4 Elliptical GalaxiesThe most massive galaxies are ellipticals, and they feature significantly in clusters of galaxies.a smooth, featureless appearancelittle gas and dustreddish color; low rate of star formation; no core- collapse supernovae.huge range of possible masses from 106 to 1013 Solar Massesvery elongated stellar orbits and little overall rotation.Classified as E0 through E7
5 Elliptical Galaxies Elliptical galaxies, classed by E0 to E7 The E stands for elliptical (obviously)The number indicates how egg-shaped the ellipse is- 0 means a ball shape- 5 a bit like a football- 7 looks like a cigarE0 is M87E1 is M49E5 is NGC 5253E7 is NGC4526The S0 Lenticular is M84.
13 SpiralsAll have spiral arms, and they are grouped by how tightly those arms are wound and how large the central bulge is - the two happen to be closely related.The name is defined by the "S" and the lower case letter after which indicates how wound up the arms are: from "a" to "c": Sa, Sb, ScThe lower branch of the tuning fork diagram is largely a copy of the upper branch, but its occupants all have a line of stars through the center - a bar. The B stands for barred: SBa, SBb, SBc
17 Spiral GalaxiesThe central bulge is similar to an elliptical galaxy (i.e. smooth and reddish in color)a surrounding, highly flattened disk in circular orbital motion about the spheroidlarge amounts of gas and dust in the disk where stars actively formspiral arms within the diskhaloes of stars and globular clusters and dark matter in which the disk and spheroid are embeddedmasses: to 3x1011 Solar Massesin some spirals, central bulge has a bar-shape: `barred’ spirals.
18 Irregular GalaxiesSome spirals are poorly defined and merge with a set classed as `irregular'. Irregulars featurevery large dust and gas fractionsvigorous star formation which gives a patchy appearance.Masses: 106 to Solar Masses
20 Environmental effects Unlike most stars, galaxies are heavily effected by their environment.When discussing stars, collisions were barely mentioned except in the dense cores of globular clusters.In the Solar neighborhood, an average main-sequence star (excluding binary stars) is separated by of order 107 times its size from its nearest neighbors (1 Solar Radius vs. 1 pc).Galaxies on the other hand have sizes ranging from 1 to 100 Kpc, but are separated by of order 1 to 10 Mpc from their neighbors, only a factor of to This means that almost all galaxies have probably had direct interactions, collisions and mergers with others during their lives.For an individual star, a galaxy collision would not mean much, however, gas clouds are likely to collide and star formation affected considerably. The result may be a much higher supernova rate and the birth of a young group of stars. It is possible that the collisions of spirals disrupt their disks and lead to elliptical galaxies.
22 Galactic Populations Population I Population II Stars with heavy elementsNew star formationFound inIrregular galaxiesSpiral galaxy disksPopulation IIStars with little or no heavy elementsOld starsElliptical galaxiesSpiral galaxy halos and bulge
23 Formation of Galaxies How do galaxies form? Structural differences Spiral versus EllipticalSeems to be largely a matter of the original rotationNo rotation of original gas cloud – EllipticalRotation – SpiralElliptical (E0 thru E7)Translation through the surrounding gasLeaves a ‘wake’Irregular -- Little translation or rotation
30 Other Galaxies Rotation Seems to be a feature, not an anomaly:
31 Galactic RotationsThe odd speed distribution does have a solution, but it adds to the mysteryThis type of speed distribution happens when there is a lot more mass out in the disk than toward the center.We can't see this mass. It is now called "Dark Matter"Estimates of the Dark Matter imply that the visible mass of the Universe is a very small percentage of what is really there.
32 Active Galaxies Active Galaxy Zoo The Central Engine Seyfert Galaxies Radio GalaxiesThe Central EngineEnergy generation efficiency of accretionHow big are the black-holes?
33 Seyfert Galaxies Bright, point-like nuclei Dust and Distance Seyfert I Broad emission line spectra like a quasarStrong X-rayLow (compared to quasars) luminositySeyfert IINarrow emission lines onlyDust and Distance
36 Radio galaxiesAt radio wavelengths, most sources are galaxies; stars are feeble emitters of radio waves in general. Some galaxies are much more powerful at radio wavelengths than normal. They can exceed the Milky Way by 103 to 107 times. These are radio galaxies. When resolved many have a double-lobe appearance in which two large lobes some hundred of kiloparsecs apart emit radio waves.Further imaging revealed that these lobes are powered by jets emanating from the nuclei of (usually) elliptical galaxies. One can achieve remarkable resolution at radio wavelengths, and yet it is never possible to resolve the source of these jets. The jets contain material moving close to the speed of light. The lobes are formed as these jets plough into the intergalactic medium.
