Classification Distribution in Space Galaxy Clusters. Formation and Evolution Hubble s Law

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1 The American astronomer Edwin Hubble in 1924, according to appearance of galaxies categorized them into four basic types. Classification Distribution in Space Galaxy Clusters Masses Formation and Evolution Hubble s Law All bright galaxies fall into one of above mentioned four classes: Spiral (ordinary or barred) Galaxies (~75%) Elliptical Galaxies (20%) Irregular Galaxies (5%) Classified by relative strength of the central bulge & tightness of the spiral arms Types: Sa, Sb, and Sc Sa: Sb: Sc: Properties: Mass: M sun Diameter: 5-50 kpc Luminosity: L sun Structure & Dynamics: Disk, bulge & halo Supported by relatively ordered, rapid rotation in the disk, but halo is puffed up by random motions. 1

2 Parallel group to the ordinary spirals: About as many barred as ordinary spirals. Feature a strong central stellar bar: Bar rotates as a unit (solid body rotation) Spiral arms emerge from the ends of the bar Same subclasses: SBa, SBb, and SBc Other properties are similar to the ordinary spiral galaxies. In SBa category bar is clearly visible, though in the case of SBc it is often hard to tell where the bar ends and the spiral arms begin. Show little internal structure: Classified by the degree of apparent flatness: E0 (circular), E 1 (slightly flattened), so on. up to E7 (flattest) Properties: Mass: M sun Diameter: 1 Kpc few Mpc (large range of sizes) Luminosity: L sun Sombrero Galaxy: a spiral Galaxy seen edge-on Bulge indicates --- type Sa Structure & Dynamics: Halo of old stars with little gas or dust Supported by random motions of stars with some very slow rotation 2

3 Intermediate between the E7 ellipticals and the Sa spirals there is one more class called S0 (if no bar) and SB0 (if with bar). Show an irregular, often chaotic structure. Little evidence of systematic rotation. Catch-all class: Proposed systematic subclasses, but many irregulars defy classification. Significant dwarf irregular population, classified as "di. Irr I tend to be smaller than spirals but somewhat larger than dwarf ellipticals. Properties: Mass: M sun Diameter: 1-10 kpc Luminosity: few x 10 9 L sun Structure & Dynamics: Chaotic structure, lots of young blue stars Moderate rotation in Irregulars, but very chaotic motions as well. Our neighbour galaxies, both are of type Irr I. 3

4 Some irregular (Irr II) galaxies Irr II galaxies are much rarer. They have distinctly explosive appearance. Spirals: Range is ~10-20% gas (Sa least, Sc most gas & dust,) On-going star formation in the disks Mix of Pop I and Pop II stars Ellipticals: Very little or no cool gas or dust but extensive x- ray halos of very hot gas Star formation ended billions of years ago See only old Pop II stars Irregulars: Can range up to 90% gas Often a great deal of on-going star formation Dominated by young Pop I stars Dwarf Irregulars: Very metal poor (<1% solar) Forming stars for the first time only now. Low-luminosity Ellipticals & Irregulars. Significant number of dwarfs (they outnumber their counterpart large ellipticals & irregulars by about 10 to 1) There are no (convincing) Dwarf Spirals. Possibilities: Small versions of their larger cousins Different population of objects despite their superficial similarities to larger E's and Irr's Galaxies of all types are the basic "units" of luminous matter in the Universe. Basic units of larger, organized structures Sites of star formation from raw gas Factories that synthesize heavy elements from Hydrogen & Helium Differences in the types of galaxies reflect the difference in their formation histories and environments. 4

5 Irr v Determining distances v Galaxy Clusters v Cepheid variables was discussed in detail in last chapter (chap 23). can only be used for nearby galaxies (up to 25 Mpc). difficult to resolve the individual stars in the distant galaxies. hence Cepheid variables simply cannot be observed well enough in distant galaxies. v Clusters of clusters 5

