what are Seyfert galaxies ??????????

Question

if possible, could u guys explain it in simpler terms plz..

Answer 1

Wikipedia says:

Seyfert galaxies are characterized by extremely bright nuclei, and spectra which have very bright emission lines of hydrogen, helium, nitrogen, and oxygen. These emission lines exhibit strong Doppler broadening, which implies velocities from 500 to 4000 km/s, and are believed to originate near an accretion disk surrounding the central black hole.

www.seds.org says:

Some galaxies, notably the Seyferts, show large quantities of gas in their nuclei which is not associated with O or B stars. Their nuclei are called Active Galactic Nuclei (AGN's); the galaxies are sometimes called Active Galaxies. While making up the biggest portion, Seyferts are not the only galaxies with AGNs: Other examples are the radio galaxies and the quasars (D.E. Osterbrock subdivides the latter in radio active quasars and radio quiet quasi-stellar objects, QSO's). They all have in common that their high luminosity is not produced by stars

Answer 2

According to Wikipedia: http://en.wikipedia.org/wiki/Seyfert_galaxies
"Seyfert galaxies are a class of galaxies with nuclei that produce spectral line emission from highly ionized gas. They are a subclass of active galactic nuclei (AGN), and they are thought to contain super-massive black holes. The galaxies are named after the astronomer Carl Keenan Seyfert, who first identified this class of galaxies in the 1940s."

Most galaxies contain a super-massive black hole at its center, these are active galaxies and the Milky Way (our galaxy) is among them. The super-massive black hole doesn't suck in all matter for an unknown reason, these black holes are still active, but they are not hungry. If you put something too close then they will suck it in (thus making them active), but they don't try to suck the entire contents of the galaxy in for some reason.

The nuclei of a galaxy is the core, in a Seyfet galaxy the core has a lot of dust that hides the super-massive black hole at its core, making us think that there were fewer super-massive black holes in the universe. We have since found out through study of X-ray astronomy that these dust shrouded centers do hold super-massive black holes.

So how do you detect a black hole? The only gravity detector we have is if you drop something and measure its rate of fall. You can do that on a planet, but no where else. So we have to see what the black hole's effects are.

When a black hole sucks in matter it pulls it in faster than it can get out of the way of itself. As the matter gets pulled in around the equator it has more space to get pulled in, but at the poles it has less space so the poles have a lot of action. This action generates x-rays, a huge burst of x-rays. An x-ray burst of this power could sterilize the Earth as far away as our own galactic core, but luckily we rotate around the equator.

When we see these kinds of galaxies that are broadcasting x-rays from the super-massive black hole at the core we see a brilliant object. In fact the brightest object known in the universe, a quasar has been found to be such a super-massive black hole. We see the energy because it is pointed near to us, if it was pointed directly at us then it would create something stronger that scientists would call a blazer, but no galaxy that we can observe, is pointed directly at us. The discovery that dust can hide the effects of the super-massive black holes let us understand that a huge number of galaxies have these stellar items and prompted them to look at the core of our own galaxy discovering that indeed there is a super-massive black hole lurking there.

We are finding that most of astronomy can’t be done in the visible light spectrum, too much of the light is blocked and the spectrum is too narrow. Also our own atmosphere blocks a lot of the light. The single best way to do astronomy is to do it from a satellite, which is what made the Hubble Space Telescope so revolutionary and so important. Plans are to orbit new space telescopes, including a new x-ray space telescope (due to be launched in 2013) will expand our view of the universe and increase our understanding.

Answer 3

It is a galaxy with a bright center and swirls of stars around it.
Our Milky Way and the Andromeda galaxies are the Serfert type. See the reference for a photo.

