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Astro 101 Fall 2013 Lecture 10 Galaxies (part I) T. Howard PowerPoint PPT Presentation


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Astro 101 Fall 2013 Lecture 10 Galaxies (part I) T. Howard. The Galaxy – part I -- Overview. New distance unit: the parsec (pc). Using Earth-orbit parallax, if a star has a parallactic angle of 1", it is 1 pc away. . If the angle is 0.5", the distance is 2 pc. 1

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Astro 101 Fall 2013 Lecture 10 Galaxies (part I) T. Howard

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Astro 101 fall 2013 lecture 10 galaxies part i t howard

Astro 101

Fall 2013

Lecture 10

Galaxies (part I)

T. Howard


Astro 101 fall 2013 lecture 10 galaxies part i t howard

The Galaxy – part I -- Overview


Astro 101 fall 2013 lecture 10 galaxies part i t howard

New distance unit: the parsec (pc).

Using Earth-orbit parallax, if a star has a parallactic angle of 1",

it is 1 pc away.

If the angle is 0.5", the distance is 2 pc.

1

Parallactic angle (arcsec)

Distance (pc) =

Closest star to Sun is ProximaCentauri. Parallactic angle is 0.7”, so distance is 1.3 pc.

1 pc = 3.3 light years

= 3.1 x 1018cm

= 206,000 AU

1 kiloparsec (kpc) = 1000 pc

1 Megaparsec (Mpc) = 10 6 pc


Astro 101 fall 2013 lecture 10 galaxies part i t howard

The Milky Way Galaxy

This is NOT the Milky Way galaxy! It’s a similar one: NGC 4414.


Astro 101 fall 2013 lecture 10 galaxies part i t howard

All-sky projection

1.2, 1.6, 2.2

microns

Atlas Image [or Atlas Image mosaic] courtesy of 2MASS/UMass/IPAC-Caltech/NASA/NSF.


Astro 101 fall 2013 lecture 10 galaxies part i t howard

The Milky Way Galaxy


Astro 101 fall 2013 lecture 10 galaxies part i t howard

Core of Galaxy

(NASA – Spitzer)

(false color)


Astro 101 fall 2013 lecture 10 galaxies part i t howard

How was Milky Way size and our location determined?

Herschel (late 18th century): first map of Milky Way. Sun near

center.

Herschel’s Milky Way drawing

.

Sun

3 kpc


Astro 101 fall 2013 lecture 10 galaxies part i t howard

Shapley (1917) found that Sun was not at center of Milky Way

Shapley used distances to Globular Clusters to determine that Sun was 16 kpc from center of Milky Way. Modern value 8 kpc.


Astro 101 fall 2013 lecture 10 galaxies part i t howard

Take a Giant Step Outside the Milky Way

Artist's Conception

Example (not to scale)


Astro 101 fall 2013 lecture 10 galaxies part i t howard

Artist's Conception

from above ("face-on")

see disk, with spiral and

bar structure, and bulge (halo too dim)

.

Sun

from the side ("edge-on")


Astro 101 fall 2013 lecture 10 galaxies part i t howard

The Three Main Structural Components of the Milky Way

1. Disk

- 30 kpcdiameter

- contains young and old stars, gas, dust. Has spiral structure

- vertical thickness roughly 100 pc - 2 kpc (depending on component. Most gas and dust in thinner layer, most stars in thicker layer)


Astro 101 fall 2013 lecture 10 galaxies part i t howard

2. Halo

- at least 30 kpc across

- contains globular clusters, old stars, little gas and dust, much "dark matter"

- roughly spherical

3. Bulge

- About 4 kpc across

- old stars, some gas, dust

- central black hole of 3 x 106 solar masses

- spherical


Astro 101 fall 2013 lecture 10 galaxies part i t howard

young stars

Gas, dust

(ISM)

Type: SBc

~ 1011 stars

(mass ~ 1012Msun)

Age ~ 13.2 GYr

8.3 kpc


Astro 101 fall 2013 lecture 10 galaxies part i t howard

Morphology of the Milky Way

Disk:- 30 kpcdiameter

- contains young and old stars, gas, dust. Has spiral structure

- vertical thickness roughly 100 pc - 2 kpc

(depending on component. Most gas and dust in thinner layer, most stars in thicker layer)

Halo:- at least 30 kpc across

- globular clusters, old stars, little gas and dust, "dark matter”

- roughly spherical

Bulge:- About 4 kpc across

- old stars, some gas, dust

- central black hole of 3 x 106 solar masses

- spherical


Astro 101 fall 2013 lecture 10 galaxies part i t howard

Galaxies (believed to be) structurally similar

M31

NGC 891

NGC 891

Close up

[STSCI, HST]


Astro 101 fall 2013 lecture 10 galaxies part i t howard

Spiral Structure of Disk

Spiral arms best traced by:

Young stars and clusters

Emission Nebulae

Atomic gas

Molecular Clouds

(old stars to a lesser extent)

Disk not empty between arms, just less material there.


