Presentation on theme: "Galaxies in the Universe"— Presentation transcript:
1 Galaxies in the Universe Chapter 4Our backyard: the Local GroupYin Jun 09/20/06
2 Outline Brief introduction 4.1 Satellites of the Milky Way 4.2 Spirals of the Local Group4.3 How did the Local Group galaxies form ?4.4 Dwarf galaxies in the Local Group4.5 Past and future of the Local Group
4 MembersLocal Group contains roughly three dozen galaxies within a sphere about a Mpc in radiusThree most prominent: M31, MW, M33They emit 90% of the visible light of Local Group.Only one small elliptical : M32Irregular galaxies, less luminous dwarf irregulars ,dwarf ellipticals, dwarf spheroidals
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.
8 Sub-Structure in the Local Group M31M32M Andromeda subgroupNGC 147NGC185M33IC10NGC 6822IC 1613WLMLMCSMC Galaxy subgoupGalaxy Only galaxies with Mv<-14.0 listed
9 DistributionGalaxies within about 30 Mpc form a roughly flattened distribution. They lie near the super-galactic plane, approximately perpendicular to the MW’s diskAbout 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.
10 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.
11 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.
14 Members Main companion: LMC & SMC Dwarf spheroidal companions easily visible to the naked eyegas rich and forming new stars and star clusters in abundancecontain variable stars and calibrate for use as “standard candles” in estimating distance to galaxies beyond the Local GroupDwarf spheroidal companionsdiffuse, almost invisible on skyalmost no gas to make fresh stars
16 The Magellanic Clouds Large Magellanic Cloud 15°×13°on the sky, so its long dimension is about 14kpcL~1.7×109L⊙, about 10% ofthe MW’s luminosity.a flat disk, tilted by about 45°.It has a strong bar, with only one stubby spiral armrotation 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.
19 The Magellanic Clouds Small Magellanic Cloud 7°×4°on the sky, extending roughly 8kpcL~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
21 The Magellanic Clouds Common Characteristic (both Irr?) Have a profusion of young starsLess dust to block starlight than in MWClouds 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.
22 The Magellanic CloudsThere 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.
23 The Magellanic CloudsThe 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 earlyMW: M(HI)/LB~0.1LMC: M(HI)/LB~0.3SMC and Irrs: M(HI)/LB~ late
24 Problem 4.1Use 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 4.5A0; K0
25 Magellanic StreamA “bridge” of gas, containing young star clusters ,connects the two Cloudswraps 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
27 Magellanic StreamThe 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)
28 Clusters in MCsThe 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)
29 Clusters in LMCLMC 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 GyrThere 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.
30 Clusters in SMCThey 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.
31 Variable stars as “standard candles” RR Lyrae stars (section 2.2)low-mass helium-burning starsL~50L⊙periods ~ half a dayCepheid variable starsmassive helium-burning starsL rang up to 1000L⊙periods ~ from one to fifty daysbrighter stars, longer periods
32 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.
33 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 candlesBut 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.
34 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.
35 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)
37 Dwarf spheroidal galaxies In the past, often thought as merely large, low density globular clustersdetailed 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.
38 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.
39 Dwarf spheroidal galaxies There is evidence that these satellite galaxies can be disrupted 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 .
40 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 theoremM/L is much greater than that in GCsFor lowest-luminosity dSphs, is even higher than that measured for MW or spiral galaxiesdSph 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).
41 Problem 4.2The 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/ KE+PE=0PE ≈ -GM2/2rc M ≈ 3σr2rc/G (3.43)Carina dSph σr 40rc L M=?mCentauri GC 3σr rc l =5L mM ≈ 3σr2(40rc)/Gm ≈ 3(3σr)2rc/G M/m=40/9≈4 M/L=20m/l
42 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
43 Life in orbit: the tidal limit Jacobi constant EJRotating frame:whereEJ does not change along the star’s path.Inertial frame:Write the Jacobi constant in terms of E and L
44 Life in orbit: the tidal limit Simplest problemxΩDx=DM/(M+m)mSatellitecenter of massMmain galaxy
45 Life in orbit: the tidal limit Φeff has three maxima: Lagrange pointsL2L3L1mMxThe 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.
