galaxies in the universe chapter 4 our backyard: the local group yin jun 09/20/06

Galaxies in the Universe

Chapter 4 Our backyard: the Local Group

Yin Jun 09/20/06

Outline

Brief introduction4.1 Satellites of the Milky Way4.2 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

Brief Introduction

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 : M32Irregular galaxies, less luminous dwarf irregular

s ,dwarf ellipticals, dwarf spheroidals

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.

M31M32M110 Andromeda subgroup NGC 147NGC185M33IC10NGC 6822IC 1613WLMLMCSMC Galaxy subgoupGalaxy Only galaxies with Mv<-14.0 listed

Sub-Structure in the Local Group

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.

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.

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.

4.1 Satellites of the Milky Way

MembersMain companion: LMC & SMC

easily visible to the naked eye gas rich and forming new stars and star clus

ters in abundance contain variable stars and calibrate for use a

s “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

LMC

SMC Image by Roger Smith/NOAO/AURA/NSF

The Magellanic CloudsLarge 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.

HI gas distribution in LMC

The Magellanic CloudsSmall 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

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 bri

ght in the ultraviolet. Star-forming regions are spread throughout,

and they are rich in hydrogen gas, the raw material of star formation.

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.


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


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

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

(Putman et al. 1998) (Gardiner & Noguchi 1996)

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.

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 th

ose in Galactic GCs, and from its variable stars)

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.

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.

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

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.

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 lu

minosities, 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.

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 >8

Gyr 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.

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 I dSph( 狮子座矮星系 I)Leo II dSph( 狮子座矮星系 II)

Sextans dSph ( 六分仪座矮星系 ) Carina dSph ( 船底座矮星系 )

Ursa Minor dSph ( 小熊座矮星系)

Draco dSph ( 天龙座矮星系 )

Dwarf spheroidal galaxies In the past, often thought as merely large, low density globul

ar clusters detailed studies over the last 20 years or so have revealed th

at 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.

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.

Dwarf spheroidal galaxies There is evidence that these satellite galaxies can be disrup

ted 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 .

Dwarf spheroidal galaxiesStellar random speeds are similar to GC, but star

s 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 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).

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 40rc L M=?mCentauri GC 3σr rc l =5L m M ≈ 3σr

2(40rc)/G

m ≈ 3(3σr)2rc/G M/m=40/9≈4 M/L=20m/l

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 o

n 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


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

)'('2

1 2 xvE effJ

)'('2

1 2 xvE effJ

2)'(2

1)'()'( xxxeff

LEvxtxvxvE effJ

)(),(2

1)(

2

1 22


Simplest problem

mSatellite

Mmain galaxy

center of mass

x=DM/(M+m)

22

2)(

mM

DMx

x

Gm

xD

GMxeff

DΩx


Φeff has three maxima: Lagrange points

mM

DMx

x

Gm

xD

GM

xeff 2

22)(0

L2 L1L3

xm M

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.


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, th

e 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; i

f the outer envelope of one star expands beyond L1, its mass begins to spill over onto the other.

Jrx 3/1

3

mM

mDrJ

Problem 4.3

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

earth sun

moon

1 AU

rEM rSM


When M>>m, the mean density in a sphere of radius rJ surrounding the satellite, 3m/4πrJ

3, 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.

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>>a

H 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+aH

2) (2.9)

M(<r)=∫ 4πρH(r)r2dr= rVH2/G

3/1

)(2

DM

mDrJ


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.

Problem 4.5

The 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.

3/1

3

mM

mDrJ

4.2 Spirals of the Local Group

Three spiral galaxies: MW, M31, M33M31 (770kpc) is the most distant object that can

easily be seen with the unaided eyeM33 (850kpc) is much harder to spot Comparing these three systems with each

other, we see what properties spiral galaxies have in common, and how they differ.

