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A new instrument, SIMBA (“SEST IMaging Bolometer Array”), has been
installed at the Swedish-ESO Submillimetre Telescope (SEST) at the ESO
La Silla Observatory in July 2001. It records astronomical images at a
wavelength of 1.2 mm and is able to quickly map large sky areas. In
order to achieve the best possible sensitivity, SIMBA is cooled to only
0.3 deg above the absolute zero on the temperature scale.

SIMBA is the first imaging millimetre instrument in the southern
hemisphere. Radiation at this wavelength is mostly emitted from cold
dust and ionized gas in a variety of objects in the Universe. Among
other, SIMBA now opens exciting prospects for in-depth studies of the
“hidden” sites of star formation, deep inside dense interstellar nebulae.
While such clouds are impenetrable to optical light, they are transparent
to millimetre radiation and SIMBA can therefore observe the associated
phenomena, in particular the dust around nascent stars.

This sophisticated instrument can also search for disks of cold dust
around nearby stars in which planets are being formed or which may be
left-overs of this basic process. Equally important, SIMBA may observe
extremely distant galaxies in the early universe, recording them while
they were still in the formation stage.

Various SIMBA images have been obtained during the first tests of the new
instrument. The first observations confirm the great promise for unique
astronomical studies of the southern sky in the millimetre wavelength

These results also pave the way towards the Atacama Large Millimeter
Array (ALMA), the giant, joint research project that is now under study
in Europe, the USA and Japan.

PR Photo 28a/01: SIMBA image centered on the infrared source IRAS

PR Photo 28b/01: SIMBA image centered on the infrared source IRAS

PR Photo 28c/01: SIMBA image centered on the infrared source IRAS

PR Photo 28d/01: View of the SIMBA instrument

First observations with SIMBA

SIMBA (“SEST IMaging Bolometer Array”) was built and installed at the
Swedish-ESO Submillimetre Telescope (SEST) at La Silla (Chile) within an
international collaboration between the University of Bochum and the Max
Planck Institute for Radio Astronomy in Germany, the Swedish National
Facility for Radio Astronomy and ESO.

The SIMBA (“Lion” in Swahili) instrument detects radiation at a wavelength
of 1.2 mm. It has 37 “horns” and acts like a camera with 37 picture elements
(pixels). By changing the pointing direction of the telescope, relatively
large sky fields can be imaged.

As the first and only imaging millimetre instrument in the southern
hemisphere, SIMBA now looks up towards rich and virgin hunting grounds in
the sky. Observations at millimetre wavelengths are particularly useful for
studies of star formation, deep inside dense interstellar clouds that are
impenetrable to optical light. Other objects for which SIMBA is especially
suited include planet-forming disks of cold dust around nearby stars and
extremely distant galaxies in the early universe, still in the stage of

During the first observations, SIMBA was used to study the gas and dust
content of star-forming regions in our own Milky Way Galaxy, as well as in
the Magellanic Clouds and more distant galaxies. It was also used to record
emission from planetary nebulae, clouds of matter ejected by dying stars.
Moreover, attempts were made to detect distant galaxies and quasars
radiating at mm-wavelengths and located in two well-studied sky fields,
the “Hubble Deep Field South” and the “Chandra Deep Field” [1].

Observations with SEST and SIMBA also serve to identify objects that can
be observed at higher resolution and at shorter wavelengths with future
southern submm telescopes and interferometers such as APEX (see MPG Press
Release 07/01 of 6 July 2001) and ALMA.

SIMBA images regions of high-mass star formation

ESO PR Photo 28a/01

Caption: This intensity-coded, false-colour SIMBA image is centered on
the infrared source IRAS 17175-3544 and covers the well-known high-mass
star formation complex NGC 6334, at a distance of 5500 light-years. The
southern bright source is an ultra-compact region of ionized hydrogen
(“HII region”) created by a star or several stars already formed. The
northern bright source has not yet developed an HII region and may be a
star or a cluster of stars that are presently forming. A remarkable,
narrow, linear dust filament extends over the image; it was known to
exist before, but the SIMBA image now shows it to a much larger extent
and much more clearly. This and the following images cover an area of
about 15 arcmin x 6 arcmin on the sky and have a pixel size of 8 arcsec.

