Lecture 40

Galaxies (continued).

Evolution of the Universe.

• Characteristics of different galaxies

• Redshifts

• Unusual Galaxies

Chapter 18.6 18.9

Galaxy Types

Spiral galaxies – flat with bulges and spiral arms.Elliptical galaxies – redder and rounder.Irregular galaxies – strange-shaped.

Sizes of galaxies: from dwarf (~100 million stars) to giant (~1 trillion stars).

Spiral galaxies have disks, bulges, and halos.Bulge and halo make the spheroidal component.The disk component is the galaxy midplane.

Some galaxies have disks, but no spiral arms (lenticular galaxies).

Elliptical Galaxies

Ellipticals lack a significant disk component.

Thus, they have only spheroidal components.Most of their interstellar medium consists of low-density, hot, X-ray emitting gas.They also have very little dust.

Some ellipticals have small rotating disks at their centers (perhaps remnants of a collision with a spiral galaxy).

Irregular Galaxies

A small percentage of large galaxies are neither spiral nor elliptical.

The irregulars are mostly small and “peculiar”.

Their star systems are usually white and dusty, like the disks of spirals.

Distant galaxies are more likely to be irregular than those nearby (they were more common in younger universe).

Hubble’s Work

In 1924, Edwin Hubble discovered Cepheid variables in the Andromeda Galaxy using the 100-inch telescope at Mt. Wilson.

He used photographs taken at different time and measured brightness of stars.

Using the period-luminosity relation for galactic Cepheids, he measured the distance toward M31.

Hubble’s Law Since 1910’s, astronomers knew that spectra of most of the “spiral nebulae” were redshifted.Hence, the nebulae were moving away from us.

In 1929, after measuring distances to a number of spirals, Hubble announced that:The more distant a galaxy, the greater its redshift.

v = H0 x d

v – velocity, d –distance, H0 – Hubble’s constant

Hubble’s Law

The Hubble constant’s units are kilometers per second per megaparsec (km/s/Mpc).

If H0=100 km/s/Mpc, then a galaxy located at d=10 Mpc is moving away at a speed of 1000 km/s.

There are 2 major problems:Not all galaxies obey the Hubble lawThe distances from the law are as accurate as the constant itself.

Thus, the universe is expanding!

Universal Expansion

The Bubble Constant

Let us measure distances on an expanding surface of a balloon.

3 sec after the beginning: A – B = 3cm and v=1cm/s D – B = 6cm and v=2cm/s

1 cm/sv = ---------- d 3 cm

1v = ------ d 3 s

Hubble Constant and Age

The universe expansion rate is related to its age.

v = H0 x d

The inverse Hubble constant, 1/H0, tells us how long the universe has been expanding.

The Hubble constant changes with time, but stays roughly equal to 1/(age of the universe).

Current value of H0 corresponds to ages 1218 billion years.

Cosmological Redshift

The redshifts tell how quickly they are moving from us.

Another explanation of the redshifts is that the expansion of the universe stretches out all the photons within it (cosmological redshift).It tells us how much space has expanded during the lookback time to a galaxy.

The universe does not have an edge.But it has a horizon, a place beyond which we cannot see. It is a boundary in time, not in space.

Cosmological Redshift

Quasars and Active Galactic Nuclei

Some galaxies display incredible phenomena:- extreme amounts of radiation- jets of material

The unusually bright galactic centers are called active galactic nuclei.The brightest of them are called quasars.

Quasars

In 1963, a radio source 3C 273 was found to have bright emission lines in its optical spectrum.The lines turned out be redshifted UV lines.

The distance to 3C 273 was computed from its redshift using the Hubble law.It turned out to be ~1 trillion of solar luminosities or hundreds of that of the Milky Way.

Most quasars lie very far away from us.They formed when the universe was <1/3 of its present age.

Energy Generation in Quasars

Quasars look star-like, because their nuclei radiate a lot more energy than the surrounding material. The light-emitting regions in the nuclei are very small, a few light days or even hours across.

The only current explanation of this incredible amount of energy from quasars and active galactic nuclei is matter falling into a supermassive black hole.

Supermassive Black Holes

Gravity converts the potential energy into kinetic energy.Collisions between particles convert the kinetic energy into thermal energy.Photons carry the thermal energy away.

~40% of the mass-energy (E=mc2) can be converted into radiation before the matter crosses the event horizon.Nuclear fusion converts into radiation only ~1%.

Limitations of the Theory

We do not know: what would create such giant black holes why quasars stop shining

Since quasars formed early in the universe history, the formation of supermassive black holes seems to be connected with galaxy formation.

Clusters of neutron stars from extremely dense starbursts at the centers of galaxies might coalesce to form an enormous black hole.

Evidence for Supermassive Black Holes

The nearby galaxy M87 has a bright nucleus and a jet that emits both radio and visible light.

The spectra show blueshifted emission on one side of the nucleus and redshifted on the other.

The high-speed orbital motion indicates the presence of a 2-3 billion solar mass object in the center.

SummaryThe inverse Hubble constant gives us an estimate of the age of the universe.

The expansion of the universe makes photons redder.

The complete story of galaxy formation is not known yet. They seem to grow from protogalactic clouds of gas, but collisions affect many of them.

Quasars and active galactic nuclei are probably powered by gas accreting onto supermassive black holes.