Large survey of galaxies yields new findings on star formation (Forwarded)

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University of California-San Diego

Contact:
Tim Stephens, (831) 459-2495

January 9, 2006

Large survey of galaxies yields new findings on star formation

New findings from a large survey of galaxies suggest that star formation
is largely driven by the supply of raw materials, rather than by
galactic mergers that trigger sudden bursts of star formation. Stars
form when clouds of gas and dust collapse under the force of gravity,
and the study supports a scenario in which exhaustion of a galaxy's gas
supply leads to a gradual decline in the star-formation rate.

The results, presented this week at the American Astronomical Society
(AAS) meeting in Washington, D.C., come from the Extended Groth Strip
Survey, a collaborative effort using major ground-based and space-based
telescopes to focus on one patch of sky that offers a clear view of the
distant universe.

By analyzing data from a combination of powerful instruments,
researchers derived information on galaxy weights and star formation
rates, as well as the numbers of stars already formed, for more than
3,500 galaxies. They found that the weight (or mass) of a galaxy is an
important factor determining how fast it makes stars and how the star
formation rate evolves over time, said Kai Noeske, a postdoctoral
researcher at the University of California, Santa Cruz.

"The picture we're getting is that heavy galaxies form stars early and
rapidly, whereas smaller galaxies form their stars over longer
timescales," said Noeske, who presented the group's findings at the AAS
meeting on Monday, January 9.

The study's findings shed light on ongoing debates over the physical
mechanisms that activate star formation in galaxies -- in particular,
the importance of starbursts triggered by mergers of similar galaxies.

"What we see is consistent with mostly undisturbed galaxies using up
their gas over time, like firewood burning down," Noeske said.

The Extended Groth Strip collaboration consists of astronomers from 16
institutions who have pooled their data and resources to create what is
now one of the most intensely studied regions of the sky, said David
Koo, professor of astronomy and astrophysics at UCSC and a member of the
team.

Light from distant galaxies takes billions of years to reach Earth,
giving astronomers a window into the past. The galaxies included in this
study cover a wide range of redshifts (a measure of distance) and
corresponding "lookback times," extending out to redshift 1.4 or as far
back in time as 9 billion years, about two-thirds of the age of the
universe. The study also encompassed galaxies with a wide range of masses.

"We have now been able to track star formation in galaxies out to modest
distances, more than half the age of the universe, and we find that all
galaxies, big or small, seem to be fading gradually so that they are
less active today than they were further back in time," Koo said.

Astronomers have found from previous galaxy surveys that star formation
activity becomes more intense as they probe farther back in time. One
proposed explanation has been that galaxy mergers were more frequent in
the past, triggering bursts of star formation due to compression of gas
clouds during the merger process.

"We are finding that mergers do not appear to play the dominant role in
star formation, because we see normal-looking, undisturbed galaxies that
are undergoing large amounts of star formation," Koo said.

"There probably are multiple mechanisms that can activate star
formation. We are asking which is dominant," he added. "Mergers do drive
star formation; they just don't seem to be the major driver."

Koo and Noeske are both members of the DEEP2 team, one of seven survey
teams involved in the Extended Groth Strip Survey. DEEP (Deep
Extragalactic Evolutionary Probe) began about 15 years ago, led by Koo
and other UCSC astronomers using the twin 10-meter Keck Telescopes at
the W. M. Keck Observatory in Hawaii and NASA's Hubble Space Telescope
to conduct a large-scale survey of distant field galaxies. Phase 2 of
the project, led by UCSC and UC Berkeley, began three years ago using
the powerful DEIMOS spectrograph on the Keck II Telescope and has now
gathered spectroscopic data from almost 40,000 distant galaxies.

DEEP2 has observed 13,000 galaxies in the Extended Groth Strip, one of
four fields surveyed by the project. Joining the DEEP2 team in the
Extended Groth Strip Survey is a broad consortium of other survey teams
that are contributing data. Infrared data from NASA's Spitzer Space
Telescope were especially important for Noeske's study, because they
enable astronomers to see through the dust that obscures much of the
star formation taking place in distant galaxies.

"Having the infrared data from Spitzer allows us to measure the star
formation rates very accurately because we are no longer blinded by
dust," Koo said.

The array of instruments trained on the Extended Groth Strip covers a
tremendous range of wavelengths, including x-rays and radio waves, as
well as infrared, visible, and ultraviolet light.

"This is an exceptional period of time for astronomy, because for the
first time we are able to combine data from almost all of the important
wavelengths," Koo said.

This work is linked to other projects that analyze Extended Groth Strip
data. The same session at the AAS meeting includes a presentation by
Kevin Bundy of the California Institute of Technology on how the
termination of star formation is related to a galaxy's weight and
environment. Projects led by Jennifer Lotz of the National Optical
Astronomy Observatory (NOAO) and Lihwai Lin of National Taiwan
University measure the frequency of galaxy mergers and their importance
in the production of new stars over the past 8 billion years.

