The Milky Way

Our home galaxy, full of stars, gas and mysterious dark mattermysterious dark matter

We decompose it into a disk and a halo and a few other parts

Almost a View of our Milky Way

NGC 4526, a spiral galaxy like the MW but about 30 Mpc away: it has a similar size, luminosity and structure

Edge-on “View” and View of MW

Key Parts of the Milky Way

• HALO Contains most globular clusters, and most Pop II stars; roughly 30 kpc (105 lt-yr) in diameter.

• THICK DISK roughly 5 kpc thick, and 30 kpc in diameter; contains Pop I stars (but low density).

• THIN DISK ~500 pc thick: contains MOST stars; includes spiral arms and great majority of luminosity.

• DUST DISK only 50 pc thick; new stars are born in the molecular clouds found within this very thin disk.

• SPIRAL ARMS are wrapped within the dust/thin disk; contain almost all hot, luminous (O and B) stars.

Overall Structure of the Milky Way

Inner Parts

• GALACTIC BULGE roughly 2 kpc in radius around center; highest concentration of stars, including

many globular clusters.• GALACTIC CENTER in

the direction of the constellation Sagittarius, some 8 kpc from the Solar System (SS).

• Multiwavelength Milky Way

MAPPING the MILKY WAY

• Dust, mainly in molecular clouds, shrouds the Disk; we see few stars beyond 2 kpc from SS in the thin disk, where the number of stars is much greater

• Originally astronomers thought the Milky Way WAS the Whole Universe & SS central to it (because of visible light extinction by dust)

• Location of Globular Clusters in halo implied center towards Sagittarius and SS actually towards one side in early 20th century.

• Atomic Hydrogen gas sends 21 cm radio waves that allow us to map the far side of the galaxy, and the outer reaches where there are few stars

Our Nearest Big Neighbor, M31, the Andromeda Galaxy

Andromeda, about 30 kpc across down to nucleus only 15pc

A Limited Conception of the MW

Herschel’s “map” of the Galaxy from star counts;More in the MW plane, but thought the Sun near the center and got the size too small: didn’t understand dust

Distribution of Globular Clusters

How do stars orbit in our galaxy?

Stars in the disk all orbit in the same direction with a little up-and-down motion

Orbits of stars in the bulge and halo have random orientations

Thought Question

Why do orbits of bulge stars bob up and down?

A. They’re stuck to interstellar mediumB. Gravity of disk stars pulls toward diskC. Halo stars knock them back into disk

Thought Question

Why do orbits of bulge stars bob up and down?

A. They’re stuck to interstellar mediumB. Gravity of disk stars pulls toward diskC. Halo stars knock them back into disk

Distances from Variable Stars

• Certain stars act as “standard candles” with fixed LUMINOSITY (M)

• So, MEASURED BRIGHTNESS (m) lets us compute their distances.

• RR Lyrae stars all have similar absolute magnitudes (around -0.5 to -1.5). Their periods are all less than one day. They can be seen in nearby galaxies outside the MW.

• Can be seen out to 10’s of Mpc. Cepheid variables are even more luminous, and have longer periods (1-50 days).

Light Curves of RR Lyrae and WW Cgyni (a Cepheid Variable)

The Instability Strip

• Both RR Lyrae and Cepheid variable stars are post-main sequence stars (subgiants and giants) whose atmospheres pulsate strongly due to opacity variations

Standard Candles via Period-Luminosity Relations

Variable Stars and

the Distance Ladder:

They take us out to

moderately distant

galaxies

Motions Near the Sun

• Measure Doppler shifts of many stars to get velocities near the Sun

• Motions are faster closer to the galactic center so, on the average, stars ahead of Sun and inside get ahead (redshifted) while those behind and outside fall behind (also redshifted)

Gas Velocities from 21 cm Lines

Rotation Gives Mass Distribution

• ROTATION CURVES plot the stellar or gas velocity (v) against distance from center of galaxy (r). Mostly measured by 21 cm emission from H I gas

• Rigid body curve: v r (like CD in a player or a rigid arm swinging)

• Keplerian curve: v 1/r1/2 most mass centrally concentrated. This would be like Mercury orbiting fastest and Neptune slowest around the Sun.

