So DM, whatever it ends up being, has to be uncharged, right? Otherwise it would be interacting with light?

Also, if DM has any sort of energy dissipation mechanism, would we expect to see clumps of DM, mutually attracted by gravity? (Well, not "see", but you know what I mean...). Would it be denser around a star-system, in stellar or planetary gravity wells than elsewhere?

__________________
http://amssolarempire.blogspot.com

These are interesting questions.

It might be useful to explore them, separately, in several different regimes - galaxy halos, clusters, and elsewhere.

For example, in the case of (rich) clusters, any DM that was charged would interact with the hot (or warm) plasma that pervades such clusters, so its 'footprint' would likely be 'seen' in that plasma.

Generally, I'm not sure what you are referring to re 'it would be interacting with light', could you clarify please?As an unqualified generalisation, the answer would seem to be 'yes'.

However, a quick look at some possible regimes shows that the generalisation needs to be somewhat tempered.

For example, if the DM is 'hot', then the details of the 'energy dissipation mechanism' would be important (in deteriming the extent to which 'clumps' might form). If the 'energy dissipation mechanism' is weak, or slow, cf timescales of baryonic matter, very different sorts of effects would be expected than if it were strong or fast.Exactly - the extent to which it would would depend quite significantly on the details!

If you just click the ApJ abstract where it says "PDF" on the linke you provided you'll get the paper for free. No subscription needed. In addition you can do a search on ADS and any papers over 3 years old that were published in most of the major journals can be downloaded for free. Between ADS and ArXiv it is possible to get the majority of the published papers.

__________________
"The scientist who asks the right question reconnoiters a new patch of the unknown, and may, with luck, bring it within the constricted but expanding boundaries of the known."

~Timothy Ferris (The Red Limit) 1982

Here is a related question, and I may be missing the point entirely here, so I'll explain my reasoning behind the question.

Let me make sure I'm getting this right. All of the reasons to believe in dark matter are gravitational things, correct? That is to say, every observation that does not fit with a non-dark matter theory differs (ostensibly) because of a problem with gravity. In addition, these problems all occur on a very large scale. It is never the case that, for instance, we observe this problem in the Earth-Moon system. Even with the CMB data, we are dealing with very large masses, and thus very great amounts of gravity.

If what I've said is correct, what results are there to suggest that there really is non-baryonic dark matter out there, as opposed to our theory of gravity simply being incomplete? (Another, maybe simpler, way to ask the question is this: is there a relationship between the amount of dark matter in a system and the specifics of that system? Do two systems of similar mass and density always appear to have the same amount of dark matter affecting them?)

__________________
"It's turtles all the way down."

No, there are substantial differences in the ratios of apparent matter and cold dark matter in various galaxies and clusters of galaxies. There are also variations in the distributions of dark matter in various galaxies and clusters, in ways that suggest that MOND-like theories will be difficult to make apply universally.

It is important to note that the precision of our knowledge of dark matter distribution is still pretty fuzzy, especially for individual galaxies (the strong lensing by giant clusters gives a lot of data points), so there is still wiggle-room on both sides.

__________________
Forming opinions as we speak

The two rich cluster sets of observations that I previously introduced were straight-forward, in terms of going from observation to estimates of mass (in principle; the construction of the mass maps involves some high-powered math and computing, for example).

The mass-luminosity (M/L) relationship (ratio) is a good contrast, and also a good example of how hard you have to work (sometimes) in astronomy.

In principle, this method is also pretty straight forward - just count all the photons from a galaxy that your telescope detects (the "L"), use a well-tested value of M/L, turn the handle, and you have an estimate of the mass of the galaxy.

But what value of M/L to use? After all stars come in a wide range of both M's and L's!

We'll look at M/L in much more detail when we get to the observational basis for DM in our own Milky Way, so for now this summary will do:

- from the work of a great many astronomers, over a long time, we now have a pretty well-constrained understanding of the M/L relation for stars; for example, a Main Sequence G star will have an M/L that is close to 1 (where the units are sols).

- after several hundred years of work by thousands of astronomers, we now have a pretty good idea of the distribution of stars, by type, in different types of galactic environments.

