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Galaxies What is the Galaxy made of ? StarsStellar remnants: Clouds of gas (nebulae)found in the interstellar medium. Composed of : 70 % hydrogen 28 % helium 2 % ‘other stuff’ Heavy elements Galaxy Formation •We know much less about galaxies than stars, because we can't see the universe before galaxies formed. The evolution is slower, more complex - but we can compare cases. < 1 Byr Galactic formation occurred in dense areas of H & He gas in the early universe ~ 2 Byr Protogalactic clouds merge with a high rate of star formation. 4.5-5 Byr Mature-looking galaxies appear. Anatomy of Our Galaxy 1. Galactic Disk 2. Galactic Bulge 3. Galactic Halo Disk • young and old stars • open star clusters • gas & dust clouds (ISM) 100,000 lyrs Bulge • old stars Halo • old stars • globular clusters open cluster globular cluster The Disk Thin Disk surrounds nuclear bulge Most new star formation goes on in the disk. Types of stars – all colors) Stars are much younger (Population I, younger metal rich stars). Much free-floating gas & dust in the disk. The Halo A halo of stars and globular clusters surrounds the entire galaxy. Contains almost exclusively VERY old stars (population II stars). Population ll stars are low in heavy elements, meaning that they’re as old as the universe Galactic Bulge in center Contains much of the visible mass in the galaxy. Made of old (mostly yellow & red) Population ll stars Both the Galactic Bulge and the halo have more or less 3-dimensional spherical symmetry with the center. Motion in the Galaxy •The disk: •Fairly ordered motion •The spiral arms do not move with the stars •The halo •Much more random Population II stars are found in the halo, and are metal poor and old. The spectra of these stars are simple without many metals. Population I stars are mainly found in the disk, are metal rich and young. Their spectra contains many more lines of metals. Population II stars formed first,they have fewer metals. Population I formed after more stars exploded so that they have more metals. Types of Galaxies Elliptical galaxies are a big blob Spiral Thin disk with lots of gas and dust Central bulge and halo; Spiral arms extend from most spiral galaxies Barred spirals have extended central bulges Lenticular galaxies ( lens shaped) possess characteristics of spirals except that they lack spiral arms (len tik u lar) Irregular Galaxies- no definite shape Ellipticals Spirals Lenticular Originally conceived as an evolutionary diagram idea not correct, but classification scheme Barred Spirals The colors of late-type galaxies tend to be bluer. “early type” “late type” Rounder appearance Early-type galaxies turned almost all of their gas into stars, very quickly, very early in their lives! On average •Older Stars Gas Poor More Massive Larger bulge, less dusty gas, tighter spiral arms Late-type galaxies turn gas into stars slowly, and have lots of gas left. Presently forming stars. •On-going Star Formation Gas Rich Less Massive Elliptical Galaxies No structure - just a big elliptical blob Population II (older stars, yellow red, metal poor) with very little free dust & gas and little star formation. Elliptical galaxies range from the biggest to the smallest galaxies in the universe. The smallest are called dwarf elliptical galaxies which make up the most and contain only a few million stars making them hard to see. How did elliptical galaxies form? Rapidly Elliptical Galaxies contain little gas or dust, they’re just big balls of stars Giant Elliptical Galaxies may form by Cannibalism Ellipticals are classified according to how stretched out they are…E0 – E7 Spirals Two main types : Those with bars & Those without bars In all except one spiral galaxies,the arms trail around behind as the galaxy rotates. NGC 1288 They are called trailing arm spirals. Spiral Galaxies Hubble Sequence (Sa , Sb , Sc) according to : • Size of nuclear bulge vs disk & Tightness of spiral arms • Sa tightest pattern & largest bulge • Sb medium pattern & medium bulge Sc open pattern & smallest bulge • S0 Lenticular (lens shapped) -Have disk but no arms Spiral galaxies Spiral Galaxies formed more slowly. Spiral galaxies have a nuclear bulge, and at least 2 spiral shaped arms, with a halo of, stars, and globular clusters. Population I stars are found in the disk. Population II stars are found mostly in the halo and the bulge. Disks of spiral galaxies include young stars Spiral Galaxies The arms of spiral & barred spiral galaxies are sites of active star formation. Differential rotation of a galaxy stretches the star forming regions into long arches of stars and nebulae, that we see as spiral arms. NGC 1232 S0 Spiral Galaxies have dust lanes but no arms Sa Spirals have tightly wound arms and a large nucleus M104 Sa because of large nuclear bulge. Sb Spirals have looser arms & a smaller nucleus NGC 891 is an Sb, because of the small nucleus Sc Spirals have very loose arms and a small nucleus M-31 Andromeda Galaxy & satellite galaxies 2.4 million ly away & 100,000 ly in diameter M-51 Galaxy & satellite galaxy Barred Spirals Barred Spirals are like normal spirals except with a bar across their center Barred Spirals There is some evidence to suggest our galaxy might be a barred spiral Bar across central region is made of stars, gas, and dust Spiral arms begin at both ends of the bar M- 61 The bar is funneling material into the hub, which triggers star formation and feeds the bulge Barred Spiral galaxies (SB) are classified just like ordinary spiral galaxies: SBa, SBb, SBc. The bar is very noticeable in this SBc type. Sba Galaxies Sbb Galaxies M91 SBc Galaxies M109 NGC 1097 NGC 4123 