Andrew Yee[_1_]
January 9th 08, 03:30 PM
McDonald Observatory
University of Texas
Fort Davis, Texas
Press Officer Contacts:
Rebecca Johnson, University of Texas at Austin
512-475-6763
Ray Villard, NASA Space Telescope Science Institute
410-338-4514
Whitney Clavin, NASA Jet Propulsion Lab
818-354-4673
Science Contacts:
Dr. Shardha Jogee, University of Texas at Austin
512-471-1395
Dr. Hans-Walter Rix, Max Planck Institute of Astronomy, Germany
rix @ mpia-hd.mpg.de
Dr. Daniel McIntosh, University of Massachusetts, Amherst
dmac @ hamerkop.astro.umass.edu
Dr. Christian Wolf, Oxford University, England
cwolf @ astro.ox.ac.uk
Dr. Catherine Heymans, University of British Columbia, Canada
heymans @ phas.ubc.ca
Dr. Chien Peng, Herzberg Institute of Astrophysics, Canada
cyp @ nrc-cnrc.gc.ca
09 January 2008
Study of 'GEMS' from Hubble, Spitzer Space Telescopes Reveals Cosmic
Fireworks Fizzled Out at Universe Reached Mid-Life
AUSTIN, Texas -- We all start to party less around middle age, and new
studies by a team led by University of Texas at Austin astronomer Shardha
Jogee now finds that the universe, as a whole, is no exception. Jogee
discussed her results today at a news conference at the 211th meeting of the
American Astronomical Society.
According to the current models of galaxy formation, dubbed "hierarchical
lambda cold dark matter" models, galaxies built up to their current masses,
shapes, and sizes through the successive mergers of less massive
protogalaxies made of gas, stars, and dark matter. In the first quarter of
the universe's lifespan, the cosmic landscape was dominated by violent
galaxy mergers, which could radically transform the shape of a galaxy and
convert its gas into stars at an extreme rate. More than half of bright
galaxies were indulging in such violent "partying."
New research is showing that all changed when the universe hit middle age.
"Our study finds that over the last 7 billion years, after the universe hit
its mid-forties, so to speak, it transitioned from a violent merger-driven
mode into a quieter mode," Jogee said.
She and her team find that over each billion-year interval, only 10% of
galaxies are typically involved in strong interactions and mergers.
Jogee's research team includes University of Texas at Austin students Sarah
Miller and Kyle Penner, as well as her colleagues in the international GEMS
collaboration, whose principal investigator is Hans-Walter Rix of the Max
Planck Institute for Astronomy. Jogee's team has analyzed more than 5,000
galaxies imaged by Hubble Space Telescope as part of GEMS, one of the
largest-area surveys conducted with Hubble in two filters.
"With Hubble's spectacular resolution," Penner said, "we could discern
amazing tell-tale clues of the mergers and interactions -- huge tails,
warps, ripples, double nuclei -- in galaxies billions of light-years away."
"It's been exciting to apply different complementary techniques in this
large survey," Miller said, "and to sift through the merger history of the
universe during this elusive era." Miller is the recipient of a 2008 Rhodes
scholarship from Oxford University.
In addition to estimating the frequency of mergers, Jogee and her colleagues
found that contrary to what is commonly assumed, the average star formation
rate in these interacting and merging galaxies is only enhanced by a modest
factor of two to three compared to that in normal non-interacting galaxies.
"While extreme bursts of star formation, so-called cosmic fireworks, may
happen in some galaxy mergers or interactions, they are not the norm in the
vast majority of galaxy interactions taking place over the last 7 billion
years," Jogee said.
The findings of Jogee's team result from a powerful synergy of data from
NASA's Hubble and Spitzer space telescopes. "Mid-infrared observations from
the Spitzer Space Telescope, taken by George Rieke of The University of
Arizona, were key for tracing hidden star formation, obscured by dust,"
Jogee said. "The exquisite resolution of the GEMS Hubble data in turn
allowed us to identify strongly interacting and merging galaxies at much
earlier cosmological times than conventional ground-based telescopes," said
team member Daniel McIntosh of the University of Massachusetts, Amherst.
Jogee and her team, in fact, find that only 20% of the total cosmic star
formation that took place over the last 7 billion years appears to come from
strongly interacting and merging galaxies. These results extend the similar
trend found for a smaller sample of about 1,500 galaxies over a narrower
time interval by fellow team members Christian Wolf from Oxford University
and Eric Bell of the Max Planck Institute of Astronomy.