Astro 101 fall 2013 lecture 10 galaxies part i t howard

from above ("face-on")

see disk and bulge (halo

too dim)

Perseus arm

Orion spur

Sun

Cygnus arm

Sagittarius arm

Carina arm

from the side ("edge-on")


Astro 101 fall 2013 lecture 10 galaxies part i t howard

Stellar Orbits

Halo: stars and globular clusters swarm around center of Milky Way. Very elliptical orbits with random orientations. They also cross the disk.

Bulge: similar to halo.

Disk: rotates.


Astro 101 fall 2013 lecture 10 galaxies part i t howard

Globular Clusters

- few x 10 5 or 10 6 stars

- size about 50 pc

- very tightly packed, roughly spherical shape

- billions of years old

Clusters are crucial for stellar evolution studies because:

1) All stars in a cluster formed at about same time (so all have same age)

2) All stars are at about the same distance

3) All stars have same chemical composition


Astro 101 fall 2013 lecture 10 galaxies part i t howard

How Does the Disk Rotate?

Sun moves at 225 km/sec around center. An orbit takes 240 million years.

Stars closer to center take less time to orbit. Stars further from center take longer.

=> rotation not rigid like a phonograph record. Rather, "differential rotation".

Over most of disk, rotation velocity is roughly constant.

The "rotation curve" of the Milky Way


Astro 101 fall 2013 lecture 10 galaxies part i t howard

So if spiral arms always contain same stars, the spiral should end up like this:

Real structure of Milky Way (and other spiral galaxies) is more loosely wrapped.


Astro 101 fall 2013 lecture 10 galaxies part i t howard

Problem: How do spiral arms survive?

Given differential rotation, arms should be stretched and smeared out after a few revolutions (Sun has made 20 already):

The Winding Dilemma


Astro 101 fall 2013 lecture 10 galaxies part i t howard

Proposed solution:

Arms are not material moving together, but mark peak of a compressional wave circling the disk:

A Spiral Density Wave

Traffic-jam analogy:


Astro 101 fall 2013 lecture 10 galaxies part i t howard

Now replace cars by stars. The traffic jams are due to the stars' collective gravity. The higher gravity of the jams keeps stars in them for longer. Calculations and computer simulations show this situation can be maintained for a long time.

Traffic jam on a loop caused by merging

Not shown – whole pattern rotates


Astro 101 fall 2013 lecture 10 galaxies part i t howard

Gas clouds pushed together in arms too => high density of clouds => high concentration of dust => dust lanes.

Also, squeezing of molecular clouds initiates collapse within them => star formation. Bright young massive stars live and die in spiral arms. Emission nebulae mostly in spiral arms.

So arms always contain same types of objects. Individualobjects come and go.


Astro 101 fall 2013 lecture 10 galaxies part i t howard

observed curve

Milky Way Rotation Curve

Curve if Milky Way ended where radiating matter pretty much runs out.

90% of Matter in Milky Way is Dark Matter

Gives off no detectable radiation. Evidence is from rotation curve:

10

Solar System Rotation Curve: when almost all mass at center, velocity decreases with radius ("Keplerian")

Rotation

Velocity

(AU/yr)

5

1

30

1

10

20

R (AU)


Astro 101 fall 2013 lecture 10 galaxies part i t howard

Rotation curves of many normal galaxies are similar

-- allows galaxy mass to be estimated


Astro 101 fall 2013 lecture 10 galaxies part i t howard

Not enough radiating matter at large R to explain rotation curve => "dark" matter!

Dark matter must be about 90% of the mass!

Composition unknown. Probably mostly exotic particles that hardly interact with ordinary matter at all (except gravity). Small fraction may be brown dwarfs, dead white dwarfs.

Most likely it's a dark halo surrounding the Milky Way.

Mass of Milky Way

6 x 1011 solar masses within 40 kpc of center.


Astro 101 fall 2013 lecture 10 galaxies part i t howard

Galactic Rotation curve – velocity vs. galactocentric radius


Astro 101 fall 2013 lecture 10 galaxies part i t howard

Rotation curve of M31

The M31 major axis mean optical radial velocities and the rotation curve, r <120 arcmin, superposed on the M31 image from the Palomar Sky Survey. Velocities from radio observations are indicated by triangles, 90< r <150 arcmin. Rotation velocities remain flat well beyond the optical galaxy, implying that the M31 cumulative mass rises linearly with radius.

