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Fig. 1: A "slice" through the 3-dimensional distribution of galaxies
in the Sloan Digital Sky Survey. The observer on Earth is located
at the centre of the circle. The distance from the centre of the circle
represents the redshift of the galaxy. The galaxies within 6 degrees
of the equator are plotted in this diagram.
The large scale structures are clearly seen from this figure.
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Fig. 2: The contours of redshift-space two-point correlation
function (2PCF) for galaxies in the Sloan Digital Sky Survey (colour
lines), compared to that (black lines) obtained by assuming an
isotropic distribution of galaxies. The rp and π
directions are respectively perpendicular and parallel to the line of
sight. Both the effect of redshift space distortions on small scales
(often called the Finger-of-God effect) and the infall effect on large
scales are clearly visible: on small scales the 2PCF is stretched in
the π-direction and on large scales the contours are squashed along
the line-of-sight direction.
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Our understanding of the large-scale structure (LSS) of the Universe
has come primarily from studies of redshift survey of nearby galaxies.
The large redshift surveys assembled in recent years, for example the
Two Degree Field Redshift Survey ( 2dFGRS) and the Sloan
Digital Sky Survey (SDSS),
have provided angular positions and redshifts for samples of hundreds
of thousands of galaxies and have allowed astronomers to make detailed
maps of how galaxies are distributed in the local Universe. These maps
have shown that the galaxies are not distributed homogeneously, but in
filamentary structures that surround large empty regions, or voids
(Fig. 1)
The MPA-Chinese collaboration have been measuring the degree to which
a galaxy of given type is associated spatially with other galaxies of
the same type (this is termed the "clustering strength" of the
galaxy). The two-point correlation function (2PCF, see Fig.2) of
galaxies has long served as the primary way of quantifying the
clustering properties of galaxies. As the fundamental lowest order
statistic, the 2PCF is simple to compute and provides a full
statistical description, if the large-scale structure in the Universe
developed from a Gaussian initial density field as is commonly
believed. It can also be easily compared with the predictions of
theoretical models.
The MPA-Chinese team have found that the more massive the galaxy, the
more strongly it is clustered. Elliptical galaxies, which contain
very little gas and mainly old stars, are also more strongly clustered
than spiral galaxies, which contain a lot of gas and many young
stars. These results were known previously, but the surprise from the
team's analysis is that the preference for older galaxies to be found
in association with other galaxies extends over very large distances
-- up to scales of ten Megaparsecs or more. The full power of the
Sloan Survey was required in order to accurately measure the
clustering on such large scales. The reason why this result is
surprising is that 10 Megaparsecs is much larger than the distance
over which different galaxies could have exerted any influence on each
other on a timescale comparable to the entire age of our Universe.
What this implies, is that the ages of stars in a galaxy are somehow
imprinted at birth.
According to standard theory, large scale structures in the Universe
were gravitationally amplified out of of tiny density perturbations
generated in the earliest moments of the Universe following the Big
Bang. Regions in the early Universe where the initial perturbations
were large evolve into regions that have a high overdensity of
galaxies at the present day. Conversely, regions where the initial
density fluctuations are very weak, evolve into underdense regions or
voids. The MPA-Chinese team is currently exploring how galaxies are
predicted to assemble in these models in an attempt to ascertain
whether the results of their observational study can be accommodated
within this standard picture.
Cheng Li, Guinevere Kauffmann, Gerhard Börner, Yipeng Jing
Further information
http://www.mpa-garching.mpg.de/~leech/papers/clustering/wrp.pdf
Original publications
The dependence of clustering on galaxy properties, MNRAS, in press,
astro-ph/0509873
The dependence of PVD on galaxy properties, MNRAS, submitted,
astro-ph/0509874
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