46 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:,whereStars that cannot stray further from the satellite than rJ, the Jacobi radius, will remain bound to itL1 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.
47 moonrEMrSMProblem 4.31 AUearthsunShow 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 < rJFGS/ FGE=(M/m)(rEM/rSM)2≈2.2So, the gravitational pull of the Sun is stronger.rJ=D(m / 3M+m)1/3=1AU(6/3* ) 1/3=0.01AUrEM=(3.84*105km) / (1.5*108km/AU)= AU<rJSo, moon remains in orbit about the Earth
48 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=3ρMIgnoring 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.
49 Problem 4.4 4πGρH(r)=VH2/(r2+aH2) (2.9) M(<r)=∫ 4πρH(r)r2dr= rVH2/G 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 have4πGρH(r)=VH2/(r2+aH2) (2.9)M(<r)=∫ 4πρH(r)r2dr= rVH2/G
50 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≈11kpcThe 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.
51 Problem 4.5The Sagittarius dwarf spheroidal galaxy is now about 15kpc from the Galactic center: find the mass of the Milky Way within that radius, assuming that the rotation curve remains flat with V(R) ≈200km s-1. Show that this dwarf galaxy would need a mass of about 1010M⊙ if stars at 5kpc from its center are to remain bound to it. What mass-to-light ratio M/L in the V band would this require? (It is much larger than those listed in Table 4.2)M(<15kpc)=RV2/G=15000×2002/(4.5×10-3)=1.3×1011 M⊙rJ=5kpc, D=15kpc, M= 1.3×1011 M⊙so we can get m≈ 1010 M⊙m/L= 1010 M⊙/ 107 L⊙=1000!so m must less than 1010 M⊙ and rJ less than 5kpc.
53 Three spiral galaxies: MW, M31, M33 M31 (770kpc) is the most distant object that can easily be seen with the unaided eyeM33 (850kpc) is much harder to spotComparing these three systems with each other, we see what properties spiral galaxies have in common, and how they differ.
55 The Andromeda galaxy Compare with MW ~50% more luminous; disk scale length hR~6-7kpc, twice as large as MW’srotates faster: V(R)~260km s-1, 20-30% higher than MW’s~300 known GCs, over twice as many as MW’shas its own satellite galaxies: M32, 3 dwarf ellipticals, and at least 6 dwarf spheroidals: Andromeda I, II, III, V, VI, VII, Cassiopeia dSph
57 The Andromeda galaxy Bulge Larger in proportion than MW’s, providing 30-40% of the measured luminosity;Apparent long axis of the bulge dose not line up with the major axis of the disk further out;Either the bulge is not axisymmetric and would look somewhat oval if seen from above the disk,Or its equator must be tipped relative to the plane of the disk.Stars are all at lease a few gigayears old, generally rich in heavy elements.Contains dilute ionized gas, a few denser clouds of HI & dust (dark nebulae)At its center is a compact semistellar nucleus
58 The Andromeda galaxy Nucleus In HST images the nucleus proves to have 2 separate concentrations of light about 0.5’’ or 2pc apart;One harbors a dense central object, probably a black hole of mass ~106M⊙;The other may be a star cluster which has spiralled in to the center under the influence of dynamical frictionUnlike MW, the nucleus of M31 is impressively free of either gas or dust
59 The Andromeda galaxy Globular clusters: Metal-poor GCs of M31 follow deeply plunging orbits;The cluster system shows little or no ordered rotation. However, the bulge also continues smoothly outward as a luminous spheroid.Most of the stars a few kpcs above the disk plane are not those of a metal-poor halo; they are relatively metal rich, and they probably form a fast-rotating system. It is as if M31’s bulge has ‘overflowed’, largely swamping the metal-poor halo
60 The Andromeda galaxy Ring ~10kpc, star-forming ‘ring of fire’ Most of the young disk stars lie in this ring or just outside it;Stars is forming at s slower rate (~1M⊙yr-1) than that of MWJust outside this ring, dark dust lanes and strings of HII regions in the disk trace segments of fairly tightly wound spiral arms, where gas, dust, and stars have been compressed to a higher density.There is no clear large-scale spiral pattern.
