The Large Magellanic Cloud: What is it and why is it important?
The Large Magellanic Cloud
, along with its neighbor, the Small Magellanic Cloud, was named after
the explorer Ferdinand Magellan, whose crew used it as a rough
navigation tool while sailing through the southern hemisphere. On a
dark night, this
irregular galaxy
appears in the Southern sky as a
blurry spot
, subtending an angle of about 5 degrees (~10 times the apparent
diameter of the moon). Orbiting the Milky Way at about 179,000 light
years away, the Large Magellanic Cloud is one of our closest
neighbors.
The Large Magellanic Cloud has a mass of approximately 20 billion
suns, or 1/20th of the estimated mass of the Milky Way. Because of
its low mass, the LMC has had a very different history than our own
galaxy; it is therefore very useful for studying types of stars,
clusters and nebulae not often found in the Milky Way, and has
provided an opportunity for applying theories to a wider range of
circumstances. The LMC has probably become most famous, however,
because of the study of
Cepheid
variable stars,
an important group of stars used for measuring distances in the
universe. More recently, stars in the LMC have been searched for
gravitational lensing effects,
a way of identifying dark matter in the Milky Way.
The Tilt of the LMC: What does this mean and what is its significance?
Assuming a galaxy is approximately disc-shaped, the tilt of a galaxy
is simply the inclination of the disc as viewed from the Earth. For a
circular galaxy, an inclination of 0 looks like a circle, while an
inclination of 90 looks like a line. Seen below is the change in the
appearance of a circular disk as its tilt is increased from 0 to 90.
The tilt of the LMC is one of the parameters necessary to model the
interactions between the Milky Way, the LMC and the SMC (Small
Magellanic Cloud). The tilt of the LMC also has a great impact on
interpretation of microlensing survey results. The greater the tilt,
the more likely it is that LMC material (and not just galactic
material) may be lensing LMC stars.
Measuring the Tilt: Previous Methods and Results
There are three basic techniques for measuring the tilt of the LMC:
- Assume the galaxy's disc is circular, and fit an ellipse to its observed shape. (This is demonstrated in the diagram shown above.)
- Measure velocities of stars in different parts of the galaxy and model the shape of the cloud.
- Determine the relative distances of stars by observing magnitude differences.
Previous studies using these three methods have yielded results
differing by as much as ~20 degrees! What could be wrong? Well,
perhaps the most likely error is that the first two methods above use
the assumption that the galaxy is roughly circular, when, in fact,
there is no reason to believe it should be. There are many galaxies
that are NOT circular and the LMC has both a distinctive bar structure
and is likely to be distorted by forces from the SMC and Milky Way.
The third method does NOT make this assumption, and previous studies
have used various "standard candles" (objects of known magnitudes) to
measure the tilt of the LMC. Unfortunately, these standard candles
tended to be rather sparse throughout the cloud, which did not well
ensure that all observed magnitude differences were due to distance
differences.
Measuring the Tilt Using Red Clump Stars
Our method for determining the tilt of the LMC is an untried version
of the third technique mentioned above. The standard candles we've
chosen are known as
red clump stars
because they are red stars which form a "clump" of approximately
equal magnitudes on color-magnitude diagrams. They have masses
similar to our sun, but have had more time to evolve. They are
burning helium and are considered to be in the "little giant" phase of
stellar evolution. Just below is a plot of color vs. V magnitude for
one section of stars in the LMC. The red-clump region has been
circled.
Red clump stars are the ideal relative distance indicator because they
are both bright and numerous. (Red clump stars are creating a bit of a
controversy
at the moment. However, the controversy resides in their use as an
absolute indicator, while we are using them as relative
distance indicators). They all have approximately the same magnitude
and, therefore, the average magnitude of red clump stars in a field
can be used as a measure of relative distance from an observer on
earth: the dimmer the red clump, the further the stars are from the
observer.
So, how does the distance of the red clump tell us about the tilt of the
cloud? Well, rougly-speaking, if we know the distance between the
closest and further stars, and also measure the distance between these
same stars in the plane of the sky, we can form a triangle as shown below.
The angle shown is the inclination, or tilt, of the cloud.
