Untitled - ESO

Cut from Different Cloth
A large survey has shed light on our Galaxy’s
ancestry. After determining the
chemical composition of over 000 stars
in four of the nearest dwarf galaxies
to our own, astronomers have demonstrated
fundamental differences in their
make-up, casting doubt on the theory
that these diminutive galaxies could ever
have formed the building blocks of our
Milky Way Galaxy.
Our Milky Way Galaxy is surrounded by
a number of dwarf satellite galaxies,
which because of their loosely rounded
shape are referred to as ‘dwarf spheroidal’
galaxies. Faint and diffuse, these
dwarf galaxies are a thousand times
fainter than the Milky Way itself, making
them the least luminous galaxies known.
Modern cosmological models predict
that small galaxies form first, and later
assemble into larger systems like our Galaxy.
Since the Universe initially only
The Topsy-Turvy Galaxy
This FORS image of the central parts of
NGC 1313 shows a stunning natural
beauty. The galaxy bears some resemblance
to some of the Milky Way’s closest
neighbours, the Magellanic Clouds.
NGC 1313 has a barred spiral shape,
with the arms emanating outwards in a
loose twist from the ends of the bar.
The galaxy lies just 15 million light years
away from the Milky Way – a mere skip
on cosmological scales. The spiral arms
are a hotbed of star-forming activity, with
numerous young clusters of hot stars
being born continuously at a staggering
rate out of the dense clouds of gas and
dust. Their light blasts through the
surrounding gas, creating an intricately
beautiful pattern of light and dark
nebulosity.
ESO Annual Report 006
contained hydrogen and helium (most of
all other chemical elements being
synthesized inside stars), dwarf galaxies
should have the lowest heavy element
content. Not so, reveal the observations.
As part of a large observational programme,
called the Dwarf galaxies Abundances
and Radial-velocities Team
(DART), astronomers from institutes in 9
different countries used FLAMES on the
VLT to measure the amount of iron in over
000 individual giant stars in the Fornax,
Sculptor, Sextans and Carina dwarf
spheroidals.
Their data unearthed fundamental differences
in the dwarf galaxy stars’ chemical
composition compared with those
in our galactic halo, calling into question
the merger theory as the origin of large
galaxies’ haloes. Whilst the average
abundances of elements in the dwarf
spheroidals is comparable with that seen
NGC 1313’s appearance suggests it
has seen troubled times: its spiral arms
look lop-sided and gas globules are
spread out widely around them. Moreover,
observations with ESO’s 3.6-m
telescope at La Silla have revealed that
its ‘real’ centre, around which it rotates,
does not coincide with the central bar.
Its rotation is therefore also out of kilter.
Strangely enough, NGC 1313 seems to
be an isolated galaxy. It is not part of a
group and has no neighbour, and it is not
clear whether it may have swallowed a
small companion in its past. So what
caused its asymmetry and stellar baby
boom?
Chemical abundance in several dwarf galaxies.
in the Galactic halo, the former are
lacking the very metal-poor stars that are
seen in the Milky Way – the two types
of systems, contrary to theoretical predictions,
are essentially of different descent.
Amina Helmi et al. 006, Astrophysical Journal
Letters, 651, L1 1-L1 4.
The Topsy-Turvy Galaxy NGC 1313.