ESO PR Photo 28b/01

Caption: This SIMBA image is centered on the object IRAS 18434-0242. It
includes many bright sources that are associated with dense cores and
compact HII regions located deep inside the cloud. A much less detailed
map was made several years ago with a single channel bolometer on SEST.
The new SIMBA map is more extended and shows more sources.

ESO PR Photo 28c/01

Caption: Another SIMBA image is centered on IRAS 17271-3439 and includes
an extended bright source that is associated with several compact HII
regions as well as a cluster of weaker sources.

Some of the recent SIMBA images are shown above; they were taken during test
observations, and within a pilot survey of high-mass starforming regions.

Stars form in interstellar clouds that consist of gas and dust. The denser
parts of these clouds can collapse into cold and dense cores which may form
stars. Often many stars are formed in clusters, at about the same time.

The newborn stars heat up the surrounding regions of the cloud. Radiation is
emitted, first at mm-wavelengths and later at infrared wavelengths as the
cloud core gets hotter. If very massive stars are formed, their UV-radiation
ionizes the immediate surrounding gas and this ionized gas also emits at
mm-wavelengths. These ionized regions are called ultra compact HII regions.

Because the stars form deep inside the interstellar clouds, the obscuration
at visible wavelengths is very high and it is not possible to see these
regions optically. The objects selected for the SIMBA survey are from a
catalog of objects, first detected at long infrared wavelengths with the
IRAS satellite (launched in 1983), hence the designations indicated in
Photos 28a-c/01.

>From 1995 to 1998, the ESA Infrared Space Observatory (ISO) gathered an
enormous amount of valuable data, obtaining images and spectra in the broad
infrared wavelength region from 2.5 to 240 _m (0.025 to 0.240 mm), i.e.
just shortward of the millimetre region in which SIMBA operates. ISO
produced mid-infrared images of field size and angular resolution
(sharpness) comparable to those of SIMBA.

It will obviously be most interesting to combine the images that will be
made with SIMBA with imaging and spectral data from ISO and also with those
obtained by large ground-based telescopes in the near- and mid-infrared
spectral regions.

Some technical details about the SIMBA instrument

ESO PR Photo 28d/01

Caption: The SIMBA instrument — with the cover removed — in the SEST
electronics laboratory. The 37 antenna horns to the right, each of
which produces one picture element (pixel) of the combined image. The
bolometer elements are located behind the horns. The cylindrical
aluminium foil covered unit is the cooler that keeps SIMBA at extremely
low temperature (-272.85 C, or only 0.3 deg above the absolute zero)
when it is mounted in the telescope.

SIMBA is unique because of its ability to quickly map large sky areas due to
the fast scanning mode. In order to achieve low noise and good sensitivity,
the instrument is cooled to only 0.3 deg above the absolute zero, i.e., to
-272.85 ¢XC.

SIMBA consists of 37 horns (each providing one pixel on the sky) arranged in
a hexagonal pattern, cf. Photo 28d/01. To form images, the sky position of
the telescope is changed according to a raster pattern — in this way all of
a celestial object and the surrounding sky field may be “scanned” fast, at
speeds of typically 80 arcsec per second. This makes SIMBA a very efficient
facility: for instance, a fully sampled image of good sensitivity with a
field size of 15 arcmin x 6 arcmin can be taken in 15 minutes. If higher
sensitivity is needed (to observe fainter sources), more images may be
obtained of the same field and then added together.

Large sky areas can be covered by combining many images taken at different
positions. The image resolution (the “telescope beamsize”) is 22 arcsec,
corresponding to the angular resolution of this 15-m telescope at the
indicated wavelength.


[1} Observations of the HDFS and CDFS fields in other wavebands with other
telescopes at the ESO observatories have been reported earlier, e.g. within
the ESO Imaging Survey Project (EIS) (the “EIS Deep-Survey”). It is the ESO
policy on these fields to make data public world-wide.