The Andromeda galaxy

Compare with MW ~50% more luminous; disk scale length hR~6-7kpc, twice as large as MW’s rotates faster: V(R)~260km s-1, 20-30% higher than M

W’s ~300 known GCs, over twice as many as MW’s has its own satellite galaxies: M32, 3 dwarf ellipticals, a

nd at least 6 dwarf spheroidals: Andromeda I, II, III, V, VI, VII, Cassiopeia dSph


Bulge Larger in proportion than MW’s, providing 30-40% of th

e 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


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 friction Unlike MW, the nucleus of M31 is impressively free of

either gas or dust


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


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 MW

Just 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.


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 MW

Gas extends to larger radii than the stellar disk, as in the Milky Way


Holes In the region of the spiral arms, high-resolution maps s

how holes in the HI disk, up to a kpc across; At their edges, shells containing 103-107M⊙ of dense HI

gas are moving outward at 10-30 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.


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 woun

d 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.

M33: a late-type spiral

M33 is definitely an Sc or Scd galaxy Bulge is tiny The spiral arms: more open , not so smooth, c

onsist mainly of bright blue concentrations of recently formed stars.

smaller and much less luminous (5.5×109L⊙) Scale length ~ 1.7kpc , small Rotation speed V(R) rises only to 120 km s-1


Complex network of loops, filaments, and shells, like LMC

SN 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


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, ~30kpc

Little CO emission • Lack of molecular gas or• A 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


At 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 0.4pc The density of stars exceeds 107L⊙pc-3

Nucleus 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 LG An unusual optical spectrum, with strong emission lines

of nitrogen.


M33 is only 2 or 3 times more luminous than the LMC

It has a much more symmetrical spiral pattern.Low-L galaxies are in general more likely than

larger systems to resemble LMC have a strong central bar brightest parts of the galaxy lying off center form the

disk.

However, the morphology clearly depends on factors other than the galaxy’s luminosity alone

4.3 How did the Local Group galaxies form ?

A picture for the formation

300 000 years after Big Bang, photons no longer had enough energy to ionize H and He

nuclei + 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 galaxies

If its gravity was strong enough, a region that was denser than average collapse inward

The denser the gas, the earlier cosmic expansion would halt, giving way to contraction.

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 rotation

Clouds within each protogalaxy collide with each other lose part of energy fall inward rotation increases (angular momentum approximately conserves)

Earliest stars

Observe 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.

Making the Milky Way

The first star born: pure H and He lived: in a smaller lumps of gas, with masses perhaps 106-108M⊙

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 clusters Stars in each clusters generally have very closely the sam

e composition we think that GCs formed in smaller parcels of gas, where the nucleosynthetic products of earlier stars had been thoroughly mixed.

Stars

Making the Milky Way When gas clouds ran into each other, fell together to for

m 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

Halo (GCs)


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 system

8-10Gyr ago, earliest thin-disk stars were born, heavy elements enriched the gas, perhaps to 10-20% of the solar abundance.

Disk


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 constant

this 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.

Disk gas


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.

Bulge


The 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 developing

Once the dense central bulge formed, the gravitational force of the whole Galaxy would have helped it to hold onto its gas, trap debris from SN

the 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.


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.

Dark matter


The MW is still under construction today Stars of the Sagittarius dSph galaxy are being added to MW’s

halo Near the Sun, groups of young metal-poor halo stars have bee

n 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.

The buildup of heavy elements

A “clock” for galactic aging AMR: old disk stars in general contain little iron;

incomplete mixing could explain the large dispersion OCs of thin disk : younger and more metal rich

stars and GCs of the thick disk: in the middle

GCs of the halo: oldest & poorest in metal

Pop I: young metal-rich stars in disk

Pop II: old metal-poor stars in bulge & halo

oversimplification: The bulge of M31 & MW (at lease a few Gyrs ) are metal rich dIrr galaxies and the outer parts of normal spirals contain young

metal-poor stars born within the past 100Myr.