6 v Standard candles These are objects that are easily recognizable (by their light curves, spectra, or some other directly observable characteristic) and whose luminosities are already known. Those luminosities are use to determine distance and luminosity of the objects in question. Standard candles must Objects used as standard candles: novae, emission nebulae, planetary nebulae, globular clusters, type I supernovae, entire galaxy. Type I supernovae are particularly useful as standard candles because they are bright and have narrow range of luminosity. v Tully-Fisher relation Determines a galaxy s luminosity by observing how fast it rotates. A galaxy s rotation speed is determined by measuring the broadening it causes in its spectral lines (especially in 21-cm lines). Can measure spiral galaxies up to 200 Mpc away. Hierarchy of Structure of the Universe: q Groups: q Clusters: q Superclusters: q Voids, Filaments, & Walls Most galaxies are found in groups and clusters. Galaxy clusters consist of a collection of galaxies orbiting one another, bound together by their own gravity. Basic Properties: Groups: bright galaxies Clusters: bright galaxies Sizes: Mpc across Often contain many more dwarfs than bright galaxies. Total masses of to >10 15 M sun About 3000 clusters have been cataloged to date. 6

7 Group of some 45 galaxies including the Milky Way and Andromeda (M31): Size (radius): ~1 Mpc 5 bright galaxies (M31, MW, M33, LMC and IC10) 3 Spirals (MW, M31, & M33) 22 Ellipticals (4 small Es & 18 des) Remainder are Irregulars of various sizes Total Mass ~ 5 x M sun The Milky Way and Andromeda are by far the largest members of the Local group. Nearest sizable cluster to the Local Group. Relatively loose cluster, centered on two bright Ellipticals: M87 & M86 Properties: Distance: ~18 Mpc from the MW Size: ~ 3 Mpc across 2500 galaxies (mostly dwarfs) Mass: ~10 14 M sun The inset shows several galaxies surrounding the giant elliptical known as M86. M 87 M 86 7

8 Contain 1000's of galaxies: Extend for 5-10 Mpc Masses up to ~10 15 M sun One or more giant Elliptical Galaxies at center. Ellipticals found near the center. Spirals found at the outskirts % of the mass is hot ( K) intracluster gas seen in X-ray emission. Examples: Virgo Cluster, Coma Cluster Credit: O. Lopez-Cruz and I. K. Shelton (U. Toronto), Kitt Peak National Obs. Clusters of Clusters Properties: Sizes up to Masses of M sun empty space (voids) Often long and filamentary in shape Superclusters are the largest coherent structures seen in the Universe Roughly centered on the Virgo Cluster. Properties: Contains several 10s of 1000s of galaxies Size: ~ Mpc across Mass: ~ M sun very irregular in shape only ~ 5% of the volume occupied by galaxies The Local Group is on the outskirts of the Local Supercluster, about 18 Mpc from the center. 8

9 The Universe looks foamy on large scales Filaments: Occupy ~ Voids: Empty bubbles than in superclusters Found in a large-scale galaxy survey. Sheet of superclusters: 150 Mpc long 60 Mpc "high" 5 Mpc thick Mass is ~2x10 16 M sun One of the largest structures known. Within about 200 Mpc from Milky Way, covering 1057 galaxies. 9

10 Within about 150 Mpc from Milky Way Within about 300 Mpc from Milky Way The masses of nearby spiral galaxies can be determined by studying their rotation curves. (For detail see the determination of mass of Milky Way Galaxy, chap 23.) For more distant spirals, masses can be inferred from observations of the broadening of their spectral lines. (For detail see the Tully-Fisher relation.) On larger scales, astronomers use studies of binary galaxies and galaxy clusters to obtain statistical mass estimates of the galaxies involved. Typical mass for large galaxies is M sun and for dwarf elliptical and irregular can contain as little as M sun. Spiral galaxies tend to have flat or rising rotation curve. --- thus as in the Milky Way mass continue to increase as you move outward Sprial galaxies may contain 3 to 10 times more mass than can be accounted for in the form of visible matter. Study of elliptical galaxies also show the similar trend. We must accept that Unlike stars, galaxies are large compared to the distances between them: Most galaxies are separated by only ~ 20 times their diameters. By comparison, most stars are separated by ~10 7 times their diameters. Galaxies are likely to encounter other galaxies a few times over their histories. Is there any evolutionary sequence that links spiral, elliptical, and irregular galaxies? Collisions and mergers of smaller ones result into large galaxies. Galactic Rotation Curve (Contrast this process with that of star formation, in which a large cloud fragments into smaller piece that eventually become star) 10

