Notre Dame astronomers find massive supply of gas around modern galaxies
Galaxies
have a voracious appetite for fuel -- in this case, fresh gas -- but
astronomers have had difficulty finding the pristine gas that should be falling
onto galaxies. Now, scientists have provided direct empirical evidence for
these gas flows using new observations from the Hubble Space Telescope. The
team led by Nicolas Lehner, research associate professor at the University of
Notre Dame, is presenting its work January 11 at the meeting of the American
Astronomical Society in Long Beach, Calif.
The team's
observations using Hubble's two ultraviolet spectrographs, the Cosmic Origins
Spectrograph and the Space Telescope Imaging Spectrograph, show large
quantities of cool gas with very low quantities of heavy elements in the
gaseous cocoons surrounding modern galaxies. The lack of heavy elements
indicates this gas in the circumgalactic medium of the galaxies has not been
strongly processed through stars. The members' work, "The Bimodal
Metallicity Distribution of the Cool Circumgalactic Medium at z<1," has
been submitted to the Astrophysical Journal.
Led by
Lehner, the team of astronomers identified gaseous streams near galaxies
through the absorption they imprint on the spectra of distant, bright
background quasars. The atoms in the gas remove small amounts of the light, and
as the light from the quasars passes through the gas around galaxies, the
chemical elements leave characteristic spectral "fingerprints" that
allow astronomers to study the physical and chemical properties of the gas.
Lehner and collaborators searched for the signature of gas within about
100,000-300,000 light-years of galaxies, identifying this gas due to its strong
hydrogen absorption, a known signature of circumgalactic gas. They subsequently
determined the amount of "metals" -- all elements heavier than
hydrogen and helium -- in this gas to test whether the circumgalactic matter
was being newly accreted from intergalactic space and lacking in metals or
being ejected from the galaxies themselves and strong in metals.
"Astronomers
have been searching for this in-falling gas for a while," notes Lehner.
"However, due to observational limitations, they had to search for
metal-poor gas using the metals themselves. Since there is a tiny amount of
metals in this gas, it was difficult to find in that way." The new work
uses ultraviolet spectroscopy to identify the gas through its hydrogen
absorption, which is independent of the metal content. This has allowed the
team for the first time to determine how heavy elements are distributed around
galaxies in an unbiased manner.
Lehner
and colleagues estimated the amount of metals in the circumgalactic medium of
galaxies over the last six billion years. They found that the distribution of
heavy elements abundances in circumgalactic gas has two different
characteristic values, around 2 percent and 40 percent of the heavy element
content of the sun. Both branches of the metal abundance distribution have a
nearly equal number of gas clouds. Meanwhile, the circumgalactic gas probed in
this study was also found to have a mass comparable to that of all the gas
within the galaxies themselves, thus providing a substantial reservoir for
fueling continued star formation in modern galaxies. This study confirms the
earlier finding by the same team that metal-enriched gas is widespread even far
from the galaxies themselves, likely sent there by strong outflows driven by
supernovae. The metal-rich gas likely traces winds and recycled gas from
outflows and galaxy interactions. The metal-poor gas is in quantities of metals
too low to trace even in very low-metallicity galaxies that are six billion
years old or older. It very likely traces cold streams onto galaxies; its
properties are in very good agreement with those seen in the computer
simulations of galaxy formation and evolution.
"One
of the big questions remaining from our study is what types of galaxies are
associated with these gas clouds," remarks Lehner. The luminous components
of most of the galaxies in the current study have not yet been identified. This
team will use the Large Binocular Telescope, Keck and other ground-based
telescopes to reveal the nature of the galaxies.
"Independent
of the interpretation, our findings place new constraints on our understanding
of how elements are distributed around galaxies," Lehner concludes.
"There is not only a large mass of metal-rich gas around galaxies in the
modern-day universe, but also a significant mass of metal-poor gas that may
become available for star formation." This new work also implies the more
diffuse intergalactic medium far from galaxies in the modern universe may be
far more metal deficient than previously thought.
This
research has been funded by NASA and the National Science Foundation, and has
made use of the Hubble, Keck and Magellan telescopes. Co-authors include J.
Christopher Howk from Notre Dame; Todd Tripp from the University of
Massachusetts; Jason Tumlinson from the Space Telescope Science Institute
(STScI); J. Xavier Prochaska from the University of California, Santa Cruz;
John O'Meara from St. Michael's College; Chris Thom from STScI; Jess Werk from
the University of California, Santa Cruz; Andrew Fox from STScI; and Joe
Ribaudo from Utica College.
Source: University
of Notre Dame
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