Galaxies in the Universe

Galaxies: From Here to the Horizon Thus the explorations of space end on a note of uncertainty… Eventually, we reach the dim boundary – the utmost limits of our telescopes. There we measure shadows, and we search among ghostly errors of measurement for landmarks that are scarcely more substantial. The Starburst Galaxy M82 shows a frenzy of star formation in its central regions and jets of gas perpendicular to the galaxy. Both activities are believed to have been initiated by passage of the galaxy near its neighbor, M81. (Courtesy Mark Westmoquette, University College London; Jay Gallagher, UW-Madison; Linda Smith, University College London; WIYN/NSF; NASA/ESA) Edwin Hubble (1889 – 1953) American astronomer

Types of Galaxies Spiral Galaxies – 2 or more arms winding out from the center Barred Spirals - have an elongated central region (or bar) Elliptical Galaxies – No spiral structure, smooth featureless appearance, generally elliptical shape Irregular Galaxies – No arms, nor a smooth appearance, have random patches

Spirals Variations? FIGURE 16-1 Spiral Galaxies (Nearly Face-on Views) Edwin Hubble classified spiral galaxies according to the tightness of their spiral arms and the sizes of their nuclear bulges. Sa galaxies have the largest nuclear bulges and the most tightly wound spiral arms, whereas Sc galaxies have the smallest nuclear bulges and the least tightly wound arms. The images are different colors because they were taken through filters that pass different colors. (a: NASA/Hubble Space Institute; b: Robert Gendler; c: Anglo- Australian Observatory) Variations?

Spirals have a disk component and bulge & halo disk contains gas & dust relative sizes of bulge/disk & amount of gas/dust vary appear white because they contain both blue & red stars FIGURE 16-1 Spiral Galaxies (Nearly Face-on Views) Edwin Hubble classified spiral galaxies according to the tightness of their spiral arms and the sizes of their nuclear bulges. Sa galaxies have the largest nuclear bulges and the most tightly wound spiral arms, whereas Sc galaxies have the smallest nuclear bulges and the least tightly wound arms. The images are different colors because they were taken through filters that pass different colors. (a: NASA/Hubble Space Institute; b: Robert Gendler; c: Anglo- Australian Observatory)

Spirals Star Types Mix of old and new stars (pop I & pop II) Young, blue stars in the spiral This is cause by the high amounts of interstellar mass (ISM) clouds FIGURE 16-1 Spiral Galaxies (Nearly Face-on Views) Edwin Hubble classified spiral galaxies according to the tightness of their spiral arms and the sizes of their nuclear bulges. Sa galaxies have the largest nuclear bulges and the most tightly wound spiral arms, whereas Sc galaxies have the smallest nuclear bulges and the least tightly wound arms. The images are different colors because they were taken through filters that pass different colors. (a: NASA/Hubble Space Institute; b: Robert Gendler; c: Anglo- Australian Observatory)

Spirals “Edge-on” FIGURE 16-3 Spiral Galaxies Seen Nearly Edge-on from the Milky Way (a) Because of its large nuclear bulge, this galaxy (called the Sombrero Galaxy) is classified as an Sa. If we could see it face-on, the spiral arms would be tightly wound around a voluminous bulge. (b) Note the smaller nuclear bulge in this Sb galaxy. (c) At visible wavelengths, interstellar dust obscures the relatively insignificant nuclear bulge of this Sc galaxy. (a: European Southern Observatory; b: © Malin/IAC/RGO; c: Brand Ehrhorn/Adam Block/NOAO/AURA/NSF)

FIGURE 16-4 Variety in Spiral Arms The differences in spiral galaxies suggest that at least two mechanisms create spiral arms. (a) This flocculent spiral galaxy has fuzzy, poorly defined spiral arms. (b) This grand-design spiral galaxy has well-defined spiral arms. (a: NASA; b: Gemini Observatory/AURA)

Spiral Arms as Density “Traffic Jams” (Density waves) FIGURE 16-7 Compression Wave in Traffic Flow When normal traffic flow is slowed down, cars bunch together. In a grand design galaxy, a density wave moves through the stars and gas. The wave is merely a region of slightly denser matter, which, in turn, creates more gravitational force. This force compresses the gas and enhances star formation, which highlights the spiral density wave.

Barred Spirals FIGURE 16-9 Barred Spiral Galaxies As with spiral galaxies, Edwin Hubble classified barred spirals according to the tightness of their spiral arms (which correlates with the sizes of their nuclear bulges). SBa galaxies have the most tightly wound spirals and largest nuclear bulges, SBb have moderately tight spirals and medium-sized nuclear bulges, and SBc galaxies have the least tightly wound spirals and the smallest nuclear bulges. (a: Johan H. Knapen and Nik Szymanek, University of Hertfordshire; b: ESO, European Southern Observatory; c: Jean- Charles Cuillandre/CFHT/Photo Researchers, Inc.)

