Galaxies Twice As Bright As Previously Thought

Astronomers led by Simon Driver of Scotland's University of St. Andrews have discovered that interstellar dust shades us from as much as 50% of the light emitted by stars and galaxies. The scientists compared the number of galaxies we could see "edge-on" against the number which were "facing us," reasoning that dust would obscure more of the former, since we already receive less light from them. SPACE.com notes, "In fact, the researchers counted about 70 percent fewer edge-on galaxies than face-on galaxies." A NYTimes report provides some additional details: "Interstellar dust absorbs the visible light emitted by stars and then re-radiates it as infrared, or heat, radiation. But when astronomers measured this heat glow from distant galaxies, the dust appeared to be putting out more energy than the stars. 'You can't get more energy out than you put in, so we knew something was very wrong,' said Dr. Driver. The results also mean that there is about 20 percent more mass in stars than previously thought."

Stars versus dark matter

[anonieuweling]

More mass in stars, so less dark matter...

Re:So there's more dust than previously thought...

[BadAnalogyGuy]

Dark matter is non-interacting. It only exerts a gravitational force. It would not obscure the light of galaxies (except to bend the light through gravitious pull).

Re:So there's more dust than previously thought...

[Anonymous Coward]

Not exactly. Try the wikipedia article on dark matter [wikipedia.org] first.

The composition of dark matter is unknown but may include ordinary and heavy neutrinos, recently postulated elementary particles such as WIMPs and axions, astronomical bodies such as dwarf stars and planets (collectively called MACHOs), primordial black holes and clouds of nonluminous gas. Also, matter that might exist in another universe but might affect ours via gravity would be consistent with some theories of brane cosmology. Current evidence favors models in which the primary component of dark matter is new elementary particles, collectively called nonbaryonic dark matter.

This article suggests a new model where much more of it might be dust and stars.

Re:So there's more dust than previously thought...

[Anonymous Coward]

'Exotic' dark matter, which interacts only gravitationally, is a purely theoretical phenomenon. It was invented to try to account for the discrepancy between mass observed through gravitational effects, compared to optical methods.

With optical methods we now observe more mass through optical methods, and that discrepancy is smaller. The need for the theoretical, exotic 'dark matter', which has never been observed, has been decreased, if this study's results turn out to be accurate.

Re:So there's more dust than previously thought...

[IdahoEv]

What this will do is reduce the amount of dark matter that is necessary to explain the observed gravitational effects.

Dark matter is theorized to exist because galaxies behave gravitationally as if they have more mass than we can account for based on the light we see; dark matter makes up the difference. Since this result demonstrates that there is more light-emitting matter than we previously believed, it explains a slightly larger proportion of the observed gravity. Hence, a slightly smaller amount of dark matter exists than previously believed.

It's not remotely enough of an increase to explain away all of the missing mass. IIRC there is a lot more dark matter than luminous matter, so an increase of 20% in the amount of luminous matter will only make a small difference.

Re:So there's more dust than previously thought...

[zippthorne]

He wasn't suggesting that the DUST is the dark matter. He was suggesting that the stars' unaccounted-for mass is, at least part of, the "dark" matter: the matter that we cannot observe except by it's gravitational effects.

The article suggests two things by stating that the dust is obscuring galaxies more than previously thought:

1) there is more mass in the galaxies than previously thought (to be generating the light we don't see)

2) there is more mass in the dust than previously thought.

"dark" matter is in it's essence, unaccounted for matter. In a sense, Neptune was a "dark" planet until it was observed. Astronomers have suggested that the reason we haven't observed the "missing mass" is that it is not observable. The article does, in fact, suggest that at least part of the missing mass may be unobservable for mundane reasons rather than new physics.

Re:So there's more dust than previously thought...

[cnettel]

Just to make things clear, even a doubling of the amount of mass in stars would only reduce the amount of dark matter by a few percent (and then we have the dark energy...).

Re:So there's more dust than previously thought...

[FooAtWFU]

Have you measured the effect of gravity on light recently? You ever notice how your flashlight beam actually falls towards the ground when you aim it straight out? No? That's because it's trivially small.

To obscure light, matter would need to absorb it. Assuming that it cannot, the closest to "obscuring" that gravitational interactions could do is to bend it a little so it's facing a different direction. Lensing, and all that fun. I suppose in the worst case, a patch of dark matter could act to randomly diffuse the light going through it, but since it IS matter and it is gravitationally bound, it tends to form clusters like other matter, and you're not going to see diffusion over the million-light-year gaps between the galaxies being observed.

Re:Baryonic dark matter...

[Anonymous Coward]

Wow, your post is complete disinformation. I've never seen that on slashdot. Normally only a part of it is wrong.

The CMB *has* the blackbody signature of an object at 2.725 kelvin. It is even the most precise blackbody ever found. The "shift" you're talking about is more accurately a multiplicative factor of about 1000. Multiplicative factors map a theoretical blackbody signature to another one with no distinction possible.

Re:...shooting in the dark, so to speak...

