GalaxiesGalaxiesGalaxiesGalaxies

Learning Outcome (Student will…):•compare characteristics & classification of various galaxies

The basic types of galaxiesThe basic types of galaxiesShape & parts of the Milky Way GalaxyShape & parts of the Milky Way GalaxyHow galaxies are grouped into clusters How galaxies are grouped into clusters

and larger structuresand larger structuresWhat happens when galaxies collideWhat happens when galaxies collideWhat observations indicate the presence What observations indicate the presence

of dark matter in other galaxiesof dark matter in other galaxiesHow galaxies formed and evolvedHow galaxies formed and evolved

In this unit, you will learn

Hubble’s Tuning Fork DiagramHubble’s Tuning Fork DiagramHubble’s Tuning Fork DiagramHubble’s Tuning Fork Diagram

Spiral GalaxiesSpiral GalaxiesSpiral GalaxiesSpiral Galaxies

Spiral Galaxies•Spiral galaxies have ‘arms’ emerging from a bright central nucleus•Spirals are classified according to how tightly or loosely wound the arms are•brightness of the central nucleus is correlated to the tightness of the arm

Barred Spiral GalaxiesBarred Spiral GalaxiesBarred Spiral GalaxiesBarred Spiral Galaxies

Barred Spiral Galaxies•The spiral galaxies M 91 (left) and M 109 (right) have bars across their nuclei from which spiral arms unwind.•In nearly all spirals (barred or not) the galaxies rotate such that the spiral arms trail behind in the rotation•Bars form when stellar orbits in a spiral galaxy become unstable and deviate from a circular path; they are a sign of mature spiral galaxies•The Milky Way is thought to be a barred spiral galaxy.

Elliptical GalaxiesElliptical GalaxiesElliptical GalaxiesElliptical Galaxies

Elliptical Galaxies•Elliptical galaxies lack spiral arms and dust•They contain older stars than spirals.•The elliptical galaxies M 32 (below) and M 110 (right) show varying degrees of ellipticity.

Lenticular GalaxiesLenticular Galaxies have a disk but no discernible spiral armshave a disk but no discernible spiral arms

Lenticular GalaxiesLenticular Galaxies have a disk but no discernible spiral armshave a disk but no discernible spiral arms

Irregular GalaxiesIrregular GalaxiesIrregular GalaxiesIrregular Galaxies

Irregular Galaxies•Irregular galaxies lack any specific form and contain stars, gas and dust generally associated with a youth.•The irregular galaxy at right is the Large Magellanic Cloud, a satellite of the Milky Way located about 180,000 light years from the sun.•The LMC is about 60,000 light years across. The bright reddish feature in the upper right is the “Tarantula Nebula” a region of star formation in the LMC.

Regions of the Milky Way GalaxyRegions of the Milky Way Galaxy

radius of disk = 50,000 l.y. (20 kpc)

number of stars = 200 billion

thickness of disk = 1,000 l.y. (300 pc)

Sun is in disk, 8kpc out from center

The Galactic Disk:

• Most stars are here. Nearly all the interstellar gas.

• Old Stars (1010yrs) to Younger Stars (106yrs)

• Star Formation is occurring now.

• Composition: Old Metal Poor to Young Metal Rich stars.

• Motions – coplanar, direct, elliptical orbits.

• Spiral Arms density waves

The Galactic Bulge & Halo

The Galactic Halo

• Thin scattering of stars & clusters

• Stars (OLD), globular clusters (OLD), no interstellar material.

• Metal Poor material (mostly H, He, very little else)

• Random eccentric orbits

The Galactic Bulge

• Like the halo, only more crowded

Hubble Space Telescope Image

Active Galaxies

•The galaxy NGC 7742 is an otherwise normal spiral galaxy except for its extraordinarily bright nucleus that outshines the rest of the galaxy•Such galaxies, i.e. spirals with extremely bright nuclei, form a class of active galaxies known as Seyfert galaxies.

Active Galaxies•The elliptical galaxy M87, shown below in a wide-field ground-based image, has a very bright, point-like nucleus from which a jet of material emanates.•The jet is seen in great detail from an HST image at right.

