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PH607 – Galaxies, General Relativity & Cosmology Reading Material Carroll & Ostlie, An Introduction to Modern Astrophysics, 2nd edition Plan: 1: Historical perspective. The Milky Way. size, morphology, ingredients, populations, dark matter, Galactic centre. kinematics. 2: Structure of Galaxies, History, Hubble classification of galaxies, types. Mass and dynamics of galaxies, elliptical galaxies, mass distribution of spherical galaxy, spiral galaxies, mass distribution. Tully-Fisher, Faber-Jackson relations. Luminosity functions. 3: Galactic evolution, interaction, mergers, starbursts, formation 4: Structure of the Universe:Overview, galaxies, clusters, Hubble’s Law, redshift. 5: Active galactic Nuclei, Quasars, Jets. Superluminal motions. 6: General Relativity, Black Holes, Lensing 7: Cosmology 8: The Early Universe, Inflation 9: A workshop 10 A test (final lecture period) Assignments? Galaxies: Major Question: How did our Galaxy form? Monolithic or Hiearchical? Top-down or Bottom-up? The aim of this course is to explore the continuing evolution of the universe. The scales examined will range from the structure of individual galaxies up to the geometry of the universe as a whole. The course begins by considering the large-scale structure of our own Galaxy, the Milky Way. Components: Almost 90% of its mass cannot be accounted for (the "dark matter" problem). The Local Group: It then goes on to consider how the Milky Way fits in with what we see in other galaxies, and what the morphologies of these systems tell us about their life histories. Evolution: Galaxies are not isolated, especially in the past…….. AGN: The course then discusses active galaxies, which far outshine our own. The different types of active galaxies --Seyferts, radio galaxies and BL Lac objects --- will be studied in some detail. These studies suggest a coherent model in which all these objects are ordinary galaxies whose excess luminosity is powered by central supermassive black holes. The course then turns to quasars, whose redshifts imply that they lie at huge distances from us. These objects are probably extreme active galaxies, although alternative interpretations of their properties will also be presented. A good internet reference site is: http://nedwww.ipac.caltech.edu/level5/ and http://nedwww.ipac.caltech.edu/ and a very complete set of (more advanced) notes at: http://www.astr.ua.edu/keel/galaxies/ Web sites containing of pictures of Nebulae and Galaxies can be found at: http://seds.lpl.arizona.edu/messier/Messier.html and http://astro.princeton.edu/~frei/galaxy_catalog.html Definition? A galaxy is a self-gravitating system composed of an interstellar medium, stars,and dark matter. Interstellar Medium molecular gas dust warm gas (104 K) hot gas (106 K) magnetic fields cosmic rays Stars Dark Matter main-sequence stars black holes brown dwarfs massive black holes giant stars stable neutral particles supergiant stars ??? white dwarf stars ??? neutron stars Structure of the Universe: from beginning to end, requires GR to understand spacetime. Here we shall apply GR to black holes, the early universe and the present Universe. A Historical Timeline of Galaxy studies As we look out into the night sky, we see an enormous number of stars fairly uniformly distributed across the sky Additionally, on a clear, DARK night we see the Milky Way – a faint band of light cut by a dark rift stretching around the sky In 1610, Galileo (1564-1642) pointed his telescope at the Milky Way and discovered it could be resolved into “innumerable” faint stars – thus it is not a “celestial fluid” but is a stellar system 1750: Thomas Wright proposes that the Milky Way is a stellar disk system. 1755 Immanuel Kant speculates that there may exist "Island Universes" like our Milky Way. 1785 William & Caroline Herschel studies star counts along several hundred lines of sight in the galaxy. Concluded that Sun lies near the center of a flattened, roughly elliptical system which is five times wider in the direction of the plane (assumes uniform distributed, same absolute mags, no ISM). Herschel's model of the Milky Way obtained from "star gauges" along many lines of sight in the Galaxy. The Sun is the yellowr star to the right of centre. Herschel, Kapteyn and Shapley were unaware of the presence of dust in the Galaxy which causes extinction and reddening of starlight . f The Optical View (above) is dominated by emission from stars and extinction by dust. 1845 William Parsons, Earl of Rosse, using a 72-inch home constructed telescope in Ireland with a metal mirror (size unsurpassed until the 100-inch Mount Wilson telescope in 1917) He discovers the "Spiral Nebulae" (Messier 51), speculates that they may be Kant's Island Universes. 