The buildup of heavy elements One-zone instantaneous recycling model

Gas is well mixed, same composition everywhere Stars return the nuclear fusion products rapidly, faster than the ti

me to form a significant fraction of the stars. Close-box model: no gas escapes, no gas infall

Mg(t): the mass of gas in the galaxy at time t

Ms(t): the mass in low-mass stars and remnants of high-mass stars. The matter in these objects remain locked within them

Mh(t): the total mass of elements heavier than He in gas

Metal abundance: Z(t)=Mh(t)/Mg(t)

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


Heavy element in the interstellar gas alters:

△Mh=p M△ s-Z M△ s=(p-Z) M△ s

Metallicity of the gas increases by an amount:

for close-box, M△ s+ M△ g=0, so it will be Z=-p M△ △ g/Mg

If p is independent of Z, we can integrate the equation:

g

gss

g

h

M

MMZMp

M

MZ

)(

Z ’Ms△

Z Ms△+…

Z( ’Ms- Ms)△ △+p Ms△

)(

)0(ln)0()(

tM

tMptZtZ

g

g

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)

pZZ

gs MZM)0(

exp1)0()(

The buildup of heavy elements Once all the gas is gone, the mass of stars with metallicit

y between Z and Z+ Z△

For 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.

ZZdZ

ZdM pZtZ

s )0()(

exp)(

The buildup of heavy elements In other cases, the simple model is clearly wrong.

GC: no gas, and stars are all metal-poor• These 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 30-100 times lower than Galactic bulge

• Maybe it formed very few massive stars, so produced only few metals

The buildup of heavy elements Effective yield p: take into account metals lost.

For GC, p~0 p is always less than the true yield of metal produced.

Solar neighborhood: Stars: 30-40 M⊙pc-2

Gas: 13 M⊙pc-2

Total: ~50 M⊙pc-2

The 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.

The buildup of heavy elements G-dwarf problem

Close-box model:

Observation: a sample of 132 G-dwarf stars in solar neighborhood• 33 below 25% of the solar fraction of iron (25%)• 1 below 25% of the solar fraction of oxygen (<0.01%)

Possible solution Pre-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


Stars with low metal abundance have more oxygen relative to the amount of iron than stars like the Sun

These elements are made in stars of differing mass SN II (>10M⊙):

release back mainly lighter element such as O, S, Mg Most 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 reasonable Not 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.


SN Ia (binary system): main source of iron heats the interior, triggering nuclear burning, blows the star apart No remnant is left; all the Fe, Ni, and elements of similar atomic w

eight are released back to the interstellar gas. Stars that explode as SN Ia only do so at ages of a Gyr or more. S

o 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.

4.4 Dwarf galaxies in the Local Group

2 main types of dwarf galaxies Dwarf ellipticals & much diffuse dwarf spheroidals

Almost all the stars are at least a few Gyrs old; Contain little gas to make any new stars.

Dwarf irregulars Diffuse systems; Tiny, gas-rich, active star formation, a profusion of recently forme

d blue stars

All the dEs and most of dSphs orbit either MW or M31 But many of dIrrs are ‘free fliers’

dEs and dSphs dSph:

not much more luminous than a GC (table 4.2) but so diffuse as to be almost invisible on the sky

dE: more luminous versions of the dSph (L>3×107M⊙ or MV<-14)representation: 3 M31’s satellites: NGC147, NGC185, NGC205 but 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 radii

Both 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.

dEs and dSphs NGC 205 and NGC 185

a few patches of dust small amounts of cool gas by its HI & CO emission most of the stars date from at least 5Gyr ago But near the center, a small number is 100-500 Myr age

Gas lost by the old stars may have supplied the raw material

NGC 147 no sign of very recent star formation Its nuclear region contains a very few stars in early middle-age, bo

rn only a few Gyrs ago In the outer parts, the overwhelming majority is at least 5Gyr old

dEs and dSphs M32 (NGC221, E2)

