Kamuela, HI -- Astronomers have detected cold streams of primordial hydrogen, vestigial matter left over from the Big Bang, fueling a distant star-forming galaxy in the early Universe. Profuse flows of gas onto galaxies are believed to be crucial for explaining an era 10 billion years ago, when galaxies were copiously forming stars. To make this discovery, the astronomers – led by Neil Crighton of the Max Planck Institute for Astronomy and Swinburne University – made use of a cosmic coincidence: a bright, distant quasar acting as a "cosmic lighthouse" illuminates the gas flow from behind. The results were published October 2 in the Astrophysical Journal Letters.
The systematic survey of absorption systems comprises
observations with the Large Binocular Telescope and from data taken with the W.
M. Keck Observatory’s
HIRES echelle spectrograph installed on the 10 meter Keck I telescope on the
summit of Mauna Kea, Hawaii. The foreground galaxy was discovered by Charles
Steidel, Gwen Rudie (California Institute of Technology) and collaborators
using the Keck Observatory's LRIS spectrograph on the same telescope.
In the current narrative
of how galaxies like our own Milky Way formed, cosmologists postulate they were
once fed from a vast reservoir of pristine hydrogen in the intergalactic
medium, which permeates the vast expanses between galaxies.
Approximately ten
billion years ago when
the Universe was one-fifth its current age, early proto-galaxies were in a
state of extreme activity, forming new stars nearly one hundred times their
current rate. Because stars form from gas, this fecundity demands a steady
source of cosmic fuel. In the past decade, supercomputer simulations of galaxy
formation have become so sophisticated that they can actually predict how
galaxies form and are fed: gas funnels onto galaxies along thin "cold
streams" which, like streams of snow melt feeding a mountain lake, channel
cool gas from the surrounding intergalactic medium onto galaxies, continuously
topping up their supplies of raw material for star formation.
However, testing these
predictions has
proven to be extremely challenging, because such gas at the edges of
galaxies is so
rarefied that it emits very little light. Instead, the team of
astronomers
systematically searched for examples of a very specific type of cosmic
coincidence. Quasars constitute a brief phase in the galactic
life-cycle, during which they shine as the most luminous objects in the
Universe, powered by the infall of matter
onto a supermassive black hole. From our perspective on Earth, there
will be
rare cases where a distant background quasar and a stream of primordial
gas
near a foreground galaxy are exactly aligned on the night sky. As light
from
the quasar travels toward Earth, it passes by the galaxy and through the
primordial gas, before reaching our telescopes. The cosmic gas
selectively
absorbs light at very specific frequencies which astronomers refer to as
"absorption lines". The pattern and shape of these lines provide a
cosmic barcode, which astronomers can decode to determine the chemical
composition, density, and temperature of the gas.
Using this technique, a
team of astronomers led by Neil Crighton (Max Planck Institute for Astronomy;
now at Swinburne University of Technology, Melbourne) has found the best
evidence to date for a flow of pristine intergalactic gas onto a galaxy. The
galaxy, denoted Q1442-MD50, is so distant that it took 11 billion years for its
light to reach us. The primordial infalling gas resides a mere 190,000
light-years from the galaxy –
relatively nearby on galactic length-scales – and is revealed in silhouette in the
absorption spectrum of the more distant background quasar QSO J1444535+291905.
A crucial element of their discovery is the
detection of the spectral signature of cosmic deuterium, a stable isotope of
hydrogen (with an extra neutronin
the nucleus). Cosmologists have demonstrated that hydrogen and helium and their
stable isotopes like deuterium were all synthesized just minutes after the Big
Bang, when the Universe was hot enough to power nuclear reactions. All heavier
elements like carbon, nitrogen, and oxygen were created much later in the hot
nuclear furnaces of stars. Because the hostile physical conditions in the
centers of stars would destroy the fragile deuterium isotope, the discovery of
deuterium in the gas confirms that the gas falling onto the galaxy is indeed
pristine material left over from the Big Bang.
“This
is not the first time astronomers have found a galaxy with nearby gas, revealed
by a quasar. But it is the first time that everything fits together,"
Crighton said. "The galaxy is vigorously forming stars, and the gas properties
clearly show that this is pristine material, left over from the early universe
shortly after the big bang.”
This discovery of this
system is part of a large survey for quasar sightlines which pass near
galaxies, which is coordinated by Joseph Hennawi, the leader of theENIGMA research group at the Max Planck
Institute for Astronomy.
"Since this
discovery is the result of a systematic search, we can now deduce that such
cold flows are quite common," Hennawi said. "We only had to search 12
quasar-galaxy pairs to discover this example. This rate is in
rough agreement with the predictions of supercomputer simulations, which
provides a vote of confidence for our current theories of how galaxies
formed."
The astronomers’ long-term goal is to find about ten
similar examples of these cold flows, which would allow for a much more
detailed comparison of their observations with the predictions of numerical
models.
"Previous studies
of these galaxies had shown evidence for gas flowing out of them, something we
also see evidence for," said J. Xavier Prochaska (University of California
at Santa Cruz), a collaborator on the survey. "However with Neil's much
more precise analysis, wecan
also detect the raw material fueling galaxies, and thereby trace how much gas
they take in, and when. That is a key piece in the puzzle of galaxy formation.”
Avishai Dekel (Hebrew
University, Jerusalem) was instrumental in theoretically and numerically
establishing the current model of cold-flow accretion onto galaxies. While not involved in this research, he
commented on the results. "This is a very interesting finding," Dekel
said. "It is consistent with the theoretical prediction, based both on
physical analysis and on cosmological simulations, for the feeding of
high-redshift galaxies by cold streams from the cosmic web. The low metallicity
makes this case for inflow more convincing than earlier detections."
The W. M. Keck
Observatory operates the largest, most scientifically productive telescopes on
Earth. The two, 10-meter optical/infrared telescopes on the summit of Mauna Kea
on the Island of Hawaii feature a suite of advanced instruments including
imagers, multi-object spectrographs, high-resolution spectrographs,
integral-field spectroscopy and a world-leading laser guide star adaptive
optics system. The Observatory is a private 501(c) 3 non-profit organization
and a scientific partnership of the California Institute of Technology, the
University of California and NASA.
Science Contacts
Neil H. M. Crighton (first author)
Max Planck Institute for Astronomy (until August 2013)
Swinburne University of Technology
+61 (0) 3 9214 5536
ncrighton@swin.edu.au
Joseph F. Hennawi
Max Planck Institute for Astronomy
+49 (0)6221-528 263
joe@mpia.de
J. Xavier Prochaska
University of California, Santa Cruz
+1 831 459 2135
xavier@ucolick.org
Science Contacts
Neil H. M. Crighton (first author)
Max Planck Institute for Astronomy (until August 2013)
Swinburne University of Technology
+61 (0) 3 9214 5536
ncrighton@swin.edu.au
Joseph F. Hennawi
Max Planck Institute for Astronomy
+49 (0)6221-528 263
joe@mpia.de
J. Xavier Prochaska
University of California, Santa Cruz
+1 831 459 2135
xavier@ucolick.org
