A color image of the Large Magellanic Cloud galaxy combining maps of neutral atomic hydrogen gas (red), hydrogen ionized by nearby young stars (blue), and new data from Wong’s team which roughly trace dense clouds of molecular hydrogen (green). Image: Tony Wong
An international team of astronomers has mapped in detail the star-birthing
regions of the nearest star-forming galaxy to our own, a step toward
understanding the conditions surrounding star creation.
Led by University
of Illinois astronomy professor Tony
researchers published their findings in the Astrophysical
Journal Supplement Series.
The Large Magellanic Cloud (LMC) is a popular galaxy among astronomers both
for its nearness to our Milky Way and for the spectacular view it provides, a
big-picture vista impossible to capture of our own galaxy.
“If you imagine a galaxy being a disc, the LMC is tilted almost face-on so
we can look down on it, which gives us a very clear view of what’s going on
inside,” Wong says.
Although astronomers have a working theory of how individual stars form,
they know very little about what triggers the process or the environmental
conditions that are optimal for star birth. Wong’s team focused on areas called
molecular clouds, which are dense patches of gas—primarily molecular hydrogen—where
stars are born. By studying these molecular clouds and their relationship to
new stars in the galaxy, the team hopes to learn more about the metamorphosis
of gas clouds into stars.
“When we study star formation, an important question is, what is the
environment doing? How does the location of star formation reflect the
conditions of that environment? There’s no better place to study the wider
environment than the LMC.”
Using a 22-m-diameter radio telescope in Australia, the astronomers mapped
more than 100 molecular clouds in the LMC and estimated their sizes and masses,
identifying regions with ample material for making stars. This seemingly simple
task engendered a surprising find.
Conventional wisdom states that most of the molecular gas mass in a galaxy
is apportioned to a few large clouds. However, Wong’s team found many more
low-mass clouds than they expected—so many, in fact, that a majority of the
dense gas may be sprinkled across the galaxy in these small molecular clouds,
rather than clumped together in a few large blobs.
“We thought that the big clouds hog most of the mass,” Wong says, “but we
found that in this galaxy, it appears that the playing field is more level. The
low-mass clouds are quite numerous and they actually contribute a significant
amount of the mass. This provides the first evidence that the common wisdom
about molecular clouds may not apply here.”
The large numbers of these relatively low-mass clouds means that
star-forming conditions in the LMC may be relatively widespread and easy to
achieve. The findings raise some interesting questions about why some galaxies
stopped their star formation while others have continued it.
To better understand the connection between molecular clouds and star
formation, the team compared their molecular cloud maps to maps of infrared
radiation, which reveal where young stars are heating cosmic dust.
For the comparison, they exploited a carefully selected sample of newborn
heavy stars compiled by U.
of I. astronomy professor
You-Hua Chu and resident scientist Robert Gruendl, who also were co-authors of the
paper. These stars are so young that they are still deeply embedded in cocoons
of gas and dust.
“It turns out that there’s actually very nice correspondence between these
young massive stars and molecular clouds,” Wong says. “That’s not entirely
surprising, but it’s reassuring. We assume that these stars have to form in
molecular clouds, and it tells us that the molecular clouds do hang around long
enough for us to see them associated with these massive young stars.”
Wong hopes to continue to study the relationship between molecular clouds
and star formation in greater detail. If researchers can determine the relative
ages of young stars, they can correlate these against molecular clouds to
figure out which clouds have star formation, how long the clouds live and what
eventually leads to their destruction. They also plan to use a newly
constructed array of telescopes in Chile to see the cloud environment
in higher resolution, pinpointing exactly where inside the molecular cloud star
formation will occur.
“This study provides us with our most detailed view of an entire population
of clouds in another galaxy,” Wong says. “We can say with great confidence that
these clouds are where the stars form, but we are still trying to figure out
why they have the properties they do.”