- while there is much that is not well understood yet, we also have a reasonable understanding of the process of star formation, in terms of the ranges of distributions of stars (by type), from various formation environments.

- the evolution of almost all types of stars is well-understood (at least in-so-far-as we need to understand it, for the purpose of this post).

- confounding factors, such as gas and dust, can at least be constrained, in terms of how they influence the estimate of the mass of a galaxy, from an estimate of its luminosity.

So, one way to produce an estimate of the total mass of the stars in a galaxy is to 'count' all the photons your telescope detects, out to a certain threshold, to get an 'integrated magnitude'. From the spectrum of this galaxy - both its continuum and lines (emission and absorption) - and a set of distributions of stars by type, estimate the relative proportions of each kind of star which contribute to produce this spectrum. Add an estimate of the distance, turn the handle, and you have an estimate of the mass of the stars in the galaxy.

Of course, there are many uncertainties with this; for example:
> what about the gas and dust (we know these are constituents of at least several kinds of galaxy)?
> how to deal with any SMBH that may reside in the nucleus (of galaxies that have nuclei)?
> what about the faint end of the (stellar) M/L relation (brown dwarfs, L and T stars - what proportion of the mass of a newly-formed cluster of stars do they comprise)?

The good news is that we can constrain these uncertainties quite well, so while our estimates of the total mass of the stars in a galaxy may be off by ± 20% or so - averaged over all the bright galaxies in a rich cluster - the answer is still the same ... stars in the galaxies in rich clusters make up no more than a few, typically around 1 or 2, percent of the total cluster mass!

Just to finish this post - the gas and dust content of galaxies can be estimated using a variety of techniques, and for bright galaxies in rich clusters it turns out these constituents are quite minor - there's less mass in the gas and dust in galaxies in rich clusters than there is in the stars in those galaxies.

(BTW, this - or these - M/L techniques can also be applied to other galaxies - those in poor clusters or groups; dwarf galaxies; LSB (low surface brightness) galaxies - as well as to globular clusters and (with less certainty) to intra-cluster stars.)

Patience please! I've still got two or three sets of observations on rich clusters to go, and then all the others.

But to give a quick answer - scientists are an incredibly conservative lot, not at all adventurous, and most definitely not given to flights of fancy unconstrained by consistency.

So, even if they imagine the Einstein version of gravity is 'incomplete', they cannot find a way to tweak it (or radically overthrow it) to produce the kinds of mass maps of rich clusters that are derived from strong lensing observations. So, there's definitely some mass there, distributed in a way that's at least somewhat similar to what the mass maps show (can you imagine trying to change gravity so that it produces the lumps and bumps in those mass maps, without there being something there to make the lumps and bumps?)

But does it have to be non-baryonic mass? Stay tuned!

The lumps and bumps pretty much do it for me. But what <i>is</i> it anyway? And if it doesn't interact with light (and presumably by extension, electricity and magnetism), then how will we detect it close up?

__________________
http://amssolarempire.blogspot.com

Nereid is in the process of laying out that case ... and she will be attempting to show us that it is probably non-baryonic matter ... specifically CDM. I'm doing as she requests and staying tuned.

__________________
"The scientist who asks the right question reconnoiters a new patch of the unknown, and may, with luck, bring it within the constricted but expanding boundaries of the known."

~Timothy Ferris (The Red Limit) 1982

R. A. Sunyaev and Ya. B. Zel'dovich published a paper, in 1972, entitled "The Observations of Relic Radiation as a Test of the Nature of X-Ray Radiation from the Clusters of Galaxies", which has since been cited hundreds of times.

The SZE (Sunyaev-Zel'dovich effect), first identified in this paper, is very cool, and has the potential to provide lots of independent checks on a range of astronomical and cosmological phenomena (and theories, hypotheses, models, etc). For example, it provides - in principle - a way of determining H₀ (the Hubble constant), quite independently of the standard distance ladder.

For the purposes of this thread, it is the 'thermal SZE' that is of interest.

