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Galaxies in the Universe

Galaxies in the Universe. Chapter 4 Our backyard: the Local Group. Yin Jun 09/20/06. Outline. Brief introduction 4.1 Satellites of the Milky Way 4.2 Spirals of the Local Group 4.3 How did the Local Group galaxies form ? 4.4 Dwarf galaxies in the Local Group

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Galaxies in the Universe

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  1. Galaxies in the Universe Chapter 4 Our backyard: the Local Group Yin Jun 09/20/06

  2. Outline • Brief introduction • 4.1 Satellites of the Milky Way • 4.2 Spirals of the Local Group • 4.3 How did the Local Group galaxies form ? • 4.4 Dwarf galaxies in the Local Group • 4.5 Past and future of the Local Group

  3. Brief Introduction

  4. Members • Local Group contains roughly three dozen galaxies within a sphere about a Mpc in radius • Three most prominent: M31, MW, M33 They emit 90% of the visible light of Local Group. • Only one small elliptical : M32 • Irregular galaxies, less luminous dwarf irregulars ,dwarf ellipticals, dwarf spheroidals

  5. 208

  6. Distribution • Distance: measure m, estimate M, get d. • Centred between the MW and M31: • MW has 11 known satellites (close to a plan); • M31 has its brood of satellites; • Many small systems are “free fliers” • As is typical of groups, it is rich in ‘late type’ galaxies, spirals, and irregulars, and poor in the ‘early type’ giant ellipticals and S0 galaxies.

  7. Sub-Structure in the Local Group M31 M32 M110 Andromeda subgroup NGC 147 NGC185 M33 IC10 NGC 6822 IC 1613 WLM LMC SMC Galaxy subgoup Galaxy Only galaxies with Mv<-14.0 listed

  8. Distribution • Galaxies within about 30 Mpc form a roughly flattened distribution. They lie near the super-galactic plane, approximately perpendicular to the MW’s disk • About half of all galaxies are found in clusters or groups, which are dense enough that their gravity has by now halted the cosmological expansion. • The other half lie in looser clouds and associations within large walls and long filaments. These structures are collapsing, or at least expand much more slowly than the Universe as a whole.

  9. Motion • MW and M31 are approaching each other at a speed of 120 km s-1; • The satellites’ radial velocities are almost within 60 km s-1 of common motion of the MW and M31; • The Local Group galaxies have too little kinetic energy to escape.

  10. Just as the Sun is a typical star, intermediate in its mass and luminosity, so the Local Group represents a typical galactic environment: it is less dense than a galaxy cluster like Virgo or Coma, but contains enough mass to bind the galaxies together.

  11. 4.1 Satellites of the Milky Way

  12. Members • Main companion: LMC & SMC • easily visible to the naked eye • gas rich and forming new stars and star clusters in abundance • contain variable stars and calibrate for use as “standard candles” in estimating distance to galaxies beyond the Local Group • Dwarf spheroidal companions • diffuse, almost invisible on sky • almost no gas to make fresh stars

  13. LMC SMC Image by Roger Smith/NOAO/AURA/NSF

  14. The Magellanic Clouds • Large Magellanic Cloud • 15°×13°on the sky, so its long dimension is about 14kpc • L~1.7×109L⊙, about 10% of the MW’s luminosity. • a flat disk, tilted by about 45°. It has a strong bar, with only one stubby spiral arm • rotation speed measured from the HI gas reaches 80 km/s. The orbits are centred about 0.9kpc or 1°to the Northwest of the brightest region.

  15. HI gas distribution in LMC

  16. The Magellanic Clouds • Small Magellanic Cloud • 7°×4°on the sky, extending roughly 8kpc • L~0.34×109L⊙, about ten times fainter. • It is an elongated “cigar” structure seen roughly end-on, with a depth of about 15kpc along the line of sight. • Its stars show no organized motions

  17. The Magellanic Clouds • Common Characteristic (both Irr?) • Have a profusion of young stars • Less dust to block starlight than in MW • Clouds are blue in visible light and very bright in the ultraviolet. • Star-forming regions are spread throughout, and they are rich in hydrogen gas, the raw material of star formation.