1912 Vesto Slipher at Lowell Observatory observes brighter spiral nebulae spectroscopically. Spectra show emission lines from hot gas, absorption lines from stars. Radial velocities are nearly all positive with values up to several hundred km/s - later to be determined to be due to the expansion of the Universe. 1915: Harlow Shapley (1885-1972) used RR Lyrae variables found in globular clusters (evolved Population II stars). Assumed that the clusters were uniformly distributed in space. Every RR Lyrae variable star has a luminosity of about L = 80 Lsun. Shapley estimated that the Sun was 15 kpc from the galactic center. 1901-1922 Jacobus C. Kapteyn (1851-1922) made extensive star counts from photographic plates to determine the structure of the Milky Way. His model has become known as Kapteyn's Universe. A flattened spheroidal system, with the Sun only 650 parsecs from the centre. Why is it so heliocentric?? Kapteyn's Universe; the Sun is slightly off centre. Both were wrong because of interstellar extinction; Kapteyn was looking into the Galactic plane – only nearby stars observed. Shapley took a falsely calibrated P-L relation because of extinction. 1917 G. W. Ritchey observes novae in spiral nebulae. Unable to reconcile the "nova" S Andromedae observed in 1885 in the Andromeda nebula with other novae in spiral nebulae - perhaps it is a particularly powerful nova - a "supernova"? 1920: The Shapley-Curtis Debate Curtis believed that the spiral nebulae are galaxies like our own lying at distances ranging from 150 kpc (M31) to 3000 kpc for the most distant systems. Shapley believed the spirals were part of our Galaxy. 1923 Edwin Hubble resolves the disks of two nearby spiral galaxies (M31 and M33) into stars He discovers Cepheid Variable stars in Messier 31 - the Great Nebula in Andromeda, estimating its distance as nearly 0.3 Mpc (modern value is about 0.7 Mpc), well outside our Galaxy. Hubble’s Law. Best estimates today – Sun is 8 kpc from Galactic center, with diameter of 50 kpc. The Sun's place in the Milky Way is crucial to various galactic calculations. Measurements to the center of the Milky Way have varied greatly from 8.5±0.5 kpc to 7.9±0.2 kpc (one of the most recent measurements in 2005). The orbital speed is 217 km/s, i.e. 1 light-year in ca. 1400 years, and 1 AU in 8 days. It would take the solar system about 225250 million years to complete one orbit ("galactic year"), and so is thought to have completed about 20-25 orbits during its lifetime. The Material in our Galaxy: The Stars… DISK: The most prominently visible part of our galaxy is its thin disk. The disk is about 50,000 parsecs in diameter, but only about 600 parsecs thick – approximately. Stars in the disk are fairly rich in heavy elements. This indicates that they are young stars, made from recycled gas into which planetary nebulae and supernovae have dumped heavy elements (carbon, oxygen, iron, and so forth). In the jargon of astronomers, these young stars, rich in heavy elements, are called ``Population I'' stars. Note: the Galactic plane lies at an angle of 63 degrees to the Celestial/Equatorial plane. The North galactic Pole lies at Dec 12h 51m, RA 27 degrees. BULGE: At the centre is a relatively small central bulge. The bulge is about 2000 parsecs in diameter. Some stars in the bulge are young population I stars. Other stars are ``Population II'' stars, meaning that they are relatively old, and are poor in heavy elements, having been created before the interstellar gas had been seriously polluted with elements heavier than helium. A good view of the central bulge of our Galaxy is given by the nearinfrared picture below, which also shows the disk extending to either side. The picture was obtained using the COBE satellite. Infrared images show stellar emission relatively un-obscured by dust, allowing us to obtain a clear overall view of our galaxy for the first time: [Image credit: NASA & the Cosmic Background Explorer] HALO: Surrounding both the disk and bulge is an enormous spherical halo. The halo is about 100,000 parsecs in diameter, twice the diameter of the disk. The stars in the halo are widely scattered. (The Milky Way that we see in the sky is made of disk stars, not halo stars). The stars in the