This page shows results, considerations, speculations and crazy ideas about Large Scale Structure (LSS) as made and thought by the students and researchers at the Copenhagen University Observatory, Copenhagen.
1. 3D visualization of Large Scale Structure
During my thesis work I found visualization of galaxy redshifts in 3D a wonderful tool to experience Large Scale Structure in the Universe. Be my guest - Try yourself!
Man has always had a profound urge to explore his surroundings. In the middle-age the expeditions went to the furthest corners of the globe. In our days a lot of efford is being put into unveiling the nature of the remote parts of the Universe.
The subject under investigation here is the largest structures in the Universe. In the last decade a wealth of redshift surveys of the Universe has made it possible to map the distribution of galaxies over most of the sky with respect to their spatial location - as in the CfA+SSRS redshifts surveys and the Las Campanas redshift survey seen below
and not only their projected position onto the celestial sphere - as "yesterdays" surveys like the APM survey and the Lick survey seen below.
As it turns out it is extremely difficult to measure distances in the Universe in general, so we astronomers find ourselves in the usual position: We have to measure something which is almost impossible because everything else relies on it. And a lot relies on distances in the Universe, its fate, just to name one thing.
Well, the "easiest" (cheapest) way to measure distances, or something close to distances, are the redshifts of galaxies. The galaxy redshifts can as a first approximation be interpreted as distances through the Hubble law - The universe expands and makes all galaxies move apart from each other with velocity differences proportional to their distances from each other. The expansion makes the wavelength of emitted light larger or redder - the so called redshift - and this gives us a nice well-defined relationship between the redshift and distance of a given object.
Now, the Universe is not a simple place and it is not supposed to be an easy job to be an astronomer. So this is only exactly true under the assumption that galaxies do not have masses and therefore do not attract each other thereby giving rise to peculiar motions - e.g. motions on top of the Hubble expansion. But this is ok as a first approximation.
In this approximation, where redshifts are proportional, we see that galaxies are distributed in a non-isotropic way placed in a manifold of structural elements - clusters, sheets, filaments and voids. Quite interesting in itself - One might refer to this kind of cosmological mapping as cosmography (see also [Christensen, 1996]). When working with this approximation of a direct mapping between redshifts and distances we work in the so called redshift space, which as mentioned neglects galaxy motions. Since Kaiser published his memorable paper in 1987 [Kaiser, 1987] there has been quite a fuzz about deconvolving the effects of this funny chunk of phase space having two spatial and one velocity component. We will leave that part of the story for now, but refer the interested reader to the thesis.
Before getting to the fun part I really think you should take a look at the creation of the Universe, and in particular the creation of structure - Structure which we among other things see as galaxies today. In order not to expand this already very large page, I refer you to the thesis.
The data used on this page are redshifts and positions for 46383 galaxies. The data are gathered from the two CfA redshift catalogues in the NASA Astrophysical Data System (ADS) together with the ENACS cluster catalogue, the Southern Redshift Catalogue and the ENACS Literature Compilation. The data are from many different authors, and include both wide angle and pencil beam surveys. The total sample is not infinitely far away from fairness at a distance of approximately z = 0.04 (i.e. v = 12000 km/s), but it includes galaxies as far as z= 3.80. The sample is perhaps the largest collection of redshifts in the World, but it suffers severe selection effects and is very difficult to use for statistical analyses (one of the conclusions in my thesis).
Here is the angular distribution of the large sample of galaxy redshifts. It shows a very nice view of the large scale structures we know of in our "neighbourhood".
Here we look from the furthest outskirts of the sample and inwards towards our position in the center. The sampling outside the central region (our "neighbourhood") is very sparse. The zone of Galactic obscuration is clearly visible in the picture above.
This is the wellknown northern CfA slice. Seen from this angle, it looks almost like a traditional 2D plot. Another look at this slice:Here we reproduce the galaxies as polygons (spheres). Is is easier to grasp the subtle 3D structure in this way, but the CPU load gets at least two orders of magnitude larger.The sampled section in the two snapshots is 8h <= alpha <= 17h, 8.5 deg. <= delta <= 44.5 deg., and z < 0.04. If we make a thinner slice in declination, like this, we reduce contamination of the structure due to projections.
Above we look closer at the Microscopium region (left) and the Coma region (right) in 3D. The voids and filamentary structure are only first visible when looking at the galaxy distributions in 3D.
The picture on top of the page is a 15 degree slice of a sample of the CfA Catalogue ranging to $z=0.04$. This image is a snapshot from a animation made by means of IDL.
I can offer you a total look around in the distribution of galaxies in projection. This may give also give you a nice impression of the complicated filamentary structure the galaxies are distributed in: Slices
For fun I have tried to plot the positions of the nearest 15.000 galaxies (white) as they look from our viewpoint on Earth, among the brightest 6.000 stars (yellow from the Henry Draper Catalogue). Here is the area near Orion.
HI hope you liked the tour!
Cheers
2. The Surface Brightness Test for the Expansion of the
UniverseThe Surface Brightness Test for the Expansion of the Universe by Per Kjaergaard Rasmussen
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