N44



Beautiful Nebulae in the Large Magellanic Cloud

New Images from the Wide Field Imager at La Silla

The Wide Field Imager (WFI), a 67-million pixel digital camera at the
MPG/ESO 2.2-m telescope at the La Silla Observatory continues to
produce remarkable images for astronomical research. At the same time,
many of these are of great aesthetic value and provide impressive
views into sky regions with unsual objects.

Earlier this year, the WFI recorded a number of fields in the
direction of the Large Magellanic Cloud (LMC), a satellite galaxy to
our own Milky Way Galaxy, deep down in the southern sky. It is located
at a distance of about 170,000 light-years and contains many nebulae
and stellar clusters.

Two colour composite photos of "H II regions" in the LMC are shown
here, centred near N44 and N119 [1]. They are nebulae in which (some
of) the gas is ionized, i.e. the atoms have lost one or more electrons
by the action of energetic ultraviolet radiation emitted by very hot
and luminous stars in this area. Both of these nebulae are seen in
front of rich star fields in this galaxy in which there are also
several stellar clusters.

These photos are based on exposures made through three optical
filtres, including a narrow-band one centred on the H-alpha spectral
line from hydrogen (wavelength 656.2 nm in the red part of the
spectrum; light of this wavelength is emitted when protons and
electrons combine to form hydrogen atoms). The red colour therefore
traces the extremely complex distribution of ionized hydrogen within
the nebulae. The present reproductions enhance this colour in order to
show the fainter structures. Note also the very different colours of
individual stars, mostly reflecting their temperature, from the
hottest (blue) to the coolest (red).

Stars and supernova remnants in the N44 area
  
 PR Photo 26a/99 of the spectacular N44 H II region in the LMC
 displays (very nearly) the full extent of the WFI field. PR Photo
 26b/99 shows the centre of the associated complex of nebulae in more
 detail [2].


N44 is a very bright, rich and well-studied complex in the LMC,
somewhat apart from the other nebulae in this galaxy. The central
region, the structure of which can be best appreciated in PR Photo
26b/99, is dominated by a large, ringshaped nebula that includes a
bright stellar association of very luminous stars. It emits X-rays -
this is interpreted as a sign that several heavy stars in this area
have exploded as supernova during the last few million years.

The general appearance (in astronomical terminology: the "morphology")
is well explained by a combination of fast outflow from stars (stellar
winds) and supernova remnants, as well as sequential star
formation. However, according to Annie Laval and her collaborators at
Observatoire de Marseille (France), the origin and interpretation of
the individual features of this nebula is still enigmatic. She also
remarks that the motions of some of the ionized gas in N44 is unusual:
it is not clear whether there are internal motions in dust clouds or
if there are several distinct gas layers.

The new observations with the WFI of the distribution of the different
objects (stars, gas condensations, dust clouds) will help to
understand better what is really happening in this very complex
environment.

The "spiral" nebula N119
  
 PR Photo 26c/99 of the spectacular N119 H II region in the LMC
 displays (very nearly) the full extent of the WFI field, while PR
 Photos 26d-f/99 show smaller areas in more detail [2].


N119 is another "H II region" in the LMC. The most remarkable
characteristic is its pronounced spiral shape that is reminiscent of a
barred spiral galaxy [3]. It is quite large, about 400 x 600
light-years, and it is situated at the northern side of the stellar
bar of the Large Magellanic Cloud, near the centre of rotation of the
neutral hydrogen in this galaxy. It is this bar that is responsible
for the much higher star density in the lower half of the full-field
photo (PR Photo 26c/99).

N119 is the only nebula in the LMC in which the gas is distributed
according to such a spiral structure. The distribution of the ionizing
stars cannot well explain such an intriguing shape. The measured
motions indicate the action of stellar winds and that the ionized gas
forms a rotating system. Research by Annie Laval (Observatoire de
Marseille, France) and Patricia Ambrocio-Cruz (Universidad Nacional
Autonoma de Mexico) and their respective collaborators suggest that
the reason for such a peculiar shape and motion pattern is probably a
collision between two interstellar clouds. The gas in the clouds is
highly compressed during the collision, triggering in this way the
formation of the luminous stars that are now ionizing the nebula. The
two teams are now developing a new model, based on numerical computer
simulations, to understand better the intricacies of the collision
process, and thus how N119 became such a beautiful spiral nebula.

PR Photo 26d/99 shows in more detail the central region of N119,
highlighting the different distributions of ionized gas and luminous
stars. The area depicted in PR Photo 26e/99 is located in the
northeastern (top left) arm of the spiral, where the intricate
filamentary structure of the nebulosity is well seen. PR Photo 26f/99
is centered on the bright, rich and compact cluster (NGC 1916) that
can be seen near the bottom center of the full-field picture, clearly
showing the richness of the field surrounding it, as well as the wide
variety of stellar colours.

Technical information: The two colour composites are based on 12
exposures each, obtained on 25 and 26 March 1999 in three colours,
B(lue), V (green-yellow) and H-alpha (red), and with total exposures
times of 2, 2 and 20 min, respectively. The observation conditions
were mediocre and the image quality is about 2 arcsec. The images in
each filter are composites of 4 individual frames that were obtained
with the telescope pointing at slightly different positions in the
sky, so that the parts of the sky falling in the gaps between the 8
individual 2k x 4k CCDs in any given frame are recorded on the
others. The monochromatic images were then produced by superimposing
the individual frames, correcting for the telescope offsets; this
ensures that the complete field is well covered. However, this
procedure is not perfect, as observing conditions may change slightly
from exposure to exposure, resulting in small differences in the
background intensity. Finally, the combined images in each filter are
aligned and colour-coded to produce the resulting colour picture. The
alignment process is able to correct quite well for a number of small
distortions across the field of view, but in order to obtain a uniform
quality over all the entire field, some slight misalignment may be
introduced in certain narrow strips. The field size of Photos 26a/99
and 26c/99 is about 32 x 32 arcmin2. North is up and East is left.

Notes

[1] The letter "N" (for "Nebula") in the designation of these objects
indicates that they were included in the "Catalogue of H-alpha
emission stars and nebulae in the Magellanic Clouds" compiled and
published in 1956 by American astronomer-astronaut Karl Henize (1926 -
1993).

[2] In PR Photos 26a/99 and 26c/99, the resolution has been degraded
by reducing the number of pixels in one direction from about 8000 to
2000 in the "High-Resolution version", in order to make the complete
image transportable over the web without incurring excessive transfer
times. Still it is very large, even in the highly compressed
jpeg-format, reflecting the great amount of details visible in this
extremely rich star field. The "Normal" and "Preview" versions are
smaller and may be acquired faster, but with a corresponding loss of
detail. The "Full-Resolution" versions of PR Photos 26b/99 and PR
Photos 26d-f/99 retain the original pixel structure and image
sharpness.

[3] VLT images of a typical barred spiral galaxy (NGC 1365) have been
published as ESO Press Photos 08a-e/99.



This is the caption to ESO PR Photos 26a-f/99. They may be reproduced,
if credit is given to the European Southern Observatory.


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                         Department Karl-Schwarzschild-Strasse 2,
                         D-85748 Garching, Germany