Galaxies are quite close to each other! Galaxy size ~ 100 kpc Separation between neighboring galaxies ~ 1 Mpc or less for galaxies for stars Conclusion: galaxies should interact and collide very often! They collided even more often before size separation > 0.1 size separation ~ 10 -7
Fig. 13-9a, p. 269 The Mice are a pair of colliding galaxies with peculiar tails False-color image
Note intense starburst regions! Mice modeling Stars do not collide; however, their orbits are strongly disturbed Galactic nuclei coalesce Spiral arms are destroyed Collisions trigger star formation
Mergers of Galaxies NGC 7252: Probably result of merger of two galaxies, ~ a billion years ago: Small galaxy remnant in the center is rotating backward! Radio image of M 64: Central regions rotating backward! Multiple nuclei in giant elliptical galaxies
Mergers and galactic cannibalism Giant elliptical in the cluster center gets bigger by eating neighbors
Collisions of large with small galaxies often result in complete disruption of the smaller galaxy.
Small galaxy is “swallowed” by the larger one.
This process is called “galactic cannibalism”
The Farthest Galaxies The most distant galaxies visible by HST are seen at a time when the universe was only ~ 1 billion years old. Many small galaxies Spirals are more abundant than now
Initial proto-galaxies were probably very small: 10 6 solar masses Large galaxies were produced by mergers of many small lumps Collisions and mergers drive the evolution of galaxies Collisions trigger intense star formation, consuming all gas in a short time Spirals can be destroyed in the collision. This could explain larger amount of spirals in the past Dwarf ellipticals and irregulars are produced in the collisions Giant elliptical galaxies are produced by consuming many small galaxies
Coma cluster. Note giant ellipticals in the center: probably the result of mergers
Active Galaxies Galaxies with extremely violent energy release in their nuclei (pl. of nucleus). “ Active Galactic Nuclei” (= AGN) Up to many thousand times more luminous than the entire Milky Way; energy released within a region approx. the size of our solar system!
Active galaxies Contain extremely active nuclei AGN – Active Galactic Nuclei
The Spectra of Galaxies Taking a spectrum of the light from a normal galaxy: The light from the galaxy should be mostly star light, and should thus contain many absorption lines from the individual stellar spectra.
Seyferts Consider NGC 4151, a spiral galaxy 15 Mpc away. Photographs by Carl Seyfert in the 1940s showed a very bright point-like nucleus. Its spectrum is very unusual: in addition to continua + absorption lines from normal stars, Seyfert galaxy nuclei have very strong emission lines. Some are common lines (e.g. H-alpha, H-beta) but others are weird (e.g. twice-ionized oxygen lines), requiring hot gas far out of equilibrium. The lines are very broad, requiring that the gas be Doppler shifted in all directions up to ~20,000 km/s. The nuclei vary in brightness on timescales of months, requiring them to be < 1 parsec in size. The total luminosity of the nucleus can be equivalent to 10 10 L sun !! What is this bizarre object in the center of Seyfert's spiral galaxies?
Seyferts Very bright nucleus Broad emission lines
Seyfert Galaxies NGC 1566 Circinus Galaxy Unusual spiral galaxies:
Very bright cores
Emission line spectra.
Variability: ~ 50 % in a few months
Most likely power source: Accretion onto a supermassive black hole (~10 7 – 10 8 M sun )
Later in the 1940s, astronomers began scanning the skies with radio telescopes. They found strange radio structures on opposite sides of radio galaxies, plus a tiny source of radio emission at the nucleus. The nuclei of these radio galaxies shoot out narrow beams of extremely energetic electrons, producing synchrotron radiation. The radio components include: the compact core at the galaxy nucleus, jets, lobes, and a hot spot where the jet slams into the interstellar medium. Radio Galaxies
Cosmic Jets and Radio Lobes Many active galaxies show powerful radio jets Radio image of Cygnus A Material in the jets moves with almost the speed of light (“Relativistic jets”). Hot spots: Energy in the jets is released in interaction with surrounding material
Radio galaxies are best known for their extensive double radio sources, shining by synchrotron radiation as electrons spiral through magnetic fields at relativistic speeds. These objects show a remarkable variety of forms and symmetries, as shown in this montage of radio images of radio galaxies. These are of so-called Fanaroff-Riley (FR) type I, with radio lobes decreasing smoothly in intensity outwards from the central source - for the contrasting case of an FR II source, see the Cygnus A slide. Ordinary, symmetric double structure is seen in Fornax A, 3C 219, and 3C 285. Hydra A (3C 218) exhibits an interesting corkscrew form, sometimes seen as suggesting a long-term precession of the jets feeding outwards from the nucleus, while 3C 449 shows a very long and extended set of helical twists more or less symmetric about the core. The radio source of 3C 315 is so tightly twisted that it takes on the shape nicknamed by Jacques Vallee and his Francophone collaborators as Papillon - the butterfly. In each case, the optical galaxy spans only a small part of the range of the radio source. In Fornax A, it fills the gap between the two lobes, and in the other cases the visible galaxies are much smaller compared to the radio source extent.
Radio Galaxies Radio image superposed on optical image Centaurus A (“Cen A” = NGC 5128): the closest AGN to us. Jet visible in radio and X-rays; show bright spots in similar locations. Infrared image reveals warm gas near the nucleus.
AGN variability To change in one hour, the source needs to have a size less than Velocity of light x 1 hour ~ 7 AU. AGNs are very compact!!
