Matt A. Wood, Professor Dept. of Physics & Space Sciences Florida Institute of Technology

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2 Matt A. Wood, Professor Dept. of Physics & Space Sciences Florida Institute of Technology

3 Overview An Update on Pluto Getting The Boot-o! The Scale of Things Observations Mapping the Universe Observations Galaxies that go Bump in the Night Simulations: Galactic Evolution via Mergers Milky Way, Meet Andromeda Conclusions

4 First, An Update From Prague! The Vote was held and here s the results from the International Astronomical Union meeting IAU Resolution: Definition of a Planet in the Solar System Contemporary observations are changing our understanding of planetary systems, and it is important that our nomenclature for objects reflect our current understanding. This applies, in particular, to the designation 'planets'. The word 'planet' originally described 'wanderers' that were known only as moving lights in the sky. Recent discoveries lead us to create a new definition, which we can make using currently available scientific information.

5 RESOLUTION 5A The IAU therefore resolves that planets and other bodies in our Solar System be defined into three distinct categories in the following way: (1) A planet 1 is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood around its orbit. (2) A dwarf planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape 2, (c) has not cleared the neighborhood around its orbit, and (d) is not a satellite. (3) All other objects 3 orbiting the Sun shall be referred to collectively as "Small Solar System Bodies". 1 The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. 2 An IAU process will be established to assign borderline objects into either dwarf planet and other categories. 3 These currently include most of the Solar System asteroids, most Trans- Neptunian Objects (TNOs), comets, and other small bodies.

6 IAU Resolution: Pluto RESOLUTION 6A The IAU further resolves: Pluto is a "dwarf planet" by the above definition and is recognized as the prototype of a new category of trans- Neptunian objects Note: These only apply to planets in our Solar System they re still working on the definition for extra-solar planets! This is probably for the best really fixing an error made when originally classified as a planet, and similar to the demotion of the asteroid Ceres 150 years ago, which also was originally classified as a planet when first discovered, before it was realized how small it really is.

7 The New Classification

8 The Scale of Things (Reprise) If Earth is scaled down to a 10 cm ball (softball) Person ~14 nanometers (3000 times smaller than hair) Moon diameter ~2.7 cm (ping pong ball) Moon distance ~3 m (~10 ft) Sun diameter ~10 m Sun distance ~1.2 km (~3/4 mi) Size of Solar System ~80x this, or ~60 miles Now scale sun down by 1/100, to softball size 5 LY typical interstellar distance ~3600 km (~2200 mi)! So stars are very far apart relative to their sizes! Now if we scale our Galaxy to be dinner plate size

9 The Scale of Things Now if we scale our Galaxy to be dinner plate size then we ll find several other galaxies close by Andromeda is only 15 ft away, for example If grains of sand spread out as stars in the Galaxy, though, distances between grains would be a few miles!!! So you d need to spread the grains in a disk extending from here to the moon! Highly unlikely that any 2 grains of sand would hit each other if 2 sand galaxies pass through each other!

10 The Nature of our Galaxy and Local Universe: How Do We Know This Stuff? Casual observation of the night sky shows an almostcontinuous band of light circling Earth Galileo first realized this was a vast collection of individual stars Immanuel Kant (1955) and Thomas Wright proposed that the Galaxy* must be a rotating stellar disk held together by gravity, and our Sun system is just one star within the disk Kant also suggested that some nebulae were separate galaxies

11 Mapping the Galaxy William Herschel in 1780s produced a map of the Galaxy obtained by crude star counts in 683 regions of the sky Assumed: All stars have roughly same brightness, no obscuration, the space density is roughly constant, and that he could see to the edge of the distribution

12 Mapping the Galaxy Kapteyn confirmed and refined this model, again using star counts Now called The Kapteyn Universe

13 Another Copernican Step M2 About the same time ( ), Harlow Shapley was studying the spatial distribution of globular clusters Estimating distances using variable stars (period-luminosity relationship), he showed GCs are not distributed around us, but around the region of sky in the constellation Sagittarius, with a distance of 15 kpc (~50,000 ly) Modern determination ~25,000 LY to center of the Galaxy

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15 Island Universes and The Great Debate Kant had originally suggested that the faint observed elliptical nebulae might be very distant disklike systems similar to the Galaxy and called them Island Universes On April 26, 1920, at Nat. Acad. of Sciences, The Great Debate between Harlow Shapley and Heber Curtis. Shapley: Nebulae belong to our Galaxy Curtis: nebulae are extragalactic Both had data supporting their positions, and the issue was not settled at the time

16 Hubble s Observations Edwin Hubble in 1923 settled the matter Cepheid variable stars are distance indicators, or standard candles by the Period-Luminosity relationship Cepheids in spiral nebulae firmly established their distances and their identity as galaxies in their own right The next step was to classify the different galaxy types by their appearance or morphology

17 Classification of Galaxies

18 Observations of Different Types

19 Hubble Expansion Law Again using Cepheids, Hubble showed that more distant galaxies have a higher redshift, resulting from the expansion of the universe: v = H 0 d (H 0 = 70 km/s/mpc or ~20 km/s/mly) Once H 0 is calibrated, observations of a galaxy s redshift tells you the distance Harvard CfA Survey initally ~1100 galaxies (painstaking process one galaxy at a time) Result: Distribution not random! Superclusters and voids