• Flat curve: v constant M rises significantly; specifically: M r

Galactic Rotation Curve

DARK MATTER SEEMS TO

REALLY MATTER • For the MW a FLAT rotation curve implies there is

MISSING MASS or • DARK MATTER that isn't Stars or Gas seen out to 20

kpc from galactic center.• Essentially ALL other Spiral galaxies for which Rotation

Curves can be measured ARE ALSO FLAT, so DM is EVERYWHERE!

• More evidence for DM comes from CLUSTERS OF GALAXIES; we'll discuss this later.

• Yet more evidence comes from COSMOLOGICAL measurements of the structure of the universe as a whole – as discussed by Mike Kavic last week

Dark Matter Candidates

• Missing Red Dwarfs (not enough; next slide)• Planets or Brown Dwarfs on the loose (unlikely to be

enough: gravitational lensing) • Isolated black holes (very unlikely to be enough) • Massive neutrinos (evidence for then having a tiny mass

makes them a good candidate, but very unlikely that they dominate the DM)

• SnowballsSnowballs (very difficult to form them, unpopular choice) • As yet undiscovered particles; (Axions; Supersymmetric

particles; WIMPs: Weakly Interacting Massive Particles) MOST popular now BUT no convincing detections yet.

Few Red Dwarfs Seen in Globular Cluster 47 Tucanae

Gravitational Lensing by Brown Dwarfs

• Temporary increase in star’s brightness due to a dark mass moving in front

• A rare detection is shown in the right

Key Properties of MW

• We are about r = 8 kpc from the center.• We orbit the center at v = 220 km/s• That makes for a galactic year (circumference

divided by velocity) of • (2 ) x 8,000 x (3.0857 x 1013 km) / 220 km/s =

7.1 x 1015 s = 2.24 x 108 yr. • So, roughly 225 million years is ONE

GALACTIC YEAR. • How old is the solar system in galactic years?• At nearly 4.6 billion years of age, the SS is only

about 20 galactic years old!

Weighing the Galaxy

Orbital speed depends on mass inside at a particular radius.This can be used with any galaxy for which motions can be measured. Mass vs. Distance Applet

Orbital Velocity Law

• The orbital speed (v) and radius (r) of an object on a circular orbit around the galaxy tells us the mass (Mr) within that orbit€

Mr =r × v 2

G

Mass of the Milky Way

• Mgal r3/P2 from Newton’s laws.• This is dominated by DARK MATTER, but total

mass can be estimated by the velocity of stars at different distances.

• Out to solar distance (about 8 kpc) the mass is about 1 x 1011 M (mostly stars)

• Out to ~15 kpc, (the visible radius) a good estimate for the mass is nearly 4 x 1011 M (now mostly DM).

• Out to about 70 kpc (> 90% dark matter): 2 x 1012 M

Spiral Galaxies

M101 is seen face on (similar to MW); NGC 4565 is edge on

Stellar Populations

• Pop I Stars: Have compositions like the sun: 70% H, 28% He, 2% "metals"; these metals are mostly Carbon, Oxygen and Nitrogen

• Use the CNO cycle to generate Main Sequence energy if M > 1.5 M

• Are almost all younger than 8 billion years.• Most are in the thin disk; the rest are in the

thick disk.

Stellar Populations, 2

• Pop II Stars: Have compositions with much less heavy elements than the Sun: 72%H, 28% He, 0.2% metals is typical

• Use the pp-II on the MS if M > 1.5 M • Are almost all older than 8 billion years.• Most are in the halo and galactic bulge; however plenty pass

through the thick disk too.• Pop III Stars: The very earliest born; • they have essentially NO METALS, very massive • formed from only H and He made in the BIG BANG;• Only a few possible detections.

Thought question

If the rotation curve of the MW looked like

v ~ 1/r½ we would think that:

A)There is more dark matter than we now believe is present

B)There is the same amount of DM

C)There is much less DM present.

Thought question

If the rotation curve of the MW looked like

v ~ 1/r½ we would think that:

A)There is more dark matter than we now believe is present

B)There is the same amount of DM

C)There is much less DM present.

