HEASARC has a web site (the source of the above image) describing Active Galactic Nuclei. Another HEASARC web page is the source of these optical and radio images of NGC 4261:
Bradley Peterson has written a book, An Introduction to Active Galactic Nuclei (Cambridge 1997). Peterson says: "... The two largest subclasses of AGNs are Seyfert galaxies and quasars, and the distinction between them is to some degree a matter of semantics. The fundamental difference between these two subclasses is in the amount of radiation emitted by the compact central source; in the case of a typical Seyfert galaxy, the total energy emitted by the nuclear source at visible wavelengths is comparable to the energy emitted by all of the stars in the galaxy (i.e., ~ 10^11 Lsun), but in a typical quasar the nuclear source is brighter than the stars by a factor of 100 or more. ...".
CannonBall pulses of Accreting Matter could form some Gamma Ray Bursts.
According to the Hubble Heritage website: "... one of the brightest radio sources in the sky, Virgo A, ...[is]... associated with M87 (also known as NGC 4486 ... an ordinary giant elliptical galaxy; one of many ellipticals in the nearby Virgo cluster of galaxies ...)... and its jet. ... Lying at the center of M87 is a supermassive black hole, which has ... a mass equivalent to 2 billion times the mass of our Sun. ... The jet originates in the disk of superheated gas swirling around this black hole and is propelled and concentrated by the intense, twisted magnetic fields trapped within this plasma. The light that we see (and the radio emission) is produced by electrons twisting along magnetic field lines in the jet, a process known as synchrotron radiation, which gives the jet its bluish tint. ...
... At a distance of 50 million light-years, M87 is too distant for Hubble to discern individual stars. The dozens of star-like points swarming about M87 are, instead, themselves clusters of hundreds of thousands of stars each. An estimated 15,000 globular clusters formed very early in the history of this galaxy and are older than the second generation of stars, which huddle closer to the center of the galaxy. ...".
According to astro-ph/0008063, by Biermann, Ahn, Medina-Tanco, and Stanev:
"... a simple Galactic wind model patterned after the solar wind ...[shows]... that back-tracing the orbits of the highest energy cosmic events suggests that they may all come from the Virgo cluster, and so probably from the active radio galaxy M87. ...
... a very simple model for a Galactic wind rather analoguous to the Solar wind ... may allow particle orbits at 10^20 eV to be bent sufficiently to allow "super-GZK" particles to get here from M87, and also explain the apparent isotropy in arrival directions. ...
... we will consider for reference a model which has a field strength near the Sun of 7 microGauss; this is close to the best estimates for the total local field ... The assumption of the symmetry of the magnetic field above and below the Galactic disk is important. The value of the magnetic field, here adopted as 7 microGauss, for the wind near the Sun, isa key parameter. If the magnetic field were considerably weaker, the focussing would be largely removed. ...
... The ... distance to which this wind extends ... is more uncertain: Our Galaxy dominates its near environment well past our neighbor, M31, the Andromeda galaxy, and might well extend its sphere of influence to half way to M81. Therefore we will adopt as outer the halo wind radius half the distance to M81, 1.5 Mpc. ... The scale of the Galactic wind here 1.5 Mpc, is not a critical parameter, since the calculations show that most of the bending happens within the first few 100 kpc. ...
... To follow the particle trajectories in the Galactic halo we trace protons backwards, e.g., ... from their arrival direction at Earth. We use the 14 published cosmic ray events above 10^20 eV ... There is a big uncertainty with the energy estimate of the highest energy Yakutsk ... event, which we therefore exclude from the present analysis, and hence we arrive at a final tally of 13 events used. ... the directions of all tracks point North. All events are consistent with arising originally from Virgo A. Since these particles are assumed to be accelerated out of cosmic gas, about 1/10 of all particles may be Helium nuclei with the same energy per particle. If the two highest energy events are in fact He nuclei, all 13 events point within 20 degrees of Virgo A. If Virgo A is indeed the acceleration site of the highest energy cosmic ray events, they all require additional systematic bending at a ten to twenty degree level. Such bending could be easily accomodated within the plausible magnetic field strength within the supergalactic sheet from here to Virgo ...
