Young Galaxies Grow Up Together in a Nest of Dark Matter (Forwarded)

Subaru Telescope
National Astronomical Observatory of Japan
Hilo, Hawaii

December 21, 2005

Young Galaxies Grow Up Together in a Nest of Dark Matter

Astronomers have found clear indications that clumps of dark matter are
the nursing grounds for new born galaxies about twelve billion light years
away. A single nest of dark matter can nurture several young galaxies.
These results from researchers at the Space Telescope Science Institute,
the National Astronomical Observatory of Japan, and the University of
Tokyo confirm predictions of the currently dominant theory of cosmology
known as the Cold Dark Matter model.

Recent studies suggest that dark matter out weighs ordinary matter by a
factor of seven. (Reference 1) Although dark matter cannot be seen
directly through a telescope, it reveals itself to astronomers by its
strong gravitational pull on nearby stars and gas, and even galaxies.

Galaxies are often clustered together and how they cluster is determined
mostly by gravity. (Figure 1)

By studying how galaxies cluster, it is possible to determine how dark
matter is distributed and how it affects the birth and growth of galaxies.
In the past, it was extremely difficult to study the clustering of young
galaxies. Young galaxies appear faint due to their great distances, and
finding enough of them to study how they cluster was an observational
challenge.

Masami Ouchi from the Space Telescope Science Institute and colleagues
used the Subaru telescope and its Suprime-Cam camera to study a piece of
the sky in the constellation Cetus (the Whale) called the
Subaru/XMM-Newton Deep Survey Field (SXDS; Reference 2). This piece of sky
covers an area five times the size of the full moon. By taking deep and
sensitive images of the field in three colors of visible light, the SXDS
team was able to find about seventeen thousand (17,000) young galaxies
twelve billion light years away. This number is ten times larger than
previous studies of such young galaxies. Figure 2 shows the location of
the galaxies, and Figure 3 shows the relative strength of the correlation
between pairs of galaxies with different separations. (Note 1)

Based on these data, the team found that:

(1) There are many pairs of galaxies with separations less than eight
hundred thousand (800,000) light years.
(2) Even at large distances, galaxies are strongly clustered.

Both of these results are expected if the galaxies are nestled within
clumps of dark matter. (Note 2 and 3) The SXDS team compared the
observational results in detail to theoretical predictions based on a Cold
Dark Matter model by team member Takashi Hamana (Reference 3) and found
that the average clump of dark matter nests weighs as much as six hundred
billion (600,000,000,000) Suns, and that a single clump of dark matter
harbors multiple young galaxies.

Independently, Nobunari Kashikawa from the National Astronomical
Observatory of Japan and colleagues also used Subaru's Suprime-Cam camera
to study an area of sky in the constellation Coma Berenices (Berenice's
Hair) called the Subaru Deep Field (SDF; Reference 4). This field is only
the size of one full moon but the data available are twice as sensitive as
the SXDS field data. The SDF team found about five thousand (5,000) young
galaxies at a distance of twelve billion light years (Figure 4), and eight
hundred (800) even younger galaxies at a distance of twelve billion five
hundred million light years. The SDF team was also able to double check
the identities of the young galaxies by taking spectral data of the
galaxies with the Subaru and Keck telescopes. The SDF team independently
obtained the results (1)+(2) described above, and concluded that some
single clumps of dark matter harbours multiple young galaxies. In the SDF
images, it is possible to see several new born galaxies huddled together
in a small area (Figure 5). By comparing the SDF data in detail to high
precision computer simulations of the growth of clumps in Cold Dark Matter
by team member Masahiro Nagashima of Kyoto University (Reference 5), the
SDF team concludes that heavier clumps of dark matter have more bright
galaxies, and that this preference produces the correlations found in real
observation (Note 4).

The two teams together have found the first concrete evidence that young
galaxies in the early universe (Note 5) are nestled within clumps of dark
matter, and that a single clump of dark matter nurses several young
galaxies. Both teams took advantage of the Subaru telescope's unique
ability to take deep sensitive images over a large area of sky.

Results from the SXDS team were published in the December 20, 2005 edition
of the Astrophysical Journal (Reference 6). Results from the SDF team will
be published in the February 1, 2006 edition of the Astrophysical Journal
(Reference 7).

Notes

Note 1: Figure 3 is a graph of the correlation coefficient which shows the
strength of the clustering of galaxies at different separations. The
horizontal axis shows the distance between a pair of galaxies. The
vertical axis shows how many galaxies are separated by a particular
distance. A correlation coefficient of zero means that galaxies are
distributed randomly. In Figure 3, galaxy pairs of separations of up to
one hundred million (100,000,000) light years have positive correlation
coefficients. This indicates that galaxies are clustered together up to
distance scales of one hundred million light years.