halo are all population II stars, very old and containing few heavy elements. The globular clusters surrounding our galaxy are part of the halo, and are very old, with ages greater than 10 billion years. (That is, in the globular clusters, all stars more massive than the Sun have evolved off the main sequence.) Spiral Arms. The disk contains stars, gas, and dust, and displays spiral arms. Maps of hydrogen gas reveal that the gas is not spread evenly throughout the disk, but is concentrated in a few spiral arms. (The Sun is located in the Orion arm.) The spiral arms of our Galaxy contain a large fraction of: atomic hydrogen gas, giant molecular clouds, hot O and B stars. Since spiral arms contain giant molecular clouds (the material from which stars are made), and also contain O and B stars (newly made, short-lived stars), it is apparent that spiral arms are where star formation takes place. Because O and B stars are very luminous, spiral arms are very prominent in snapshots of galaxies similar to our own. For instance, the picture below is of a galaxy called the Whirlpool Galaxy (also known by its catalogue number of M51). Note the two long, bright arms spiraling outward from its bulge. (HST image) Stellar Populations: Population I: objects closely associated with spiral arms – luminous, young hot stars (O and B), Cepheid variables, dust lanes, HII regions, open clusters, metal-rich Population II: objects found in spheroidal components of galaxies (bulge of spiral galaxies, ellipticals) – older, redder stars (red giants), metal-poor Note several different fundamental properties affect observed color: Metallicity (metal poor stars are bluer than metal rich stars) Age (younger stars are bluer) Dust (makes stars redder) Property Population Orbits Circular Shape spiral arms Thickness(pc) 120 Metals (%) 3-4 Mass (Msun) 2x109 Age (yr) 108 Typical objects Open clusters, HII regions, OB stars Iintermediate Population II Elongated Very elliptical disk spherical/halo 400 2000 0.4-2 0.4 or less 10 5x10 2x1010 109 1010 Sun Globular Clusters Globular clusters, RR Lyrae stars Galactic dust Our galaxy contains ~several 107 M of dust. o The dust is mostly found concentrated in a very thin layer (~100 pc thick) in the galactic plane, although thin clouds of dust termed "cirrus" can also be found well away from the plane of the Galaxy. View of the Galaxy showing the main features Galactic Atomic & Molecular Hydrogen Hydrogen gas is a major constituent of the Milky Way --- there is ~5 109 M of atomic hydrogen in our galaxy – also known as HI, is ~ 100K, and the average temperature of the molecular and the solid (dust) material is ~ 15K In 1944, van de Hulst pointed out that there is a hyperfine transition in hydrogen in which the relative spins of the proton and electron change direction: This transition is observable via the 21cm radiation that it produces, and has proved crucial in developing our understanding of the galactic rotation curve. These radio waves are too long in wavelength to be absorbed by dust, so they provide an excellent way of peering through the dust Multi-wavelength view of the Galactic Plane Age The age of the Galaxy is currently estimated to be about 13.6 billion (109) years, which is nearly as old as the Universe itself. This estimate is based on Very Large Telescope measurements of the beryllium content of two stars in globular cluster NGC 6397. This allowed astronomers to deduce the elapsed time between the rise of the first generation of stars in the entire Galaxy and the first generation of stars in the cluster, at 200 million to 300 million years. By including the estimated age of the stars in the globular cluster (13.4 ± 0.8 billion years), they estimated the age of the Galaxy at 13.6 ± 0.8 billion years. Detailed structure Observed structure of the Milky Way's spiral arms As of 2005, the Milky Way is thought to comprise a large barred spiral galaxy of Hubble type SBbc (loosely wound barred spiral) with a total mass of about 1012 solar masses, comprising 200-400 billion stars. A BARRED SPIRAL: It was only in the 1980s that astronomers began to suspect that the Milky Way is a barred spiral rather than an ordinary spiral, which observations in 2005 with the Spitzer Space Telescope have since confirmed, showing that the galaxy's central bar is larger than previously suspected . The galaxy's bar is thought to be about 27,000 light years long, running through the center of the galaxy at a 44±10 degree angle to the line between our sun and the