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PH607 – Galaxies Plan: L1: The Milky Way Galaxy, Historical perspective, size, morphology, ingredients, kinematics. Overview, galaxies, clusters, Hubble’s Law, redshift. L2: 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. Structure of galaxy, Interpretation of spiral and elliptical galaxies. L3: Evolution of Galaxies Distant Galaxies and Clusters. Galaxy evolution: mergers, interactions. Starbursts, sub-mm galaxies. L4: The Galactic Centre, Active Galaxies & Radio Galaxies L5: Quasars & Unified Models L6: Tutorial/Examples/Revision Other Topics: Luminosity function. Distribution of galaxies in space: local group, clustering, cluster stability - missing mass. Galaxy interaction, Starbursts, Infrared, Submillimetre galaxies. Seyfert galaxies, quasars, observational properties, central black holes and accretion disks. Aims: 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/ Major Revision Topics: Types of galaxies and their contents. Formation/evolutionary scenarios. Galaxy Luminosity Function. The Schecter function. Types of active/radio galaxies. The radio emission. Jet outflows. 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 Herschel's model of the Milky Way obtained from "star gauges" along many lines of sight in the Galaxy. The sun is the brighter 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 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. 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 0.3Mpc (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). It would take the solar system about 225-250 million years to complete one orbit ("galactic year"), and so is thought to have completed about 20-25 orbits during its lifetime. The orbital speed is 217 km/s, i.e. 1 light-year in ca. 1400 years, and 1 AU in 8 days. Shapley’s estimate too large because he neglected dust absorption 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 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. 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 catalog number of M51). Note the two long, bright arms spiraling outward from its bulge. 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) 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 Very Large Telescope to 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 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. 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 have given added dimension to our knowledge of the structure of the Milky Way. 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: 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) 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 o Optical emission lines from hotter gas o Optical absorption lines from the stellar component Most rotation curves look very similar: 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!