A short course in

The Milky Way and the ISM

Dr. Maura McLaughlinWest Virginia UniversityMaura.McLaughlin@mail.wvu.edu

July 10 2008

Pulsar Search Collaboratory

Outline

1. Introduction to the Milky Way

2. The Milky Way in the universe

3. Stellar populations in the Milky Way

4. Dynamics of the Milky Way

5. The interstellar medium

6. Dispersion, scattering and scintillation of radio signals

Almost everything we see in the night sky belongs to

the Milky Way.

We see most of the Milky Way as a faint band of light

across the sky.

Greek word for milk is “galact” -> galaxy!!

The Milky Way

The Milky Way

How in the world do we know this?!?!

First Studies of the Galaxy

First attempt to unveil the structure of the galaxy by William

Herschel (1785), based on optical observations.The shape of the Milky Way was believed to resemble a grindstone, with the Sun

close to the center.

First Studies of the Galaxy

First attempt to unveil the structure of the galaxy by William

Herschel (1785), based on optical observations.

Did not know about gas and dust!!

Determining the Structure of the Milky Way

Galactic Plane

Galactic CenterThe structure of our Milky Way is hard to

determine because:1) We are inside.

2) Distance measurements are difficult.3) Our view towards the

center is obscured by gas and dust.

Exploring the Galaxy Using Star

ClustersTwo types of clusters of stars:

1) Open clusters = young clusters of recently formed stars; within the

disk of the Galaxy

2) Globular clusters = old, centrally concentrated

clusters; mostly in a halo around the galaxy

Globular Cluster M13

Open cluster NGC 1983

Globular Clusters

• Dense clusters of 50,000 – a million stars

• Approx. 200 globular clusters in our Milky Way

• Old (~ 11 billion years), lower-main-sequence stars

Globular Cluster M80

Locating the Center of the Milky Way

In early 1900s, Shapley shows that the

distribution of globular clusters is not centered on the Sun!

Locating the Center of the Milky Way

Their distribution is centered on a location which is heavily obscured

from direct (visual) observation.

Using Cepheid distances, he measured the

distance to the center of the distribution of

20,000 parsecs (too big!)

Hubble’s breakthrough

Hubble identified a Cepheid variable in M31the Andromeda Galaxy in 1923 using the100” telescope at Mount Wilson.

Distance to M31 is 780 kpc

Our Galaxy Cluster: The Local Group

Milky Way Andromeda Galaxy

Small Magellanic Cloud

Large Magellanic Cloud

Our Galaxy Cluster: The Local Group

Milky Way Andromeda Galaxy

Small Magellanic Cloud

Large Magellanic Cloud

Our local group is a poor cluster:

> 30 galaxies

1 Mpc diameter

Of bright galaxies,

14 elliptical 3 spiral 4 irregular

Most of galaxies are dwarf ellipticals.

Our Galaxy Cluster: The Local Group

Milky Way Andromeda Galaxy

Small Magellanic Cloud

Large Magellanic Cloud

Largest members are:

Milky Way

Andromeda (M31)

Triangulum (M33)

Andromeda is the largestbut we think MW may bethe most massive.

Our Galaxy Cluster: The Local Group

Milky Way Andromeda Galaxy

Small Magellanic Cloud

Large Magellanic Cloud

Largest members are:

Milky Way

Andromeda (M31)

Triangulum (M33)

Our Galaxy Cluster: The Local Group

Milky Way Andromeda Galaxy

Small Magellanic Cloud

Large Magellanic Cloud

Largest members are:

Milky Way

Andromeda (M31)

Triangulum (M33)

Magellanic Clouds: Local group dwarfs

Mergers of Galaxies

Milky Way and Andromeda are moving towards each other at 500,000 km/hour and are expected to merge in about 3 billion years.

About nomenclatureNumbers with “M” in front of them are Messier objects,

cataloged by Charles Messier between 1758 to 1782. These were about 100 diffuse structures often

mistaken for comets.

About nomenclatureNGC means New General Catalog of nebulae and star clusters, compiled by John Dreyer in 1888.

