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Early Star-Forming Galaxies
Taken from:
Hubble 2012: Science Year in Review
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and the Space Telescope Science Institute.
Hubble
2012: Science Year in Review
The full contents of this book include Hubble science articles, an overview of
and more.
complete
Producedthe
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NASA Goddard
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Center volume and its component sections are
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and the Space
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Institute.
www.hubblesite.org/hubble_discoveries/science_year_in_review
The full contents
of this book include Hubble science articles, an overview of
the telescope, and more. The complete volume and its component sections are
available for download online at:
www.hubblesite.org/hubble_discoveries/science_year_in_review
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Characterizing Early Star-Forming Galaxies
A current topic of interest among astronomers is identifying what fueled the epoch of peak star formation in galaxies from
9 billion to 11 billion years ago. Scientists would particularly like to know whether a quasi-steady, long-lasting process in
isolated galaxies or a more rapid, starburst mode in large, merging galaxies increased the production of stars. Recent findings
from a study using Hubble and the European Space Agency’s Herschel Space Observatory now present a clear answer. Data
from these complementary telescopes indicate that young, quiescent, spiral galaxies created the bulk of the stars on their own.
The study, led by Italian astronomer Giulia Rodighiero, involved the statistical analysis of more than 19,000 galaxies. The
results show that 90 percent of the stars produced 11 billion years ago were formed within young, spiral galaxies. Massive
and merging galaxies accounted for the remaining 10 percent.
Star formation was more vigorous in the past than it is today because early galaxies contained more gas, the raw material
needed to make stars. The more gas a galaxy contains, the more stars it can form. Long-wavelength observations, such as those
made in the far-infrared and submillimeter spectral regimes, can detect the glow of warm dust heated by hot stars nearby. These
stars cannot be seen in visible light because they are buried inside dense clouds of molecular hydrogen that are laced with dust.
Previous observations by ground-based telescopes made at submillimeter wavelengths found only a few massive galaxies
within the epoch of interest. Researchers calculated that these galaxies formed stars at the remarkable rate of 1,000 Sun-like
stars per year—hundreds of times the rate observed today in galaxies similar to our own.
These ground-based, submillimeter surveys convinced many astronomers that massive galaxies undergoing mergers with
other galaxies fueled the star-formation epoch. Rodighiero pointed out, however, that a selection effect was at work—the
instruments used were capable of finding only the brightest galaxies in the early universe. She and her team reasoned that
the bulk of the star-making galaxies were not seen because the galaxies were smaller, dimmer, and enshrouded in dust.
Early galaxies are thought to have formed stars slowly and steadily over hundreds of millions of years by accreting mass from filaments of
cold, intergalactic gas. In this artist’s concept, these filaments are shown streaming into a galaxy, offering a continuous flow of material to
create stars at a sustained pace. (Illustration credit: ESA/AOES Medialab)
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Rodighiero used Herschel ’s far-infrared camera to look for galaxies hidden from visible-light observations because of
their intervening dust. This allowed the astronomers to assemble a more complete picture of star birth than ever before.
The team targeted two well-known regions of the sky that had been observed by Hubble and other telescopes. These were
the Cosmological Evolution Survey (COSMOS), and the Great Observatories Origins Deep Survey South (GOODS) fields.
Analyzing the data, Rodighiero found 600 galaxies exhibiting vigorous star formation.
However, at the target distance (corresponding to the epoch 9 billion to 11 billion years ago), Herschel could only spot bright
galaxies producing stars at a rate of 50 or more Sun-like stars a year. To analyze galaxies creating stars below this threshold,
the astronomers added more than 18,000 star-forming galaxies to the census. These galaxies were selected from near-infrared
observations of the same COSMOS and GOODS-South fields made by several additional telescopes. Images of the COSMOS
field were taken with the Subaru Telescope and the Canada-France-Hawaii Telescope, both atop Mauna Kea in Hawaii. Data
from the GOODS-South region came from Hubble, the Very Large Telescope in Chile, and Spitzer Space Telescope .
The team discovered that 98 percent of these galaxies fell within a predictable sequence, manifesting star-formation rates
based upon the size of their total mass. The remaining two percent showed unusually high star-formation rates that did not
represent the norm.
Rodighiero’s team subsequently used images taken by Hubble ’s Advanced Camera for Surveys to examine the morphology
of 28 of the study’s most massive galaxies. They were surprised to learn that only half these galaxies possessed the elongated
tails of stars and gas indicative of mergers. The other 14 appeared compact and lacked any of the distorted shapes associated
with colliding galaxies. Additional analysis, now underway, may reveal why these objects—if not merging—have such high
star-formation rates. One possibility is that they are nearing the end of a merger and simply do not manifest the telltale traits
seen earlier in the merger process.