Spiral Arms

• Fundamentally produced by Gravitational Perturbations to the galactic disk

• Produced either by a CENTRAL BAR or by a COMPANION GALAXY

• TWO ROUTES to Spiral Arms: • First, DENSITY WAVES • Think: traffic jam in space: • Second, STAR FORMATION CHAIN

REACTION• Neither provides a full explanation!

Density Wave Analogy to Traffic Jam• Small extra density holds stars/gas up, like a broken down truck

on the side of the road --Molecular clouds compressed, stars born --This best explains beautiful smooth

("grand design") spirals

Density Waves Can Make Spiral Arms

NGC 1566 shows density wave features with dust lanes and nearby young star clusters

Spiral arms are waves of star formation

1. Gas clouds get squeezed as they move into spiral arms

2. Squeezing of clouds triggers star formation

3. Young stars flow out of spiral arms

So Can Stochastic Star Formation

• Random birth of Massive Stars • Their SN explosions compress nearby clouds

& make new stars • Differential rotation of galaxy yields spiral

appearance by streching the stars out• This best explains "rattier", broken-up spirals

(like the Milky Way, though some Density Wave contribution is OK.)

Stochastic or Self-Propagating Star Formation

Spiral Arm Facts

• Typically, spiral arms have dark, DUSTY CLOUDS on their edges.

• Some of these are compressed enough to form bright O-B STAR CLUSTERS, which can in turn ionize and light up parts of the clouds into H II regions.

• Stars older than about 20-30 Myr are usually outside the arms.

• NOTE: the arms are barely denser in stars than the rest of the disk but they stand out because they have nearly all the hot, bright, young stars.

• Spiral Arms Applet

Halo Stars: 0.02-0.2% heavy elements (O, Fe, …), only old stars

Disk Stars: 2% heavy elements, stars of all ages

Halo stars formed first, then stopped

Disk stars formed later, kept forming

THE GALACTIC CENTER

• Until the past 20 years, it was very mysterious, mainly because: VISIBLE light CANNOT PENETRATE all the DUST in the DISK

• UV light is absorbed even more strongly!• Confused by stars between us and the Center

New Tools: Radio

• RADIO maps show H I gas, supernova remnants, & synchrotron emission from filaments of strong magnetic fields

Infrared Shows Fast Moving Stars

• penetrates dust much better,

• IR from tall mountains, planes, satellites

• some emission from very center and also quite a few INDIVIDUAL STARS (RGs, mostly)

• over the past decade the ORBITS of some such RGs have been determined

Stars appear to be orbiting something massive but invisible … a black hole?

Orbits of stars indicate a mass of about 4 million MSun

IR Movie of Stellar Orbits

• Milky Way Center Zoom

This is from the UCLA/Keck telescope group based on their measurements over 15 years.Similar results have come from a Max-Planck Institute for Extraterrestrial Physics in Germany, Caltech group.

Movie of stellar orbits aroud Galactic Centerl

X-rays from the Galactic Center

• Since the earth's atmosphere blocks them, we need SATELLITES!

• The lower energy (soft) X-rays are absorbed by gas,• BUT Higher energy (hard) X-rays can penetrate out

to us; • Some are from Supernova Remnants near the

Galactic Center, but a modest amount from the very center, also seen as the strong radio source: Sagittarius A*

• Q.: WHAT DO ALL THESE MEASUREMENTS TELL US?

A.: THERE'S A SUPERMASSIVE BLACK HOLE in the Galactic Center, Sgr A*

• gas moving very fast (from radio measurements)• orbits of some nearby RGs very fast; those further away

are slower; • X-rays consistent with weak emission from accretion disk• MSMBH = 3.6 x 106 M�• Right now, little mass falls into the SMBH in the MW, BUT

in the past it probably grew fast and was VERY LUMINOUS.

• Such an ACTIVE GALACTIC NUCLEUS (AGN) were more common in the earlier days of the universe -- we'll discuss them later!

• The MW's center does NOT (currently) house an AGN.

Best Evidence for a Supermassive BH at the Core of the Milky Way

1) Radio core of Sgr A* is unresolved at 43 GHz, very close to RS for a 3.6 million solar mass BH, 2) as “weighed” by orbits of stars measured over a decade in the infrared.