Galaxies Similar to the Milky Way
S. Patel (Leiden University), and the 3D-HST Team
NASA's Hubble Space Telescope has provided the first visual evidence
showing how our home galaxy, the Milky Way, assembled itself into the
majestic pinwheel of stars we see today.
Perusing Hubble's deep-sky surveys, astronomers traced 400 galaxies
similar to our Milky Way at various stages of construction over a time
span of 11 billion years.
"For the first time we have direct images of what the Milky Way
looked like in the past," said study co-leader Pieter G. van Dokkum of
Yale University in New Haven, Conn. "Of course, we can't see the Milky
Way itself in the past. We selected galaxies billions of light-years
away that will evolve into galaxies like the Milky Way. By tracing the
Milky Way's siblings, we find that our galaxy built up 90 percent of
its stars between 11 billion and 7 billion years ago, which is
something that has not been measured directly before."
The Hubble telescope's superb resolving power allowed the researchers
to study how the structure of the Milky Way changed over time. A scale
model of the Milky Way can be imagined by envisioning a fried egg. The
egg white is the disk, where the Sun and Earth reside. The yoke
represents the central bulge of older stars, home to a supermassive
black hole that must have also grown along with the galaxy.
The Hubble images suggest that our galaxy's flat disk and central
bulge grew simultaneously into the majestic spiral galaxy of today.
"You can see that these galaxies are fluffy and spread out," said study
co-leader Shannon Patel, of Leiden University, the Netherlands. "There
is no evidence of a bulge without a disk, around which the disk formed
later." Team member Erica Nelson, of Yale University, added: "These
galaxies show us that the whole Milky Way grew at the same time, unlike
more massive elliptical galaxies, in which the central bulge forms
first."
The survey reveals that billions of years ago, the Milky Way was
likely a faint, blue, low-mass object containing lots of gas, the fuel
for star birth. The blue colors of the Milky Way ancestors are a
signpost of rapid star formation. At the peak of star birth, when the
universe was about 4 billion years old, the Milky Way-like galaxies were
pumping out about 15 stars a year. By comparison, our galaxy today is
creating only one star a year.
To identify the far-flung galaxies and study them in detail, the
research team used three of the largest Hubble programs, the 3D-HST
survey, the Cosmic Assembly Near-infrared Deep Extragalactic Legacy
Survey or CANDELS, and the Great Observatories Origins Deep Survey or
GOODS. These surveys of the distant universe combined spectroscopy with
visible and near-infrared imaging by Hubble's Wide Field Camera 3 and
Advanced Camera for Surveys. The research team's analysis involved
measuring the galaxies' distances and sizes. The astronomers calculated
the mass of each galaxy from its brightness and colors. They selected
the galaxies in their census from a catalog they compiled of over
100,000 galaxies. The survey galaxies are consistent with computer
models, which show that the bulges, and presumably the black holes, of
spiral galaxies at early stages were largely built up at the same time
as the disks.
"In these observations, we're capturing most of the evolution of the
Milky Way," explained team member Joel Leja of Yale University. "These
deep surveys allow us to see the smaller galaxies. In previous
observations we could only see the most luminous galaxies in the
distant past, and now we can look at more normal galaxies. Hubble gives
us the shapes and colors of these spirals as well as their distances
from Earth. We also can measure the rates at which each part of the
galaxies grew. All of this is difficult to do from the ground."
Exploring these galaxies back to their infancy will take the infrared
eyes of NASA's James Webb Space Telescope, scheduled to launch in 2018.
The Hubble images also reinforce the idea that major mergers between
spiral galaxies were not important in building them up. Computer
simulations have shown that mergers would have destroyed the disks.
Instead, this census reveals that spirals grew through star formation.
This galaxy-formation scenario is different from the way massive
elliptical galaxies develop.
"These observations show that there are at least two galaxy-formation
tracks," van Dokkum said. "Massive ellipticals form a very dense core
early in the universe, including a black hole, presumably, and the rest
of the galaxy slowly accretes around it, fueled by mergers with other
galaxies. But from our survey we find that galaxies like our Milky Way
show a different, more uniform path of growing into the majestic spirals
we see today."
The team's results appeared on July 10, 2013, in The Astrophysical
Journal Letters. A second paper appears in the Nov. 11 online edition of
The Astrophysical Journal.
CONTACT
Donna Weaver / Ray VillardSpace Science Telescope Institute, Baltimore, Md.
410-338-4493 / 410-338-4514
dweaver@stsci.edu / villard@stsci.edu
Pieter van Dokkum
Yale University, New Haven, Conn.
203-432-3000
pieter.vandokkum@yale.edu
Shannon Patel
The Observatories of the Carnegie Institution for Science, Pasadena, Calif.
626-304-0292
patel@carnegiescience.edu