Amber Miller's SZE page gives a succinct summary of the SZE (my bold):The extent to which observations of the SZE estimate the mass of (rich) clusters depends upon a determination of the mass of the baryons that the the number of electrons (indirectly) observed points to.

As the quote from Amber Miller makes clear, observations of the SZE need to be bolstered by X-ray observations; in particular, it is important that the electrons producing both X-rays and the SZE be (predominently) 'thermal', and it is the X-ray observations (examined next) that seal this.

Note that, on its own, the SZE has only an indirect relation to gravity, so it provides an observational basis for the baryonic mass in rich clusters that has (essentially) nothing to do with gravity.

Here and here are two pages giving some of SZE observational programs, and (hundreds of) observations of the SZE

Finally, two good, general reviews of the SZE, including both cluster mass and cosmological aspects, are Mark Birkinshaw's 1999 paper, and Carlstrom, Holder, and Reece's 2002 one. BAUT readers may be interested to understand the SZE better; in particular, what it is (in more detail), what the underlying physics is (e.g. "inverse Compton scattering", "the Rayleigh-Jeans side of the blackbody spectrum", and "the Wien side" (of the blackbody spectrum). Or, in terms of its (potential) importance in cosmology, including the 'kinetic' SZE, independent means of estimating H, and so on. If so, perhaps someone could start a new Q&A thread? I'd be happy to contribute to it.

It sure is! Now things are starting to make sense for me... I can see roughly how it's all going to tie together, as though all of my questions are beginning to coalesce into something measurable against the chaotic background of data.

Sorry, I just liked the metaphor. I have tons of questions, but I'll save them until the end.

Eagerly awaiting the next installment. (And thanks for this thread, by the way. I love it!)

__________________
"It's turtles all the way down."

Is there an estimate for the distance from earth to the center of the "Great Attractor"?

__________________
°
°
My invisible elf ??? Why he is made of dark matter and lives off of dark energy !!!
°
°

http://en.wikipedia.org/wiki/Great_Attractor

It is about 10¹⁶ solar masses, about 65 megaparsecs away in the direction of the constellation Centaurus.

__________________
Forming opinions as we speak

As far as the great attractor being dark matter - it may well be - but I can not see any relation to the great attractor's presence and the other phenomenon that dark matter was postulated to solve: gravitational anomylies within galaxies.

The great attractor would help out in the universal scope though as far as it being open or closed, flat or curved. and other stuff that I know little of.

__________________
°
°
My invisible elf ??? Why he is made of dark matter and lives off of dark energy !!!
°
°

In 1965, instruments carried on an Aerobee rocket detected X-rays from M87. Not long afterwards, X-rays from (rich) clusters of galaxies were detected, and a new branch of astronomy got underway.

Today, Chandra and XMM-Newton devote a considerable fraction of their observing time to galaxy clusters.

This short summary, on the Chandra website, gives a popular overview.

Rich clusters are prodigious X-ray emitters; only GRBs outshine them, and only for seconds.

Most of the rich cluster x-ray emission comes from the diffuse, hot gas (a fully ionised plasma actually) that forms 'the atmosphere' of rich clusters.

Detailed observations of the x-ray emission from a rich cluster can be used to determine the abundance of elements, the density (of the electrons), the temperature, and the structure of the plasma, and the profiles of these, across the cluster (seen projected on the sky).

This 3.5 MB PDF is a 44-page overview, from a university course; here is Craig L. Sarazin's 1988 book "X-ray Emission from Clusters of Galaxies" ... online, for free (!).

And the bottom line?

The mass in the cluster plasma, responsible for the x-ray emission, is only ~10-20% of the total cluster mass, for rich clusters studied.

This ~10-20% is consistent with estimates of the plasma mass, derived from SZE observations.

There are three independent ways to estimate total cluster mass: galaxy motions and the virial theorem; strong gravitational lensing, and x-ray profiles (basically, assume the plasma is in, or near, hydrostatic equilibrium; from its temperature and density, the mass needed to keep it in equilibrium can be estimated). The three independent methods produce consistent estimates.

So, most of the mass of a rich cluster is 'dark' - it's not in the stars, gas, and dust of the constituent galaxies, and it's not in the hot 'cluster atmosphere'.