Note 2: Astronomers call clumps of dark matter that surround galaxies
"dark matter halos."

Note 3: The new result of (1) is the first to show that many pairs of
galaxies have separations of less than eight hundred thousand (800,000)
light years. This distance scale corresponds to clumps of dark matter with
mass scales of one hundred billion (100,000,000,000) solar masses.
Similar, yet less precise results were reported by other research groups
(References 8 and 9) around the same time as the new results. Previous
studies had already suggested the result of (2). The new results confirm
this with greater precision and reliability. The strength of clustering
matches theoretical predictions of dark matter halos with mass scales of
one hundred billion solar masses (Reference 10).

Note 4: Why do heavier clumps of dark matter have more young galaxies?
There are no clear answers to this question yet, but in the framework of
the modern scenario for galaxy formation which involves repeated
collisions are mergers between galaxies, such correlations between visible
matter and dark matter are very significant. Finding multiple galaxies
within a single halo is still common at the present era, and has been
reported in large surveys of nearby galaxies such as the Sloan Digital Sky
Survey (Reference 11). It is worth noting that it is now possible to study
such small scale properties even the early universe.

Note 5: Light for galaxies twelve billion light years away were emitted
twelve billion years ago. This corresponds to a little over one billion
years after the birth of the universe thirteen billion and seven hundred
million years ago, or ten percent of the current age of the universe.

References

[1] Spergel et al. 2003, Astrophysical J. Supplement Series, 148, 175-194

[2] http://subarutelescope.org/Pressrele.../01/index.html

[3] Hamana et al. 2004, Monthly Notices of the Royal Astronomical Society,
347, 813-823.

[4] http://subarutelescope.org/Pressrele.../03/index.html

[5] Nagashima et al. 2005, Astrophysical J., 634, 26-50 (Part of this
simulation can be seen as a movie on the website of the Four Dimensional
Digital Universe Project at the National Astronomical Observatory of
Japan. (http://4d2u.nao.ac.jp/)

[6] Ouchi et al. 2005, Astrophysical J., 635, L 117-L 120

[7] Kashikawa et al. 2006, Astrophysical J., February issue

[8] Hamana et al., Monthly Notices of the Royal Astronomical Society
Submitted (astro-ph/0508536)

[9] Lee et al., Astrophysical J. in press (astro-ph/0508090)

[10] Sheth & Tormen 1999, Monthly Notices of the Royal Astronomical
Society,308, 119

[11] Sloan Digital Sky Survey (http://www.sdss.org/)

IMAGE CAPTIONS:

[Figure 1:
http://subarutelescope.org/Pressrele.../21/fig1_l.jpg (108KB)]
A scientifically accurate artistic image of galaxies twelve billion light
years away. The blue nebulosity is dark matter. Denser regions are white.
The blue-white regions correspond to the dark matter clumps or dark matter
halos where young galaxies are forming. (Image created by Naomi Ishikawa
and Takaaki Takeda, National Astronomical Observatory of Japan)

[Figure 2:
http://subarutelescope.org/Pressrele...21/fig2e_l.jpg (77KB)]
(Left) Visible light image of the Subaru/XMM-Newton Deep Survey field in
the constellation Cetus. (Right) The distribution within the SXDS field of
the seventeen thousand galaxies that are twelve billion light years away.

[Figure 3:
http://subarutelescope.org/Pressrele...21/fig3e_l.jpg (54KB)]
A graph showing the average level of clustering between galaxies twelve
billion light years away. The horizontal axis shows the separation between
galaxy pairs. The vertical axis shows the correlation coefficient
indicating the relative number of galaxy pairs with a particular
separation. At separations of less than eight hundred thousand (800,000)
light years, the number of galaxy pairs increases dramatically. The graph
also shows that there is clustering of galaxies even up to separations of
one to ten million (1,000,000 to 10,000,000) light years.

[Figure 4:
http://subarutelescope.org/Pressrele...21/fig4e_l.jpg (112KB)]
The distribution of galaxies twelve billion light years away in the Subaru
Deep Field in the constellation Coma Berenices. Colored circles indicate
the location of the galaxies. Denser regions are red, sparser regions are
blue. The new results are based on detailed studies of the uneven
distribution of galaxies visible in this figure.

[Figure 5:
http://subarutelescope.org/Pressrele...21/fig5e_l.jpg (76KB)]
Four examples of gatherings of galaxies twelve billion light years away in
the Subaru Deep Field data. Each gathering is shown in three different
wavelengths, B-band (0.45 micrometers), R-band (0.65 micrometers), and
i'-band (0.77 micrometers). Each galaxy twelve billion light years away is
circled in yellow in the i'-band image. The size of each panel roughly
corresponds to the size of the dark matter clump (dark matter halo) that
harbors the galaxies.