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.