A short course in The Milky Way and the ISM Dr. Maura McLaughlin West Virginia University July 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

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 Center The 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 Clusters Two 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 M31 the Andromeda Galaxy in 1923 using the 100” 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 largest but we think MW may be the 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 nomenclature Numbers with “M” in front of them are Messier objects, cataloged by Charles Messier between 1758 to These were about 100 diffuse structures often mistaken for comets.

About nomenclature NGC means New General Catalog of nebulae and star clusters, compiled by John Dreyer in Contains 8000 objects.

The Structure of the Milky Way Disk 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 Way We 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 M sun M = 25 billion M sun M = 100 billion M sun

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 masses Additional mass in an extended halo: Total: Approx. 1 trillion solar masses Most 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 Populations Population 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 Stars Absorption lines almost exclusively from Hydrogen: Population II Many absorption lines also from heavier elements (metals): Population I At 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 Way Quasi-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 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 because a) dense interstellar clouds are the birth places of stars b) 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/cm 3 ); size: ~ 100 pc Hot (T ~ a few 1000 K), ionized hydrogen (HII); low density (n ~ 0.1 atom/cm 3 ); 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/cm 3 ); gas can remain ionized because of very low density. If we have ionized hydrogen then we will also have…. FREE ELECTRONS!

Pulse dispersion Pulsars 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 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

Amount of scattering  f ? ?

Scattering vs DM

Pulsar scintillation Twinkle twinkle little pulsar Degree of ISS (interstellar scintillation) will depend on distance, medium and velocity of pulsar.