  18. The Magellanic Clouds • There are holes, loops, and filaments centred on sites of recent starbirth, where supernovae and the winds of hot stars have given the surrounding interstellar gas enough momentum to push the cooler HI gas aside, forming a large hot bubble.

  19. The Magellanic Clouds • The ratio of the HI mass to the luminosity in blue light is a useful measure of a galaxy’s progress in converting gas into stars. • Dwarf spheroidal galaxies: M(HI)/LB~0 early • MW: M(HI)/LB~0.1 • LMC: M(HI)/LB~0.3 • SMC and Irrs: M(HI)/LB~1 late

  20. Problem 4.1 Use the data of Tables 1.2, 1.3, and 1.4 to estimate approximate spectral types for the brightest stars of the LMC, in the right panel of Figure 4.5 A0; K0

  21. Magellanic Stream • A “bridge” of gas, containing young star clusters ,connects the two Clouds • wraps a third of the way around the sky, approximately on a Great Circle through l=90°and l=270° • contains a further 2×108M⊙ of HI gas

  22. Magellanic Stream • The Magellanic Clouds are in orbit about each other, and they also orbit the MW in a plane passing almost over the Galactic pole, with a period of ~2Gyr. • The centers of the Large and Small Clouds are now about 20kpc apart, but they probably came within 10kpc of each other during their last perigalactic passage. At that time, the gravitational pull of the LMC pulled out of the SMC the gas that now forms the Magellanic Stream. • The combined gravity of the MW and the LMC has obviously distorted the SMC, and perhaps even destroyed it as a bound system; the different pieces are now drifting slowly apart. (Putman et al. 1998) (Gardiner & Noguchi 1996)

  23. Clusters in MCs • The MCs are extremely rich in star clusters. We can use CMDs of these clusters to find their ages, distances, and chemical compositions. • LMC: 50kpc from the Sun (apparent brightness of MS stars in LMC’ cluster vs Galactic open clusters) • SMC: ~60kpc (giant branch in SMC’s old clusters vs those in Galactic GCs, and from its variable stars)

  24. Clusters in LMC • LMC has some globular clusters similar to MW. They are old(>10Gyr) and poor in heavy elements (<1/100 of solar), but they lie in a thickened disk. • Almost none of the LMC’s clusters have ages in the range 4-10 Gyr • There are many younger clusters and associations (some formed ~50Myr ago, when the LMC & SMC had their last close passage.) • Some are 100 times more populous than most Galactic open clusters; they may be young versions of the LMC’s globular clusters.

  25. Clusters in SMC • They cover the same age range as in the LMC, but there is no gap in time during which few clusters were formed. • The bulk of their stars may have intermediate ages, between a few gigayears and ~12Gyr. • The gas and youngest star clusters are poorer in metals than those of the LMC, with only about 10% of the solar proportion of heavy elements.

  26. Variable stars as “standard candles” • RR Lyrae stars (section 2.2) • low-mass helium-burning stars • L~50L⊙ • periods ~ half a day • Cepheid variable stars • massive helium-burning stars • L rang up to 1000L⊙ • periods ~ from one to fifty days • brighter stars, longer periods

  27. Variable stars as “standard candles” • Care is needed: • Cepheids in the disk of the MW, where the metal abundance is high, are brighter than stars with the same period but fewer metal. • Correct for the effect of interstellar dust in dimming and reddening the stars. • RR are useful within 2-3 Mpc; Cepheids are useful to about 30 Mpc.