Furthermore, the results reported by Jogee and her team on the modest
fraction (about 20%) of merger-induced star formation, and the frequency of
galaxy mergers over the last 7 billion years, are in remarkably good
agreement with prevailing theoretical cold dark matter models of galaxy
evolution.
According to team member Rachel Somerville of the Max Planck Institute of
Astronomy, "Mergers are thought to be a crucial process in transforming
galaxies, causing bursts of star formation, and perhaps even feeding gas to
the supermassive black holes lurking in the galaxy's nucleus.
"Although the frequency of mergers predicted by the models agrees quite well
with the observed frequency," Somerville said, "these observations can also
teach us much more about the effect these violent episodes have on
galaxies."
In fact, Jogee said, "Our results raise many additional questions which can
only be addressed with next generation facilities. For example, the cosmic
star-formation rate is declining in normal galaxies, but it remains unclear
what drives this decline. Are galaxies using up their internal cold gas
supply, or is the accretion rate of gas from external filaments declining?"
Next-generation radio facilities, such as ALMA [the Atacama Large
Millimeter/Sub-millimeter Array] will be critical for exploring how the cold
gas content of galaxies changes over the last seven billion years, she said.
"Another key thing to note is that some of our results starkly disagree with
prevailing hierarchical models of galaxy evolution," Jogee said. According
to these models, the frequency of pure disk galaxies or so-called "bulgeless
galaxies" is expected to be extremely low, because a past major merger in
the life of every galaxy invariably builds a bulge.
Contrary to such predictions, postdoctoral fellow Fabio Barazza, formerly
working with Jogee at The University of Texas and now at Geneva
Observatory's Ecole Polytechnique Federale de Lausanne, found that about 20%
of present-day spiral galaxies are bulgeless or disk-dominated, based on the
analysis of about 1,000 galaxies from the Sloan Digital Sky Survey.
"We also see striking super-thin bulgeless galaxies in GEMS, at earlier
epochs," Jogee said. "We yet have to characterize the frequency and origin
of these enigmatic bulgeless galaxies at different epochs, but there is no
denying their prevalence in the local universe."
All in all, "We have made important headway in piecing part of the cosmic
puzzle of galaxy evolution, but daunting challenges loom ahead for both
observers and theorists, " she said.
[NOTE: Images supporting this release are available at
http://mcdonaldobservatory.org/news/releases/2008/0109.html ]
University of Texas
Fort Davis, Texas
Press Officer Contacts:
Rebecca Johnson, University of Texas at Austin
512-475-6763
Ray Villard, NASA Space Telescope Science Institute
410-338-4514
Whitney Clavin, NASA Jet Propulsion Lab
818-354-4673
Science Contacts:
Dr. Shardha Jogee, University of Texas at Austin
512-471-1395
Dr. Hans-Walter Rix, Max Planck Institute of Astronomy, Germany
rix @ mpia-hd.mpg.de
Dr. Daniel McIntosh, University of Massachusetts, Amherst
dmac @ hamerkop.astro.umass.edu
Dr. Christian Wolf, Oxford University, England
cwolf @ astro.ox.ac.uk
Dr. Catherine Heymans, University of British Columbia, Canada
heymans @ phas.ubc.ca
Dr. Chien Peng, Herzberg Institute of Astrophysics, Canada
cyp @ nrc-cnrc.gc.ca
09 January 2008
Study of 'GEMS' from Hubble, Spitzer Space Telescopes Reveals Cosmic
Fireworks Fizzled Out at Universe Reached Mid-Life
AUSTIN, Texas -- We all start to party less around middle age, and new
studies by a team led by University of Texas at Austin astronomer Shardha
Jogee now finds that the universe, as a whole, is no exception. Jogee
discussed her results today at a news conference at the 211th meeting of the
American Astronomical Society.
According to the current models of galaxy formation, dubbed "hierarchical
lambda cold dark matter" models, galaxies built up to their current masses,
shapes, and sizes through the successive mergers of less massive
protogalaxies made of gas, stars, and dark matter. In the first quarter of
the universe's lifespan, the cosmic landscape was dominated by violent
galaxy mergers, which could radically transform the shape of a galaxy and
convert its gas into stars at an extreme rate. More than half of bright
galaxies were indulging in such violent "partying."
New research is showing that all changed when the universe hit middle age.
"Our study finds that over the last 7 billion years, after the universe hit
its mid-forties, so to speak, it transitioned from a violent merger-driven
mode into a quieter mode," Jogee said.
She and her team find that over each billion-year interval, only 10% of
galaxies are typically involved in strong interactions and mergers.
