Figure 1: 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) (Larger Image)

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.

Figure 2: (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. (Larger Image)

Figure 3: 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. (Larger Image)

Figure 4: 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. (Larger Image)

Figure 5: 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. (Larger Image)

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).

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/Pressrelease/2004/06/01/index.html

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

[4] http://subarutelescope.org/Pressrelease/2003/03/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/)