Roses in the Southern Sky (Forwarded)

ESO Education and Public Relations Dept.

Contacts:
Fernando Comerón
European Southern Observatory
Garching, Germany
Phone: +49-89-3200-6531
email:

Nausicaa Delmotte
European Southern Observatory
Garching, Germany
Phone: +49-89-3200-6418
email:

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Information from the European Southern Observatory

For immediate release: 3 November 2003

ESO Press Photos 31a-e/03

Roses in the Southern Sky

The Wide-Field-Imager at La Silla Unveils Intricate Structures
Illuminated by Hot Stars

ESO PR Photo 31a/03 ESO PR Photo 31b/03

PR Photo 31a/03 shows the southern part of the spectacular
N44 region in the Large Magellanic Cloud. The green colour
indicates areas that are particularly hot. The field measures
27.5 x 26.5 arcmin2. PR Photo 31b/03 shows the centre of the
associated nebular complex in more detail. The field size is
8.5 x 8.5 arcmin2. In both photos, North is up and East is left.

The two best known satellite galaxies of the Milky Way, the
Magellanic Clouds, are located in the southern sky at a
distance of about 170,000 light-years. They host many giant
nebular complexes with very hot and luminous stars whose
intense ultraviolet radiation causes the surrounding
interstellar gas to glow.

The intricate and colourful nebulae are produced by ionised
gas [1] that shines as electrons and positively charged
atomic nuclei recombine, emitting a cascade of photons at
well defined wavelengths. Such nebulae are called "H II
regions", signifying ionised hydrogen, i.e. hydrogen atoms
that have lost one electron (protons). Their spectra are
characterized by emission lines whose relative intensities
carry useful information about the composition of the emitting
gas, its temperature, as well as the mechanisms that cause the
ionisation. Since the wavelengths of these spectral lines
correspond to different colours, these alone are already very
informative about the physical conditions of the gas.

N44 [2] in the Large Magellanic Cloud is a spectacular example
of such a giant H II region. Having observed it in 1999 (see
ESO PR Photos 26a-d/99), a team of European astronomers [3]
again used the Wide-Field-Imager (WFI) at the MPG/ESO 2.2-m
telescope of the La Silla Observatory, pointing this
67-million pixel digital camera to the same sky region in
order to provide another striking -- and scientifically
extremely rich -- image of this complex of nebulae. With a
size of roughly 1,000 light-years, the peculiar shape of N44
clearly outlines a ring that includes a bright stellar
association of about 40 very luminous and bluish stars.

These stars are the origin of powerful "stellar winds" that
blow away the surrounding gas, piling it up and creating
gigantic interstellar bubbles. Such massive stars end their
lives as exploding supernovae that expel their outer layers
at high speeds, typically about 10,000 km/sec.

It is quite likely that some supernovae have already exploded
in N44 during the past few million years, thereby "sweeping"
away the surrounding gas. Smaller bubbles, filaments, bright
knots, and other structures in the gas together testify to
the extremely complex structures in this region, kept in
continuous motion by the fast outflows from the most massive
stars in the area.

The new WFI image of N44

The colours reproduced in the new image of N44, shown in PR
Photo 31a/03 (with smaller fields in more detail in PR Photos
31b-e/03) sample three strong spectral emission lines. The blue
colour is mainly contributed by emission from singly-ionised
oxygen atoms (shining at the ultraviolet wavelength 372.7 nm),
while the green colour comes from doubly-ionised oxygen atoms
(wavelength 500.7 nm). The red colour is due to the H-alpha
line of hydrogen (wavelength 656.2 nm), emitted when protons
and electrons combine to form hydrogen atoms. The red colour
therefore traces the extremely complex distribution of ionised
hydrogen within the nebulae while the difference between the
blue and the green colour indicates regions of different
temperatures: the hotter the gas, the more doubly-ionised
oxygen it contains and, hence, the greener the colour is.

The composite photo produced in this way approximates the real
colours of the nebula. Most of the region appears with a
pinkish colour (a mixture of blue and red) since, under the
normal temperature conditions that characterize most of this
H II region, the red light emitted in the H-alpha line and
the blue light emitted in the line of singly-ionised oxygen
are more intense than that emitted in the line of the
doubly-ionised oxygen (green).

However, some regions stand out because of their distinctly
greener shade and their high brightness. Each of these regions
contains at least one extremely hot star with a temperature
somewhere between 30,000 and 70,000 degrees. Its intense
ultraviolet radiation heats the surrounding gas to a higher
temperature, whereby more oxygen atoms are doubly ionised
and the emission of green light is correspondingly stronger,
cf. PR Photo 31c/03.

A selection of fields in the N44 complex

ESO PR Photo 31c/03 ESO PR Photo 31d/03

PR Photo 31c/03 shows the ionised region DEM L 159 and
two clusters with hot stars named KMHK 840 (top left) and
KMHK 831 (bottom right). PR Photo 31d/03 shows a region
with pink-green shades that has been designated DEM L 144.
It is a region of ionised hydrogen.

ESO PR Photo 31e/03

PR Photo 31e/03 shows a part of the central nebula, known as
N44C. The green colour indicates areas that are particularly
hot. The nature of the exciting source that delivers the
necessary energy has been the subject of studies during two
decades but is still not known with certainty.

By contrast, ESO PR Photo 26a/99 was a three-colour composite
produced by means of two broad-band filter exposures in blue
and green light and one H-alpha exposure, and therefore
outlining the general appearance of the ionised region. Says
Fernando Comerón, leader of the team: "Thanks to the use of
a UV-filter and two narrow-band filters isolating the emission
of specific ions, the new picture provides much more
information about the complicated physics of the excited
interstellar gas in the region."

Nausicaa Delmotte, member of the team, adds: "With its
colourful beauty, N44 is a display of the violent phenomena
that occur as the heaviest known stars unfold their power on
the parental gas out of which they were born."

Technical information

The colour picture of N44 in the LMC is based on three
monochromatic images taken on 6 and 7 December 2001 with the
Wide-Field-Imager (WFI) at the ESO/MPG 2.2-m telescope, using
the U-band filter (containing the forbidden line of
singly-ionised oxygen, [OII], at 372.7 nm) and two narrow-band
filters centred, respectively, on the wavelengths of the
forbidden line of doubly-ionised oxygen ([OIII], at 500.7 nm)
and hydrogen (H-alpha line, at 656.2 nm). Each single-colour
image is in turn composed of four individual frames of 20
minutes of exposure time each. The WFI detector system is
composed of eight individual 2k x 4k CCDs with small gaps
between them; for this reason, the individual frames in each
filter were obtained with the telescope pointing at slightly
different positions in the sky, so that the parts of the sky
falling in the detector gaps in any given frame are recorded
on the others. A problem with one of the detector chips
causes double stellar images to appear over a small, narrow
strip near the upper left edge of the full field image. The
monochromatic images were produced by superimposing the
individual frames, correcting for the telescope offsets.
Finally, the combined images in each filter were aligned and
colour-coded to produce the resulting colour picture. North
is up and East is left.

Notes

[1]: A gas is said to be ionised when its atoms have lost one
or more electrons -- in this case by the action of energetic
ultraviolet radiation emitted by very hot and luminous stars
close by.

[2]: 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).

[3]: The team is composed of Fernando Comerón and Nausicaa
Delmotte from ESO, and Annie Laval from the Observatoire de
Marseille (France).

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