Zlender Images

Active, Starburst, Seyfert... Galaxies

Active galactic nuclei (AGN) are among the most luminous objects, the most powerful energy sources in the universe and hence they are the most distant markers that we can observe in space and time. An AGN emits unusually large amounts of energy from a very compact central source. This central powerhouse may be observed directly in the cores of Seyfert galaxies, BL Lac objects, QSOs or quasars. There is evidence that QSOs and Seyferts are located in spiral host galaxies, while radio galaxies, BL Lac objects, and radio-loud quasars have their homes in elliptical host galaxies.
Not at least HST-observations of the motions of stars and gas in galactic cores, in addition to other arguments ( see quasar, power-law spectrum, X-rays), strongly suggest that the energy output is derived from the gravitational potential of  a supermassive black hole: the energy would arise from an accretion disc of gas, spiraling into the black hole. This gas could come from the interstellar medium of a spiral galaxy (especially when perturbed by gravitational effects in interacting galaxies), from the tidal disruption of stars near the black hole, or from flows of intergalactic gas on to the central galaxy of a cluster of galaxies, as the gas cools. The X-ray continuum spectrum of many AGN features components of emission that are thought to be reflected off the accretion disc of the black hole, producing a reflection spectrum .
 

The table below is from Osterbrock 1989 and shows the fractions of galaxies per unit volume.
 

Type Number Mpc-3
Field galaxies 10-1
Luminous spirals 10-2
Seyfert galaxies 10-4
Radio galaxies 10-6
QSOs 10-7
Quasars 10-9
Approximate space densities


  M87, Center of  giant elliptical galaxy in Virgo with Jet, 04/16/99, jpeg, 10k
This image was made with my CG11 at F=2800mm. It is a composition of 5 single shots of 20 seconds each. Jet structures are a common phenomenon among radio galaxies (Virgo A in this case, is one of the brightest sources). The knotty appearance results when relativistic particles crash into gas which is encapsulated inside the jet. The particles are accelerated by a giant black hole of roughly 3 billion solar masses. The  jet has a length of 4000 light-years. It was detected in 1918 by H. Curtis.  Ultra high resolution images from HST and VLBI.

Another image from 2003 was made with my 10" newtonian and the ST8-camera in 2003. Beside M87 and the jet it shows a larger field around this giant galaxy dominating the Virgo cluster.

M82, Starburst galaxy in Ursa Major, 12/17/98, jpeg, 8k
  Images of M82 made in the light of H-alpha show explosive filaments blowing away from the nucleus of this galaxy. The temperature of H-alpha emitting gas is around 10000 K and may be heated by shocks and hot stars. The large extent (around 6 kpc) of the emission results from a superwind blowing out of the galaxy. The high supernovae rate in M82 (probably one every ten years) is thought to be the power source driving this superwind. The assumed distance to M82 is 3.6 Mpc.

  NGC 3628, Starburst galaxy in Leo, 03/19/03,jpeg,78k

A starburst galaxy is, as the name indicates, a galaxy in which a massive burst of star formation is currently taking place: it is characterized by an infrared luminosity that is considerably larger than its optical luminosity, sometimes by a factor of 50 or more. The luminosity of starburst galaxies can compare to the bolometric luminosity of quasars, but is powered by a fundamentally different mechanism. The starburst occurs in a region over a kilo parsec in size, thus distinguishing these galaxies from active galaxies, which have a tiny central powerhouse. A few nearby starburst galaxies (e.g. M 82) have long been known from their disturbed optical appearance, but their widespread occurrence was only established when the infrared satellite IRAS revealed thousands of them.
They are basically spirals in which the star formation is proceeding at a rate that cannot be sustained for much of the lifetime of the galaxy. The trigger for the burst of star formation is unclear although in some cases the gravitational effect of a companion galaxy may be responsible ( see interacting galaxies). The new stars are still enveloped in the galaxy's dense molecular clouds; their ultraviolet radiation is absorbed by dust in the clouds and reradiated as infrared.
 