Irregular Galaxies Irregular galaxies have no definite shape. These galaxies are full of regions of new star formation, dusty. Irregular galaxies are often the result of collisions between galaxies. Collisions between galaxies are common •Galaxy collisions can produce: •Elliptical galaxies •Irregular galaxies •Intense activity in galaxies Every now and then galaxies collide especially those in large groups. Mergers are devastating events, they change the galaxy. Mergers can transform two spirals into an elliptical galaxy. Some astronomers think that galaxies are always born as spirals, and they can become ellipticals only via mergers. Two galaxies in a cluster or from nearby clusters can occasionally pass through one another. There is so much space that the chances of stars colliding is extremely small, but huge clouds of gas and dust do collide. Collisions can merge galaxies together or hurl stars and gases out into space. These collisions can produce strong shock waves and new star formation. Starburst galaxies Stars are forming in our galaxy Star formation rate: R* ~ 1 star / year In some galaxies R* ~ 100 stars / year- Known as starburst galaxies Often have double nuclei Likely explanation: A collision of two spiral galaxies Why spirals? Need gas / dust for star formation Cosmic Capitalism: Mergers and acquisitions Galaxy Clusters Galaxies occur in clumps called clusters and these are in clumps called superclusters. Typical superclusters contain dozens of individual clusters. Most Clusters in superclusters are drifting away. Between superclusters are voids, areas of few galaxies. Galaxy Clusters Poor Clusters 10-100 galaxies Spirals, Irregulars and lastly dwarf ellipticals Local Group - our galaxy cluster Rich Clusters Thousand + galaxies Ellipticals, SO, with spirals at edges only Giant Ellipticals at the center Galaxies come in groups The Coma cluster is a rich, regular cluster The Virgo Cluster is a rich, regular cluster A Poor, Irregular Group Measuring distance :The Tully-Fisher Relationship First, more massive galaxies rotate faster than less massive ones. Tully and Fisher then made the plausible assumption that brighter galaxies are more massive than dimmer ones. The relationship works like this: As a galaxy rotates, some of the stars move toward us and the light is blue shifted. Other stars are moving away the light is red shifted Taken together, the the 21 cm emission line is spread out. The wider the emission line, the faster the galaxy rotates, so the more massive it must be. Because the line widths can be measured accurately, the luminosities ( absolute magnitudes) of spiral galaxies can be measured and we can calculate the distance. Hubble Space Telescope image of nearly “empty” part of sky on 1/30 of the size of the moon. Over 1000 galaxies seen Likely around 100 billion galaxies in observable universe!!! The Hubble Deep Field Dark Matter Rotation Curve Now consider the Milky Way galaxy. Using radio observations, it is possible to measure the orbital speed as a function of the distance from the center. This is not a single gravitational source like the Sun, so the speed near the center will be small and rise quickly. As you get to the edge of the visible galaxy, the velocity should drop down. But, in our galaxy the orbital speeds continue to climb well above the visible edge of the galactic disk, so there must be more gravitational force acting on the stars & clouds. Rotation Curve Compare the two graphs: The rotation curve for the Solar system is falling with radius. The rotation curve for the Milky Way is flat or even rising with radius. The mass distribution of the Milky Way is not a simple “point mass” distribution. Stars in the outer parts are orbiting at a much higher velocity than expected, based on the amount of visible matter (e.g. stars). There must be dark matter in order to account for this. IfWhat we trust theory of gravity... there may isour Dark Matter? be 10 times more dark than luminous matter in our Galaxy Dark matter is found in the halo and far beyond the luminous disk There must be sufficient matter to provide gravitational force to bind galaxies together in clusters. No clusters of galaxies contain enough visible matter to keep them bound together. Dark Matter can be anything that gives off no light. Possibilities : 1. Ordinary matter is called baryonic and consists of MACHOS (Massive Compact Halo Objects). These objects could be Brown Dwarfs, White Dwarfs, Jupiters , protons and neutrons. Protons and neutrons are composed of baryons. 2. Non-baryonic matter called WIMPS (Weakly Interacting Massive Particles). Neutrinos are now thought to have some mass, so they are a possibility. There may be some sub-atomic particles yet to be discovered that could be the answer. 3.The answer could be a combination of both. 4. Maybe our gravity works differently for massive galaxies. The other question about the nature of dark matter is, the matter: 1. “hot” fast moving like neurtinos 2. “cold” slow moving The type of dark matter determines when structure could actually form. It is hard to form structures in a hot Universe and easier in a cold Universe. So we need to look at when structure was formed. 0 z 0 v 0 Z c 0 If z=1 , then v = c , if z = 3 then v=3c . Can’t be moving faster than light. Yet, there are objects with z = 7 or 8. The answer is you must use the Relativistic form below. If Z > .1, you need to use the Relativistic form. cv z 1 cv v z 1 1 2 c z 1 1 2 Or more useful Redshift z 0 Recessional velocity v/c 0 Distance Bly 0 0.2 0.180 2.41 0.4 0.324 4.26 0.75 0.508 6.57 1 0.6 7.73 2 0.8 10.3 3 0.882 11.5 4 0.923 12.1 5 0.946 12.5 10 0.984 13.2 Infinite 1 13.7