Elliptical Galaxies only have a spheroidal component; no disk FIGURE 16-11 A Elliptical Galaxy This nearby E4 galaxy, called Leo I, is about 600,000 light-year from Earth. It is only 3000 light-year in diameter and is so sparsely populated with stars that you can see right through its center. It is a satellite of the Milky Way. (NASA) FIGURE 16-10 Giant Elliptical Galaxies The Virgo cluster is a rich, sprawling collection of more than 2000 galaxies about 50 million light-years from Earth. Only the center of this huge cluster appears in this photograph. The two largest galaxies in the cluster are the giant elliptical galaxies M84 and M86. (Royal Observatory, Edinburgh) only have a spheroidal component; no disk very little gas/dust, little active star formation appear red because they contain mostly red stars

Elliptical Galaxy Star Types FIGURE 16-12 Elliptical Galaxies Hubble classified elliptical galaxies according to how round or elongated they appear. An E0 galaxy is round; a very elongated elliptical galaxy is an E7. Three examples are shown. (a: J. D. Wray, McDonald Observatory; b, c: 1999 Princeton University Press/Zsolt Frei and James E. Gunn) Mostly old stars (pop II)

Irregular Galaxy Variations “none of the above”; neither spiral nor elliptical appear white & dusty: lots of gas & dust have more in common w/ disk component of spirals distant galaxies more likely to be irregular So…more common when the Universe was young FIGURE 16-13 Irregular Galaxies (a) At a distance of only 179,000 light-years, the Large Magellanic Cloud (LMC), an Irr I irregular galaxy, is the third closest known companion of our Milky Way Galaxy. (The Milky Way’s closest known companion, the Canis Major Dwarf, is shown in Figure 15-16.) About 62,000 light-years across, the LMC spans 22° across the sky, about 44 times the angular size of the full Moon. Note the huge H II region (called the Tarantula Nebula or 30 Doradus). Its diameter of 800 light-years and mass of 5 million solar masses make it the largest known H II region. (b) The small irregular (Irr II) galaxy NGC 4485 interacts with the highly distorted Sc galaxy NGC 4490, also called the Cocoon Galaxy. This pair is located in the constellation Canes Venatici. (a: Anglo-

Irregular Galaxy Star Types Young blue stars are common due to high ISM Has a large percentage of ISM left (will last a long time – lots of “star fuel”) FIGURE 16-13 Irregular Galaxies (a) At a distance of only 179,000 light-years, the Large Magellanic Cloud (LMC), an Irr I irregular galaxy, is the third closest known companion of our Milky Way Galaxy. (The Milky Way’s closest known companion, the Canis Major Dwarf, is shown in Figure 15-16.) About 62,000 light-years across, the LMC spans 22° across the sky, about 44 times the angular size of the full Moon. Note the huge H II region (called the Tarantula Nebula or 30 Doradus). Its diameter of 800 light-years and mass of 5 million solar masses make it the largest known H II region. (b) The small irregular (Irr II) galaxy NGC 4485 interacts with the highly distorted Sc galaxy NGC 4490, also called the Cocoon Galaxy. This pair is located in the constellation Canes Venatici. (a: Anglo-

Edwin Hubble (1889-1953) Discovered Cepheid variables in Andromeda galaxy. (a pulsating star’s period) Calculated distance to Andromeda galaxy. 2.2 million light years Developed a classification scheme for galaxies.

Hubble’s Classification System FIGURE 16-14 Hubble’s Tuning Fork Diagram Hubble summarized his classification scheme for galaxies with this tuning fork diagram. Elliptical galaxies are classified by how oval they appear, whereas spirals and barred spirals are classified by the sizes of their central bulges and the correlated winding of their spiral arms. An S0 or SB0 galaxy, also called lenticular galaxy, is an intermediate type between ellipticals and spirals. It has a disk but no spiral arms.

Hubble Law Hubble observed a redshift in almost all galaxies (a few exceptions of nearby galaxies), that increased for more distant galaxies What does this mean?? The universe is expanding!

Hubble Law Hubble discovered that a galaxies recession velocity (speed at which they move away) increased with distance from the MW. V = HD V = Velocity (in km/s) H = 71 km/sec/Mpc (The Hubble Constant) D = Distance (megaparsecs)

Hubble’s Law: Velocity of Distant Galaxies Increases with Distance Farther Galaxies move away from us faster FIGURE 16-32 The Hubble Law The distances and recessional velocities of distant galaxies are plotted on this graph. The straight line is the “best fit” for the data. This linear relationship between distance and speed is called the Hubble law. For historical reasons, distances between galaxies, clusters of galaxies, and superclusters of galaxies are usually given in megaparsecs, Mpc, rather than millions of light-years. Closer Galaxies move away from us

FIGURE 16-32 The Hubble Law The distances and recessional velocities of distant galaxies are plotted on this graph. The straight line is the “best fit” for the data. This linear relationship between distance and speed is called the Hubble law. For historical reasons, distances between galaxies, clusters of galaxies, and superclusters of galaxies are usually given in megaparsecs, Mpc, rather than millions of light-years.

