Lecture 27: Spiral Galaxies

Key Ideas:

Disk & Spheroid Components

Rotation of the Disk:

• Differential Rotation Pattern
• Measurement of Galaxy Masses

Spiral Arms:

• Outlined by O&B Stars, HII Regions, & Gas
• Spiral Density Waves in the Disk
• Sites of recent star formation.

Spiral Galaxies

The Milky Way & Andromeda are examples of Spiral Galaxies.

All spirals share a common structure:

• Thin Disk of stars, gas, and dust.
• Thick Spheroid of stars with little gas or dust.

All Spirals have disks

Spheroids vary greatly in size.

Spheroid Structure

Bulge: where inner spheroid & disk merge

• Many RR Lyrae stars
• A little gas & dust

Halo: sparse outer spheroid

• Old metal-poor stars
• Globular clusters
• RR Lyrae Stars

Disk Structure

Thick disk of Stars (~1000 pc thick)

• Open Clusters & loose Associations of stars
• Mix of young & old stars
• Cepheid Stars in young clusters

Thin disk of Gas & Dust (~100 pc thick)

• Mostly cold atomic Hydrogen gas
• Dusty Giant Molecular Hydrogen (H₂) Clouds

Rotation of the Disk

Measure using the Doppler Effect

Stars: Doppler shifts of stellar absorption lines

Ionized Gas: emission lines from HII regions

Atomic Hydrogen (HI) Gas:

• Cold H clouds emit a radio emission line at a wavelength of 21-cm
• Can trace nearly the entire disk beyond where the stars have begun to thin out.

Rotation Curves

The disk rotates about the center of the galaxy

Inner Parts: Solid-Body Rotation

• speed rises with radius.
• orbit period is about constant.

Outer Parts: Differential Rotation

• speed is about constant with radius.
• orbit period increases with radius.

Rotation Speeds

Inner Parts: Rise from Zero to few 100 km/sec

Outer Parts: Nearly constant at a few 100 km/sec

Example in the Milky Way:

• Sun has V_rot=220 km/sec at R=8.5 kpc
• Orbital Period: 240 Myr

Measuring Masses of Galaxies

Star or Gas cloud is held in its orbit by the mass interior to the orbit.

Newton's Gravity gives:

Where:

M(R) = mass interior to radius R

V_rot = rotation speed

Example: Milky Way

Sun:

• R=8.5 kpc, V_rot=220 km/sec
• Gives: M = 9.6x10¹⁰ M_sun inside R=8.5 kpc

Gas Cloud in outer disk:

• R=16 kpc, V_rot=275 km/sec
• Gives: M=2.8x10¹¹ M_sun inside R=16 kpc

Provides us with a way to measure the masses of Galaxies.

Spiral Arms

Pattern of hot stars, star clusters, gas & dust crossing the disk.

Tracers:

• O&B Stars
• HII Regions (star forming regions)
• Giant Molecular Clouds
• Hydrogen Gas and Dust Clouds

Sites of Active Star Formation

Recall: Sun takes 240 Myr orbit Galaxy

• Sun lives ~12 Gyr, so can make ~50 orbits

O&B Stars only live ~10 Myr

• Only move ~10-20^o around before dying
• Don't move far from their birthplace.

See O&B Stars and HII Regions strung along the Spiral Arms like "Beads on a String."

What are Spiral Arms?

Spiral Arms are Density Waves in the Disk.

Density Waves are a kind of orbital traffic jam

• Orbits crowd together in the arms, stars pile up and make the regions look brighter.
• Gas clouds pile up, collide, fragment, and form new stars.
• O&B Stars ionize leftover gas (HII Regions), then die before moving far from the waves.

Density Waves

Density waves pass through the disk like water waves pass over the ocean.

• Stars move through the spiral arms.
• Gas clouds try to move through, but some are induced to form stars (collision or compression)

We are not sure how the waves are excited:

• Tidal disturbance from a nearby companion?
• Excited by a stellar bar in the central regions?