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Connecting the physical properties of galaxies with their clustering

Using the Sloan Digital Sky Survey (SDSS), the most ambitious optical imaging and spectroscopic survey carried out to date, members of a collaboration between the Max Planck Institute for Astrophysics and Chinese researchers have explored how the spatial distribution of galaxies depends on properties such as their stellar mass, the rate at which they form stars and whether they have elliptical or spiral morphologies.

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.

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.

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 (linkPfeilExtern.gif2dFGRS) and the Sloan Digital Sky Survey (linkPfeilExtern.gifSDSS), 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


Original publications

The dependence of clustering on galaxy properties, MNRAS, in press,

The dependence of PVD on galaxy properties, MNRAS, submitted,

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