[Tacvek]

The dark matter theory comes from the caculated amount of total matter that should exist. As it is, a significant change in the amount of luminous matter would not change the amount of dark matter needed to reach that total by very much. However, what exactly is that total amount of energy based on? Presumably the amount of matter needed to correct the orbits of large systems. However, this throws distance measurements into doubt. My understanding is that distance measurements are based in part on observed brightness. Distances to objects of known intensity are calculated from the apparent intensity of objects of known actual intensity. However the calculations are surely based on the inverse square law. However the amount of light lost to this dust means that the inverse squares law is not really accurate. That means that the distances to those reference objects have been overestimated. That in turn means the distances to the other objects are incorrect too. If our distance measurements are incorrect, it seems quite reasonable that out calculated orbits are incorrect too. The orbits may be many, many, many times closer to what they should be based on luminous matter alone. That means the total matter needed could drop an enormous amount. If the total amount of matter needed is then quite close to the amount of luminous matter needed, it may be that we do not need to invoke the existence of enormous amounts of exotic matter.

Not Intergalactic Dust...

[florescent_beige]

From reading TFA, the dust they are talking about is *within* the galaxies. Because of it galaxies don't emit as brightly edge-wise.

But perpendicular to the plane there is little dust absorption. So the brightness of galaxies viewed this way shouldn't need much correction. Since most galaxies are viewed this way due to the bias caused by this effect, why would there need to be a major rethink of stellar brightness? I'm not getting it.

Maybe it's galactic density that needs correction.

Re:So there's more dust than previously thought...

[no1home]

It's not so much obscuring as mildly redirecting. It's called gravitational lensing. (http://en.wikipedia.org/wiki/Gravitational_lensing) This is what causes effects like the halo around a distant back lit object or the optical illusion of two copies of the same object (star, galaxy).

Re:So there's more dust than previously thought...

[shma]

There seem to be a lot of questions about dark matter, so I'll do my best to answer them.

1)Dark matter is indeed postulated to account for the discrepancy between gravitational measurements of the mass distribution of galaxies vs evidence from other sources.

2)We know that dark matter can't be accounted for by large mass objects (like planets, asteroids, dust, etc) because CMB measurements tell us that the total amount of baryonic matter ('normal' matter made up of protons and neutrons) is a small fraction of the total matter in the universe (around 15%). So it must be made of heavy non-baryonic particles. This, by the way, is the reason why the discovery mentioned in TFA has little impact on dark matter. There is already an upper limit on the amount of baryonic mass in the universe, irrespective of what we see with telescopes.

3) We know that these particles can't interact electromagnetically or with the strong force, otherwise they would end up in atoms (either as part of the nucleus or orbiting the nucleus). In this case, these atoms would be much heavier than normal atoms and we would see evidence of them in the spectral lines of stars.

4)That leaves us with particles which interact only through the weak force, like neutrinos. We have also found that dark matter plays an important role in the formation of structure in the universe, and in order for structure to form in the way it has, the dark matter must be moving at non-relativistic speeds at that time. This rules out the neutrino, which would be moving at speeds very close to the speed of light at that time.

Re:big shake-up

[khallow]

I believe you've nailed the major implication of this research. Assuming it turns out to be true, it may provide an alternate explanation for why distant supernova (type 1A) appear more distant than expected from their red shift.

Re:So there's more dust than previously thought...

[The_Wilschon]

Fine, but that's pretty clearly not what TFA meant by "obscured". So context-ignoring semantic wrangling aside, gravitational lensing is not particularly relevant here.

Re:So there's more dust than previously thought...

[scratchpaper]

Hi everyone, I teach astronomy, and I see this all the time: the term "dark matter" is almost always misconstrued to be some strange, exotic form of matter. In reality, its just an umbrella term meaning ANY kind of matter that, for one reason or another, is obscured from our observations. So yes, IS dust clouds could be a significant contributor to the "missing matter" that we think is out there. Really, anything we can't directly observe. Think about it: no detector is 100% efficient, and no observation equipment can scan ALL the frequencies of the EM spectrum. We can cover good portions of it, but not all...so there's some vital information missing. Also, cool objects emit vastly less broadband radiation, so objects like old dead dwarfs (red, white and black), "failed" stars like brown dwarfs, exoplanetary systems just to name a few do not contribute much to the "light" we receive from the rest of the universe. And light is really ALL we have to go on in observational astronomy. And let's not get started on neutrino mass... :)

Re:Same to you...

[hxnwix]

How can pointing to a picture which claims to observe the unobservable not be funny?

Dark matter is postulated to be observable solely by its gravitational interaction with directly observable matter and energy. In this case, the Bullet cluster image represents empirical data: matter not visible in the image but within the frame of the image and in front of other visible objects is deflecting electromagnetic radiation emitted from those background objects.

Similarly, you can't see electrons, but if you collect a large amount of them, you can observe the force caused by the static charge upon observable objects. If you move them, you can detect the generated magnetic field.

And here's your CMB [dfi.uem.br], predicted long before Big Bang cosmology, and more accurately, too.

That's a non sequitur. The +5 rated post to which you originally replied pointed out that CMB anisotropy indicates the presence of cold dark matter; the link you supply deals with average CMB black body spectra and does not mention dark matter even once.