Hubble Space Telescope Image

Active Galaxies•This image shows the spiral galaxy NGC 4319 and the quasar Markarian 205.•The distance to NGC is 80 million light years, which Mkn 205 is 14 times farther away at a distance of 1 billion light year.•The very distant quasar is nearly as bright as the much closer galaxy. The extraordinary brightness of quasars, which is a blending of the term quasi-stellar radio source, indicates that some incredibly powerful mechanism must be producing enormous amounts of energy from a small volume of space. Hubble Space Telescope Image

Mkn 205NGC 4139

•Rather than occurring individually in space, galaxies are grouped in clusters ranging in size from a few dozens to thousands of galaxies. •The Coma Cluster, shown at right, is 300 million light years from the Milky Way and contains more than 1,000 (and possibly as many as 10,000) galaxies.•The Milky Way is a member of a small cluster called the Local Group which contains about 40 galaxies. The largest member of the Local Group is M 31, with the Milky Way coming in second in size.

Clusters of Galaxies

The Local The Local GroupGroup

The Local The Local GroupGroup

Galaxies in Collision & Mergers•In this close encounter between two spiral galaxies, their arms are dramatically warped and massive star formation is triggered when the hydrogen gas clouds in the two collide.•It is believed the Milky Way may have “cannibalized” small galaxies in the past through collision.

http://burro.cwru.edu/JavaLab/GalCrashWeb/

Key Ideas The Hubble Classification:The Hubble Classification: Galaxies can be grouped Galaxies can be grouped

into four major categories: spirals, barred spirals, into four major categories: spirals, barred spirals, ellipticals, and irregulars.ellipticals, and irregulars.

The disks of spiral and barred spiral galaxies are sites of The disks of spiral and barred spiral galaxies are sites of active star formation.active star formation.

Elliptical galaxies are nearly devoid of interstellar gas Elliptical galaxies are nearly devoid of interstellar gas and dust, and so star formation is severely inhibited.and dust, and so star formation is severely inhibited.

Lenticular galaxies are intermediate between spiral and Lenticular galaxies are intermediate between spiral and elliptical galaxies.elliptical galaxies.

Irregular galaxies have ill-defined, asymmetrical shapes. Irregular galaxies have ill-defined, asymmetrical shapes. They are often found associated with other galaxies.They are often found associated with other galaxies.

Key Ideas Clusters and Superclusters:Clusters and Superclusters: Galaxies are grouped into Galaxies are grouped into

clusters rather than being scattered randomly throughout clusters rather than being scattered randomly throughout the universe.the universe.

Our Galaxy is a member of a poor, irregular cluster Our Galaxy is a member of a poor, irregular cluster called the Local Group.called the Local Group.

Key Ideas Galactic Collisions and Mergers:Galactic Collisions and Mergers: When two galaxies When two galaxies

collide, their stars pass each other, but their interstellar collide, their stars pass each other, but their interstellar media collide violently, either stripping the gas and dust media collide violently, either stripping the gas and dust from the galaxies or triggering prolific star formation.from the galaxies or triggering prolific star formation.

The gravitational effects during a galactic collision can The gravitational effects during a galactic collision can throw stars out of their galaxies into intergalactic space.throw stars out of their galaxies into intergalactic space.

Galactic mergers may occur; a large galaxy in a rich Galactic mergers may occur; a large galaxy in a rich cluster may tend to grow steadily through galactic cluster may tend to grow steadily through galactic cannibalism, perhaps producing in the process a giant cannibalism, perhaps producing in the process a giant elliptical galaxy.elliptical galaxy.

Key Ideas The Dark-Matter Problem:The Dark-Matter Problem: The luminous mass of a The luminous mass of a

cluster of galaxies is not large enough to account for the cluster of galaxies is not large enough to account for the observed motions of the galaxies; a large amount of observed motions of the galaxies; a large amount of unobserved mass must also be present. This situation is unobserved mass must also be present. This situation is called the dark-matter problem.called the dark-matter problem.

Gravitational lensing of remote galaxies by a foreground Gravitational lensing of remote galaxies by a foreground cluster enables astronomers to glean information about cluster enables astronomers to glean information about the distribution of dark matter in the foreground cluster.the distribution of dark matter in the foreground cluster.

Key Ideas Formation and Evolution of Galaxies:Formation and Evolution of Galaxies: Observations Observations

indicate that galaxies arose from mergers of several indicate that galaxies arose from mergers of several smaller gas clouds.smaller gas clouds.

Whether a protogalaxy evolves into a spiral galaxy or an Whether a protogalaxy evolves into a spiral galaxy or an elliptical galaxy depends on its initial rate of star elliptical galaxy depends on its initial rate of star formation.formation.