61 The Andromeda galaxy Gas HI ~4-6×109M⊙, about 50% more than MW, concentrated at the “ring of fire”;H2 is probably a smaller fraction of the total, so the ratio of gas mass to stellar luminosity is lower than in MWGas extends to larger radii than the stellar disk, as in the Milky Way
62 The Andromeda galaxy Holes In the region of the spiral arms, high-resolution maps show holes in the HI disk, up to a kpc across;At their edges, shells containing M⊙ of dense HI gas are moving outward at km s-1. Need a few megayears to reach their present sizes.Winds from massive O & B stars which lies within the hole, and recent SN explosions, have blown away the cool gas.Holes in the inner parts of the disk tend to be smaller, perhaps because the gas is denser or the magnetic field stronger, making it harder to push cool material out of the way.
63 The Andromeda galaxy The outer part of the gas disk is “S” shape; The stellar disk is visibly warped in the same sense;“S” warp is quite common in spirals;As in our MW, the HI layer flares out to become thicker at greater distances from the center.Because of its large bulge and moderately tightly wound spiral arms, and the relative paucity of gas and recent star formation in the inner disk, we classify M31 as an Sb galaxy, while our Milky Way is Sbc or Sc.
65 M33: a late-type spiral M33 is definitely an Sc or Scd galaxy Bulge is tinyThe spiral arms: more open , not so smooth, consist mainly of bright blue concentrations of recently formed stars.smaller and much less luminous (5.5×109L⊙)Scale length ~ 1.7kpc , smallRotation speed V(R) rises only to 120 km s-1
66 Radio & Optical Combination Image of M33 M33: a late-type spiralComplex network of loops, filaments, and shells, like LMCSN explosions, and the winds from stars, heat the surrounding gas and drive it away, thus affecting the location and rate of future starbirth.Such feedback has a strong effect on the way that galaxies came into being from lumps of primordial gas, and on their subsequent develop-ment.Radio & Optical Combination Image of M33
67 M33: a late-type spiral Gas relatively richer in HI gas than M31 or MW Compared to the MW, relatively more of the HI gas in the warm component and less in cold dense clouds.HI layer has large holes, often centred on SF regions.HI gas disk is very extended, ~30kpcLittle CO emission Lack of molecular gas orA smaller ratio of CO to H2 than in the MW(more likely, stars born in the dense cores of H2 clouds)outer disk is warped, possibly by tides from M31
68 M33: a late-type spiralAt the center of M33, we find a dense nuclear star cluster, with no more than a small bulge around it.Luminous than any Galactic globular (2.5×106L⊙)Its core is less than 0.4pcThe density of stars exceeds 107L⊙pc-3Nucleus contains old, middle-aged, and young stars, not a single generation of stars.No sign of a black hole with MBH~106M⊙, but we do see evidence for a power source other than ordinary stars.M33’s nucleus is the single brightest X-ray source in LGAn unusual optical spectrum, with strong emission lines of nitrogen.
69 M33: a late-type spiralM33 is only 2 or 3 times more luminous than the LMCIt has a much more symmetrical spiral pattern.Low-L galaxies are in general more likely than larger systems to resemble LMChave a strong central barbrightest parts of the galaxy lying off center form the disk.However, the morphology clearly depends on factors other than the galaxy’s luminosity alone
71 A picture for the formation years after Big Bang, photons no longer had enough energy to ionize H and Henuclei + electrons gas of neutral atoms.Light could freely propagate;Universe became transparent.The gas was no longer supported by pressure of photons trapped within it form galaxiesIf its gravity was strong enough, a region that was denser than average collapse inwardThe denser the gas, the earlier cosmic expansion would halt, giving way to contraction.