The following teams contributed to the study presented by Noeske at AAS:

DEEP2 team: Marc Davis (principal investigator), Jeffrey Newman, and
Michael Cooper at UC Berkeley; Sandra Faber (co-PI), Kai Noeske (lead
author), David Koo, and Susan Kassin at UCSC; Benjamin Weiner at the
University of Maryland; Christopher Willmer and Alison Coil at the
University of Arizona; and Jennifer Lotz at NOAO.

Spitzer IRAC team (Infrared Array Camera on the Spitzer Space
Telescope): Giovanni Fazio (PI), Jiasheng Huang, and Pauline Barmby at
the Harvard-Smithsonian Center for Astrophysics, and Gillian Wilson at
the Spitzer Science Center, Caltech.

Spitzer MIPS team (Multiband Imaging Photometer for Spitzer): George
Rieke (PI), Emeric Le Floc'h, and Casey Papovich at the University of
Arizona.

Palomar Near Infrared Survey: Richard Ellis (PI) and Kevin Bundy at
Caltech, and Christopher Conselice (co-PI) at the University of
Nottingham, UK.

GALEX team (Galaxy Evolution Explorer satellite): Christopher Martin
(PI) and Todd Small at Caltech, and David Schiminovich at Columbia
University.

The EGS collaboration also includes radio astronomers observing this
region with the NRAO Very Large Array and the James Clerk Maxwell
Telescope. Institutions involved in the collaboration include, in
addition to those listed above, Imperial College of London, Space
Telescope Science Institute, Herzberg Institute of Astrophysics in
Canada, National Taiwan University, UCLA, Oxford University, and the
Royal Observatory of Edinburgh.

This work was supported by the National Science Foundation, NASA, and
the Space Telescope Science Institute.

Note to reporters: You may contact Noeske at (831) 459-3387 and Koo at
(831) 459-2130.


IMAGE CAPTIONS:

[Image 1:
http://www.ucsc.edu/news_events/pres...s_galaxies.jpg (72KB)]
Field galaxies in the Extended Groth Strip survey, seen at a redshift of
approximately 0.7, or 6.5 billion years back in time.

The two images on the left show galaxies in the process of merging. Such
merger events can cause the birth of many stars at once. In the past,
big galaxies formed stars more quickly than today, and scientists have
explored whether frequent mergers caused most of this rapid star formation.

The two images on the right show spiral galaxies similar to our own
galaxy, the Milky Way. New data from the Extended Groth Strip have now
revealed that the increased pace of star formation mostly happened in
such spiral galaxies.

Undisturbed spiral galaxies form their stars at a more steady pace than
mergers. However, billions of years ago, this pace was much higher than
today: galaxies still had a bigger supply of gas from which many stars
could form at the same time. Today, much of this gas has already been
turned into stars and the remaining gas can only support the birth of
stars at a slower pace.

Images were taken with the Hubble Space Telescope Advanced Camera for
Surveys, and measure 9 arcseconds or 1/200 of the full moon's diameter
on a side. At the distance of the galaxies, this corresponds to roughly
210,000 lightyears. The color images were created from exposures in two
filters, F606W (red) and F814W (near infrared).

This material was presented to the American Astronomical Society meeting
in Washington, DC on January 9, 2006.

PHOTO CREDIT: The DEEP2 Team, UC Berkeley and UC Santa Cruz, K. Noeske
and J. Lotz, NASA Hubble Space Telescope.

[Images 2 & 3:
http://www.ucsc.edu/news_events/pres...gs_cutout1.jpg (35KB)
http://www.ucsc.edu/news_events/pres...gs_cutout2.jpg (45KB)]
These images show parts of the Extended Groth Strip, a sky region in the
vicinity of the Big Dipper. This patch of sky has very few bright stars,
and opens the view to extremely faint, distant galaxies. Light from
distant galaxies traveled billions of years until it reached Earth, and
gives astronomers a window into the past. Astronomers have observed the
Extended Groth Strip using a variety of powerful telescopes on Earth and
satellite telescopes from space, creating a pool of information on the
history of galaxies and the Universe.

These images were taken with the Hubble Space Telescope Advanced Camera
for Surveys. The image egs_cutout1.jpg measures 1 arcminute or 1/30 of
the full moon's diameter on a side; egs_cutout2.jpg measures 2/3 of an
arcminute on a side, or 1/50 of the full moon's diameter. The full
region photographed by the Hubble Space telescope is a rectangle of 1
degree by 10 arcminutes (two by 1/3 full moon diameters), and contains
more than 80,000 galaxies.

The color images were created from exposures in two filters, F606W (red)
and F814W (near infrared).

This material was presented to the American Astronomical Society meeting
in Washington, DC on January 9, 2006.

PHOTO CREDIT: The DEEP2 Team, UC Berkeley and UC Santa Cruz, J. Lotz and
K. Noeske, NASA Hubble Space Telescope.