Unlike quasars, Seyfert galaxies are often found quite close to home.http://en.wikipedia.org/wiki/Seyfert_ga ... cteristics wrote:
The two largest subclasses of active galaxies are quasars and Seyfert galaxies, the main difference between the two being the amount of radiation they emit. In a typical Seyfert galaxy, the nuclear source emits at visible wavelengths an amount of radiation comparable to that of the whole galaxy's constituent stars, while in a quasar, the nuclear source is brighter than the constituent stars by at least a factor of 100.[1][23]
Seyfert galaxies and quasars are believed to be powered by similar mechanisms, though on vastly different scales.http://en.wikipedia.org/wiki/Seyfert_galaxy#Discovery wrote:
More recent surveys that count galaxies with low-luminosity and obscured Seyfert nuclei suggest that the Seyfert phenomenon is actually quite common, occurring in 16% ± 5% of galaxies; indeed, several dozen galaxies exhibiting the Seyfert phenomenon exist in the close vicinity (≈27 Mpc) of our own galaxy.[3]
Ultraviolet light is associated with the nuclei of Seyfert galaxies.http://en.wikipedia.org/wiki/Seyfert_galaxy wrote:
These galaxies have supermassive black holes at their centres which are surrounded by accretion discs of in-falling material.
Bystander recently referred to a NASA/ESA press release about well-studied Seyfert galaxy NGC 5548. According to the press release, astronomers have discovered a clumpy gas stream quickly flowing outwards and blocking 90% of the X-rays from the black hole. And it would seem that ultraviolet radiation from the accretion disk is responsible for for blowing gas clouds away which can later "return" to block X-rays from the black hole:http://en.wikipedia.org/wiki/Seyfert_galaxy wrote:
The accretion discs are believed to be the source of the observed ultraviolet radiation.
I looked up NGC 5548 to check out its colors, and I was extremely surprised by them. The B-V index of NGC 5548 is 0.700, which is quite unremarkable. It is exactly the same as the B-V index of large Virgo spiral M100: its B-V index is also 0.700. But the U-B index of NGC 5548 is -0.150. For a galaxy with a B-V index as "moderately red" as 0.700, a U-B index is -0.150 is extremely blue (or rather, it is extremely ultraviolet). For comparison, the U-B index of M100 is -0.010, and that value is in itself pretty blue (or ultraviolet) for a galaxy with as "red" a B-V index as 0.700. I checked a number of well-known galaxies, and none of them can match the combination of a slightly "reddish" B-V index and a strikingly blue U-B index. Amazingly, my software says that the "effective U-B index" (whatever that is) of NGC 5548 is -0.490!!!! That's amazing!!!As matter spirals down into a black hole it forms a flat disc, known as an accretion disc. The disc is heated so much that it emits X-rays, near to the black hole, and less energetic ultraviolet radiation further out. The ultraviolet radiation can create winds strong enough to blow gas away from the black hole, which otherwise would have fallen into it. But, the winds only come into existence if their starting point is shielded from X-rays.
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The newly discovered gas stream in the archetypal Seyfert galaxy NGC 5548 — one of the best-studied sources of this type over the past half-century — absorbs most of the X-ray radiation before it reaches the original cloud, shielding it from X-rays and leaving only the ultraviolet radiation.
Actually, the colors of NGC 5548 become even more remarkable when its Hubble type is taken into account. Most galaxies with with B-V colors around 0.700 are either somewhat evolved spirals with some star formation or dwarf spheroidals made up of an old, metal-poor population. According to my software, NGC 5548 belongs to Hubble type S0-a. These are typically quite red galaxies, with B-V indexes considerably redder than 0.700.
Seyfert galaxies are typically normal or barred spirals:
But NGC 5548 is not a typical spiral galaxy:http://en.wikipedia.org/wiki/Seyfert_galaxy#Discovery wrote:
By 1977, it was found that very few Seyfert galaxies are ellipticals, most of them being normal or barred spiral galaxies.[18]
http://en.wikipedia.org/wiki/NGC_5548]Wikipedia wrote:
Morphologically, this is an unbarred lenticular galaxy with tightly-wound spiral arms, while shell and tidal tail features suggest that it has undergone a cosmologically-recent merger or interaction event.
So it could be that the moderately blue B-V colors (too blue for a typical S0-a galaxy) could be the result of a cosmologically recent merger or interaction. Perhaps the Seyfert nucleus also owes something to such a merger or interaction.
The picture does show little blue knots of star formation near the nucleus. Larger versions of the picture also show brilliant blue-white spikes emerging from the bright nucleus.
For me, the color nerd, the picture of the core of NGC 5548 is interesting. But it is the color indexes of this galaxy that reveal something truly remarkable.
Ann