by neufer » Wed Nov 10, 2010 8:00 pm
bystander wrote:Ann wrote:
... A similar phenomen is the famous "Hanny's Voorwerp":
This strange green (but really red) hydrogen gas cloud was expelled by the quasar at the center of the large galaxy in the picture. However, the quasar has now "died", and the center of the galaxy is now quiet. But the expelled gas cloud still glows green (really red) with hydrogen emission, because it was ionized by the jet of the quasar before the jet disappeared. So the ionizing source is gone, but the "punch" it packed still affects the gas cloud thousands of light years from the galaxy.
I do not think the gas cloud was expelled from the galaxy, but that the "jet" ionized an existing gas cloud.
Not only that but the strange green "Hanny's Voorwerp" is actually a blue object redshifted to green.
http://www.astr.ua.edu/keel/research/voorwerp.html wrote:
<<This page is set up to summarize basic data and ongoing study of the curious blue object known as "Hanny's Voorwerp". The "Hanny's Voorwerp" spectrum shows emission lines typical of star-forming regions spread across the object, at a redshift associating it with IC 2497, but shows, in addition, features indicating unusually high temperature and higher ionization than seen in H II regions. I wrote up this description for the GalaxyZoo blog: First, the gas is hot (even by the standards of ionized nebula). The ratio of [O III] lines between 4363 and 4959+5007 is sensitive to temperature. To have the 4363 line even detectable, the gas has to be unusually hot, more like 15-20,000 K. Even odder are some of the other lines. He II is produced when an electron joins a bare helium nucleus, and requires high enough temperature or radiation with enough energy to tear both electrons from helium (four times harder than for hydrogen). We don't see this in star-forming regions. The only stars hot enough to produce He II in surrounding nebulae are the central stars of planetary nebulae (which are the hottest stars known, but only for a few thousand years) and a handful of X-ray-bright stars usually associated with accretion onto black holes or neutron stars. On top of that, at the blue end of the spectrum is [Ne V]. If it's hard to rip two electrons from helium, it's that much harder to pull four from neon. This requires 97 electron volts (eV), compared to 54 to make He II and 13.6 to ionize hydrogen. [Ne V] does sometimes show up in planetary nebulae, but even there calculations suggest that it's not the UV starlight that's responsible, but that high-speed shock waves may be the culprit. This line is also common in the spectra of active galaxies - Seyfert nuclei and their kin, where we know that there are abundant X-rays interacting with the gas.
At this point, we know that the object is rich in highly ionized gas. There is continuum light, especially at the northern tip, but the emission lines are so strong that we can as yet say little about its continuum structure. The high ionization might suggest shock ionization or photoionization by an active galactic nuclei, which would have to be much brighter than any we see in the neighborhood. If the AGN is in IC 2497, it must be highly obscured from our direction but not toward the gas. (It may be significant that the cloud lies near the galaxy's projected minor axis).
Our working hypothesis is that Hanny's Voorwerp consists of dust and gas (maybe from a tidally disrupted dwarf galaxy) which is illuminated by a quasar outburst within IC 2497, an outburst which has faded dramatically within the last 100,000 years.>>
[quote="bystander"][quote="Ann"][float=left][img2]http://apod.nasa.gov/apod/image/0806/hannysvoorwerp_wht.jpg[/img2][/float]
... A similar phenomen is the famous "Hanny's Voorwerp":
This strange green (but really red) hydrogen gas cloud was expelled by the quasar at the center of the large galaxy in the picture. However, the quasar has now "died", and the center of the galaxy is now quiet. But the expelled gas cloud still glows green (really red) with hydrogen emission, because it was ionized by the jet of the quasar before the jet disappeared. So the ionizing source is gone, but the "punch" it packed still affects the gas cloud thousands of light years from the galaxy.[/quote]
I do not think the gas cloud was expelled from the galaxy, but that the "jet" ionized an existing gas cloud.[/quote]
Not only that but the strange green "Hanny's Voorwerp" is actually a blue object redshifted to green.
[quote=" http://www.astr.ua.edu/keel/research/voorwerp.html"]
[float=right][img3="2D optical spectrum crossing the cloud and IC 2497.
William Herschel Telescope"]http://www.astr.ua.edu/keel/research/WHTSpecLabelled.jpg[/img3][/float]<<This page is set up to summarize basic data and ongoing study of the curious blue object known as "Hanny's Voorwerp". The "Hanny's Voorwerp" spectrum shows emission lines typical of star-forming regions spread across the object, at a redshift associating it with IC 2497, but shows, in addition, features indicating unusually high temperature and higher ionization than seen in H II regions. I wrote up this description for the GalaxyZoo blog: First, the gas is hot (even by the standards of ionized nebula). The ratio of [O III] lines between 4363 and 4959+5007 is sensitive to temperature. To have the 4363 line even detectable, the gas has to be unusually hot, more like 15-20,000 K. Even odder are some of the other lines. He II is produced when an electron joins a bare helium nucleus, and requires high enough temperature or radiation with enough energy to tear both electrons from helium (four times harder than for hydrogen). We don't see this in star-forming regions. The only stars hot enough to produce He II in surrounding nebulae are the central stars of planetary nebulae (which are the hottest stars known, but only for a few thousand years) and a handful of X-ray-bright stars usually associated with accretion onto black holes or neutron stars. On top of that, at the blue end of the spectrum is [Ne V]. If it's hard to rip two electrons from helium, it's that much harder to pull four from neon. This requires 97 electron volts (eV), compared to 54 to make He II and 13.6 to ionize hydrogen. [Ne V] does sometimes show up in planetary nebulae, but even there calculations suggest that it's not the UV starlight that's responsible, but that high-speed shock waves may be the culprit. This line is also common in the spectra of active galaxies - Seyfert nuclei and their kin, where we know that there are abundant X-rays interacting with the gas.
At this point, we know that the object is rich in highly ionized gas. There is continuum light, especially at the northern tip, but the emission lines are so strong that we can as yet say little about its continuum structure. The high ionization might suggest shock ionization or photoionization by an active galactic nuclei, which would have to be much brighter than any we see in the neighborhood. If the AGN is in IC 2497, it must be highly obscured from our direction but not toward the gas. (It may be significant that the cloud lies near the galaxy's projected minor axis).
Our working hypothesis is that Hanny's Voorwerp consists of dust and gas (maybe from a tidally disrupted dwarf galaxy) which is illuminated by a quasar outburst within IC 2497, an outburst which has faded dramatically within the last 100,000 years.>>[/quote]