center of the galaxy. It is composed primarily of red stars, believed to be ancient. The bar is surrounded by a ring called the "5-kpc ring" that contains a large fraction of the molecular hydrogen present in the galaxy and most of the Milky Way's star formation activity. Observed and extrapolated structure of the spiral arms Each spiral arm describes a logarithmic spiral (as do the arms of all spiral galaxies) with a pitch of approximately 12 degrees. There are believed to be four major spiral arms which all start at the Galaxy's center. These are named as follows, according to the image at right: 2 and 8 – 3 kpc and Perseus Arm 3 and 7 - Norma and Cygnus Arm (Along with a newly discovered extension - 6) 4 and 10 - Crux and Scutum Arm 5 and 9 - Carina and Sagittarius Arm There are at least two smaller arms or spurs, including: 11 - Orion Arm (which contains the solar system and the Sun - 12) Outside of the major spiral arms is the Outer Ring or Monoceros Ring, a ring of stars around the Milky Way, which consists of gas and stars torn from other galaxies billions of years ago. Summary: The galactic disk is surrounded by a spheroid halo of old stars and globular clusters. While the disk contains gas and dust obscuring the view in some wavelengths, the halo does not. Active star formation takes place in the disk (especially in the spiral arms, which represent areas of high density), but not in the halo. Open clusters also occur primarily in the disk. Recent discoveries; extended structure. 1. The Andromeda Galaxy (M31) extends much further than previously thought. The disk of the Milky Way extends further is a clear possibility and is supported by evidence of the newly discovered Outer Arm extension of the Cygnus Arm. 2. With the discovery of the Sagittarius Dwarf Elliptical Galaxy came the discovery of a ribbon of galactic debris as the polar orbit of Sagittarius and its interaction with the Milky Way tears it apart. Similarly, with the discovery of the Canis Major Dwarf Galaxy , a ring of galactic debris from its interaction with the Milky Way encircles the galactic disk. 3. On January 9, 2006 Mario Juric and others announced that the Sloan Digital Sky Survey of the northern sky has found a huge and diffuse structure within the Milky Way that does not seem to fit within our current models. The collection of stars rises close to perpendicular to the plane of the spiral arms of the Milky Way. The proposed likely interpretation is that a dwarf galaxy is merging with the Milky Way. This galaxy is tenatively named the Virgo Stellar Stream and is found in the direction of Virgo about 30,000 light years away. Spiral galaxies in general: Spirals are complex systems, more complex than ellipticals Wide range in morphological appearance Fine scale details – bulge/disk ratios, structure of arms, resolution into knots, HII regions, etc. Wide range in stellar populations – old, intermediate, young, and currently forming Wide range in stellar dynamics: “cold” rotationally supported disk stars “hot” mainly dispersion supported bulge & halo stars Significant interstellar medium (ISM) Kinematics: Spiral Galaxy Rotation Curve As is typical for many galaxies, the distribution of mass in the Milky Way is such that the orbital speed of most stars in the galaxy does not depend strongly on its distance from the center. Away from the central bulge or outer rim, the typical stellar velocity is between 210 and 240 km/s. Hence the orbital period of the typical star is directly proportional only to the length of the path travelled. This is unlike the solar system where different orbits are also expected to have significantly different velocities associated with them. Like the Milky Way, external spiral galaxies are supported against collapse by their rotation. (c.f. elliptical galaxies, which are not). By using the Doppler shifts in spectral lines to measure galaxies' line-of-sight velocity as a function of position, we can measure their rotation curves (speed of material following circular orbits around the centre of the galaxy as a function of radius). We can derive this quantity from: o 21cm emission from atomic hydrogen Optical emission lines from hotter gas o Optical absorption lines from the stellar component Most rotation curves look very similar: o For our own galaxy, it is possible to obtain the "rotation curve" --- the circular velocity as a function of radius --- of the inner part of the Milky Way using the line-of-sight velocity of atomic hydrogen as measured from the Doppler shift in its 21cm emission. Combining the results from these methods, we obtain the following estimate for the Milky Way rotation curve: Although the enclosed mass, M(r), continues to grow apparently without limit, the enclosed luminosity, L(r), tends to a finite limit as we reach the edge of the luminous material in the galaxy. There must therefore be significant amounts of dark matter which continue to contribute to M(r) out to very large radii. o Out to the furthest point measured, typical galaxies have a luminosity of L ~ 1010 L , and a typical enclosed mass of M ~ 1011 M . The "mass-to-light ratio" M / L is hence ~ 10 solar units. 