust clouds or IRAS far infra-red flux. This survey should complement the radio maps from the ATNF and MOST, those of the new Parkes HI multibeam survey as well as those from mm wave telescopes here and overseas. The prospects for collaboration and comparison from studies in other wavebands is excellent.
The Scientific Aims of the Survey
H-alpha emission lines from HII regions are one of the most direct optical tracers of current star formation activity. These lines also trace out the distribution of ionized gas in the ISM in general revealing for example: stellar outflows in regions masked by strong reflection nebulae; shocks from high velocity galactic HI clouds; the optical couterparts of supernova remnants; stellar wind-blown bubbles, shells, sheets and filaments and emission nebulosity close to young stellar sources. The spatial extent and detailed morphology of HII regions, OB associations and the wide variety of structures (shells, rings, holes, bubbles, filaments and arcs) over a range of scales from a few arcseconds to tens of degrees can be particularly well studied by H-alpha imaging.
Survey planning and logistics
The new survey, timely in respect of telescope loading, commenced in April 1997.A map of the proposed Galactic-Plane area that will comprise the new survey is given HERE
The coloured fields denote the survey area. The code is: blue (field yet to be observed), yellow (A-grade film available), light blue (B-grade film available) and pink (C-grade film available). The map represents the survey status as of the end of April 1998.A better quality postscript version of the survey field map can be obtained HERE
where black represents fields yet to be observed, light grey (A-grades), and darker grey (B-grades). C-grades are not plotted in this version.Given the filter's 5.5 degree circular field, normal UKST 5 degree field centres could not provide full contiguous H-alpha sky coverage due to the 1.5degree overlap between the 6.5x6.5 sized fields. A small 1degree area in the overlap regions was missed. We have thus adopted a conservative 4 degree field centre separation which ensures no gaps in H-alpha coverage. Consequently 233 such fields are needed to cover the Southern Galactic Plane. This will then be extended to the outer regions of the Galactic Plane and to declinations from +0 to +15 degrees. Exposures will be of the order of 3 hours and the initial survey region will take about 3 years to complete. Although not sky-limited, the 3 hour exposure times are a good compromise between survey progress and exposure quality due to the increased likelihood of seeing variations, cloud interruptions and effects of field rotation.
The narrow-band nature of the H-alpha filter means that the survey could continue in good seeing grey/bright time when the sky is too bright for normal observations.
The photometric integrity of the survey is currently being assessed via independent narrow band CCD photometry from the Curtis Schmidt at CTIO and with reference to previously studied objects over a range of UKST fields.
Survey Availability & Announcement of Opportunity
To maximise the availability and usefulness of the new survey, the AAO, Schmidt Telescope Panel and H-alpha survey consortium have agreed to upgrade the Galactic plane/Magellanic Clouds proposal to a fully fledged AAO survey.
This will ensure rigorous quality control of the survey and its immediate accessibility by the community.There will be no proprietary period on the survey data.
The community is invited to submit proposals for particular fields that are not part of the survey area if they would like H-alpha imaging in special non-galactic plane/Magellanic cloud regions. Potential users of the survey material are encouraged to contact the AAO for further details.
In addition, the consortium intend to produce a fully calibrated digitised database of 10micron resolution pixel data which will be released to the astronomical community as a CD-ROM atlas from SuperCOSMOS scans as soon as practicable. The CD-ROM atlas may be released in installments for faster community access. Film copies of the survey will also be available.
UKST photographic application forms can be obtained HERE
H-alpha International Workshop
(took place April 16-18th 1997)
A very successful 3 day International Workshop was held in Sydney, Australia
from April 16-18, 1997 to highlight the science that can be expected from
the new UKST H-alpha survey. Around 50 participants, half from overseas,
gathered at CSIRO Radiophysics laboratory in Epping.The aims of the workshop
were primarily to showcase the new survey, to discuss the science likely
to be produced and hopefully to generate new ideas and collaborations,
particularly in the radio and millimetre regimes (and hence to tie in with
these associated surveys). Some preliminary results from the survey were
presented as well as an assessment of the H-alpha imaging work performed
on other telescopes. It became clear that our new survey would set the
benchmark for wide-field high resolution imaging of our Galaxy in ionized
gas. The workshop proceedings should appear around April 1998 in a special
issue of `Publications of the Astronomical Society of Australia' (PASA)
which is a refereed journal (Volume 15, No.1). However, all accepted papers
can already be accessed on the www via electronic PASA which can be accessed
HERE.
