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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.'"
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 ...
The correlation between ignorance of statistics and using "correlation is not causation" as an argument is close to 1.
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.
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.
Suddenly, the hairy finger of a familiar monkey tapped me on the shoulder. It was time.--G. T.