Contains 8000 objects.

The Structure of the Milky WayDisk contains stars, open

star clusters and lots of dust and gas.

Sun is in disk at 8.5 kpc from center of Galaxy (D =

25 kpc).

Halo contains only 2% as many stars as the disk, and very little gas and dust. We can’t detect

halos of other galaxies.

Nuclear bulge has radius of 2 kpc and contains little gas and dust.

Observing Neutral Hydrogen:

The 21-cm (radio) line (1)Electrons in the ground state of neutral hydrogen

have slightly different energies, depending on their spin orientation.

Magnetic field due to electron spin

Opposite magnetic

fields attract => Lower energy

Equal magnetic

fields repel => Higher energy

Magnetic field due to proton spin

Observing Neutral Hydrogen:

The 21-cm (radio) line (2)

Observing Neutral Hydrogen:

The 21-cm (radio) line (3)

21 cm emission maps out spiral arms

Infrared View of the Milky Way

Interstellar dust (absorbing

optical light) emits mostly infrared.

Near-infrared image

Infrared emission is not strongly absorbed and provides a clear view throughout the

Milky Way

Nuclear bulge

Galactic plane

Infrared View of the Milky Way

Near-infrared image

Nuclear bulge

Galactic plane

Spitzer Space Telescope view of Milky Way

Orbital Motions in the Milky Way (1)

Disk stars:

Nearly circular

orbits in the disk of the

galaxy

Halo stars:

Highly elliptical orbits; randomly oriented

The mass of the Milky WayWe use binary star systems to find

the masses of stars.

We can measure orbits of stars in the galaxy to find the mass of the

galaxy.

Orbital Motions in the Milky Way (2)

Differential Rotation Sun orbits

around galactic

center at 220 km/s.1 orbit takes

approx. 240 million years.

We have completed roughly 20 orbits.

Mass determination from orbital velocity:

The more mass there is inside the

orbit, the faster the Sun has to

orbit around the Galactic center.Combined mass:

M = 4 billion Msun

M = 25 billion Msun

M = 100 billion Msun

The Mass of the Milky Way

If all mass was concentrated in the center, rotation curve would follow a modified version of Kepler’s 3rd law.

Rotation Curve = orbital velocity as function of radius.

The Mass of the Milky Way (2)

Total mass in the disk of the Milky

Way:

Approx. 200 billion solar

massesAdditional mass in an extended

halo:

Total: Approx. 1 trillion solar

massesMost of the mass is not emitting any radiation:

dark matter!

Possible dark matter sources

• Neutrinos• Massive compact halo objects

– Brown dwarfs– Black holes

• Gas• Planets• Other exotic objects

How old is the Galaxy? Stellar

PopulationsPopulation I: Young stars:

metal rich; located in spiral arms and disk

Population II: Old stars: metal poor; located in the halo (globular clusters) and

nuclear bulge

How old is the Galaxy? Stellar

Populations

Our Sun is an intermediate Population 1

star.

Metal Abundances in the Universe

Logarithmic Scale

All elements heavier than He are very

rare.

Linear Scale

Metals in StarsAbsorption lines almost exclusively from Hydrogen:

Population II

Many absorption lines also from heavier elements (metals):

Population IAt the time of

formation, the gases forming the Milky Way consisted exclusively

of hydrogen and helium. heavier

elements (“metals”) were later only

produced in stars.=> Young stars contain more metals than older stars.

The History of the Milky

WayQuasi-spherical gas cloud fragments into

smaller pieces, forming the first, metal-poor stars

(pop. II).Rotating cloud collapses into a disk-

like structure.Later populations of stars (pop. I) are

restricted to the disk of the Galaxy.

Oldest GCs are 13 billion years old.

Exploring the structure of the Milky Way with O/B

Associations

Distances to O/B Associations determined using Cepheid

variables.

O/B Associations trace out 3 spiral arms near the Sun.