Details concerning the workshop and proceedings can be found HERE
The high quality H-alpha interference filter
As no suitable AAO H-alpha filter was currently available a vendor (Barr associates, USA) was identified to supply a high specification, unusually large monolithic interference filter for use at the UKST's focal surface.Very stringent optical requirements were necessary as the filter was to be used in a converging f/2.48 beam and the excellent imaging capabilities of Tech Pan must not be compromised.
This filter, probably the largest of its type for use in astronomy, has a circular aperture of about 305 mm permitting a field of about 5.5 degrees diameter to be observed. This circular aperture is coated on a 356x356 mm RG610 glass substrate which permits imaging over the entire UKST field.The CSIRO National Measurement Laboratory in Sydney have quantitatively confirmed that the filter meets the stringent optical specifications set, especially the 70 Angstroms FWHM filter bandwidth and 6590Angstroms central wavelength in collimated light.
The optical quality of the filter has now been demonstrated on the telescope by the first few properly focussed H-alpha exposures taken in good seeing during April 1997. These preliminary 2 hour exposures exhibit excellent imaging and uniformity across the entire field and confirmed the clear (circular) aperture of excellent H-alpha sensitivity of close to 300 mm, or a little over five degrees on the sky. Figure.3a-b gives 5x6arcminute areas taken from one of the first 2 hour test exposures of field 213 from an area in the SW corner (Figure.3a left) and NE corner (Figure.3b right). No gross distortions, image-splitting or other serious image defects or aberrations are seen unlike those exhibited near the edges of previous UKST H-alpha images taken with earlier mosaiced or lower quality filters (Elliot & Meaburn 1976). These peripheral images can be compared directly with a central region from the same film in figure 2b reproduced on the same scale. The smallest visible images are estimated at 25 micron in diameter.
Figure 3. Peripheral Images from the NE and SW corners of the test H-alpha exposure
The full filter specifications can be found HERE
Preliminary H-alpha image comparisons
New images from the new H-alpha filter
An impression of the significant information gains from the new Tech-Pan H-alpha filter combination compared to previous UKST H-alpha imaging is presented in figure 4a-e of an identical 6x5arcmin area in an interesting LMC emission bubble from 5 different UKST H-alpha exposures taken with a range of filters/emulsions and exposure times. Figure 4a (top left) is data from a 2 hour Tech-Pan exposure with the new filter whilst figure 4b (top right) is an 80minute equivalent (same emulsion and filter). There is a dramatic increase in information in the longer exposure with no degradation in image quality. Figure 4e (bottom left) gives a 90 minute exposure through the old 120 Angstrom FWHM AAO656 10inch H-alpha filter with the fast, coarse-grained 098-04 emulsion. This is the worst performing combination for tracing/discovering the fine detailed structure visible in the new exposures. Finally Figure 4e (bottom right) is a 3 hour exposure taken with the same AAO656 filter with the standard IIIaF red-sensitive emulsion. Despite the wider passband and longer exposure the depth and detail of the Tech-Pan new filter equivalent in Figure 4a is far superior.Figure 4a-d. H-alpha imaging comparisons between new and old H-alpha data
The Consortium
Principal Investigators: Q.A.Parker (AAO) and S.Phillipps (Bristol)
Prof.W.J.Zealey, + Stacy Mader & Andrew Walker, (University of Wollongong)Dr.A.Green et al (Univ.Sydney),
Mr.M.Hartley, Dr.J.Bland-Hawthorn, Dr.D.Malin, Dr.R.D.Cannon (AAO)
Dr.M.Fillipovic, Dr.G.White + students (Univ.Western Sydney)
Dr.M.Mashedar & John Precious (Univ.Bristol), Dr.M.G.Edmunds (Univ.Cardiff)
Dr.D.Morgan (Royal.Obs.Edinburgh).
References
Acker A., Ochsenbein F., Stenholm B., Marcout J. & Schohm C. 1992, Strasbourg-ESO Catalogue of Galactic Planetary Nebulae, ISBN 3-923524-41-2Davies,R.S., Elliot,K.H., & Meaburn,J. 1976, Mem.RAS,81,89.
Parker,Q.A.,Phillipps,S., & Morgan,D.H. 1995, IAU colloq.No.148, ASP Conf.Ser.84, ed.J.M.Chapman et al, 129.
Kogure T., Kobayashi Y., Sasaki T., Sakka K., Miyajima K. & Nakano M., 1982. Contribution from the Department of Astronomy, University of Kyoto, No.133, ISSN0388-0230.
UK Schmidt Telescope, Anglo-Australian Observatory schmidt@aaocbn.aao.gov.au
24-Jan-2000
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