Answer 4

Seyfert galaxies have been among the most intensively studied objects in astronomy, primarily because they are thought to be nearby, low-luminosity versions of the same phenomenon observed in quasars. A massive black hole in the nucleus of a galaxy, accreting gas from its surrounding environment, is thought to power all these objects. Of course, we do not see the black hole itself, but the UV continuum radiation is generally presumed to be thermal emission from the hot gas that forms an accretion disk surrounding the black hole. In addition, very broad emission lines are observed, which are thought to come from clouds somewhat farther away, moving at velocities of order . These broad-line clouds are photoionized and heated by the extreme-UV radiation from the central source, resulting in the strong, broad resonance line emission observed from hydrogen Lyman-, CIV (1550 Å), and other elements. The permitted lines also sometimes show narrower cores, and there are also narrow forbidden lines, which are thought to arise from more distant, lower density, photoionized gas in a narrow-line region.

The broad-line component dominates the spectra of quasars and type 1 Seyfert galaxies, while the narrow-line component dominates in type 2 Seyferts. It is widely believed that the objects may be basically similar, but that obscuration of the central region as viewed from certain directions may hide the continuum and broad-line regions in type 2 Seyfert galaxies, leaving a clear view of only the narrow-line region. The luminosity of Seyferts is typically , where is the luminosity of the Sun, making the tiny nuclear region as luminous as an entire galaxy of stars, and the inferred mass of the central black hole is to , where is the mass of the Sun. The luminosity is proportional to the mass-accretion rate, which is 1 yr. Small values of these parameters are associated with low-luminosity Seyferts, and large values are thought to characterize the much rarer, high-luminosity quasars.

The far-UV spectral region is of fundamental importance in determining the nature of all these active galactic nuclei. The UV continuum radiation may arise in an accretion disk very close to the black hole, while UV emission and absorption lines provide the best diagnostics of the surrounding material in the broad- and narrow-line regions. Consequently, observations of Seyfert galaxies and quasars were a goal of one of the major observational programs for HUT on Astro-1.

One of the brightest and best-studied Seyfert galaxies is NGC 4151. It has been classified as type 1.5, showing the characteristic features of both types 1 and 2 (Osterbrock & Koski 1976). We obtained a high quality spectrum of NGC 4151 in a 2200 s observation with HUT (Figure 3). Below 1200 Å, a region in which no Seyfert galaxy has previously been observed, we find strong emission in the OVI doublet and a very complex absorption-line spectrum. The Lyman- line and the OVI line are both found to have broad wings with full width at half maximum identical to the CIV feature, but overlying absorption by numerous lines tends to obscure this fact. Kriss et al. found that the broad lines have relative intensities similar to those seen in quasars (where the large redshift of quasar radiation makes this spectral region accessible to other telescopes) and to theoretical photoionization calculations. All of the permitted lines also have similar cores, with FWHM = 1500 km s

The strongest absorption lines include the Lyman series of hydrogen, as well as features due to CIII and NIII and higher ionization states, up to NV and OVI. All of the absorption lines are blueshifted, with respect to the galaxy rest frame, by several hundred km s, and they appear to have intrinsic widths of about . The UV continuum disappears completely below 924 Å, owing to strong absorption by overlapping Lyman lines. The ratio of the strengths of the CIII 977 Å line and the 1176 Å line (which arises in an excited state) indicates densities in the absorbing gas greater than . Such high densities are characteristic of the gas in the clouds that yield broad emission lines. Kriss et al. (1993) conclude that the absorption may arise in the disintegrating remnants of outflowing, radiatively accelerated, broad-line clouds. Furthermore, this same material may be responsible for producing the narrow cores of the permitted emission lines. Finally, this material may collimate the ionizing radiation from the central source, explaining the bipolar cone-like appearance of the narrow emission-line region (Kriss et al. 1993). The absorption lines seen by HUT in the far-UV thus provide an important new means for studying conditions in active galactic nuclei.


Seyfert Galaxies are named for Carl K. Seyfert in 1943

Answer 5

try this site:

http://www.seds.org/~spider/spider/ScholarX/seyferts.html