Where is this 'dark mass'?

... called CL0024+1654*. The techniques used include strong lensing and weak lensing ('shear') - I will cover the latter later.

Here is the pretty picture, from an ESA site; here is the ArXiV preprint (this is paper II - "The Cluster Mass Distribution"; the other paper is I - "morphological distributions to 5 Mpc radius". The cluster has also been studied by ISOCAM, and in x-rays ... and likely radio and microwave bands).

*If you go searching, you might find it also called "CL0024+16" and "CL0024+17".

Nereid, what is next? Still working on clusters or are you moving on to the next item on the list?

__________________
"The scientist who asks the right question reconnoiters a new patch of the unknown, and may, with luck, bring it within the constricted but expanding boundaries of the known."

~Timothy Ferris (The Red Limit) 1982

I have been looking at web sites concerned with MACHO detection and micro-lensing.

I seem to be developing into a bit of a skeptic on many fronts.

Only 3 microlensing events ascribed to galactic halo MACHOs were detected, and this is where the conclusion we need non-baryonic matter comes from, as 3 events translates into: only 20% max of dark-matter is ordinary matter in the form of brown dwarfs etc.

Now the first thing that struck me with the microlensing detection was, there are truly masses of data to wade through, so much that it's completely impossible for a human to do it, computer software is necessary.

So we have an automated system, following algorithms.

The crux of the matter is that only those events that have a high probability of being microlensing events are selected by the software. That's laudable from many points of view of course. When they designed the experiment they knew they would have to stand up in front of their peers and defend the events selected, so it's quite understandable.

However, from a statistical point of view that might not be what's required. If the critieria for selection were relaxed somewhat, so that ON BALANCE, it COULD be a microlensing event, we might end up with a lot more MACHOs detected.

That's one comment I have, there's more to come. I'm going through them one at a time, as I've noticed most of the time only ONE point per post is noticed by most people.

There's a bit of an interlude - "so, what, then, can the (dark) matter (in clusters) be?" ... a look at what sorts of (baryonic, non-baryonic) things could comprise the ~80-90% of a cluster's mass.

After that? Probably a somewhat historical detour - DM in the Milky Way disk/"near" us - and some background on tracing/detecting mass in the environs of galaxies (etc), before diving into DM in the MW halo. Or perhaps it will be DM in galaxies (in general), followed by a closer look at DM in the MW halo.

This next part - DM in galaxies - is both much richer (a lot more observations, over big ranges of objects and scales, longer history, ...) and poorer (the observational basis isn't so clean and clear as it is for rich clusters). The net is: rather than a "house of cards", the observational basis for DM in galaxies is a fortress, built on solid foundations, with multiple, interlocking, self-reinforcing structural elements.

After that? We still have DM in cosmology - GR, CMB, large-scale structure, etc. And, if anyone's still interested, a quick look at the dozens of "DM" experiments under way in Earthly labs (including the one which the team feels has an unambiguous DM signature).

You have used the "plasma" word a lot in this thread, is there a difference in your use of "plasma" and the Arp use of "plasma"?

It seems like "plasma" is a dirty word when discussing the universe (among mainstreamers) and yet there also seems to be an appropriate use of the plasma word.

Plasma is not a dirty word in astronomy. Most astronomers are aware that stars are made of plasma, and that much of the matter in interstellar space is also a very rarified plasma. The problem comes when supernatural properties are atributed to this plasma, such as the ability to somehow segragate massive amounts of particles by charge, or to somehow create energy.

__________________
Forming opinions as we speak

To add to antoniseb's post:

I have no idea where you got this idea from, Squashed - have you been spending rather too much time, perhaps, surfing crackpot websites?

Perhaps you could start a Q&A thread, along the lines of "what is the role of plasmas in modern astrophysics?" or "how much plasma physics do undergraduate and graduate astrophysics courses, at leading universities, include?"

In the meantime, perhaps a quick skim of this BAUT (General Science) thread might serve to provide some context.

This post is a bit of an interlude, finishing off a look at the observational basis for DM in (rich) clusters, before moving to the next topic*.