... If the model proposed here could be confirmed, then it would constitute strong evidence that all powerful radiogalaxies produce high energy cosmic rays, and that they do this at a good fraction of their total power output. ... ".
( adapted from the program Deep Space Explorer 1.0, for Mac ) shows our
and ( reddish, near the lower right border of the image ) the Large Magellanic Cloud satellite galaxy. The green arrow points to the location of our Solar System.
According to a 6 January 2003 BBC web article: "... A ring of ... several hundred million stars - about 1% of the total number of stars in our galaxy ... stars surrounds the Milky Way ...
... The ring, which has the appearance of a giant doughnut, could be the remains of a satellite galaxy. ...". According to an article by D. T. on page 22 of the April 2003 issue of Sky and Telescope: "... Astronomers ... have discovered a new part of our galaxy: a dim, sparse, but enormous ring encircling the Milky Way some 60,000 light-years from its center, just beyond the outer edge of the galaxy's disk ... most likely the remnant of a small galaxy that collided with and merged with the Milky Way in the very distant past. ... the ring contains 20 million to 500 million stars in all - perhaps as many as in the Small Magellanic Cloud. ... To maintain the stars' observed fast motions along the ring's wide, circular orbit, immense amounts of dark mass must be present around the Milky Way ..." [ and/or MOND might be an accurate theory of gravity on the galactic scale ].
According to an article by Angelle Tanner on pages 44-50 of the April 2003 issue of Sky and Telescope:
"... astronomers ... can now see clear to the Milky Way's center ...
... the thin "threads" ..., observed only at the galactic center, glow in radio by means of synchrotron radiation: emission from charged particles spiraling at relativistic speeds arond magnetic-field lines ...
... symbols show the positions of .. six stars for [2002 and] each of the previous 7 years ... The white orbits are best fits to these positions ... SO-16 swung a mere 60 astronomical units from the black hole ... at more than 9,000 km per second! Kepler's laws of motion tell us that the mass ...[of Sgr A*]... amounts to at least 3 million solar masses ... Sgr A* ... X-ray variations ... in as little as 10 minutes prove that the X-ray source can be no bigger than about 10 light-minutes (about 1 a.u.) wide ... such a large mass in such a small volume cannot avoid ending up as a black hole. ...".
According to a 20 September 2000 article by David Whitehouse of the BBC:
"... Sagittarius A* (Sgr A*), about 26,000 light-years from Earth ...
... is located at the exact point around which our galaxy revolves. ... the 10-metre Keck telescope atop the dormant Mauna Kea volcano on Hawaii, ... took snapshots of Sgr A* ... This showed three stars orbiting about 16 billion km (10 billion miles) from Sgr A*. Their movement around the suspected black hole revealed not only their velocity but allowed their acceleration to be deduced. And this gave a better analysis of the mass of the central black hole at Sgr A*, which is now estimated to be 2.6 million times that of the Sun. This extraordinary object is calculated to be about 400 million km (640 million miles) across. ... Supermassive black holes are believed to be common at the cores of galaxies. In 1994, the Hubble space telescope saw evidence for such an object at the centre of the galaxy M87. ... This was calculated to have a mass equal to two to three billion Suns and occupy a space no larger than our Solar System. Sgr A* is nothing like as big. It would extend from the Sun to the orbit of Mars. ... the three stars seen orbiting Sgr A* were accelerating so quickly that their complete orbit of the black hole may be as short as a few decades and in one case as little as 15 years. ...".