Sagittarius

arm

Orion-Cygnus

arm

Perseus arm

Star Formation in Spiral Arms (1)

Shock waves from supernovae, ionization fronts initiated by O and B stars, and the shock fronts forming spiral arms trigger

star formation.Spiral arms are stationary

shock waves,

initiating star

formation.

Density wave

theory.

Star Formation in Spiral Arms

(2)Spiral arms are

basically stationary shock

waves.Stars and gas clouds orbit around the

galactic center and cross spiral arms.

Shocks initiate star formation.

Star formation self-sustaining through

O/B ionization fronts and supernova shock

waves.

The Galactic Center (1)

Wide-angle optical view of the GC region

galactic center

Our view (in visible light) towards the Galactic center (GC) is heavily obscured by

gas and dust:Extinction by 30 magnitudes

Only 1 out of 1012 optical photons makes its way from the GC towards

Earth!

Radio View of the Galactic Center

The galactic center contains a supermassive black hole of approx. 4

million solar masses.

Sgr A*: The center of our galaxy

Many supernova remnants; shells and

filaments.

Measuring the Mass of the Black Hole in the Center of

the Milky Way By following the orbits of individual stars near the center of the Milky Way, the mass of the

central black hole could be determined to be ~ 4 million solar masses.

Question

Which part of the Milky Way contains mostly Population II stars and globular clusters ?

A) the disk B) the halo C) the bulge D) the spiral arms

Question

Which part of the Milky Way contains mostly luminous O and B stars?

A) the disk B) the halo C) the bulge D) the spiral arms

Question

Stars with more metals are likely to be _______ than stars with fewer metals.

A) younger B) older

Question

Which one of the following galaxies is not a member of the local group?

A) Milky Way B) Antenna C) Andromeda D) Triangulum

Question

The rapid rotation of the outer parts of the disk of our galaxy shows that

A) the center of the galaxy is very massive B) there are many young stars in the outer parts C) there is a lot of mass in the outer parts of the Galaxy D) the rotation of the Galaxy is Keplerian

The space between the stars is not completely empty, but filled with

very dilute gas and dust, producing some of the most

beautiful objects in the sky.We are interested in the ISM

becausea) dense interstellar clouds are the birth

places of starsb) dark clouds alter and absorb the light from

stars behind them

The Interstellar Medium (ISM)

Structure of the ISM

• HI clouds (molecular clouds)

• Hot intercloud medium:

The ISM is 99% interstellar gas and comprises 10-15% of the visible mass of MW. It occurs in two main types of clouds:

Cold (T ~ 100 K) clouds of neutral hydrogen (HI); moderate density (n ~ 10 – a few hundred atoms/cm3); size: ~ 100 pc

Hot (T ~ a few 1000 K), ionized hydrogen (HII); low density (n ~ 0.1 atom/cm3);

gas can remain ionized because of very low density.

• HI clouds (molecular clouds)

• Hot intercloud medium:

Hot (T ~ a few 1000 K), ionized hydrogen (HII); low density (n ~ 0.1 atom/cm3);

gas can remain ionized because of very low density.

If we have ionized hydrogen then we will also have….

FREE ELECTRONS!

Pulse dispersionPulsars are dispersed in frequency by free electrons in the interstellar medium.

Photons with higher frequencies travel faster through space and arrive earlier than lower frequency ones.

The total delay is proportional to the distance to the pulsar.

Using a model for the interstellar medium, we can use this property to estimate distances to pulsars.

A bright single burst

We are always searching for new radio signals in our data

Hot off the press:

- a new radio transient - extragalactic origin - note frequency dispersion discriminates against RFI! - D ~ 500 Mpc (1.7 Gly) - origin unknown (NS-NS?)

Pulsar distances

€

DM = ne0

D

∫ dl

We measure DMs in pc cm-3.

Can use measured DMs to estimate distances to pulsars!

Black and yellow points are at two different frequencies. Which color is the higher one??

Pulse scattering

Pulse scattering

Amount ofscattering f? ?

Scattering vs DM

Pulsar scintillation

Twinkle twinkle little pulsar

Degree of ISS (interstellarscintillation) will dependon distance, medium andvelocity of pulsar.