Note: Photos online at http://www.eso.org/outreach/press-rel/pr-2001/phot-32-01.html

Photo Caption: ESO PR Photo 32a/01 is a (false-)
colour infrared photo of the nearby barred spiral galaxy Messier
83. It is based on the combination of three images obtained in the
Ks- (wavelength 2.2 µm), J- (1.2 µm) and I-bands (0.8
µm), respectively. The first two were recorded with the SOFI
instrument at the ESO
3.5-m New Technology Telescope, the third with the DFOSC
instrument at the Danish 1.54-m
telescope, both at the La Silla Observatory. ESO PR Photo
32b/01 shows the central area in more detail. Technical information about these photos is
available below.

Messier 83 (M83) is a relatively nearby spiral galaxy with a
pronounced bar-like structure. It is located in the southern
constellation Hydra (The Water-Snake) and is also known as NGC
5236; the distance is approximately 12 million light-years.

Images of M83 obtained in visible light – like the VLT photo
published exactly two years ago (ESO PR 18/99) – show
clumpy, well-defined spiral arms that are rich in young stars while
the disk reveals a complex system of intricate dust lanes. This galaxy
is known to be a site of vigorous star formation and no less than six
supernovae (exploding stars) have been observed in M83 during the past
century. It is a fairly symmetrical object and possesses no nearby
companions.

Gas dynamics and galaxy bars

Investigations of gas motions in the nucleus and in the main
disk play a key role in understanding the structure and evolution of
barred spiral galaxies like M83. Inflow of gas towards the center
caused by a mass distribution that is not circularly symmetric is
often invoked to explain certain observed phenomena, e.g., the feeding
of Active Galactic Nuclei (AGNs, see also the report about recent
observations in three such galaxies in ESO
PR 18/01), and the fueling of bursts of star formation in the
nuclear region. Some astronomers think that this process may cause a
change of a galaxy’s (morphological) type, for instance from barred to
normal spiral galaxy.

It has also been suggested that the development of spiral
structures in galactic disks may be due to central stellar
bars. Interstellar gas that is subject to periodical perturbations by
the non-circularly symmetrical gravitational field in a barred system
will develop a “density wave” that attracts neighbouring stars and
gas. The local density increases and once a certain (“critical”)
value is reached, star formation is “ignited” in this area.

The mass distribution

In order to better understand phenomena like these, it is essential
to know in detail the distribution of matter in the galaxy disk. This
is best done by means of infrared observations.

Images obtained in the optical region of the spectrum mainly trace
the pattern of star formation as well as young and bright stars,
rather than the mass distribution in the galaxy. The regions with much
dust are also well visible on such images as dark lanes and clouds,
since they are very opaque to visible light. However, on infrared
images, the dust absorption is much smaller and the light that is
recorded mainly comes from old giant stars. Although those stars
contribute little to the mass, they have the same spatial distribution
as the much more numerous, smaller (“main sequence”) stars. Their
distribution therefore shows the mass distribution in the galaxy and
hence the gravitation field that directs the motions of the
interstellar gas.

New infrared images of Messier 83

It is for this reason that infrared images of barred galaxies, like
the ones of M83 shown here that was prepared by a group of Swedish
astronomers [1], are extremely useful for the
study of the dynamics of such galaxies and their development.

In order to produce these (false-) colour photos, three infrared
images of M83 obtained in the Ks- (wavelength 2.2 µm), J- (1.2
µm) and I-bands (0.8 µm), respectively, were combined into
one colour image. The field shown in Photo 32a/01 measures
32,000 light-years across at the distance of M83. The dust lanes
appear red since the emission in the longest waveband (Ks) is able to
penetrate the dense gas and dust clouds. Note also how the dust lanes
follow the leading edge of the bar – assuming that the arms are
trailing and that the upper right area is the side of the galaxy that
is nearest to us.

Photo 32b/01 is an enlarged view of the galactic nucleus,
processed to better show the intricate structure in this area. Here
the dust lanes merge into a nuclear ring. It is obvious that the dust
absorption in this region is so high that even Ks-photons have a hard
time passing through.

Note

[1] The group includes Andreas Andersson
and Hans Olofsson (Stockholm Observatory, SCFAB), and Tommy
Wiklind and Gustaf Rydbeck (Onsala Space Observatory). The
group is grateful to Søren Larsen for the use of an I-image for
the present work.

Technical information about the photos

PR Photos 32a-b/01 of Messier 83 are based on a
composite of three images in the filters Ks (NTT + SOFI; wavelength
2.162 µm; Full-Width-Half-Maximum (FWHM) 0.275 µm; total
exposure time on target 49 min; approx. 12 min per quadrant; here
rendered as red); J (NTT + SOFI; 1.247 µm; 0.290 µm; 22
min; 5 min; green), and I (Danish 1.54-m +
DFOSC; 0.797 µm; 0.142 µm; 20 min; full-field
coverage; blue). As indicated, the Ks and J images were created from
exposures of four neighbouring fields. These images were taken in May
2001 during a period with 1.0 arcsec average seeing. The original
frames have 1024 x 1024 pixels, each measuring 0.29 arcsec. The
resulting mosaic was created from 31 Ks-band object frames and 17
J-band object frames, each with 100 sec exposure time. From each
object frame, a sky frame with the same exposure time was subtracted
to remove the sky contribution. The final size of the mosaic image is
1905 x 1839 pixels (9.2 x 8.9 arcmin). The I image was observed in
February 1998 by Søren Larsen (more information about this
image is available in “Astronomy and Astrophysics Supplement”, Vol
139, p.393-415, 1999 and at URL: http://nedwww.ipac.caltech.edu/). The
original frame size was 2036 x 2042 pixels, each measuring 0.39
arcsec. The pixel size and the field-of-view of this image was changed
to match the characteristics of the Ks and J images. North is up and
east to the left.

Contacts

Andreas Andersson
Stockholm Observatory, SCAF
SE-10691 Stockholm, Sweden
Phone: +46-8-55378533
E-mail: andreas@astro.su.se

Tommy Wiklind
Onsala Space Observatory
SE-43992 Onsala, Sweden
E-mail: tommy@oso.chalmers.se

ESO PR Photos 32a-b/01 may be reproduced, if credit is given
to the European Southern Observatory (ESO).