(Image by Rubin and Janice Dunlap.)

Source: Physics Today, Dec. 2006


Astro 101 fall 2013 lecture 10 galaxies part i t howard

Several alternative explanations

  • Matter is “missing” (i.e., can’t be seen – nonluminous)

    • Probably view most prevalent is that of Cold Dark Matter (CDM)

  • What can’t it be?

    • Dust – no. Why??

    • Gas – no. Why??

  • What can it be?

    • Normal (i.e., baryonic) matter

      • MACHOs – MAssive Compact Halo Objects

        • White dwarfs, Red/Brown dwarfs

        • Neutron stars, BHs

    • Non-baryonic matter

      • WIMPs – Weakly Interacting Massive Particles

      • Weak interactions w/baryonic matter, no significant interactions w/light


Astro 101 fall 2013 lecture 10 galaxies part i t howard

Ruling Explanations In or Out

  • White & red dwarfs – HST can’t find enough of them

  • BHs – large – would probably leave signatures, disruption of Globulars in the halo  not seen

  • BHs – small – can’t be excluded, but how did they get there?

    • Compact gas clouds in halo  why not seen? Why don’t they form stars?

  • Cosmology theory predicts baryonic matter should be ~ 4% of the critical density of universe; luminous matter seems to be only about 0.8%

    • But when hot matter (i.e., intracluster gas) added  probably close

    • Although, error bars on this are large

  • Note, “CDM” vs. “HDM” (cold vs. hot) means:

    • Non-relativistic vs. relativistic


  • Astro 101 fall 2013 lecture 10 galaxies part i t howard

    More on Dark Matter

    • Dark matter thought to be necessary in cosmology theory to explain clumpy distributions of matter w/ enough mass  galaxies form

      • Could have been either CDM or HDM

    • Usual predictions of CDM candidates require modifications or extensions to Standard Model of particles

    • Non-baryonic candidates

      • Neutrinos (neutrino oscillation  implies neutrino mass)

      • WIMPs – hypothetical

        • Possibly “heavy” neutrinos or something equally exotic

      • Axions


    Astro 101 fall 2013 lecture 10 galaxies part i t howard

    MOND – MOdified Newtonian Dynamics

    • Introduced c. 1983, M. Milgrom (Weizmann Inst., Israel)

    • ad hoc phenomenological fit to explain two things:

      • Galactic “flat” rotation curves, and

      • Tully-Fisher relation for spirals: M = (const.)*Va, a ~ 4

    • Modifies Newtonian force for very small accelerations

    • Successfully predicts several astrophysical observations

    • Doesn’t require “missing mass”

      •  but some attempts made to derive it as an effect of CDM

      •  not completely successful

    • Not tied to any physical theory, i.e., a “fit to observations”

      • Some attempts to modify Newtonian gravity, inertia, or GR

      • Not complete

    • Not entirely popular but not ruled out; has a few vocal adherents


    Astro 101 fall 2013 lecture 10 galaxies part i t howard

    Massive Black Holes

    -- leading theory to

    explain the emission

    and properties of many

    active galaxies (AGNs)

    -- Central BH may be

    Billions of solar masses

    -- Accretion disk made

    of large clouds of gas

    And dust

    -- X-rays and gamma

    rays from BH are

    converted to longer

    wavelengths by clouds


    Astro 101 fall 2013 lecture 10 galaxies part i t howard

    Milky Way -- Evidence for Central BH

    • Dynamical – orbits of stars near

    • Galactic center, esp. S2, indicate

    • Central mass of ~ 4 million Msun

    • Only a BH dense enough to have

    • this much mass in such a small

    • volume of space

    • Corresponds to location of

    • radio source Sgr A

    • Smaller compact source Sgr A*

    • at center, diameter 0.3 AU

    • Observations also in x-rays

    • and gamma rays


    Astro 101 fall 2013 lecture 10 galaxies part i t howard

    Milky Way – Central Black Hole

    Central area

    (enlarged) 

    Center of Galaxy and

    Constellation Sagittarius (Sgr)


    Astro 101 fall 2013 lecture 10 galaxies part i t howard

    Other views of the

    Galactic Center

    Near-infrared

    X-rays (Chandra)

    infrared


    Astro 101 fall 2013 lecture 10 galaxies part i t howard

    “flare” as BH accretes gas(period ~ 48 hr)

    Hot gas (10 million K)

    BH area


    Astro 101 fall 2013 lecture 10 galaxies part i t howard

    Galaxies


    Astro 101 fall 2013 lecture 10 galaxies part i t howard

    Early drawings of

    nebulae by Herschel (1811). Stars, gas or

    both? Distances?