72 A picture for the formation Clumps near the center of a large infalling region would fall toward each other, eventually merging into a single big galaxy;Those further out might become smaller satellite galaxies.The protogalaxies lay closer together than the galaxies do now, because the Universe was smaller.Not neat spheres, but irregularly shaped lumps, tugging at each other by gravity. Mutual tidal torques would have pulled them into a slow rotationClouds within each protogalaxy collide with each other lose part of energy fall inward rotation increases (angular momentum approximately conserves)
73 Earliest starsObserve stellar light and the emission lines of hot gas ionized by early stars: z~5 (Universe was less than 1 Gyr)The first stars could not form too early :the cosmic background radiation had to cool sufficiently, so star-sized lumps of gas could radiate heat away, and collapse using equation (1.28): z≤6, hundreds of millions of years after Big Bang.
74 Making the Milky Way Stars The first star born: pure H and Helived: in a smaller lumps of gas, with masses perhaps M⊙die: contaminated the remaining gas with metals.One or two SN were enough to add elements to the gas in 1/1000 or even 1/100 of the solar proportion the oldest stars, metal-poor globular clustersStars in each clusters generally have very closely the same composition we think that GCs formed in smaller parcels of gas, where the nucleosynthetic products of earlier stars had been thoroughly mixed.
75 Making the Milky Way Halo (GCs) When gas clouds ran into each other, fell together to form the MW some GCs bron.The collisions would have compressed the gas, raising its density so that many stars formed in a short time.Stars, unlike gas, do not lose significant energy through collisions; so their formation halts the increase in ordered rotation.The orbits of the old metal-poor globulars and halo stars are not circular but elongated.Stellar orbits: in random directions.So metal-poor halo: no ordered rotation.This is probably because the material did not fall far into the Galaxy before it became largely stellar
77 Making the Milky Way Disk By contrast, the material that became the MW’s rotating disk had to lose a considerable amount of its energy.A circle is the orbit of lowest energy for a given angular momentum (section 3.3)Thin-disk stars occupy nearly circular orbits because they were born from gas that had lost almost as much energy as possible;Thick disk stars & more metal-rich GCs (predate most of the thin disk) may have born from gas clouds that lost less of their energy. But they still formed a somewhat flattened rotating system8-10Gyr ago, earliest thin-disk stars were born, heavy elements enriched the gas, perhaps to 10-20% of the solar abundance.
78 Making the Milky Way Disk gas If tidal torques gave disk gas a rotational speed only 5% of that needed for a circular orbit, the gas must fall inward until it reached an orbit appropriate for its momentum;If the MW’s V(R) is constantthis gas must have fallen in from a distance R~100kpc;the gas around galaxies must have extended much further out at earlier times;The disk material (much less dense) had to remain gaseous as it moved inward, so that it could continue to lose energy through collisions.
79 Making the Milky Way Bulge CMD shows no horizontal branch in Galactic bulge.The overwhelming majority of stars have ages less than 8-10Gyr, some may be much younger, few as old as GC.We do not yet know how the bulge stars were made.may have formed in the dense center of the protogalactic gas that was to make up the MW;might have grown out of a dense inner region of the disk;may be the remains of dense clusters that fell victim to dynamical friction, and spiralled into the center.
80 Making the Milky WayThe central kpc of galaxies such as M33 and the LMC is not dense as the inner MW; the low density may have prevented a bulge from developingOnce the dense central bulge formed, the gravitational force of the whole Galaxy would have helped it to hold onto its gas, trap debris from SNthe bulge formed large numbers of metal-rich stars.Stars of more luminous galaxies , and closer to the center of galaxies, contain a higher fraction of heavy elements: the stronger gravity prevents metal-rich gas from escaping, and it is incorporated into stars.
81 Making the Milky Way Dark matter Much in the outskirts, beyond most of the stars of the disk.If we presume that all forms of matter were mixed evenly at early times the dark matter must have less opportunity to loss energy, so it was left on orbits far from the Galactic center.If the dark matter consists of compact objects such as brown dwarfs or black holes, we would expect that these formed very early in the Milky Way’s collapse, probably predating even the globular clusters.