37 Radio GalaxiesCentaurius ARadio ImageOptical Image
40 BlazarsA Blazar is a very compact and highly variable energy source associated with a presumed supermassive black hole at the center of a host galaxy.Blazars are among the most violent phenomena in the universeBlazars are active galactic nuclei (AGN) with a relativistic jet that is pointing in the general direction of the Earth. We observe "down" the jet, or nearly so, and this accounts for the rapid variability and compact features
42 IRAS Galaxies Infrared Astronomy NASA's Spitzer Space Telescope has detected the building blocks of life in the distant universe.Training its eye on a faint object located at a distance of 3.2 billion light-years , Spitzer has observed the presence of water and organic molecules in the galaxy IRAS FWith an active galactic nucleus, this is one of the most luminous galaxies in the universe, rivaling the energy output of a quasar. Because it is heavily obscured by dust, most of its luminosity is radiated at infrared wavelengthsThe broad depression in the center of the spectrum denotes the presence of silicates (chemically similar to beach sand) in the galaxy.An emission peak (red) within the bottom of the trough is the chemical signature for molecular hydrogen.The hydrocarbons (orange) are organic molecules comprised of carbon and hydrogen, two of the most common elements on Earth.Since it has taken more than three billion years for the light from the galaxy to reach Earth, it is intriguing to note the presence of organics in a distant galaxy at a time when life is thought to have started forming on our home planet.
43 The Eye of the Beholder: What we see depends on how we see itRadio Galaxy / Seyfert 2Quasar / Seyfert 1Blazar
44 Gamma Ray Burst (GRB)Gamma-ray bursts (GRBs) are the most luminous electromagnetic events occurring in the universe since the Big Bang.They are flashes of gamma rays emanating from seemingly random places in deep space at random times.The duration of a gamma-ray burst is typically a few seconds, but can range from a few milliseconds to several minutes, and the initial burst is usually followed by a longer-lived "afterglow" emitting at longer wavelengthsMost observed GRBs appear to be caused by the collapse of the core of a rapidly rotating, high-mass star into a black hole.
45 MagnetarA magnetar is a neutron star with an extremely powerful magnetic field, the decay of which powers the emission of copious amounts of high-energy electromagnetic radiation, particularly X-rays and gamma-rays.Magnetars are somewhere around 20 kilometers in diameter. Despite this, they are substantially more massive than our Sun. Magnetars are so compressed that a thimbleful of its material is estimated to weigh over 100 million tons.Most magnetars recorded rotate very rapidly, at least several times per second.The active life of a magnetar is short.Their strong magnetic fields decay after about 10,000 years, after which point activity and strong X-ray emission cease.Given the number of magnetars observable today, one estimate puts the number of "dead" magnetars in the Milky Way at 30 million or more.Quakes triggered on the surface of the magnetar cause great volatility in the star and the magnetic field which encompasses it, often leading to extremely powerful gamma ray flare emissions which have been recorded on Earth in 1979, 1998 and 2004.[
46 The Power SourceIt is now widely believed that all active galaxies are powered by the same phenomenon: accretion onto supermassive black-holes. The various types reflect differences in viewing angle and jet activity. The evidence that suggests this model can be summarized by:high-velocity gas ( 10,000 Km/s) and relativistic jets imply a deep potential.the tiny size of the energy generation region is impossible for stable star clustersaccreting black-holes are efficient 1014 Solar Luminosities. e.g. implies 4x1024 Kg/s at 10% conversion efficiency, or 70 solar masses per year.Any stellar source would use up material at 10 times the rate
47 Energy generation efficiency of accretion Accretion is a source of power. In fact, other than matter/anti-matter annihilation (which does not play a significant role in astronomical energy generation), it is by some way the most efficient source of power.For a Neutron Star, this is about 30x more efficient than nuclear fusionBlack-holes are also efficient although less so than neutron starsThis is because black-holes have no surface so much of the energy is never released but is swallowed up by the black-hole directly and also orbits are unstable within three times the Schwarschild radius and little energy is returned inside this distance.These factors lead to an efficiency of about 10%
48 How big are the black-holes? There is an interesting physical limit that allows us to estimate a minimum mass for the black-holes that power active galaxies, if indeed they do. It is based upon the balance of gravity with radiation pressure.Material coming into the black-hole is hot and ionized. Photons radiated by the black-hole interact mostly with electrons and exert an outward force on them. The electrons are electrostatically coupled to protons which are gravitationally attracted to the black-hole.
49 How big are the black-holes? If the accretion rate and corresponding luminosity are too high, the radiation pressure will exceed gravity and mass will be pushed away from the black-hole. The higher the mass of the black-hole, the larger luminosity will be required for this to take place, but in the end we conclude thatfor a given black-hole there is a maximum accretion rate and luminosity that it can sustain.the limiting luminosity scales linearly with the black-hole massThis is known as the Eddington limit after its discoverer. It applies equally to neutron stars and white dwarfs as to black-holes
50 How big are the black-holes? It is now simple to estimate what mass we need to produce 1014 L. It comes out to be 3x109 M . Such a black-hole has a Schwarschild radius of 1010 Km, comparable to the radius of Pluto's orbit around the Sun. Although large, this satisfies the restriction upon the size of the energy generation region.There is now good evidence that most galaxies, active or not, contain large black-holes, if not always as large as a billion solar masses.