A Firestorm of Star Birth
The Growth of Milky Way-Like Galaxies Over Time
Credit: NASA, ESA, C. Papovich (Texas A&M University), H. Ferguson (STScI), S. Faber (University of California, Santa Cruz), and I. Labbé (Leiden University)
In one of the most comprehensive multi-observatory galaxy surveys
yet, astronomers find that galaxies like our Milky Way underwent a
stellar "baby boom," churning out stars at a prodigious rate, about 30
times faster than today.
Our Sun, however, is a late "boomer." The Milky Way's star-birthing
frenzy peaked 10 billion years ago, but our Sun was late for the party,
not forming until roughly 5 billion years ago. By that time the star
formation rate in our galaxy had plunged to a trickle.
Missing the party, however, may not have been so bad. The Sun's late
appearance may actually have fostered the growth of our solar system's
planets. Elements heavier than hydrogen and helium were more abundant
later in the star-forming boom as more massive stars ended their lives
early and enriched the galaxy with material that served as the building
blocks of planets and even life on Earth.
Astronomers don't have baby pictures of our Milky Way's formative
years to trace the history of stellar growth. Instead, they compiled
the story from studying galaxies similar in mass to our Milky Way,
found in deep surveys of the universe. The farther into the universe
astronomers look, the further back in time they are seeing, because
starlight from long ago is just arriving at Earth now.
From those
surveys, stretching back in time more than 10 billion years,
researchers assembled an album of images containing nearly 2,000
snapshots of Milky Way-like galaxies.
The new census provides the most complete picture yet of how galaxies
like the Milky Way grew over the past 10 billion years into today's
majestic spiral galaxies. The multi-wavelength study spans ultraviolet
to far-infrared light, combining observations from NASA's Hubble and
Spitzer space telescopes, the European Space Agency's Herschel Space
Observatory, and ground-based telescopes, including the Magellan Baada
Telescope at the Las Campanas Observatory in Chile.
"This study allows us to see what the Milky Way may have looked like
in the past," said Casey Papovich of Texas A&M University in
College Station, lead author on the paper that describes the study's
results. "It shows that these galaxies underwent a big change in the
mass of its stars over the past 10 billion years, bulking up by a factor
of 10, which confirms theories about their growth. And most of that
stellar-mass growth happened within the first 5 billion years of their
birth."
The new analysis reinforces earlier research that showed Milky
Way-like galaxies began as small clumps of stars. The diminutive
galaxies built themselves up by swallowing large amounts of gas that
ignited a firestorm of star birth.
The study reveals a strong correlation between the galaxies' star
formation and their growth in stellar mass. Observations revealed that
as the star-making factories slowed down, the galaxies' growth
decreased as well. "I think the evidence suggests that we can account
for the majority of the buildup of a galaxy like our Milky Way through
its star formation," Papovich said. "When we calculate the
star-formation rate of a Milky Way galaxy and add up all the stars it
would have produced, it is pretty consistent with the mass growth we
expected. To me, that means we're able to understand the growth of the
'average' galaxy with the mass of a Milky Way galaxy."
The astronomers selected the Milky Way-like progenitors by sifting
through more than 24,000 galaxies in the entire catalogs of the Cosmic
Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS),
taken with Hubble, and the FourStar Galaxy Evolution Survey (ZFOURGE),
made with the Magellan telescope.
They used the ZFOURGE, CANDELS, and Spitzer near-infrared data to
study the galaxy stellar masses. The Hubble images from the CANDELS
survey also provided structural information about galaxy sizes and how
they evolved. Far-infrared light observations from Spitzer and Herschel
helped the astronomers trace the star-formation rate.
The team's results will appear in the April 9 issue of The Astrophysical Journal.
Contact:
Space Telescope Science Institute, Baltimore, Md.
410-338-4493 / 410-338-4514
dweaver@stsci.edu / villard@stsci.edu
Casey Papovich
Texas A&M University, College Station, Texas
979-862-2704
papovich@tamu.edu
Source: HubbleSite
