Galaxies What is the Galaxy made of ? Stars- Stellar remnants: Clouds of gas (nebulae)f ound 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 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) Bulge old stars Halo old stars globular clusters 100,000 lyrs open cluster globular cluster Thin Disk surrounds nuclear bulge Thin Disk surrounds nuclear bulge The Disk Stars are much younger (Population I, younger metal rich stars). Much free-floating gas & dust in the disk. Most new star formation goes on in the disk. Types of stars all colors) 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 theyre as old as the universe The Halo Galactic Bulge in center Made of old (mostly yellow & red) Population ll stars Contains much of the visible mass in the galaxy. 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 I stars are mainly found in the disk, are metal rich and young. Their spectra contains many more lines of metals. 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 II stars formed first,they have fewer metals. Population I formed after more stars exploded so that they have more metals. Types of Galaxies Barred Lenticular galaxies ( lens shaped) possess characteristics of spirals except that they lack spiral arms (len tik u lar) spirals have extended central bulges Spiral T hin disk with lots of gas and dust Central bulge and halo; Spiral arms extend from most spiral galaxies Elliptical galaxies are a big blob Irregular Galaxies- no definite shape Ellipticals Spirals Barred Spirals Lenticular Originally conceived as an evolutionary diagram idea not correct, but classification scheme remains. Early-type galaxies turned bluer The colors of late-type galaxies tend to be bluer. Late-type galaxies turn gas into stars slowly, and have lots of gas left. Presently forming stars. Older Stars Gas Poor More Massive On-going Star Formation Gas Rich Less Massive On average Larger bulge, less dusty gas, tighter spiral arms early type late type almost all of their gas into stars, very quickly, very early in their lives! Rounder appearance No structure - just a big elliptical blob with very little free dust & gas and little star formation. Population II (older stars, yellow red, metal poor) with very little free dust & gas and little star formation. Elliptical Galaxies 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 Giant Elliptical Galaxies may form by Cannibalism Elliptical Galaxies contain little gas or dust, theyre just big balls of stars Ellipticals are classified according to how stretched out they areE0 E7 Those with bars & Those without bars Spirals Two main types : NGC 1288 In all except one spiral galaxies,the arms trail around behind as the galaxy rotates. 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 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 formed more slowly. Spiral Galaxies The arms of spiral & barred spiral galaxies are sites of active star formation. Differential rotation of a galaxy stretches the NGC 1232 star forming regions into long arches of stars and nebulae, that we see as spiral arms. 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 The bar is funneling material into the hub, which triggers star formation and feeds the bulge M- 61 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 The bar is very noticeable in this SBc type. Barred Spiral galaxies (SB) are classified just like ordinary spiral galaxies: SBa, SBb, SBc. Sba Galaxies NGC 1097 M91 M109 SBc Galaxies NGC 4123 Sbb Galaxies 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. Galaxy collisions can produce: Elliptical galaxies Irregular galaxies Intense activity in galaxies Collisions between galaxies are common 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 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 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 Galaxy Clusters 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. The Hubble Deep Field 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!!! 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. 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. 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. 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. What is Dark Matter? 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. If we trust our theory of gravity... there may be 10 times more dark than luminous matter in our Galaxy Dark matter is found in the halo and far beyond the luminous disk 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. 4. Maybe our gravity works differently for 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. 3.The answer could be a combination of both. massive galaxies. If Z >.1, you need to use the Relativistic form. Or more useful If z=1, then v = c, if z = 3 then v=3c. Cant be moving faster than light. Yet, there are objects with z = 7 or 8. The answer is you must use the Relativistic form below. Redshift Recessional velocity Distance Infinite z v/c Bly