The
discovery of "teenage" galaxies is giving scientists a better handle
on how galaxies transform from sexy, spiral star factories to shapeless
retirement homes for old stars.
In the
early 1900s Edwin Hubble discovered that the Milky
Way galaxy is not alone. Our galaxy is just one of many "island
universes," as Hubble dubbed them, swimming in the vast sea of space.
Now that
astronomers can measure the age of each galaxy, its star-making activity and
other related data, they are piecing together an understanding that galaxies
grow gradually like children, gliding through their visibly different teen
years before reaching adulthood.
Results being
published in a December issue of the Astrophysical Journal provide the
strongest evidence yet for this thinking, called nurture theory, in which the
elegant spirals (young galaxies) and blob-like ellipticals (old galaxies) are
evolutionarily linked.
Color-coded
Scientists
have long thought that young galaxies grow up into old ones, referred to as
blue and red galaxies, respectively. The color indicates how actively the
galaxy is churning out new stars. Younger stars shine in ultraviolet or blue
light, and so galaxies bustling with star-making activity appear blue. Older
stars emit infrared or red light. In aging galaxies, their "stellar
reproductive" capacity has begun to shut down and so the remaining stars
are just hanging out for the remainder of their lives.
About half
of all galaxies are blue and half are red. It had been postulated that the two
are linked, with the blue young'uns running out of star-making material and
maturing into passive red galaxies.
If this
theory holds true, you'd expect to see a population of "teenage"
galaxies in the process of transitioning from young to old. Finding these teens
is tricky though, because the cosmic change occurs over billions of years.
"The
nurture theory of galaxy evolution predicted that there would be galaxies in
transition," said lead author Christopher Martin, principal investigator for NASA's
Galaxy Evolution Explorer (GALEX) mission at Caltech in Pasadena, Calif. "Finding these galaxies required ultraviolet light, because they really stand out
at this wavelength."
Cosmic
history
Data from
GALEX, launched in 2003, allowed Martin and his colleagues to observe galaxies
in ultraviolet light across 10 billion years of cosmic
history. The researchers' analysis of tens of thousands of images taken by
GALEX has revealed that young, spiral galaxies do in fact first mature into
"teens" before winding down into their elderly ellipses.
The details
of the picture now emerging suggest that a spiral galaxy might merge with
another spiral or perhaps an irregularly shaped galaxy before churning out a
few bursts of newly minted stars. Eventually, the galaxy begins to exhaust its
star production and settles into later life as an elliptical.
"Our
data confirm that all galaxies begin life forming stars," Martin said.
"Then through a combination of mergers, fuel exhaustion and perhaps
suppression by black holes, the galaxies eventually stop producing stars."
The findings
also suggest that some young galaxies waltz into old age quickly, while others leisurely
stroll into their golden years.
advertisement