Yet again, it seems Zwicky was there first; it was he who first reported that there is ""an extended mass of luminous intergalactic matter of very low surface brightness" in the core of the Coma" cluster**

Of course, with spectacular images of the tidal tails of colliding galaxies, such as the Antennae, and The Mice, and the large number of galaxies in close proximity in rich clusters, it would be surprising if there weren't any stars in between the galaxies.

Observationally, getting a good handle on this intracluster light (ICL, or IcL) is challenging - to characterise it well requires detection of surface brightness light that is below 1% of the dark sky (even of such dark sites as Hawaii), and, for nearby clusters, over large areas of sky (certainly far greater than Hubble's pencil beam).

And recently, where the stars which produce this ICL came from has become an interesting research topic - tidal tails, stripping of dwarf galaxies, extended halos of the cD monster galaxies, formed at the time of the cluster, ...

Nearby clusters (such as Virgo) can be observed in detail (individual stars and planetary nebulae can even be seen!); different techniques can be used to study ICL integrated across many clusters, and so on.

The relevance, for this thread, of all this work is to address one loophole - how much (luminous, baryonic) mass is there, other than in galaxies and the hot IGM?

For stars, as determined from ICL studies, the answer is "only a little".

For example, the ICL may amount to perhaps 30% of a cluster's light, in the central ~500kpc core (vs 70% from galaxies), and much less outside the central core. Which translates to only a modest increase in the estimated mass of stars in a rich cluster (unless the M/L ratio of the stars which produce the ICL is dramatically different from that which produces the observed light in galaxies).

As colliding galaxies contain both gas and dust, as well as stars, the tidal debris from such collisions should also include gas and dust. Can we detect such wreckage? And how much dust is there in the IGM of rich clusters?

But first, the gas. Blobs of gas/plasma that are less than 100% ionised are relatively easy to detect - light which shines through such blobs will have tell-tale absorption lines. Such absorption lines are common in quasars, and they provide a good tool for research into the gas content of clusters. AFAIK, the net of this research, of relevance to how much (luminous, baryonic) mass there is, other than in galaxies and the fully ionised component of the IGM, is "not much" (I've not looked into this in much detail, so if any BAUT member reading this has more observational material to offer ....).

Returning to dust. The work of BAUT's ngc3314 has an indirect relevance; if the IGM were as dusty as spiral galaxies, (rich) clusters would be opaque (in the optical). That we can see distant galaxies and quasars on sightlines through clusters means there is little dust in the way (other than in our own Milky Way).

However, ISO did detect cold dust in the heart of the Coma cluster. As the researchers noted, this is quite interesting, because this dust could only survive immersed in the hot IGM (that generates the observed x-rays) for 100 million years or so, which in turn suggests that it is constantly being replenished, from galaxy collisions and tidal stripping.

Anyway, in terms of relevance to how much (luminous, baryonic) mass (dust, in this case) there is, other than in galaxies and the hot IGM, the answer is "very little, compared with the mass of stars etc in the observed galaxies".

So, what other forms could baryonic matter have, and still escape detection by vigilant astronomers? Perhaps there's a vast number of pebbles, rocks, boulders, asteroids, even planets and brown dwarfs wandering around, not bound to any cluster galaxy?

AFAIK, there are no direct observations which constrain the mass of such things, in the IGM; or, if there are such constraints, they are weak indeed (except, perhaps, for smaller objects composed of 'metals' - the hot IGM, though tenuous, would erode 'pebbles', and produce a diffuse gas of highly ionised 'metals' ... such a gas is not seen, so some reasonably strong observational constraints can be put on the smaller objects).

However, such vast quantities of 'pebbles to brown dwarfs' would be easily detected, unless confined to rich clusters only (we will look at these types of objects later; for now, consider these questions: how many comets have been observed, with hyperbolic orbits? ditto, meteors?), so such baryonic populations seem unlikely.

Finally, there is one class of (compact) object that may have a combined mass consistent with what we estimate the mass of DM in rich clusters to be (from the several independent kinds of observations, per earlier posts), AND which would have escaped all attempts at detection (to date): intermediate black holes (IMBH). However, while these objects, if they exist, are most certainly 'cold', I don't think anyone would count them as 'baryonic'.