  28. Variable stars as “standard candles” • This technique of finding objects in a far-off galaxy which resemble those found closer by, and assuming that the distant objects have the same luminosity as their nearby counterparts, is called the method of standard candles • But this method can lead us badly astray: • Distance of Cepheids in the MW’s disk, and thus their luminosities, had been underestimated because dust. • W Virginis stars in Galactic GCs, which were thought to be as bright as the Cepheids, are in fact much dimmer.

  29. Dwarf spheroidal galaxies • MW has 9 dwarf spheroidal galaxies; • They are effectively gas free, almost no stars younger than 1-2 Gyr; • Many of them contain RR Lyrae variables, which require >8Gyr to evolve to that stage. These systems maybe as old as “giant” galaxies like the MW; • Their surface brightness is about a hundred times less than that of the MCs. The smallest of the dSph galaxies are only about as luminous as the larger globular clusters, although their radii are much larger.

  30. Fornax dSph(天炉座矮星系) Sagittarius dSph(人马座矮星系) Sculptor dSph(玉夫座矮星系) Leo II dSph(狮子座矮星系II) Leo I dSph(狮子座矮星系I)

  31. Carina dSph (船底座矮星系) Sextans dSph (六分仪座矮星系) Ursa Minor dSph (小熊座矮星系) Draco dSph (天龙座矮星系)

  32. Dwarf spheroidal galaxies • In the past, often thought as merely large, low density globular clusters • detailed studies over the last 20 years or so have revealed that the dSph galaxies possess a more diverse set of properties and contain more complex stellar populations than the globular cluster analogy would predict. • Indeed an alternative definition of a dSph might now be a low luminosity M(V)> -14, non-nucleated dwarf elliptical galaxy with low surface brightness (fainter than 22 V magnitude per square arcsecond). • Since the individual stars in dSph galaxies can be resolved, their study will contribute to the understanding of the origin and evolution of dwarf galaxies in general.

  33. Dwarf spheroidal galaxies • Our satellite dSph galaxies are really galaxies, not just another form of star clusters within MW. • Unlike star clusters in MW, the dwarf galaxies did not form all their stars at once; they all include stars born over several gigayears (Fig.4.9), from gas with differing proportions of heavy elements. • Even the most luminous of the dSphs are only about 1/30 as rich in heavy elements as the sun, and the less luminous systems are even more metal poor. • Their low metallicity suggests that these galaxies lost much of their metal-enriched gas into intergalactic space.

  34. Dwarf spheroidal galaxies • There is evidence that these satellite galaxies can bedisrupted by the gravitational pull of the Milky Way. Theoretical work has shown that stars can be torn from galaxies orbiting the Milky Way, resulting in thin streams of debris trailing or leading the satellite around its orbit. • Interaction of a small dwarf galaxy with a Milky Way-like parent galaxy, made by Key Project team member Kathryn Johnston. • Sagittarius, the most recently discovered dSph, is clearly showing signs of having been disrupted. Nearly in the plane of the Galactic disk, it lies only 16kpc from Galactic center. It is strongly distorted and spreads over 22°×7°in the sky, corresponding 10kpc×3.5kpc .

  35. Dwarf spheroidal galaxies • Stellar random speeds are similar to GC, but stars spread over distances ten or a hundred time greater; • If they are in steady state virial theorem • M/L is much greater than that in GCs • For lowest-luminosity dSphs, is even higher than that measured for MW or spiral galaxies dSph galaxies may consist largely of dark matter, with luminous stars as merely the “icing on the cake” • Or they are not in equilibrium, but are being torn apart by the MW’s gravitational field (Sagittarius).

