Astronomy 162: Introduction to Stars, Galaxies, & the Universe Prof. Richard Pogge, MTWThF 9:30

Lecture 26: 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

All spiral galaxiess share a common structure:

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

All spiral galaxies have disks of varying sizes.

The spheroids of spirals, however, can vary greatly in size.

Spheroid Structure

The bulge is the center of the galaxy where the inner spheroid & the disk merge together. In the bulge we find

• Many RR Lyrae stars
• A little gas & dust

Halo

The halo is the low-density outer regions of the spheroid, where we find:

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

Disk Structure

This is the main body of the disk, about 1000pc thick, composed of:

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

Thin disk of Gas & Dust

This is a region about 100 pc thick embedded within the stellar disk composed of

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

Rotation of the Disk

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

Inner Parts: Solid-Body Rotation

• Orbital speed increases with radius.
• Orbital period is constant.

Outer Parts: Differential Rotation

• Orbital speed is nearly constant with radius.
• Orbital period increases with radius

Typical Rotation Speeds:

Inner Parts:

Rise from Zero to few 100 km/sec

Outer Parts:

Nearly constant at a few 100 km/sec

Measuring Masses of Galaxies

Newton's Gravity predicts:

M(R) = mass interior to radius R

V_rot = rotation speed

Example: Milky Way

• R=8 kpc, V_rot=220 km/sec
• Gives: M = 9.0x10¹⁰ M_sun inside R=8 kpc

Gas Cloud in Outer Disk:

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

Measuring the rotation speeds in spiral galaxies provides us with a good way to measure the masses of spiral galaxies, and measure how that mass is distributed within the galaxies. As we'll see in future lectures, those measurements held some surprises.

Spiral Arms

Spiral Arm Tracers:

• O &B Stars
• HII Regions (star forming regions lit up by hot O & B stars)
• Giant Molecular Clouds
• Hydrogen Gas and Dust Clouds

Sites of Active Star Formation

• The Sun lives for ~12 Gyr
• It can complete about ~50 orbits around the Galaxy before becoming a white dwarf.

By comparison, O&B Stars only live for ~10 Myr

• O & B stars will only move ~10-20° along their oribts before dying.
• This means they won't move very far from their birthplace.

When look at deep images of spiral galaxies, we see O&B Stars and HII Regions strung out along the Spiral Arms like "Beads on a String." (in the words of Walter Baade).

What are Spiral Arms?

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

• 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?