Individual young stars and star clusters in the ‘dead’ elliptical galaxy, Messier 105, detected using the Wide Field Camera 3 (WFC3) on the Hubble Space Telescope (HST). Messier 105 can be seen in the top, left corner, in an image from the Sloan Digital Sky Survey (SDSS; Data Release 8). The outlined region in the center of Messier 105 is expanded to reveal Hubble’s unique view of the inner region of Messier 105, which is further expanded to unveil several individual young stars and star clusters (denoted by dashed circles; top, right). These signposts of recent star formation are unexpected in old, ‘dead’ galaxies. Data from HST’s WFC3 and Advanced Camera for Surveys (ACS) were used in the creation of these HST images.”
of Michigan astronomers
examined old galaxies and were surprised to discover that they are still making
new stars. The results provide insights into how galaxies evolve with time.
U-M research fellow Alyson Ford and astronomy professor Joel
Bregman used the Wide Field Camera 3 on the Hubble Space Telescope and saw
individual young stars and star clusters in four galaxies that are about 40
million light years away. One light year is about 5.9 trillion miles.
“Scientists thought these were dead galaxies that had
finished making stars a long time ago,” Ford said. “But we’ve shown
that they are still alive and are forming stars at a fairly low level.”
Galaxies generally come in two types: spiral galaxies, like
our own Milky Way, and elliptical galaxies. The stars in spiral galaxies lie in
a disk that also contains cold, dense gas, from which new stars are regularly
formed at a rate of about one sun per year.
Stars in elliptical galaxies, on the other hand, are nearly
all billions of years old. These galaxies contain stars that orbit every which
way, like bees around a beehive. Ellipticals have little, if any, cold gas, and
no star formation was known.
“Astronomers previously studied star formation by
looking at all of the light from an elliptical galaxy at once, because we
usually can’t see individual stars,” Ford said. “Our trick is to make
sensitive ultraviolet images with the Hubble Space Telescope, which allows us
to see individual stars.”
The technique enabled the astronomers to observe star
formation, even if it is as little as one sun every 100,000 years.
Ford and Bregman are working to understand the stellar birth
rate and likelihood of stars forming in groups within ellipticals. In the Milky
Way, stars usually form in associations containing from tens to 100,000 stars.
In elliptical galaxies, conditions are different because there is no disk of
cold material to form stars.
“We were confused by some of the colors of objects in
our images until we realized that they must be star clusters, so most of the
star formation happens in associations,” Ford said.
The team’s breakthrough came when they observed Messier 105,
a normal elliptical galaxy that is 34 million light years away, in the
constellation Leo. Though there had been no previous indication of star
formation in Messier 105, Ford and Bregman saw a few bright, very blue stars,
resembling a single star 10 to 20 times the mass of the sun.
They also saw objects that aren’t blue enough to be single
stars, but instead are clusters of many stars. When accounting for these
clusters, stars are forming in Messier 105 at an average rate of one sun every
10,000 years, Ford and Bregman concluded. “This is not just a burst of
star formation but a continuous process,” Ford said.
These findings raise new mysteries, such as the origin of
the gas that forms the stars.
“We’re at the beginning of a new line of research,
which is very exciting, but at times confusing,” Bregman said. “We
hope to follow up this discovery with new observations that will really give us
insight into the process of star formation in these ‘dead’ galaxies.”