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Institute of Astronomy
University of Cambridge
Cambridge, U.K.

Contact information:

Robert Sharp
email: rgs{at}ast.cam.ac.uk
Tel: +44 (0)1223 337148 (In Chile, until mid-July, contactable by
e-mail)

Ian Parry
email: irp{at}ast.cam.ac.uk
Tel: +44 (0)1223 337092

Stuart Ryder
email: sdr{at}aaoepp.aao.gov.au

Johan Knapen
email: knapen{at}star.herts.ac.uk

Lisa M. Mazzuca
email : mazzuca{at}stargate.gsfc.nasa.gov

For general contact information and assistance:

Lisa Wright
email:ljw{at}ast.cam.ac.uk
Tel: +44 (0)1223 337527

26 June 2003

Astronomers find Paschen in the bar

Robert Sharp [1], Ian Parry [1], Stuart Ryder [2], Johan H. Knapen
[3] and Lisa Mazzuca [4]
[1] IoA Cambridge
[2] AAO
[3] Univ.of Hertfordshire
[4] NASA/GSFC

An international team of astronomers have used a unique instrument on
the 8m Gemini South Telescope to determine the ages of stars across
the central region of the barred spiral galaxy, M83. Preliminary
results provide the first hints of a domino model of star formation
where star formation occurs in a time sequence, driven by the
movements of gas and stars in the central bar. 

The new instrument, called CIRPASS, simultaneously produces 500
spectra, taken from across the whole region of interest, which act as
a series of 'fingerprints'. Encoded in these 'fingerprints' is not
only all the information the team required to determine when
individual groups of stars formed, but also information on their
movements and chemical properties. Dr. Johan Knapen, project
co-investigator, "The unique combination of a state-of-the-art
instrument like CIRPASS with one of the most powerful telescopes
available is now providing us with truly sensational observations."

M83 is a grand design spiral galaxy undergoing an intense burst of
star formation in its central bar region. Large scale images, of the
visible light from the galaxy, taken with ground based telescopes,
show a pronounced bar across the middle of the galaxy) seen as the
diagonal white structure in figure 1 . Astronomers believe that it is
the influence of this bar which leads a concentration of gas in the
central regions of the galaxy from which stars are born. "The central
region of M83 is enshrouded in dust, but by using CIRPASS, which
operates in the infra-red not the visible, we are able to see through
this dust and investigate the hidden physical processes at work in
the galaxy," said Dr Ian Parry, leader of the CIRPASS instrumentation
team. 

Two competing theories strive to explain the burst of star formation
in the centre of the galaxy, M83. One theory suggests that stars form
randomly across the whole nuclear region. A second model, favoured by
the observational team, proposes that star-formation is triggered by
the bar structure. In this model, the rotation of gas and stars in
the bar causes stars to be formed sequentially, in a domino manner.

Using a technique first demonstrated by Dr. Stuart Ryder and
colleagues, the team searched for a hydrogen emission feature, the
Paschen-beta line, within the galaxy's 'fingerprints'. The
measurement of this feature indicates the presence of hot young
stars. By comparing the strengths of the Paschen-beta emission with
the amount of absorption from carbon-monoxide (arising in the cool
atmospheres of old giant stars) the team are able determine the age
of the stars in each region of the galaxy. "A detailed analysis of
the data is underway but initial results hint at a complex sequence
of star formation," said Dr Robert Sharp, instrument support
scientist with CIRPASS. 

Preliminary analysis of other emission features (due to Paschen-beta
and ionized iron) revealed a potentially intriguing result. "Ionized
iron enables us to trace past supernova explosions. The observations
indicate that energy from exploding stars (supernovae) may be being
passed into regions of undisturbed gas causing further massive star
formation," said Dr. Stuart Ryder, principle investigator.

While some members of the instrument team are presenting their work
at the Royal Society Science Exhibition in London, CIRPASS is back on
the Gemini South Telescope in Chile, performing the next set of
observations. 

Scientific Publications:

Harris J. et al. 2001, ApJ, 122, 3046
Ryder S.D., Knapen J.H. and Takamiya M. 2001, MNRAS, 323, 663

Web Resources:

1. Insitute of Astronomy, Cambridge
    http://www.ast.cam.ac.uk
2. Anglo-Australian Observatory
    http://www.aao.gov.au/
3. University of Hertfordshire Astronomy Research Group
    http://star.herts.ac.uk
4. NASA Goddard Space Flight Centre
    http://www.gsfc.nasa.gov/
5. Cambridge Infra-Red PAnoramic Survey Spectrograph
    http://www.ast.cam.ac.uk/~optics/cirpass/index.html
6. The Gemini Observatory
    http://www.gemini.edu
7. Hubble Space Telescope Science Institute
    http://www.stsci.edu

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