  36. Problem 4.2 The Carina dwarf spheroidal galaxy has a velocity dispersion σthree times less than that at the center of the globular cluster ω Centauri, while Carina’s core radius is 40 times greater. Use the viral theorem to show that Carina is about four times as massive as ω Centauri, so that M/L must be 20 times larger. KE ≈ 3Mσr2/2 2KE+PE=0 PE ≈ -GM2/2rc M ≈ 3σr2rc/G (3.43) Carina dSph σr 40rcL M=?m Centauri GC 3σr rcl =5L m M ≈ 3σr2(40rc)/G m ≈ 3(3σr)2rc/G M/m=40/9≈4  M/L=20m/l

  37. Life in orbit: the tidal limit • Three-body problem: Due to the combined gravitational force changing in time, they can no longer conserve their energies according to (3.25) • Many of the possible orbits are chaotic; • A small change to a star’s position or velocity has a huge effect on its subsequent motion. • If the satellite follows a circular orbit, and the gravitational potential is constant in a frame of reference rotating uniformly about the center of mass of the combined system, we can find a substitute for the no-longer-conserved energy: effective potential Φeff

  38. Life in orbit: the tidal limit • Jacobi constant EJ • Rotating frame: where EJ does not change along the star’s path. • Inertial frame: Write the Jacobi constant in terms of E and L

  39. Life in orbit: the tidal limit • Simplest problem x Ω D x=DM/(M+m) m Satellite center of mass M main galaxy

  40. Life in orbit: the tidal limit • Φeff has three maxima: Lagrange points L2 L3 L1 m M x The middle point L1 is the lowest; the next lowest point, L2, lies behind the satellite, and L3 is behind the main galaxy. If a star has EJ < Φeff (L1), it must remain bound to either M or m; it cannot wander between them.

  41. Life in orbit: the tidal limit If the satellite’s mass is much less than that of the main galaxy, L1 and L2 will lie close to m. So L1 and L2 is: ,where • Stars that cannot stray further from the satellite than rJ, the Jacobi radius, will remain bound to it • L1 is not the point where the gravitational forces from M & m are equal, but lies further from the less massive body. • The Lagrange points are important for close binary stars; if the outer envelope of one star expands beyond L1, its mass begins to spill over onto the other.

  42. moon rEM rSM Problem 4.3 1 AU earth sun Show that the gravitational pull of the Sun (mass M) on the Moon is stronger than that of the Earth (mass m), but the Moon remains in orbit about the Earth, because its orbital radius r < rJ FGS/ FGE=(M/m)(rEM/rSM)2≈2.2 So, the gravitational pull of the Sun is stronger. rJ=D(m / 3M+m)1/3=1AU(6/3*2000000) 1/3=0.01AU rEM=(3.84*105km) / (1.5*108km/AU)=0.00256AU<rJ So, moon remains in orbit about the Earth

  43. Life in orbit: the tidal limit • When M>>m, the mean density in a sphere of radius rJ surrounding the satellite, 3m/4πrJ3, is exactly three times the mean density within a sphere of radius D around the main galaxy. M>>m, so rJ=D(m/3M)1/3 rJ3=D3(m/3M) ρm=3m/4πrJ3=3m/4π[D3(m/3M)]=9m/4πD3=3ρM • Ignoring for the moment the force from the main galaxy, equation (3.23) tell us that the period of a star orbiting the satellite at distance rJ would be roughly equal to the satellite’s own orbital period. • The satellite can retain those stars close enough to circle it in less time than its own orbit about the main galaxy, but it will lose its hold on any that are more remote.

  44. Problem 4.4 If the mass M is replaced by the ‘dark halo’ potential of Equation 2.19, show that the mass enclosed within radius r>>aH of its center is M(<r) ≈rVH2/G. A satellite orbits at distance D>>aH, show that when its mass m<<M(<D), then instead of Equation 4.10 we have 4πGρH(r)=VH2/(r2+aH2) (2.9) M(<r)=∫4πρH(r)r2dr= rVH2/G

  45. Life in orbit: the tidal limit • The LMC’s disk is now safely stable against disruption by the MW. By equation (4.11) rJ≈50kpc×[1010M⊙/2×5×1011 M⊙]1/3≈11kpc The LMC’s disk is safely within this radius. • The SMC is too distant from the LMC to remain bound to it. • The problem below shows that some dwarf galaxies are probably being torn apart by the MW’s gravitational field.

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