Hubble Law

Hubble Law Example V = HD H = 71 km/s/Mpc From a galaxy’s spectrum, its recession velocity is 56,000 km/s. How far away is the galaxy? 788 Mpc 3.262E6 LY = 1 Mpc

Hubble Law Example V = HD H = 71 km/s/Mpc If a distance galaxy is 1,109 Mpc away, how fast is the galaxy moving away from our galaxy? From a galaxy’s spectrum, its recession velocity is 37,500 km/s. How far away is the galaxy? 78,739 km/s 528 km/s 3.262E6 LY = 1 Mpc

Cosmic Distance Ladder!

Galactic “Cannibalism” http://viz.adrian.pw/galaxy/ FIGURE 16-28 Simulated Galactic Cannibalism This computer simulation shows a small galaxy (yellow stars) being devoured by a larger, disk-shaped galaxy (blue stars, white gas). Note how spiral arms are generated in the disk galaxy by its interaction with the satellite galaxy. (Lars Hernquist, Institute for Advanced Study with simulations performed at the Pittsburgh Supercomputing Center)

Evidence of Dark Matter FIGURE 16-29 The Rotation Curves of Four Spiral Galaxies This graph shows how the orbital speed of material in the disks of four spiral galaxies varies with the distance from the center of each galaxy. If most of each galaxy’s mass were concentrated near the center of the galaxy, these curves would fall off at large distances. But these and many other galaxies have flat rotation curves that do not fall off. This indicates the presence of extended halos of dark matter. See Figure 15-18 to compare these to the Milky Way’s rotation curve.

Gravitational Lenses Suggest Dark Matter FIGURE 16-30 Gravitational Lensing of Extremely Distant Galaxies (a) Schematic of how a gravitational lens works. Light from the distant object changes direction due to the gravitational attraction of the intervening galaxy and underlying dark matter. The more distant galaxy appears in two places to the observer on the right. (b) Three examples of gravitational lensing: (1) The bluer arc is a galaxy that has been lensed by the redder elliptical galaxy. (2) A pair of bluish images of the same object lensed symmetrically by the brighter, redder galaxy between them. (3) The lensed object appears as a blue arc under the gravitational influence of the group of four galaxies. (c) Superimposed in blue on this image of the galaxy cluster CL 002417 is the location of dark matter that is gravitationally lensing the galaxies behind it. (b: NASA, ESA, A. Bloton [Harvard- Smithsonian CfA] and the SLACS Team; c: NASA, ESA, and M.J. Jee [Johns Hopkins University])

FIGURE 16-30 Gravitational Lensing of Extremely Distant Galaxies (a) Schematic of how a gravitational lens works. Light from the distant object changes direction due to the gravitational attraction of the intervening galaxy and underlying dark matter. The more distant galaxy appears in two places to the observer on the right. (b) Three examples of gravitational lensing: (1) The bluer arc is a galaxy that has been lensed by the redder elliptical galaxy. (2) A pair of bluish images of the same object lensed symmetrically by the brighter, redder galaxy between them. (3) The lensed object appears as a blue arc under the gravitational influence of the group of four galaxies. (c) Superimposed in blue on this image of the galaxy cluster CL 002417 is the location of dark matter that is gravitationally lensing the galaxies behind it. (b: NASA, ESA, A. Bloton [Harvard- Smithsonian CfA] and the SLACS Team; c: NASA, ESA, and M.J. Jee [Johns Hopkins University])

FIGURE 16-34 Distant Galaxies (a) The young cluster of galaxies MS1054-03, shown on the left, contains many orbiting pairs of galaxies, as well as remnants of recent galaxy collisions. Several of these systems are shown at the right .This cluster is located 8 billion light-years away from Earth. (b) This image of more than 300 spiral, elliptical, and irregular galaxies contains several that are an estimated 12 billion light-years from Earth. Two of the most distant galaxies are shown in the images on the right, in red,at the centers of the pictures. (a, b: P. Van Dokkum, Uner of Granengen, ESA, and NASA)

Expanding Chocolate Chip Cake The Expanding Chocolate Chip Cake Analogy The expanding universe can be compared to a chocolate chip cake baking and expanding in the International Space Station. Just as all of the chocolate chips move apart as the cake rises, all of the superclusters of galaxies recede from each other as the universe expands.