Although these giant galaxies do not appear to dominate the star-formation era, they are important objects to study. Their
gas-rich composition not only feeds star formation, but also the growth of supermassive black holes at their centers. These,
in turn, can form quasars whose intense radiation pushes star-forming gases out of the galaxy, shutting down additional
star birth. The massive stars that these early galaxies produced are thought to have had short lives—about 20 million years.
Coupled with the action of a central quasar, these systems would likely transition from colliding, gas-rich spirals into large,
gas-poor elliptical galaxies. Astronomers would like to confirm this process in detail, however.
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Views from the Spitzer Space Telescope and the Herschel Space Observatory created this composite image, which reveals distant starforming galaxies in the COSMOS field. The area of the field is approximately 20 times the area of the full Moon seen with the unaided eye.
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These Hubble near-ultraviolet-light images from the COSMOS survey show 17 star-forming galaxies found in Rodighiero’s Herschel telescope study.
The Hubble images offer a view of young stellar populations in the galaxies, providing crucial information on the relative distributions of young stars
in these systems.
In addition, how normal spiral galaxies created the bulk of the stars during the star-formation epoch is something of a mystery
too. These galaxies do not look like the majestic spiral galaxies seen in our local neighborhood. They have irregular clumpy
shapes with only traces of spiral arms—not the long curving spiral arms wrapped around the well-formed discs seen today.
Today’s galaxies generate stars at a steady pace, about 10 to a few hundred solar masses a year for several billion years. Early
galaxies, however, did not have enough gas to sustain the high star-formation rate manifested during this ancient epoch.
Computer simulations of the period suggest that, unlike today, gas was funneled into these galaxies from intergalactic space.
The added gas provided the fuel to maintain their high star-formation rates.
To confirm this supposition, Rodighiero plans to measure the gas content of the early super-starburst galaxies directly using
new interferometric facilities such as the Atacama Large Millimeter/submillimeter Array (ALMA), an array of radio telescopes
in the Atacama desert of northern Chile. She will then compare her measurements with those of normal star-forming galaxies
observed at the same epoch. Eventually, her team—along with many other astronomers—should unravel the mysteries of
these early star-forming systems.
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Multi-wavelength observations often reveal differences in galaxy shapes and sizes. (Left) Two starburst galaxies, found with the Herschel observatory,
appear bright in near-infrared images taken with Hubble’s Wide Field Camera 3. (Right) These shorter-wavelength images of the same two galaxies
reveal luminous regions of ongoing star formation. Older and dimmer stars dominate the reddish outer regions.
Further Reading
Dunlop, J., “The Cosmic History of Star Formation.” Science 333, no. 178 (July 8, 2011): 178–181.
Elbaz, D., et al. “GOODS-Herschel: An Infrared Main Sequence for Star-Forming Galaxies.” Astronomy & Astrophysics 533
(September 13, 2011): 119.
Lutz, D., et al. “PACS Evolutionary Probe (PEP). A Herschel key program.” Astronomy & Astrophysics 532 (July 28, 2011): 90.
Magnelli, B., et al. “Dust Temperature and CO → H2 Conversion Factor Variations in the SFR–M* Plane.” Astronomy & Astrophysics
548 (November 2012): 22.
Mullaney, J. R., et al. “The Hidden AGN Main Sequence: Evidence for a Universal Black Hole Accretion to Star Formation Rate Ratio
since z ~ 2 Producing an M BH-M* Relation.” The Astrophysical Journal Letters 753, no. 2 (June 19, 2012): 30.
Rodighiero, G., et al. “The Lesser Role of Starbursts in Star Formation at z = 2.” The Astrophysical Journal Letters 739, no. 2
(September 2, 2011): 40.
Wuyts, S., et al. “Galaxy Structure and Mode of Star Formation in the SFR–Mass Plane from z~2.5 to vvvvvvv~0.1.” The
Astrophysical Journal 742, no. 2 (December 1, 2011): 96.
Dr. Giulia Rodighiero is a staff researcher at the Department of Physics and Astronomy “Galileo Galilei” at the
University of Padua in Italy. Her work focuses on galaxy formation and evolution, particularly the statistical
and physical properties of dusty star-forming galaxies at high redshift. Her past research has utilized mid- and
far-infrared space telescopes—such as the Infrared Space Observatory, Spitzer Space Telescope, and Herschel
Space Observatory—to look at the evolution of the luminosity functions and star-formation rates of the most
obscured objects invisible at optical wavelengths. Dr. Rodighiero received her master of science in astrophysics
and her doctorate in astrophysics from the University of Padua, where she is currently teaching astronomy
laurea degree courses. She was born and resides in Vicenza, Italy.
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