ASTRONEWS
WWW.ASTRONOMY.COM 19All disk galaxies fully rotate about once every billion
years, no matter their size or mass, astronomers
announced in a study published March 9 in the
Monthly Notices of the Royal Astronomical Society.
“It’s not Swiss watch precision,” said Gerhardt
Meurer of the International Centre for Radio
Astronomy Research in a press release. “But
regardless of whether a galaxy is very big or very
small, if you could sit on the extreme edge of its
disk as it spins, it would take you about a billion
years to go all the way round.”
To carry out the study, the researchers deter-
mined the velocity of neutral hydrogen on the
outer edges of more than 100 galaxies — ranging
from small dwarf irregulars to massive spirals.
These galaxies differed in both size and rotational
velocity by up to a factor of 30. Using the velocity
measurements, the researchers calculated the
rotational periods of their sample galaxies, which
led them to conclude that the outer rims of all disk
galaxies take roughly a billion years to complete
one rotation.
The researchers expected to find only sparse
populations of young stars and interstellar gas on
the outskirts of these galaxies, based on theoreti-
cal models. Instead, they discovered a significant
population of much older stars mingling with the
young stars and gas.
“This is an important result because knowing
where a galaxy ends means we astronomers can
limit our observations and not waste time, effort,
and computer processing power on studying data
from beyond that point,” said Meurer. “So because
of this work, we now know that galaxies rotate
once every billion years, with a sharp edge that’s
populated with a mixture of interstellar gas [and]
both old and young stars.” — J.P.Disk galaxies spin
like clockwork
Radio images reveal a bustling stellar birthplace
COSMIC CAROUSEL. The spiral galaxy NGC 1232 is just over 60 million light-years away in the constellation
Eridanus. According to new research, all disk galaxies take about a billion years to complete one full rotation.ESO/H. DRASS/ALMA (ESO/NAOJ/NRAO)/A. HACARREADY, SET, FORM STARS. Astronomers combined 296 observations
from Chile’s Atacama Large Millimeter/submillimeter Array (ALMA) and France,
Germany, and Spain’s Institut de Radioastronomie Millimétrique (IRAM) 30-meter
radio dish to create a high-resolution radio image of the Orion Nebula (M42). The
nebula is a hotbed of star formation about 1,350 light-years away. Red areas show
millimeter-wavelength data taken by ALMA and the IRAM telescope, while blue
areas show infrared imagery from the European Southern Observatory’s Very Large
Telescope. The flowing red strands represent large amounts of cold gas that, over
time, compress under their own gravity and produce protostars. Using this data,
astronomers identified 55 such filaments, which are so cold they are invisible to
optical and infrared telescopes. The blue-white points (left side of the image) are
the Trapezium Cluster, hot stars just a few million years old. With its proximity
to Earth and intense star-forming activity, the Orion Nebula serves as a tool for
astronomers looking to study star birth and evolution. — A.J.FALSE ALARM. The mysterious glow of gamma rays at the center of the Milky Way, once believed to come
from dark matter, is instead most likely due to emission from thousands of millisecond pulsars.1 billion
The diameter, in light-years, of the largest ever computer-
generated “slice” of a model universe created by cosmologists
to study galaxy formation.FORS/8.2-METER VLTANTU/ESO