82 Making the Milky Way The MW is still under construction today Stars of the Sagittarius dSph galaxy are being added to MW’s haloNear the Sun, groups of young metal-poor halo stars have been found, that may be the remnants of another partially digested galaxy.The orbit of the MCs has been shrinking, and the LMC will probably fall into the Milky Way within the next 5 to 10 Gyr.
83 The buildup of heavy elements A “clock” for galactic agingAMR: old disk stars in general contain little iron;incomplete mixing could explain the large dispersionOCs of thin disk : younger and more metal richstars and GCs of the thick disk: in the middleGCs of the halo: oldest & poorest in metalPop I: young metal-rich stars in diskPop II: old metal-poor stars in bulge & halooversimplification:The bulge of M31 & MW (at lease a few Gyrs ) are metal richdIrr galaxies and the outer parts of normal spirals contain young metal-poor stars born within the past 100Myr.
84 The buildup of heavy elements One-zone instantaneous recycling modelGas is well mixed, same composition everywhereStars return the nuclear fusion products rapidly, faster than the time to form a significant fraction of the stars.Close-box model: no gas escapes, no gas infallMg(t): the mass of gas in the galaxy at time tMs(t): the mass in low-mass stars and remnants of high-mass stars. The matter in these objects remain locked within themMh(t): the total mass of elements heavier than He in gasMetal abundance: Z(t)=Mh(t)/Mg(t)
85 The buildup of heavy elements yield p: represents an average abundance over the local stars; it depends on the IMF and on details of the nuclear burning.The distribution of angular momentum in the stellar material, its metal abundance, stellar magnetic fields, and the fraction of stars in close binaries can also affect p.at time t, △’Ms of stars is formed, left △Ms at the end,The heavy element return to interstellar medium: p △Ms
86 The buildup of heavy elements Z△’MsThe buildup of heavy elementsZ(△’Ms-△Ms)+p△MsZ△Ms+…Heavy element in the interstellar gas alters:△Mh=p△Ms-Z△Ms=(p-Z)△MsMetallicity of the gas increases by an amount:for close-box, △Ms+△Mg=0, so it will be △Z=-p△Mg/MgIf p is independent of Z, we can integrate the equation:
87 The buildup of heavy elements The mass of stars Ms(t) formed before time t, and so with metallicity less than Z(t), is just Mg(0)-Mg(t):Time does not appear explicitly here; the mass Ms(<Z) of slowly evolving stars that have abundance below the given level Z depends only on the quantity of gas remaining in the galaxy when its metal abundance has reached that value.Explains a basic fact: where the gas density is high in relation to the number of stars formed, the average abundance of heavy elements is lowIn gas-rich regions such as MCs, or outer disk of spiral galaxies, the stars and gas are relatively poor in metals (Fig. 4.15)
88 The buildup of heavy elements Once all the gas is gone, the mass of stars with metallicity between Z and Z+△ZFor the Galactic bulge, the model is good if the gas originally lacked any metals and the yield p≈0.7Z⊙.The bulge may have managed to retain all its gas, and turn it completely into stars.
90 The buildup of heavy elements In other cases, the simple model is clearly wrong.GC: no gas, and stars are all metal-poorThese clusters must have formed out of gas that mixed very thoroughly after its initial contamination with heavy elements;Any material not used in making their single generation of stars must also have been expelled promptly.Most of the heavy elements have been lost. Interstellar gas could easily escape the weak gravitational force, and perhaps only a small fraction of the hot metal-rich material from SN would have mixed with cool gas.dSph galaxies: little gas, metal abundance times lower than Galactic bulgeMaybe it formed very few massive stars, so produced only few metals
91 The buildup of heavy elements Effective yield p: take into account metals lost.For GC, p~0p is always less than the true yield of metal produced.Solar neighborhood:Stars: M⊙pc-2Gas: 13 M⊙pc-2Total: ~50 M⊙pc-2The average abundance in the gas: 0.7Z⊙So, if heavy elements were originally absent and no gas flow:Z(now)≈0.7Z⊙≈p ln(50/13), so p≈0.5Z⊙, lower than bulge. the disk had been less efficient than the bulge in retaining metal-rich gas from its supernovae.
