Dim Galaxy Could Give Clues to Dark Matter

chamblah writes "Reuters is reporting that the dimmest galaxy has been found. 'In fact, it is dimmest galaxy ever detected, which means it could give clues to the mysterious dark matter that appears to be pushing regular matter around.' Since this galaxy is '...100 times dimmer than the night sky', it could only be detected using 'instruments involved in the Sloan Digital Sky Survey, the sky-mapping project.' The galaxy is also part of the Andromeda galaxy, only 2 million light years from us. The article goes on to explain how finding these dim galaxies can be useful, 'Andromeda IX fits the profile for the small, dim galaxies that cosmic theorists predict should exist as leftovers from the formation of big galaxies.'"

If I read the article right ...

[Daniel Dvorkin]

It's not "part of" Andromeda; it's a satellite galaxy, like the Magellanic Clouds are to the Milky Way. It wouldn't make a whole lot of sense to talk about a dim galaxy that's part of a regular galaxy, anyway ...

What this means for dark matter

[Pi_0's don't shower]

Alright, I hope this doesn't come off as condescending, but IAAA (grad student, at least), and *one* dim, tiny dwarf galaxy will tell us very little about dark matter.

You can measure its velocity dispersion to infer its total mass, and you can measure its light and spectra to attempt to infer its mass in baryons (protons, neutrons, and electrons), and you can measure the spectral lines to determine its metallicity, but this has nothing to do with inferring dark matter.

Dark Matter is inferred, at least when it comes to galaxies and clusters of galaxies (to keep it simple), because the mass required to provide the galaxy/cluster with the internal velocities observed is much more than what we see in starlight. Therefore, some of the matter is non-luminous, or "dark". Dark matter exists, on AVERAGE, so that 1/7 of the total mass in a galaxy is in baryons, and 6/7 is in dark matter. This ratio varies widely for different galaxies, and I do not see how *one* galaxy is going to tell us anything?

Also, if this satellite galaxy is less than ~100 kpc from Andromeda, the main galaxy's dark halo will envelop the satellite, too, further complicating the matter.

Re:Dark matter or dark energy

[bradkittenbrink]

Also, since there is so much matter in the universe, and it was all in a very tiny place just after the big bang, we know for sure that we were inside a black hole. But nothing can escape a black hole, not even light. So we live inside a black hole. A gigantic black hole. Why don't we see the universe collapsing? Simply because time is a continuous and in the black hole event horizon, time doesn't flow. If you stay at the horizon, your clock doesn't go forward nor backward. Therefore as time is continuous, time must go backwards in the black hole, because it goes forward outside the black hole.

To my knowledge, there are several mistakes in this description. However, I have never heard that reasoning before and after correcting these technical mistakes the reasoning as a whole may still be sound if it were phrased more accurately. First of all, matter can escape from a black hole, it gets radiated away in a thermal spectrum as described here [wikipedia.org]. Secondly, if you're sitting on the event horizon, you would observe your clock to tick normally, time does flow. An observer far removed from the black hole would indeed observe your clock as stopped however. Thirdly, I don't think that you can simply say that time "flows backward" inside a black hole. Although I have never taken a GR course, I am familiar with special reletivity and I imagine the frame of reference inside the event horizon to be somewhat analagous to a frame of reference travelling faster than light (at least when comparing them to a traditional inertial frame). There's nothing theoretically wrong with such a coordinate system, but there is simply no way you can transform values measured there into a traditional frame of reference without getting nonsense for an answer. In other words, if you were sitting inside the event horizon, there is simply no way to translate the ticks you observe on your clock to what a far removed observer would observe.

Re:dimmer than night sky??

[beeplet]

Any portion of sky which doesn't have a star/galaxy in it is black, black black!

Not totally black... there is a fair bit of light that reflects off dust in the solar system (zodiacal light). So it's entirely possible for this galaxy to appear 101% as bright as the background sky.

And just for general info... there are lots of low-surface brightness galaxies out there - Malin1 [cam.ac.uk] for example.

Re:OK ...

[Aglassis]

Tthe rotation of the Galaxies doesn't follow Kepler's Laws of planetary motion (which should apply if you consider all the stars in the galaxy to be planets). The easiest way to look at this is to compare the rotational velocities of the stars in the galaxy (which can be determined by taking the red/blue shift of the galaxy in general to determine its velocity relative to you and then taking the redshift of stars in the galaxy that are roughly parallel to your line of sight). Kepler's Third Law says that the orbital period squared is proportional to the long elliptical axis of the orbit cubed. Stars in galaxies don't do this so it is supposed that there is a dark matter halo surrounding the galaxy that corrects their motion to what is observed (of roughly ten time the mass of the galaxy). It is called dark matter because it can't be seen through electromagnetic radiation.