Active Galactic Nuclei observed at high (>100 MeV) energies form a subclass known as blazars; a blazar is believed to be an AGN which has one of its relativistic jets pointed toward the Earth so that the emission we observe is dominated by phenomena occurring in the jet region. Amongst all AGNs, blazars emit over the widest range of frequencies, being detected from radio to gamma-ray. Specifically, to be classified as a blazar an AGN must be seen with one of the following properties: 1) high radio-brightness accompanied by flatness of the radio spectrum, 2) high optical polarization, 3) strong optical variability on very short timescales (less than few days). In the class of objects selected according to these criteria, there appear to be two subgroups: (1) sources showing strong and broad emission lines, such as those of quasars (called Flat Spectrum Radio Quasars), and (2) sources showing a featureless optical spectrum (called BL Lac objects). There are additional important differences between these subclasses such as they show different luminosity and redshift distributions, and a different morphology of the extended radio emission. Blazars
What engine powers observed AGNs??? A supermassive black hole?!
Formation of Radio Jets Jets are powered by accretion of matter onto a supermassive black hole Black Hole Twisted magnetic fields help to confine the material in the jet and to produce synchrotron radiation. Accretion Disk
Evidence for Black Holes in AGNs Elliptical galaxy M 84: Spectral line shift indicates high-velocity rotation of gas near the center. Visual image NGC 7052: Stellar velocities indicate the presence of a central black hole.
Cores of galaxies show an accretion disk with a possible black hole
Model for Seyfert Galaxies Accretion disk Dense dust torus Gas clouds UV, X-rays Emission lines Supermassive black hole Seyfert I: Strong, broad emission lines from rapidly moving gas clouds near the BH Seyfert II: Weaker, narrow emission lines from more slowly moving gas clouds far from the BH
Other Types of AGN and AGN Unification Radio Galaxy: Powerful “radio lobes” at the end points of the jets, where power in the jets is dissipated. Cyg A (radio emission) Observing direction
Black Holes and Galaxy Formation Interactions of galaxies not only produce tidal tails etc., but also drive matter towards the center triggering AGN activity. Such interactions may also play a role in the formation of spiral structures.
Interacting Galaxies Seyfert galaxy NGC 7674 Active galaxies are often associated with interacting galaxies, possibly result of recent galaxy mergers. Often: gas outflowing at high velocities, in opposite directions Seyfert galaxy NGC 4151
Quasars look like stars, very different from galaxies
In the 1960s it was observed that certain objects emitting radio waves but thought to be stars had very unusual optical spectra. It was finally realized that the reason the spectra were so unusual is that the lines were Doppler shifted by a very large amount, corresponding to velocities away from us that were significant fractions of the speed of light. The reason that it took some time to come to this conclusion is that, because these objects were thought to be relatively nearby stars, no one had any reason to believe they should be receding from us at such velocities. Quasars
Fig. 14-7b, p.287 Strongly redshifted EMISSION lines in the spectrum of 3C273
Most distant quasars have redshift z = 6. How can it be????
Quasar Red Shifts z = 0 z = 0.178 z = 0.240 z = 0.302 z = 0.389 Quasars have been detected at the highest red shifts, up to z ~ 6 z = / Our old formula / = v r /c is only valid in the limit of low speed, v r << c
Doppler effect: How come that z > 1 ?? First, relativistic Doppler effect is described by a different formula: Redshift z = (Observed wavelength - Rest wavelength) (Rest wavelength)
The redshift is due to the expansion of the Universe: Contrary to popular belief, this is not a Doppler shift. Instead, as a light wave travels through the fabric of space, the universe expands and the light wave gets stretched and therefore redshifted. However, cosmological redshift is not a Doppler effect!!!
Two galaxies permanently located at positions (x1 , y1 , z1 ) and ( x2 , y2 , z2 ) at one time find themselves one billion light years apart. Then a few billion years later while located at the same coordinates, they find themselves 3 billion light years apart. The galaxies have not 'moved', nevertheless, their separations have increased.
Another evidence of cosmological distance to quasars: gravitational lensing
Studying Quasars The study of high-redshift quasars allows astronomers to investigate questions of: 1) Large scale structure of the universe 2) Early history of the universe 3) Galaxy evolution 4) Dark matter
3C273 Jets and host galaxies have been resolved for “nearby” quasars
Superluminal Motion Individual radio knots in quasar jets: Sometimes apparently moving faster than speed of light! Light-travel time effect: Material in the jet is almost catching up with the light it emits
Evidence for Quasars in Distant Galaxies Quasar 0351+026 at the same red shift as a galaxy evidence for quasar activity due to galaxy interaction
Galaxies Associated with Quasars Two images of the same quasar, 1059+730 New source probably a supernova in the host galaxy of the quasar
Host Galaxies of Quasars Host galaxies of most quasars can not be seen directly because they are outshined by the bright emission from the AGN. Blocking out the light from the center of the quasar 3C 273, HST can detect the star light from its host galaxy.
Gallery of Quasar Host Galaxies Elliptical galaxies; often merging / interacting galaxies
Spectra contain strongly redshifted lines indicating large cosmological distances to the objects
Gravitational lensing also indicates huge distances
2) Broad emission line as in Seyferts, indicating rapid motion 3) Jets, intense radiation from radio waves to gamma-rays observed This means that quasars are most luminous objects in the Universe! L ~ 10 12 – 10 14 L sun 4) Host galaxies are found around nearby quasars 1)-5) indicate that quasars sit in the centers of galaxies, are extremely compact and super-luminous. 5) Rapid variability on the scale of days is observed They must be AGN!
Quasars Active nuclei in elliptical galaxies with even more powerful central sources than Seyfert galaxies Also show very strong, broad emission lines in their spectra. Also show strong variability over time scales of a few months.
Quasars were much more numerous in the early Universe than now Galaxy collisions were more frequent; they supplied more stars and gas to the central black holes Modern galaxies with central black holes are sleeping quasars! Collisions and mergers play crucial role in the AGN activity
What could be a direct observation of a black hole Could be possible for our Galaxy center?