According to a 21 February 2002 NASA Science web page:
"... Chandra ... pinpointed a source of X-rays that coincided with Sagittarius A*. ...[ According to a Harvard Chandra web page: "... This 400 by 900 light-year mosaic
of several Chandra images of the central region of our Milky Way galaxy reveals hundreds of white dwarf stars, neutron stars, and black holes bathed in an incandescent fog of multimillion-degree gas. The supermassive black hole at the center of the Galaxy is located inside the bright white patch in the center of the image. The colors indicate X-ray energy bands - red (low), green (medium), and blue (high). ... An analysis of the X-ray data showed that the temperature of the gas does not have to be 100 million degrees Celsius, as previously thought. Rather, a relatively mild 10 million degrees will do. ...". ]
... the X-rays were only a fifth the intensity that theory predicted. In other words, Sagittarius A* was faint -- strange, given that active galactic nuclei are so brilliant. ... Ten thousand years ago a supernova exploded very close to Sagittarius A*. The fast-expanding gases swept away much of the local interstellar gas and dust, preventing material from falling into the Milky Way's supermassive black hole, thereby "starving" it. Less material falling into the black hole meant fewer X-rays being emitted. Nevertheless, some material is still infalling. In 2001, right before Chandra's vigilant X-ray eye, Sagittarius A* suddenly brightened. Within minutes it was 45 times its normal intensity. Then it faded back to its pre-flare level about three hours later. The energy released corresponded to the black hole suddenly having engulfed a chunk of material with the mass of a comet or asteroid! Moreover, from the specific way the X-rays brightened and dimmed, astrophysicists calculated that Sagittarius A* was only about 15 million kilometers across-less than a quarter the diameter of the orbit of the planet Mercury around our Sun. This observational evidence of small size coupled with enormous mass seemed to clinch the case for its being a supermassive black hole. ...".
According to a University of Toronto web page: "... The nearest big spiral galaxy to the Milky Way is the
...[ According to a SEDS web page: "... M31
is the famous Andromeda galaxy, our nearest large neighbor galaxy ...". ]...
... Appearing as a smudge of light to the naked eye in the constellation Andromeda, this galaxy is about twice as big as the Milky Way but very similar in many ways. At the moment it is about 2.2 million light years away from us but the gap is closing at 500000 km/hour. Andromeda is the only big spiral galaxy galaxy moving towards the Milky Way and the best explanation is that the Milky Way and Andromeda are in fact a bound pair of galaxies in orbit around one another. Both galaxies are thought to have formed close to each other shortly after the Big Bang initially moving apart with the overall expansion of the universe. But since they are bound to one another, they are now falling back back together and one very plausible scenario puts them on a collision course in 3 billion years. ...
In about 3 billion years, the two galaxies will collide and then over about 1 billion years after a very complex gravitational dance they will merge to form an elliptical galaxy. ...".
An interesting fact on the timing is that the Sun will still be burning brightly when this collision occurs and maybe life of some sort will still be around on Earth at that time. So what would people see in the night sky during this billion year galactic dance? As Andromeda approaches, it will grow in size and just before the collision the night sky will be filled by a giant spiral galaxy. When the two galaxies intersect, our familiar Milky Way arch over the sky will be joined by a second intersecting arch of stars but this will only last for 100 million years or so and will be a very confusing state of affairs for galactic astronomers. Finally, when the two galaxies merge our view will depend on which direction the Sun is thrown.
There are two possible fates of the Sun which depend closely on the details of where it is in its galactic orbit at the time of the collision.
"... Astrophysicists Lars Hernquist and John Dubinski ... recently simulated the future collision of the Milky Way and her larger neighbor, Andromeda, using model galaxies comprised of 110 million particles ... with the help of Blue Horizon, NPACI's 1,152-processor IBM RS/6000 SP. ... ", according to an NPACI web page describing the computer simulation images shown above. Note that the computer simulation not only has fewer particles than the galaxies have stars, but it also is based on gravitational interactions among massive point-particles, so it may be an oversimplification of what actually might happen a few billion years in our future.
...
