Slashdot Mirror
Powerful Galaxies Found in Infrared
demachina writes "NASA's Spitzer Infrared space telescope has discovered 'a mysterious population of distant and enormously powerful galaxies radiating in the infrared spectrum with many hundreds of times more power than our Milky Way galaxy.' They are 80% of the way back to the big bang. They found them by comparing a visible and infrared scan of the sky and looking at the places where there was a big infrared signature and no visible one. They are shrouded in dust."
Infrared "energy" IS light.
Electromagnetic radiation takes many forms: radio, microwave, infrared, visual (what we see as "light"), UV, Xrays, gamma rays. They are all "light".
Sorry to be a pedant.
No gods, no demons, and no masters. Secular Humanism!
You can do spectral analysis to determine the original emission spectra. The stars have emission and absorption lines at certain wavelengths, and these all get shifted by a certain amount. If it was due to redshift alone, we would know it, i'm pretty sure.
IAAA (I am an astronomer).
All galaxies (with the exception of the recently discovered and dubiously titled "dark matter galaxy" mentioned here a few days ago) emit light at a wide variety of wavelengths, from radio all the way to gamma rays. The wavelengths at which a star emits is related to its temperature (google "blackbody radiation" or "planck spectrum"); other astrophysical processes can produce or modify passing emissions as well (molecular & plasma clouds, various types of "dead" stars like neutron stars, white dwarfs, etc. can create emissions due to non-blackbody radiation - google "bremstrahllung", "cerenkov", "synchrotron", etc.).
The reason that these particular galaxies are only visible in the infrared is that a) intervening dust reddens emissions across intergalactic (and, for that matter, INTRAgalactic) distances, and b) they are so far away that as the universe has expanded, the light traveling from them has been redshifted - stretched along with the spacetime through which they have been traveling. Thus, what we see as infrared now was originally of much shorter wavelength when it was emitted.
Hope that's useful, let me know if I can clarify.
No gods, no demons, and no masters. Secular Humanism!
Because these galaxies are surrounded by dust (likely from massive starbursts, which produce dust). Dust, because of it's scattering properties, preferentially lets long wavelength light pass through it (ie. infrared) but scatters shorter wavelength light (ie. visible light) into other directions. This is the same effect you see when looking at a sunset. The setting sun looks redder because there is dust (small, scattering particles of various sorts) letting more red light through to you than blue light. In these galaxies, it is more extreme.
The effect is called "dust reddening." I have some slides about it for the lastest entry (March 2) for my Astronomy 1050 class at my astronomy webpage if you want to see examples.
Professor of Astronomy, Author of Spider Star & Star Dragon (Tor)
I'll just reply to a few of the questions raised.
The hot intercluster medium IS hot, but temperture is a funny thing in some astronomical settings. In this case, the density of particles is so low, a better vacuum than you'd get in Earth laboratories, that the heat content would be pretty low. You wouldn't get incinerated, for instance. But a conventional thermometer wouldn't work either since it probably wouldn't get into thermodynamic equilibrium. It would radiate away its heat faster than the ambient gas could warm it.
Astronomers have excellent limits on the amount of normal matter, as the parent poster says. We've got an excellent idea what is out there based on emission in the far infrared, interstellar scintillation, absorption line studies, reddening studies, etc. We have very good limits on the Oort cloud density, too, from comet statistics. There are even a number of direct observations based on microlensing surveys, and there's a shadow survey, too, looking at large star fields. In short, we've got pretty good numbers and we're not going to discover that there's more normal dark baryonic matter out there than we already know about.
Professor of Astronomy, Author of Spider Star & Star Dragon (Tor)