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 is led by Nicolas Lehner, research associate professor at the University of Notre Dame.
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 team's work, "The
Bimodal Metallicity Distribution of the Cool Circumgalactic Medium at z<1 astrophysical="astrophysical" been="been" div="div" has="has" journal.="journal." nbsp="nbsp" submitted="submitted" the="the" to="to">
Image credit: ALMA (ESO/NAOJ/NRAO); ESO/Y. Beletsky
Source: The Daily Galaxy via University of Notre Dame1>
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 infalling 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.
The image at the top of the page shows Centaurus A's Clouds of Gas This
new image of Centaurus A combines ALMA and near-infrared observations of
the massive elliptical radio galaxy. The new ALMA observations, shown
in a range of green, yellow and orange colors, reveal the position and
motion of the clouds of gas in the galaxy. They are the sharpest and
most sensitive such observations ever made.
Image credit: ALMA (ESO/NAOJ/NRAO); ESO/Y. Beletsky
Source: The Daily Galaxy via University of Notre Dame1>
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