*I haven't quite decided what that should be yet - possibly a historical note, on DM in our immediate neighbourhood, an arm of the Milky Way.
**This quote is from the 57th NOAO Newsletter; the best reference for the Zwicky work, that I have found so far, is Zwicky, F. Morphological Astronomy p. 48 (Springer-Verlag, Berlin, 1957)

In order for what we can see to do what it does there has to be some stuff we can't see, at least not yet. What it is I haven't a clue.

That may be an oversimplification. In order for what we see to do what it does, it may be required for the gravitational field to be stronger in some domains than in others, resulting in a gravitational constant that is higher in some domains and lower in others. The mainstream does not consider this, and relies on the invention of additional entities.

__________________
The ether of general relativity therefore differs from that of classical mechanics or the special theory of relativity respectively, in so far as it is not 'absolute', but is determined in its locally variable properties by ponderable matter.

Albert Einstein, "On the Ether", 1924

However, upon further investigation, the idea of a variable "g" in different domains of acceleration does not seem to be a self-consistent hypothesis. Binary stars orbiting far from the galaxy center have large accelerations (so they should be in the newtonian "g" domain), but they orbit within the galaxy as if they are in the "stronger than newtonian g" domain. I'm not sure that simply changing our human point of view reconciles this apparent inconsistency.

Nonsense. Mainstream scientists have considered variable "g" theories, and if they haven't rejected them altogether, they have found dark matter as a more likely solution.

And now the dark matter solution has been strongly supported by weak lensing observations of colliding clusters -- observations that would not be expected with "modified gravity" theories. -- Article

__________________
Everyone is entitled to his own opinion, but not his own facts.

Nereid,

Thanks very much for your detailed postings. I will probably have to read them again to make sure I get it all. In any event, I appreciate the hard work on this interesting topic.

One point struck me however: (Emphasis mine)

I recently read this article http://www.sciencedaily.com/releases...0508072847.htm about cold dust found in Blue Compact Dwarf galaxies. I found this quote interesting: (Emphasis mine).

Now in another post, you write about hot plasmas making up the intersteller and intergalactic matrix (I hope I'm getting this right). I have read elsewhere that scientists anticipated the hot plasma would erode this cold dust - but that doesn't seem to be the case. Anyway, the recent Chandra observations suggest that the bullet cluster collision stripped this hot plasma from both clusters and the microlensing observed suggests proof of DM. Some suggest that the plasma made up around 90% of the ordinary matter of these clusters. http://cosmicvariance.com/2006/08/21...matter-exists/ If I understand the theory correctly, the fact that 90% of the normal matter (the plasma gas) remained behind both clusters but significant gravitational microlensing was observed near the clusters them suggests DM must exist. You've also posted that any cold baryonic dust, brown dwarves, planets, etc. can only make up a small percentage of the DM - meaning that there must be huge quantities of NBDM - right?

If the plasma constituted 90% of the ordinary matter of the clusters - wouldn't its abrupt removal have catastrophic consequences on the remaining 10%? The photos suggest that the galaxies in those clusters retained cohesive shapes. Is the NBDM component of these galaxies and clusters sufficiently large to absorb the loss of 90% of the baryonic matter without a blip?

I am not trying to be intentionally dense - but am I mistaken in assuming that dragging 90% of the mass from a galaxy wouldn't be the equivalent of a cosmic hurricane with the potential to fling the remaining 10% about willy-nilly? Or is the 90% somehow gravitationally insignificant?

The 90% can not be "gravitationally insignificant" because the very inference of dark matter is from gravitational anomalies which means dark matter has to have a gravitational effect.

Sorry to confuse, Squashed - the CosmicVariance article cited says that the plasma makes up 90% of the normal matter. I presume from reading about the Blue Dwarf Galaxies, that the interstellar gas plasma within the galaxy likewise makes up a significant percentage of the normal matter - thus, presumably 90% of the normal matter in a galaxy or cluster of galaxies is in the form of interstellar and intergalactic plasma.

That clarified: I reask my questions as if fully set forth herein (grin )