Subject: Faint dark halo around nearby galaxy contains "weird" population of
     dwarf stars, says international team of astronomers (Forwarded)
Date: Wed, 6 Jan 1999 23:23:21 -0500
From: Andrew Yee <ayee@nova.astro.utoronto.ca>
Organization: UTCC Campus Access
University of California-Berkeley

EMBARGOED FOR 9 A.M. CST WEDNESDAY, JAN. 6, TO COINCIDE WITH PRESS
CONFERENCE
AT THE MEETING OF THE AMERICAN ASTRONOMICAL SOCIETY

Faint dark halo around nearby galaxy contains "weird" population of dwarf
stars, says international team of astronomers

By Robert Sanders, Public Affairs

Berkeley -- Hubble Space Telescope imaging of the extended halo of a nearby
spiral galaxy has turned up some intriguing ideas about the nature of the
unseen "dark matter" that holds galaxies together.

Astronomers from the University of California, Berkeley, Yale University,
Cambridge University in England and universities in France and the
Netherlands took long-exposure infrared images of a small portion of the
faint halo of the galaxy NGC 5907, an edge-on spiral galaxy a mere 39
million light years away.

What they did not see -- lots of stars -- has led them to conclude that the
halo is composed of a weird population of stars, mostly dim dwarfs too faint
to see from Earth.

Most galaxies contain a mix of bright giant stars and dim dwarf stars, with
about half of the light coming from each group. If the halo of NGC 5907
contained a mix similar to that in our own galaxy, the team would have seen
hundreds of bright giants in the field of view. Instead they saw only a
handful of bright stars.

The best explanation of the team's observations is that at least 20 times
more light comes from dwarfs than giants in the halo of NGC 5907.

"Our results force us to turn to more esoteric descriptions of the stellar
content of NGC 5907's halo," said Michael Liu, a graduate student at UC
Berkeley and lead author. "In particular, our data combined with the
measured colors of the halo suggest a very metal-poor stellar population with an
enormous excess of faint dwarfs.

"This is the first direct evidence of a substantial population of stars
which is essentially all dwarf stars," Liu added. "Such a population has
been invoked in the past as a constituent of the dark matter making up galaxy
halos."

The results could mean that this faint or "dark" halo -- as distinct from
the normal stellar halo surrounding spiral galaxies -- is the dark matter within
NGC 5907, as suggested when it was first observed in 1994.

Even if the dwarfs don't have enough mass to account for the dark matter
holding this galaxy together, the fact that the dwarfs have the same profile
as the dark matter means some formation mechanism may well tie the two
together, Liu said.

"What's exciting about this is that the stars in this faint halo may have
the same kind of properties as inferred for the dark matter," said coauthor
Stephen Zepf, an assistant professor in the astronomy department at Yale
University.

Liu, Zepf and colleagues report their results in a poster session Jan. 6 at
the annual meeting of the American Astronomical Society in Austin, Texas.
Coauthors include UC Berkeley astronomy professor James Graham, Francine
Marleau of the Institute of Astrophysics at Cambridge University, Stephane
Charlot of the Institute of Astrophysics in Paris and Penny Sackett of the
University of Groningen in the Netherlands.

Four years ago, Sackett and Heather Morrison, an astronomer then at the
National Optical Astronomy Observatories in Tucson, took long-exposure
optical pictures of NGC 5907 using the 0.9 meter telescope at Kitt Peak.
The images of the galaxy, which covers an area of the sky about half the
diameter of the moon in the constellation of Draco, revealed a faint red
halo surrounding the disk.

Such measurements are difficult because the halo is about 1,000 times
fainter than the glow of the night sky. This background glow must be subtracted from
the image to view the halo.

Unlike the halo of normal stars surrounding many spiral galaxies, which
drops off very quickly away from the center, this halo dropped off more slowly --
at about the rate predicted for the mysterious dark matter. Dark matter,
composed of either normal or exotic material, must fill the galaxy in order
to explain why stars rotating around the center haven't flown away. Its mass
must drop off as the square of the distance from the center, approximately.
That is, the mass at twice the distance is only one fourth as great. (The
matter in a normal stellar halo drops off as the fourth power of the
distance from the center, so mass at twice the distance is only one sixteenth times
as great.)

Other groups have since confirmed the existence around NGC 5907 of a halo of
stars with a gradual fall-off, unlike, for example, the more rapid drop-off
of the known stellar halo of our own Milky Way galaxy. A similar halo has
since been observed around a second spiral galaxy.

Subsequent measurements suggested also that the halo stars must be
metal-rich, which doesn't fit with the type of stars expected in a stellar halo. A
normal stellar halo is thought to have little or no on-going star formation, so
halo stars should be old and have formed out of metal-poor material. For this
reason, NGC 5907's halo colors were hard to explain.

The team of American and European astronomers decided to use the Hubble
Space Telescope's near infrared camera (NICMOS, or Near Infrared Camera and Multi-
Object Spectrometer) to look for bright stars in a small area of this faint
halo. They observed at a wavelength of 1.6 microns in the infrared. The team
reasoned that if the faint red glow of the halo is from a normal population
of stars, we should see hundreds of bright, giant stars in the field, which
is about 20 arcseconds (1,000 light years) on a side.

The observations from the space telescope required 13 orbits totaling about
nine and a half hours of viewing time. In comparison, recent deep field
views from the Hubble Space Telescope required about 100 orbits. No one else has
attempted such a close look at a galactic halo.

To their surprise, the researchers saw only one or two bright stars,
hundreds of times fewer than expected.

The only way to explain the numbers and still account for the reddish color
of the halo light is to assume that the mix of giant and dwarf stars is
different from the normal galactic population, Liu said. That is, that
almost all the light from the halo is coming from dwarf stars.

It is not clear how such a population could have arisen, though Zepf said
that one speculation is that stars like our Sun formed differently billions
of years ago, yielding different proportions of giant and dwarf stars.

A group of French astronomers suggested earlier this year that the faint
halo around NGC 5907 is unrelated to the "dark matter," but rather resulted from
a long-ago collision with a companion elliptical galaxy, which eventually
merged with NGC 5907. The metal-rich stars of the companion would have
permeated NGC 5907's halo, leading to the extended light observed. This
scenario seemed plausible, Liu said, because elliptical galaxies are known
to have metal-rich stars, small elliptical companions to big galaxies are
common and galaxies often merge.

"Our HST NICMOS results argue against the merger scenario, since we don't
see the hundreds of giant stars one should see for a normal metal-rich
stellar population," he said.

The work was supported by the National Science Foundation and the National
Aeronautics and Space Administration through grants awarded by the Space
Telescope Science Institute.

---
Andrew Yee
ayee@nova.astro.utoronto.ca