Jogee's research team includes University of Texas at Austin students Sarah
Miller and Kyle Penner, as well as her colleagues in the international GEMS
collaboration, whose principal investigator is Hans-Walter Rix of the Max
Planck Institute for Astronomy. Jogee's team has analyzed more than 5,000
galaxies imaged by Hubble Space Telescope as part of GEMS, one of the
largest-area surveys conducted with Hubble in two filters.
"With Hubble's spectacular resolution," Penner said, "we could discern
amazing tell-tale clues of the mergers and interactions -- huge tails,
warps, ripples, double nuclei -- in galaxies billions of light-years away."
"It's been exciting to apply different complementary techniques in this
large survey," Miller said, "and to sift through the merger history of the
universe during this elusive era." Miller is the recipient of a 2008 Rhodes
scholarship from Oxford University.
In addition to estimating the frequency of mergers, Jogee and her colleagues
found that contrary to what is commonly assumed, the average star formation
rate in these interacting and merging galaxies is only enhanced by a modest
factor of two to three compared to that in normal non-interacting galaxies.
"While extreme bursts of star formation, so-called cosmic fireworks, may
happen in some galaxy mergers or interactions, they are not the norm in the
vast majority of galaxy interactions taking place over the last 7 billion
years," Jogee said.
The findings of Jogee's team result from a powerful synergy of data from
NASA's Hubble and Spitzer space telescopes. "Mid-infrared observations from
the Spitzer Space Telescope, taken by George Rieke of The University of
Arizona, were key for tracing hidden star formation, obscured by dust,"
Jogee said. "The exquisite resolution of the GEMS Hubble data in turn
allowed us to identify strongly interacting and merging galaxies at much
earlier cosmological times than conventional ground-based telescopes," said
team member Daniel McIntosh of the University of Massachusetts, Amherst.
Jogee and her team, in fact, find that only 20% of the total cosmic star
formation that took place over the last 7 billion years appears to come from
strongly interacting and merging galaxies. These results extend the similar
trend found for a smaller sample of about 1,500 galaxies over a narrower
time interval by fellow team members Christian Wolf from Oxford University
and Eric Bell of the Max Planck Institute of Astronomy.
Furthermore, the results reported by Jogee and her team on the modest
fraction (about 20%) of merger-induced star formation, and the frequency of
galaxy mergers over the last 7 billion years, are in remarkably good
agreement with prevailing theoretical cold dark matter models of galaxy
evolution.
According to team member Rachel Somerville of the Max Planck Institute of
Astronomy, "Mergers are thought to be a crucial process in transforming
galaxies, causing bursts of star formation, and perhaps even feeding gas to
the supermassive black holes lurking in the galaxy's nucleus.
"Although the frequency of mergers predicted by the models agrees quite well
with the observed frequency," Somerville said, "these observations can also
teach us much more about the effect these violent episodes have on
galaxies."
In fact, Jogee said, "Our results raise many additional questions which can
only be addressed with next generation facilities. For example, the cosmic
star-formation rate is declining in normal galaxies, but it remains unclear
what drives this decline. Are galaxies using up their internal cold gas
supply, or is the accretion rate of gas from external filaments declining?"
Next-generation radio facilities, such as ALMA [the Atacama Large
Millimeter/Sub-millimeter Array] will be critical for exploring how the cold
gas content of galaxies changes over the last seven billion years, she said.
"Another key thing to note is that some of our results starkly disagree with
prevailing hierarchical models of galaxy evolution," Jogee said. According
to these models, the frequency of pure disk galaxies or so-called "bulgeless
galaxies" is expected to be extremely low, because a past major merger in
the life of every galaxy invariably builds a bulge.
Contrary to such predictions, postdoctoral fellow Fabio Barazza, formerly
working with Jogee at The University of Texas and now at Geneva
Observatory's Ecole Polytechnique Federale de Lausanne, found that about 20%
of present-day spiral galaxies are bulgeless or disk-dominated, based on the
analysis of about 1,000 galaxies from the Sloan Digital Sky Survey.
"We also see striking super-thin bulgeless galaxies in GEMS, at earlier
epochs," Jogee said. "We yet have to characterize the frequency and origin
of these enigmatic bulgeless galaxies at different epochs, but there is no
denying their prevalence in the local universe."
All in all, "We have made important headway in piecing part of the cosmic
puzzle of galaxy evolution, but daunting challenges loom ahead for both
observers and theorists, " she said.
[NOTE: Images supporting this release are available at
http://mcdonaldobservatory.org/news/releases/2008/0109.html ]