20 Galaxies Cluster in Local Universe

21 Hubble Deep Field 2-week exposure on blank patch of sky revealed galaxies as they were when the universe was much younger. Galactic fragments and higher space density than in local space are evidence for mergers over time! Notice the lack of spiral galaxies - these are common in local space

22 Sloan Digital Sky Survey The Sloan Digital Sky Survey (sdss.org) is the most ambitious astronomical survey ever undertaken (current data volume is equivalent to the Library of Congress) It will provide detailed optical images covering more than a quarter of the sky, and a 3-dimensional map of about a million galaxies and quasars. As the survey progresses, the data are released to the scientific community and the general public in annual increments. See for more information and for many great images! (several of which follow)

23 The SDSS is two separate surveys in one: galaxies are identified in 2D images (right), then have their distance determined from their spectrum to create a 2 billion lightyears deep 3D map (left) where each galaxy is shown as a single point, the color representing the luminosity - this shows only those 66,976 our of 205,443 galaxies in the map that lie near the plane of Earth's equator.

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25 Sloan Digital Sky Survey First: Photometry to identify objects esp galaxies

26 SDSS Redshifts

27 Flight Through 2dF Data

28 Flight Through SDSS Data

29 Dynamical Friction A mass moving through a sea of stars accelerates those stars and creates a wake, both of which slow the moving object. This process is key to galactic mergers

30 Galaxy Interactions are common Dynamical Friction leads to mergers

31 M81 not interacting (at this time)

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34 Interactions often trigger bursts of star formation, as in the Antennae Galaxy

35 Detailed studies in the 1990's led to the remarkable discovery that the interstellar gas in the outer regions of M64 rotates in the opposite direction from the gas and stars in the inner regions. Active formation of new stars is occurring in the shear region where the oppositely rotating gases collide, are compressed, and contract. Particularly noticeable in the image are hot, blue young stars that have just formed, along with pink clouds of glowing hydrogen gas that fluoresce when exposed to ultraviolet light from newly formed stars. Astronomers believe that the oppositely rotating gas arose when M64 absorbed a satellite galaxy that collided with it, perhaps more than one billion years ago. This small galaxy has now been almost completely destroyed, but signs of the collision persist in the backward motion of gas at the outer edge of M64.

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37 Ring-shaped galaxies can form in several different ways. One possible scenario is through a collision with another galaxy. Sometimes the second galaxy speeds through the first, leaving a "splash" of star formation. But in Hoag's Object there is no sign of the second galaxy, which leads to the suspicion that the blue ring of stars may be the shredded remains of a galaxy that passed nearby. Some astronomers estimate that the encounter occurred about 2 to 3 billion years ago.

38 The unusual disk-ring structure is not yet understood fully. One possibility is that polar rings are the remnants of colossal collisions between two galaxies sometime in the distant past, probably at least 1 billion years ago. During the collision the gas from a smaller galaxy would have been stripped off and captured by a larger galaxy, forming a new ring of dust, gas, and stars, which orbit around the inner galaxy almost at right angles to the larger galaxy's disk.

39 Simulating Galaxy Collisions With N-Body Simulations Simple to program for a computer the motions of a large number of stars that are in galaxies that interact (simple in principle, complicated to do it efficiently enough so N~100,000,000~10 8 ) Every star in the galaxy interacts gravitationally with every other star (and gas cloud, etc.) Direct calculation by pairs calculations per time step, so clever approaches are used State of the art is now objects, or within about a factor of 100 from real galaxies!

40 WWF (UWF?) Smackdown: Andromeda vs. Milky Way! Sunday! Sunday! Sunday!!! well ok about 4 billion years from now and a very long string of Sundays Andromeda Galaxy and Milky Way galaxy will collide What will happen? Option 1: Wait and see Option 2: Simulate and see it now!

41 Do the simulations look like reality? You be the judge! Andromeda/MW collision The view from inside! Simulations are by John Dubinsky of University of Toronto Galaxies in Collision: The Movies

42 The title slide is a pair of galaxies called The Mice This interacting pair shows tidal tails and lots of star formation (blue young stars) The next slide has a great simulation movie The galaxies probably just had their first close pass In a billion years or so, this will be an elliptical galaxy

43 Here s a beautiful simulation By John Dubinsky of U. Toronto

44 Conclusions Galaxy clusters show striking evidence for a history of mergers Elliptical galaxies dominate the central regions, often with cd supergiant ellipticals that have sizes up to 300 million light years across Spiral galaxies (and fewer ellipticals) are found on the fringes Galaxy Cluster Abell 1689

45 Conclusions SDSS is mapping the 3D positions of close to a million galaxies, providing the clearest picture of the large-scale structure of the universe to date Galaxy interactions and mergers are an essential part of galaxy evolution galaxies merge, and big galaxies cannibalize smaller galaxies The modelers are within about 10 years of simulating the dynamics of systems with the same number of stars as contained in actual galaxies Galaxy Cluster Abell 1689

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