Most luminous satellite of M31 No cool gas and no stars younger than a few Gyr Its central brightness is the highest (Fig. 4.18) No constant-brightness inner core (HST); the density continues t

o climb, to >106L⊙pc-3 within the central pc. maybe exist a BH

M32’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 have stars at center are red, approximately as rich as the Sun

typical of more massive galaxies (stronger gravity to confine gas)

dEs and dSphs The outer regions of M32 still have an elliptical shape, bu

t its long axis is twisted away from that of inner regions tidal forces of M31 may effect the orbits of the outermost stars

slightly flattened, stars orbit in a common direction, but they also have considerable random motions

V/σ— measure the degree of ordered rotation MW’s disk: V/σ~200/30~7 cold M32: V/σ~1 dSph: V/σ<1 hot

The stronger the influence of ordered rotation, the more disklike an object must be

Not all flattened galaxies rotate fast (section 6.2); but strongly rotating galaxies must always be flattened

Dwarf Irregular galaxies Irregular galaxies (L>108L⊙)

Messy and asymmetrical appearance Starbirth occurs in disorganized patches. Even quite small Irrs can produce OB associations Their disk have low average surface brightness; so the bright concentrations of young stars stand out

Dwarf irregular systems (L<108L⊙) diffuse, and ordered rotational motion is much less important Contain gas and recently formed blue stars V/σ~ 4-5 for larger Irr, V/σ<1 in the smallest dIrr abundance is very low, <10% of the sun;

the least luminous are the most metal poor brighter than the dSphs only because of their populations of young stars contain relatively large amounts of gas (seen as HI)gas layer often extends well beyond the main stellar disk (IC 10)

Dwarf Irregular galaxies Phoenix & LGS3 are classified between dIrr and dSph

All 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 episodes

Because 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.

Dwarf Irregular galaxies

LMC: a transition class between S & dIrr LMC is basically a rotating disk (like spiral) Its lopsidedness is also common in dIrr

• Lacks regular or symmetric spiral structure

• random motions, V/σ~4 for old stars

• The brightest region (central bar) is off center with respect to the disk HI layer has “holes”, similar with IC 10 (Irr) , but smaller in proporti

on to the galaxy’s size.

Dwarf Irregular galaxies

Dwarf galaxies are not simply smaller or less luminous versions of bigger and brighter galaxies, but probably developed by different processes dwarf galaxies

• dE all have about the same physical size; the core radius is always rc~200 pc

• More luminous dwarfs have higher surface brightness (Fig. 4.18) normal or ‘giant’ ellipticals

• the most luminous galaxies are also most diffuse:

• core radius is so much larger at higher luminosity

• the central surface brightness is lower in the most luminous systems

4.5 Past and future of the Local Group

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

We can use the orbits to make an estimate of the total mass within the LGAssume: all the mass of LG lies in or very close to MW or M31 treat these two as point masses m and M now separated by r ~ 770kpc closing on each other with dr/dt ~ -120km/s

Equations 4.22 & 4.24 tell us at the present time t0

Since dr/dt<0 sinη<0 π<η<2π nearly circular orbit (e~0): the seed of approach is a ver

y small fraction of the orbital speed Imply a large total mass straight line (e~1) smallest combined mass For r ~770kpc,dr/dt ~ -120km/s,12Gyr<t0<18Gyr

we find m+M~3-5×1012M⊙

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~1

In that case, we will come close to a direct collision Some astronomers believe that many of the giant elliptical gala

xies are the remnant of galactic traffic accidents: At earlier time, Universe was denser, collisions were frequent As the disks crash into each other, their gas is compressed, swiftly conve

rting much of it into stars Material 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

Thank you!

galaxies in the universe chapter 4 our backyard: the local group yin jun 09/20/06

Documents

dwarf galaxies

local group slide

giant galaxies

stars younger

local group galaxies

dwarf spheroidal galaxies

dwarf spheroidals slide

s0 galaxies