Cambridge, MA — One of the fundamental questions astronomers are
trying to answer is: What is the Universe made of? Numerous lines of
evidence show that the Universe is about 73 percent “dark energy,” 23
percent “dark matter,” and only 4 percent normal matter. Yet this
answer raises further questions, including: Where is all the normal
matter?

Astronomers call this dilemma the “missing mass” problem. They can
see normal, baryonic matter — protons, electrons, and neutrons —
when it forms luminous stars, or when it blocks starlight as huge,
dark molecular clouds. And what they see totals only a fraction of
the normal matter they know is out there.

Now, astronomer Fabrizio Nicastro of the Harvard-Smithsonian Center
for Astrophysics (CfA) and colleagues have found evidence for the
existence of a large reservoir of baryons in our Local Group of
galaxies. This baryonic matter forms a warm fog surrounding and
enveloping the Milky Way and its neighbors.

“Our research shows that this warm fog may hold as much as two-thirds
of the normal matter within the neighborhood of the Milky Way,” says
Nicastro.

Finding The Missing Mass

This warm intergalactic fog is a challenge to find. Astronomers
cannot see it directly because it is too diffuse, despite its
temperature of 100,000 to 10 million Kelvin (10^5 — 10^7 K), which
causes it to shine faintly in X-rays. Instead, they detect the fog
using the shadow it casts. Nicastro and his team looked at
ultraviolet and X-ray wavelengths where the intergalactic fog
absorbed light from distant sources like quasars and active galactic
nuclei. They culled data from the Far Ultraviolet Spectroscopic
Explorer (FUSE) satellite to identify about 50 clouds, or fog banks,
surrounding our galaxy in every direction.

Atoms in individual clouds absorb light at specific wavelengths,
creating dark lines in the spectra of background light sources. The
motion of a cloud shifts the wavelength of its spectral line due to
the Doppler effect. Nicastro’s team used these spectral shifts to
derive radial (line-of-sight) velocities for the clouds, giving clues
to the clouds’ locations and origins. Those studies showed that the
warm clouds were almost certainly part of the Local Group of
galaxies, which is comprised of the Milky Way and Andromeda spirals,
along with about 30 smaller galaxies.

Given the amount of material they detected using FUSE and NASA’s
Chandra X-ray Observatory, Nicastro and his associates infer that the
warm fog in the Local Group contains as much mass as a million
million (10^12) Suns. This result shows remarkable agreement with the
amount of matter needed to gravitationally bind together the galaxies
within the Local Group.

A Relic Of Galaxy Formation

“Given the fact that this warm fog exists, it raises the question of
where this matter came from,” says Nicastro. “Most likely, it is
material left over from the galaxy formation process, a relic from
the early history of the Universe.”

Theories indicate that the early Universe was filled with a nearly
homogeneous mix of hydrogen and helium gas. Clumps of dark matter
within this primordial soup acted as seeds for galaxy formation. Over
several hundred million years of time, the force of gravity pulled
together some of the Universe’s normal matter to form galaxies
holding billions of stars.

However, only about one-third of the Universe’s baryonic matter was
consumed. Much of it still floats between the galaxies, invisible
except for the shadow it casts.

“Finding this leftover material provides further evidence that our
theories of galaxy formation are correct and offers clues to the
history of our own Milky Way galaxy,” says Nicastro. “This discovery,
combined with future research, also may help track dark matter
because the intergalactic filaments of baryonic matter should connect
the dark matter clumps.”

This research was reported in the February 12, 2003, issue of the
scientific journal Nature in a paper authored by Fabrizio Nicastro
(CfA); Andreas Zezas and Martin Elvis (CfA); Smita Mathur (Ohio State
University); Fabrizio Fiore (Osservatorio Astronomico di
Monteporzio); Cesare Cecchi-Pestellini, Douglas Burke, Jeremy Drake,
and Piergiorgio Casella (CfA).

Headquartered in Cambridge, Massachusetts, the Harvard-Smithsonian
Center for Astrophysics (CfA) is a joint collaboration between the
Smithsonian Astrophysical Observatory and the Harvard College
Observatory. CfA scientists organized into six research divisions
study the origin, evolution, and ultimate fate of the universe.

NOTE: contact data are near the end.

Short and Long Gamma-Ray Bursts Different to the Core

While the origin of gamma-ray bursts — the most powerful explosions
known in the universe — remains a mystery, scientists say that the
two major varieties, long and short bursts, arise from different
types of events.

