Could you please tell me roughly, how many galaxies have been observed?
Around 1011 galaxies in the observable universe, roughly the same number (to an OOM or two) as the number of stars in our galaxy.
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?
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.
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.
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.
What is causing galaxies to cluster? Is this a remnant of the bb or is it something that is happening after the fact? Like a kind of settling.(?)
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.
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?
The 2MASS survey, done in three bands in the infrared, lists some 1.6 million 'extended sources', of which some ~97% are galaxies (no spectra of course).
In the radio band, this NRAO VLA Sky Survey page (NVSS) gives a quick summary of the various large scale surveys. It notes that there are "over 1.8 million sources in the entire survey" in the NVSS catalogue.
Of course, not all those sources are galaxies, but perhaps a majority are (here is the paper describing the NVSS).
In the x-ray band, clusters (and groups) are more easily observed than individual galaxies (except for AGN, such as quasars). BAX is an online catalogue of such objects; REFLUX is a detailed survey.
Although the number of clusters observed seems quite modest - compared with the millions in the optical, IR, and radio bands - when you consider that a cluster may contain over a thousand galaxies, the total number of galaxies 'observed' in the x-ray band could be considered quite large.
It's more that the universe becomes increasingly homogeneous at larger scales, as this SDSS page 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; the Millennium simulation has some nice visuals of this.
Whoa! Thank you Nereid. Outlandish simulations. The millennium simulation warns "Beware of it's huge size!" when offering a download of the postscript file of a projected density field for a 15 Mpc/h thick slice of the redshift z=0 output. No kidding!
What determines the formation of the strands and branches of matter in this simulation of the universe? Its like a gigantic mass of capillaries or even a mass of neurons. What forces are at work that render galactic formations such as this to look like earthbound structures such as tree branches etc....? Thanks again.
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