The discussion centers around the number of observed galaxies in the universe, exploring how this number is determined and the implications of galaxy clustering. It includes inquiries about observational methods and the nature of galactic structures, touching on both theoretical and observational aspects.
Participants generally agree that the number of observed galaxies is significantly lower than the total estimated number, but there is no consensus on the exact figures or definitions of "observed." The discussion also reveals differing views on the nature and origins of galaxy clustering.
Limitations include varying definitions of "observed," dependence on observational methods, and the complexity of classifying galaxy structures. The discussion does not resolve the nuances of these definitions or the implications of the observations.
russ_watters said:The OP asked how many have been observed. That would probably be several tens of thousands - most of them in the two Hubble Deep Fields (Garth's number comes from extrapolating from the Hubble Ultra Deep Field).
Please excuse my intrusion, if any, with this question.
How do astronomers and cosmologists determine that there are "galactic clusters" and "super clusters"? Is the determination also a result of extrapolation from observations of galaxies by the two Hubble Deep Fields? Or have these structures actually been observed?
Wallace said:Yes these structures are indeed observed in several ways. Clusters are seen in galaxy redshift surveys as well as imaging campaigns such as the HDF by just noting the clumpy of the galaxies. We can also see clusters through X-rays. The presence of the deep 'potential well' of the cluster mass makes surrounding gas fall into the cluster so quickly in emits X-rays.
baywax said:That's astounding! So a cluster of galaxies can actually create a sort of super-sized gravity well that attracts the gasses of its resident galaxies.
Wallace said:Not quite, the gas that is seen falling into clusters is part of the diffuse medium, not gas being stripped off the galaxies within the cluster.
Mass clusters in the Universe due to the action of gravity on the very small inhomogeneities present in the early universe. So your second or question is actually an and, i.e. galaxies cluster because of what was left from the Big Bang and the way the mass has evolved under gravity since.
The 2MASS survey, done in three bands in the infrared, http://www.ipac.caltech.edu/2mass/releases/allsky/doc/sec1_3.html#xtsourcat", of which some ~97% are galaxies (no spectra of course).Wallace said:It depends what is meant by observed though. If we count taking spectral data but not high resolution imaging as an observation then the galaxy surveys of 2DFGRS in Australia and SDSS in the US blow this number up to more than a Million.russ_watters said:The OP asked how many have been observed. That would probably be several tens of thousands - most of them in the two Hubble Deep Fields (Garth's number comes from extrapolating from the Hubble Ultra Deep Field).
In the x-ray band, clusters (and groups) are more easily observed than individual galaxies (except for AGN, such as quasars). http://bax.ast.obs-mip.fr/" is a detailed survey.Yes these structures are indeed observed in several ways. Clusters are seen in galaxy redshift surveys as well as imaging campaigns such as the HDF by just noting the clumpy of the galaxies. We can also see clusters through X-rays. The presence of the deep 'potential well' of the cluster mass makes surrounding gas fall into the cluster so quickly in emits X-rays.baywax said:Please excuse my intrusion, if any, with this question.
How do astronomers and cosmologists determine that there are "galactic clusters" and "super clusters"? Is the determination also a result of extrapolation from observations of galaxies by the two Hubble Deep Fields? Or have these structures actually been observed?
It's more that the universe becomes increasingly homogeneous at larger scales, as http://www.sdss.org/news/releases/20031028.powerspectrum.html" makes clear.baywax said:Thank you for your patience with my ignorance about this matter. And thank you for explaining what we know about galactic clusters.
Its unimaginable to think about the number of galaxies out there when we barely know about our own. I suppose its easier to observe a galaxy that is outside of ours than to get a clear picture of this one. When you look at the arm of our galaxy we call the milky way its so huge and so detailed with stars its hard to imagine its just one arm of this monsterous spinning collection of them.
What's the possibility that there are clusters of super-clusters? Can it just go on as far as our telescopes can reach? Have we reached a point where we can say the galaxies stop and the universe ends?
Nereid said:It's more that the universe becomes increasingly homogeneous at larger scales, as http://www.sdss.org/news/releases/20031028.powerspectrum.html" makes clear.
While galaxies are relatively easy to define, as discrete objects - and they come is fairly easily classified kinds (elliptical, spiral, etc) - groups and clusters tend to be less so. Still, one can say, in principle, that they are gravitationally bound, and have had time since they were formed to undergo considerable interaction.
Superclusters are probably gravitationally bound objects, though their constituent galaxy clusters have not had time to 'relax'.
At the next largest scale - clusters of superclusters - it's unlikely there are gravitationally bound objects. At this scale, the universe has voids, walls, filaments, etc; http://www.mpa-garching.mpg.de/galform/millennium/" has some nice visuals of this.