92 The buildup of heavy elements G-dwarf problemClose-box model:Observation: a sample of 132 G-dwarf stars in solar neighborhood33 below 25% of the solar fraction of iron (25%)1 below 25% of the solar fraction of oxygen (<0.01%)Possible solutionPre-enrich: Z(0) ≈0.15Z⊙ (problem 4.8)Infall (more likely) (problem 4.9)Subsequent inflow of fresh metal-deficient gas would dilute that material, preventing abundance from rising fast.Long-lived stars formed at early times should also return metal-poor gas as their age. If enough of this gas was released, the fraction of metals in newly made stars might even decline with time
95 The buildup of heavy elements Stars with low metal abundance have more oxygen relative to the amount of iron than stars like the SunThese elements are made in stars of differing massSN II (>10M⊙):release back mainly lighter element such as O, S, MgMost of the heavier nuclei such as Fe are mainly swallowed up into the remnant neutron star or black hole.These massive stars go through their lives within 100Myr, so the IRA is reasonableNot all of the “lighter” heavy elements are produced in very massive stars: AGB.These stars often take far longer than 100Myr to contribute metal; for them, IRA is a poor approximation.
96 The buildup of heavy elements SN Ia (binary system): main source of ironheats the interior, triggering nuclear burning, blows the star apartNo remnant is left; all the Fe, Ni, and elements of similar atomic weight are released back to the interstellar gas.Stars that explode as SN Ia only do so at ages of a Gyr or more. So in stars formed over the first few Gyrs of a galaxy’s life, we expect O/Fe and Mg/Fe higher than it is in the sun.At the present day, slowly evolving stars that were born early on should be returning metal-poor gas to ISM. The abundance of heavy elements in the gas is now increasing only slowly, or may even be falling with time.
98 2 main types of dwarf galaxies Dwarf ellipticals & much diffuse dwarf spheroidalsAlmost all the stars are at least a few Gyrs old;Contain little gas to make any new stars.Dwarf irregularsDiffuse systems;Tiny, gas-rich, active star formation, a profusion of recently formed blue starsAll the dEs and most of dSphs orbit either MW or M31But many of dIrrs are ‘free fliers’
99 dEs and dSphsdSph:not much more luminous than a GC (table 4.2)but so diffuse as to be almost invisible on the skydE:more luminous versions of the dSph (L>3×107M⊙ or MV<-14)representation: 3 M31’s satellites: NGC147, NGC185, NGC205but similar sizes, so higher stellar density.vulnerable to tidal damage.NGC205 is clearly interacting with Andromeda. The random speed of its stars is greater at large radiiBoth dSph and dE appear quite oval rather than round, yet their stars show no pattern of ordered rotation. may no axis of symmetry, probably triaxial.
100 dEs and dSphs NGC 205 and NGC 185 NGC 147 a few patches of dust small amounts of cool gas by its HI & CO emissionmost of the stars date from at least 5Gyr agoBut near the center, a small number is Myr ageGas lost by the old stars may have supplied the raw materialNGC 147no sign of very recent star formationIts nuclear region contains a very few stars in early middle-age, born only a few Gyrs agoIn the outer parts, the overwhelming majority is at least 5Gyr old
101 dEs and dSphsM32 (NGC221, E2)Most luminous satellite of M31No cool gas and no stars younger than a few GyrIts central brightness is the highest (Fig. 4.18)No constant-brightness inner core (HST); the density continues to climb, to >106L⊙pc-3 within the central pc. maybe exist a BHM32’s luminosity within the normal range for dE, but its very high density suggests that M32 is a miniature version of a normal or ‘giant’ elliptical galaxy. Perhaps M32 is only the remnant center of a much larger galaxy.lacks GCs, whereas the less luminous dE do havestars at center are red, approximately as rich as the Suntypical of more massive galaxies (stronger gravity to confine gas)
102 dEs and dSphsThe outer regions of M32 still have an elliptical shape, but its long axis is twisted away from that of inner regions tidal forces of M31 may effect the orbits of the outermost starsslightly flattened, stars orbit in a common direction, but they also have considerable random motionsV/σ— measure the degree of ordered rotationMW’s disk: V/σ~200/30~ coldM32: V/σ~1dSph: V/σ< hotThe stronger the influence of ordered rotation, the more disklike an object must beNot all flattened galaxies rotate fast (section 6.2); but strongly rotating galaxies must always be flattened