M77, Seyfert galaxy in Cetus, 12/17/98, jpeg, 20k

The 9.6mag bright M77 belongs to a special class of galaxies, known as Seyfert galaxies. They have unusual bright, almost stellar-like cores. Their spectra reveal broad emission lines, almost invariably broader than those in starburst galaxies and covering a considerable larger range in ionization.
In Seyfert 1 nuclei a broad-line region (BLR) is found, in which the internal velocities of gas and dust are large (several thousand km/sec). These regions are surrounded by a narrow-line region (NLR), where speeds are considerably smaller. Seyfert 2 galaxies have spectra, in which the permitted and forbidden emission lines have approximately the same width. Their spectral features indicate that Seyfert 2 types lack the BLR region, or if they have one it must be obscured. It has been speculated by some authors that the difference between Seyfert galaxies of type1 and 2 might be a projection effect, depending on the angle of view.

In this scheme M77 was listed as a Seyfert 2 object, until Antonucci and Miller (1985) found that M77 shows in plane polarized light, the spectrum of a Seyfert 1 galaxy. Furthermore the plane of polarization is perpendicular to the axis of the radio jet in its nucleus, a situation which is interpreted as the result of a hidden BLR. No light can escape directly from the BLR to the observer. Only photons escaping along this axis of symmetry are scattered at some distance from the nucleus and can be observed. As a result of this scattering their mean plane of polarization will be perpendicular to the axis. Since this detection eight additional objects with hidden BLRs have been found.

M77 is the nearest and brightest representative of this class of active galaxies, about 60 million light years distant.  Seyferts are often  interesting variable radio objects,( here Cetus A), and they are very prominent in the infrared, where most radiation is emitted, and in X-ray, too. Strong variability on the time scale of months is observed at all wavelength, indicating a compact central object.  It is expected, that the powering machine is a huge black hole of millions of solar masses.
Seyfert galaxies are thought to be intermediate between normal galaxies and quasars (usually 100 times more powerful, although the most luminous object in the universe is a faint source IRAS galaxy (FSC 10214+4724), which shows a Seyfert 2 spectrum!).

The central spiral features in the above image are only 20"x50" small and over-exposed in most cases.
 

  M77, with faint spiral arms, 12/17/98, jpeg, 4k

On deeper images (as the one above), faint, large spiralarms can be traced out. In fact, with a diameter of  170,000 light years, M77  is one of the biggest galaxies in Messier's catalog. Other famous Seyfert-Galaxies are NGC4151 and NGC1275 (see below), more than 150 others are known to date. As Seyfert activity is probably only a transitory period in the life of a galaxy, then it is possible that all giant spiral galaxies, including our own, spend 10% of their lives in a Seyfert phase.
As not only the central region shows a disrupted appearance, M77 has also become a member of Arp's Atlas of Peculiar Galaxies.

NGC 1275, Seyfert galaxy at the center of the Perseus cluster, 12/17/98, jpeg, 13k

NGC 1275 is the central galaxy in the the Perseus cluster, 350 million light years away. Photographs in the light of H-alpha show a system of long filaments, blown out by an explosive event at 2400km/sec.
It is usually  identified as a Seyfert galaxy, because it has all the characteristics of that class of galaxies.  A long time ago, Walter Baade and Rudolph Minkowski have already suggested that it consists of two galaxies passing through each other. We nowadays know that refuelling with gas and dust might stimulate activity in AGNs, so it might be a good interpretation of what is going on here. In my opinion there is a huge elliptical cD-galaxy, which is typical as a central object of a cluster of galaxies and it has incorporated a dusty spiral, where one of it's arms pears out to the north.

A  B C

The 3 images above show NGC 1275,

This view  is supported by the fact, that NASA's Hubble Space Telescope (HST) has detected many blue, that means young  globular clusters, as in NGC 4038/39 another famous pair of colliding galaxies in Corvus. This is quite unusual, because most  globular clusters belong to the oldest objects known.
A problem might be the rare occurrence of spirals in the Perseus cluster.

NGC 3786/88, Interacting pair of galaxies, 05/16/99, 8 f/4.5, jpeg,18 k

Here the original goal was to obtain an image of supernova 1999bu, discovered near to the center of NGC 3786, (1".3 west and 3".1 south). But my equipment was unable to resolve this 17.6 mag sn in this unfavorable position. Then I learned, that NGC 3786 is also a Seyfert galaxy, being partner in a pair of interacting galaxies. The close encounter might be the reason for the activity in the AGN that we observe here.

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