90% of the material in the galaxy is dark! Example: The Sun moves at about 220 km s-1 in a circular orbit around the centre of the Galaxy, like almost all the stars near the Sun. We can assume that all the matter is at the Galactic centre, a not too bad approximation. Let the speed be V0 , the mass of the Galaxy be MG and the distance of the Sun from the Galactic centre be R0. Then the centrifugal force due to rotational speed must balance the gravitational force due to the mass of the Galaxy. GMG/R02 = V02/R0 where G is the gravitational constant. Hence MG = V02 R0/G Substituting values of 8 kpc and 220 km s-1 for the Sun and G = 0.00430 M (km s-1)2 / pc, we get MG = 1011 M . However, measurements of the rotation of the outer edge of the Milky Way show that the stars out there also rotate at 220 km s-1, out to about 20 kpc. Thus, within a radius of 20 kpc we get a mass of MG = 2x1011 M If the light distribution of the Galaxy were proportional to the mass distribution, then the two mass estimates above would imply that the amount of light emitted by the 8 kpc region would be the same as the region from 8 - 20 kpc... whereas measurements show that the 8 kpc region emits about 10 times more light than the 8 - 20 kpc region. The major conclusion is that the distribution of emitted light is not necessarily the same as the underlying distribution of matter. The Galactic Centre The nucleus of the Milky Way contains a complex of gas, dust, stars, supernova remnants, magnetic filaments, and, almost certainly, a massive black hole at the very center; it lies in the direction of Sagittarius, around R.A. 17h 46m and Dec. -28° 56'. The galactic centre harbours a compact object of very large mass, strongly suspected to be a supermassive black hole. Most galaxies are believed to have a supermassive black hole at their centre. Ever since black holes were suggested as the power sources for Active Galactic Nuclei (AGNs) such as Seyfert Galaxies and QSOs, we have speculated on whether the center of our galaxy might contain a black hole Because the center of the Milky Way is by far the closest galaxy nucleus, we can study details that will remain indistinguishable in other galaxies for a long time.. "Galactic Center" here will mean the central ~10 parsecs of the Galaxy. 1) the stellar population including evidence for star formation there in the last 50 million years or even less 2) interstellar material including both ionized gas (HII regions) and molecular clouds which orbit the Center in a ring with an inner radius of about 2 pc. Hot dust is also observed. 3) strong magnetic fields (milliGauss) as compared to elsewhere in the Galaxy 4) a compact radio source called SgrA* which is quite unlike any another radio source in the Galaxy. 5) radial velocities and proper motions of both stars and gas which imply the existence of a large, unseen, compact object. Large means a mass=~2.5x106MSun. 6) The discovery that the radio source SgrA* corresponds to the dynamical center of the Milky Way and coincides with the large, dark mass has lead to the realization that SgrA* is a black hole, albeit a puzzling one: 7) Lying dead center in the Galaxy is the Sagittarius A Complex, which is believed to be associated with a black hole, material in orbit around this object, and a nearby supernova remnant. Surrounding the galactic center are narrow threads known as nonthermal filaments (NTFs), the most prominent of which are called the Arc, the Pelican, and the Snake. These seem to consist of magnetic flux tubes filled with relativistic electrons, beaming synchrotron radiation, that have been swept up from adjacent molecular clouds and hurled along the field lines at incredible speeds. Another unusual structure in the nucleus is catalogued as 359.1-00.5 and appears to be a superbubble with a cluster of as many as 200 newborn stars at its heart.