11 Few basic processes of evolution of galaxies Tidal Interactions Galaxy Collisions Splash Encounters Starbursts Mergers Galactic Cannibalism Galaxies interact via Gravitation. Because of their large sizes, two galaxies passing near each other raise mutual tides. These tides distort the shapes of the galaxies Gives rise to dramatic effects without direct collision. Many "peculiar galaxies" are interacting pairs. Raising Tides: Tidal stretching along the encounter line. Near side feels stronger gravitational pull from the companion Far side feels weaker gravitational pull and lags behind the near side One galaxy strips halo material from other by tidal forces. An example of a tidally interacting pair. Rare direct collisions have more dramatic effects: Tides raised are stronger, giving greater tidal distortion Tear off huge "Tidal Bridges" of stars & gas Stars pass through without colliding, but Gas clouds collide, leading to a massive starburst in the galaxy disks. Credit: NOAO/AURA/NSF 11

12 An example of a directly colliding galaxy pair. Closeup of Galaxy Collision pair Credit: B. Whitmore (STScI), F. Schweizer (DTM), NASA (APOD) A fast-moving smaller galaxy collides at (or near) the center of a larger disk galaxy (hits a "bulls-eye"). Results in a tidal "splash": Circular density wave moving outward through the "target" galaxy. Wave triggers star formation in the disk gas an example of an ring galaxy caused by a bull's eye "splash" encounter. "Intruder" galaxy passes on through. Credit: NASA 12

13 A starburst galaxy may result when a galaxy experiences a close encounter with a neighbor. Case of intense star formation in a galaxy. Violent events, collisions appear to have rearranged the galaxy s internal structure and triggered a sudden, intense burst of star formation. An example of a directly colliding galaxy pair and star bursts. Millions of O & B stars greatly enhance the brightness of the galaxy. Exhausts the available gas in a few Myrs. Many supernovae can drive fast "superwinds" blowing out of the galaxies. The most intense starbursts occur in violently interacting galaxy pairs. If two colliding galaxies can dissipate enough orbital energy: Wreckage merges into a single galaxy. Gas clouds collide and form new stars. Some portion of the old stars are ejected from the system (carry off orbital energy). IC 694 and NGC 3690 Arp 220 NGC 1275 (Irr II) Mergers may play a pivotal role in the formation ("assembly") of galaxies. 13

14 Slow encounter between a large & a small galaxy: Smaller galaxy gets torn apart by the tides from the larger galaxy. Gas and stars get incorporated into the larger galaxy. Nuclei of the galaxies slowly spiral together. May be the way giant Ellipticals grow. The Milky Way (us) and Andromeda are perhaps on a collision course. Moving towards each other at ~120 km/sec In ~3-4 Gyr, they will have a close encounter Tidally distort & merge after ~1-2 Gyr Slipher discovered most spiral nebula are receding from us 1920's- Hubble discovered that the speed of recession is proportional to the distance Eventually, only 1 galaxy may remain behind, most likely an Elliptical galaxy. This idea is controversial: it depends on knowing with the tangential motion of the Milky Way is relative to Andromeda, which cannot be measured reliably until the next generation of astrometric satellites. Credit: John Wallin (GMU) 14

15 The redshift associated with the Hubble expansion is called cosmological redshift. H 0 is the Hubble constant H 0 = 65 km/s/mpc The Hubble constant very difficult to accurately determine. current values range from 50 to 80 km/s/mpc. determining H is one of the main missions of Hubble Space Telescope. latest value through WMAP is 72+/- 3 km/s/mpc What does it mean? Hubble's law was discovered with observational data not a theoretical prediction. it is an empirical law. It can be used to determine the distance to galaxies measure the recessional velocity, and calculate distance. Hubble s law enables us to measure the distance beyond 100 million pc --- so long as we can obtain the spectrum of galaxies/objects. 15

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