In an analysis of nearly 2,000 bursts, a team of researchers from
Europe and Penn State University uncovered new discrepancies in the
light patterns in bursts lasting less the two seconds and in bursts
lasting longer than two seconds.

“We can now say with a high degree of statistical certainty that the
two show a different physical behavior,” said Lajos Balazs of Konkoly
Observatory in Budapest, lead author on a paper appearing in an
upcoming issue of the journal Astronomy & Astrophysics.

The analysis supports the growing consensus that long bursts
originate from fantastic explosions of stars over 30 times more
massive than our Sun. Short bursts have been variously hypothesized
to be fiery mergers of neutron stars, black holes, or both, or
perhaps a physically different type of behavior in massive collapses.

“It is suspected that, either way, with each gamma-ray burst we wind
up with a brand new black hole,” said Peter Meszaros, professor and
head of the Penn State Department of Astronomy and Astrophysics.
“The puzzle is in trying to identify clues that would help to
elucidate whether these two types consist of essentially the same
objects with different behaviors, or different objects with somewhat
similar behavior.”

Gamma-ray bursts are like a 10^45 watt bulb, over a million trillion
times as bright as the Sun. Although common — detectable at a rate
of about one per day — the bursts are fast-fading and random, never
occurring in the same place twice. Scientists have been hard pressed
to study the bursts in detail, for they last only a few milliseconds
to about 100 seconds, with most around 10 seconds long. Most
scientists agree that the majority of bursts originate in the distant
reaches of the universe, billions of light years away.

Previous results have shown that the short bursts have “harder”
spectra, which means that they contain relatively more higher-energy
gamma-ray photons than the longer bursts do. Also, in short bursts,
the photons hitting a burst detector are closely spaced, or bunched,
compared to the longer bursts, suggesting that the source is
physically different, as well.

This type of information is valuable because it appears to contain
clues about the intrinsic physical mechanism by which the sources
produce the gamma rays, but these sources have still not been
characterized in enough detail to understand them. Balazs and his
colleagues sought to establish what, if any, correlation exists
between different pairs of properties, when one considers separately
the long and the short bursts.

The team examined the fluence and duration of 1,972 bursts and found
a new relationship. The fluence is the total energy of all the
photons emitted by the burst during its gamma-ray active stage, a
measurement incorporating both the flow and energy of individual
photons.

Within both categories, long and short, there is a correlation
between fluence and duration: the longer the burst, the greater the
fluence. Yet the degree of this relationship is statistically
different for the two categories (at a 4.5 sigma significance level).
This difference places constraints on what can cause these bursts or
how they can operate.

In long bursts, there is a direct proportionality between duration
and fluence, suggesting that the energy conversion rate into gamma
rays is, on average, more or less constant in time. For the short
bursts, there is a weaker dependence, which could, for instance, be
due to an energy conversion rate into gamma rays that drops in time,
resulting in a less efficient gamma-ray engine.

It seems unlikely that the same engine could produce both types of
bursts, the team said. Although not directly addressed in the paper,
these results support the notion that if the long bursts originate
from massive stellar explosions, then short bursts originate from
something entirely different. In the latter scenario, this event
could be either mergers or such a drastic Jekyll-and-Hyde-like switch
in the stellar explosion mode that the engine appears physically
quite different. Such drastic and well-defined differences in the
correlation between two of the major variables will need to be
addressed quantitatively in future models of the burst physics.

The 1,972 bursts were observed by the BATSE instrument on the NASA
Compton Gamma Ray Observatory, a mission active between 1991 and
2000. Coauthors also include Zsolt Bagoly, of the Laboratory for
Information Technology at Eotvos University in Budapest; Istvan
Horvath, of the Department of Physics at Bolyai Military University
in Budapest; and Attila Meszaros, of the Astronomical Institute at
Charles University in Prague.

This research was supported by the U. S. National Aeronautics and
Space Administration (NASA) and the Hungarian national research
foundation (OTKA).

For a copy of the Astronomy & Astrophysics journal article now in
press, refer to http://lanl.arXiv.org/abs/astro-ph/0301262.

[ C. Wanjek ]

CONTACTS:
Lajos Balazs at the Konkoly Observatory in Budapest: phone
(+36)1-375-4122, e-mail

Peter Meszaros at Penn State University in the United States: phone
(+1)814-865-0418, e-mail

Eva Engedi (PIO at the Konkoly Observatory): (+36)1-375-4122, e-mail

Barbara K. Kennedy (PIO at Penn State): phone (+1)814-863-4682,
e-mail