103 Dwarf Irregular galaxies Irregular galaxies (L>108L⊙)Messy and asymmetrical appearanceStarbirth occurs in disorganized patches.Even quite small Irrs can produce OB associationsTheir disk have low average surface brightness;so the bright concentrations of young stars stand outDwarf irregular systems (L<108L⊙)diffuse, and ordered rotational motion is much less importantContain gas and recently formed blue starsV/σ~ 4-5 for larger Irr, V/σ<1 in the smallest dIrrabundance is very low, <10% of the sun;the least luminous are the most metal poorbrighter than the dSphs only because of their populations of young starscontain relatively large amounts of gas (seen as HI)gas layer often extends well beyond the main stellar disk (IC 10)
104 Dwarf Irregular galaxies Phoenix & LGS3 are classified between dIrr and dSphAll their stars are more than a few Gyrs old,But, contain a little gas and a few young stars.Fornax dSph has a few young stars (~500 Myr)Carina dSph made most stars in a few discrete episodesBecause of their similar structures, small Irrs may be at an early stage, while dSphs represent the late stages, in the life of a similar type of galaxy.In the dSph (close to MW or M31), gas may have been compressed by interactions with these lager galaxies, perhaps encouraging more stars to form earlier on.By now, there dSphs have used up or blown out all their gas, while the dIrr, perhaps benefiting from a quieter life, still retain theirs.
105 Dwarf Irregular galaxies LMC: a transition class between S & dIrrLMC is basically a rotating disk (like spiral)Its lopsidedness is also common in dIrrLacks regular or symmetric spiral structurerandom motions, V/σ~4 for old starsThe brightest region (central bar) is off center with respect to the diskHI layer has “holes”, similar with IC 10 (Irr) , but smaller in proportion to the galaxy’s size.
106 Dwarf Irregular galaxies Dwarf galaxies are not simply smaller or less luminous versions of bigger and brighter galaxies, but probably developed by different processesdwarf galaxiesdE all have about the same physical size; the core radius is always rc~200 pcMore luminous dwarfs have higher surface brightness (Fig. 4.18)normal or ‘giant’ ellipticalsthe most luminous galaxies are also most diffuse:core radius is so much larger at higher luminositythe central surface brightness is lower in the most luminous systems
108 The galaxies of LG are no longer expanding away from each other The galaxies of LG are no longer expanding away from each other. Their mutual gravitational attraction strong enough to pull the group members back toward each other.MW & M31 are now approaching each other; they will probably come near to a head-on collision within few Gyrs, and could merge to form a single larger system.In Section 5.6 we will see that collisions between group galaxies are fairly common
112 We can use the orbits to make an estimate of the total mass within the LG Assume:all the mass of LG lies in or very close to MW or M31treat these two as point masses m and Mnow separated by r ~ 770kpcclosing on each other with dr/dt ~ -120km/s
113 Equations 4.22 & 4.24 tell us at the present time t0 Since dr/dt<0 sinη<0 π<η<2πnearly circular orbit (e~0): the seed of approach is a very small fraction of the orbital speed Imply a large total massstraight line (e~1) smallest combined massFor r ~770kpc,dr/dt ~ -120km/s,12Gyr<t0<18Gyrwe find m+M~3-5×1012M⊙
116 We have about 5Gyr to go until η=2π. there are no large concentrations of massive galaxies near the LG, that could have pulled on M31 & MW to give them an orbit of high angular momentum e~1In that case, we will come close to a direct collisionSome astronomers believe that many of the giant elliptical galaxies are the remnant of galactic traffic accidents:At earlier time, Universe was denser, collisions were frequentAs the disks crash into each other, their gas is compressed, swiftly converting much of it into starsMaterial from the outer disks will be stripped off as ‘tidal tails’A few Gyrs later, we would be left with a red galaxy, largely free of gas or young stars