    First “spiral nebula” found in 1845 by the Earl of Rosse. Speculated it was beyond our Galaxy.

    1920 - "Great Debate" between Shapley and Curtis on whether spiral nebulae were galaxies beyond our own. Settled in 1924 when Hubble observed individual stars in spiral nebulae.


    Astro 101 fall 2013 lecture 10 galaxies part i t howard

    The Variety of Galaxy Morphologies


    Astro 101 fall 2013 lecture 10 galaxies part i t howard

    More on bars…

    Milky Way schematic

    showing bar

    Another barred galaxy

    A bar is a pattern too,

    like a spiral.


    Astro 101 fall 2013 lecture 10 galaxies part i t howard

    bulge less prominent,

    arms more loosely wrapped

    Irr

    increasing apparent flatness

    disk and large bulge, but no spiral

    Galaxy Classification

    Hubble’s 1924 "tuning fork diagram"

    Spirals Ellipticals Irregulars

    barred unbarred E0 - E7 Irr I Irr II

    SBa-SBc Sa-Sc "misshapen truly

    spirals" irregular


    Astro 101 fall 2013 lecture 10 galaxies part i t howard

    bulge less prominent,

    arms more loosely wrapped

    Irr

    increasing apparent flatness

    disk and large bulge, but no spiral

    Still used today. We talk of a galaxy's "Hubble type"

    Milky Way is an SBbc, between SBb and SBc.

    What the current structure says about a galaxy’s evolution is

    still active research area.

    Ignores some notable features, e.g. viewing angle for ellipticals, number of spiral arms for spirals.


    Astro 101 fall 2013 lecture 10 galaxies part i t howard

    Sa vs. Sc galaxies

    Messier 81 – Sa galaxy

    Messier 101 – Sc galaxy


    Astro 101 fall 2013 lecture 10 galaxies part i t howard

    Irr I vs. Irr II

    Irr I (“misshapen spirals”)

    Irr II (truly irregular)

    bar

    poor beginnings

    of spiral arms

    Large Magellanic Cloud

    Small Magellanic Cloud

    These are both companion galaxies of the Milky Way.


    Astro 101 fall 2013 lecture 10 galaxies part i t howard

    Ellipticals

    Similar to halos of spirals, but generally larger, with many more stars. Stellar orbits are like halo star orbits in spirals.

    Stars in ellipticals also very old, like halo stars.

    An elliptical

    Orbits in a spiral


    Astro 101 fall 2013 lecture 10 galaxies part i t howard

    A further distinction for ellipticals and irregulars:

    Giant vs. Dwarf

    1010 - 1013 stars 106 - 108 stars

    10's of kpc across few kpc across

    Dwarf Elliptical NGC 205

    Spiral M31

    Dwarf Elliptical M32


    Astro 101 fall 2013 lecture 10 galaxies part i t howard

    In giant galaxies, the average elliptical has more stars than the average spiral, which has more than the average irregular.

    What kind of giant galaxy is most common?

    Spirals - about 77%

    Ellipticals - 20%

    Irregulars - 3%

    But dwarfs are much more common than giants.


    Astro 101 fall 2013 lecture 10 galaxies part i t howard

    ASTROPHYSICAL “DEMOGRAPHICS”

    • Local solar neighborhood, typical stellar distances ~ 1pc (3.26 ly)

      • -- 50% are double/multiple systems

      • -- Most in near neighborhood (local arm) are cooler/fainter

    • Most of Galaxy within ~ 20 kpc (1011 - 1012 stars)

    • 800 kpc to M31

    • Local group (about 20), radius ~ 1 Mpc

    • Local supercluster (about 2500), distance ~ 15 Mpc to Virgo cluster

    • Several other galaxy clusters to ~75 Mpc

    • Increasing numbers of AGNs out to radius ~ 100 Mpc

    • QSOs to extent of observable universe, ~10000 Mpc


    Astro 101 fall 2013 lecture 10 galaxies part i t howard

    SIZE SCALES

    MILKY WAY

    Jupiter’s Orbit

    Radius 5.2AU

    LOCAL GROUP

    DISTANCES TO:

    GALACTIC CENTER 8.5 Kpc

    LARGE MAG. CLOUD 55 Kpc

    M31 ~ 800Kpc

    LOCAL ISM


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