Galaxies

Galaxies

• Galactic Morphology• Interacting Galaxies• "Active" Galaxies

Galaxy TypesHubble Classification

· Elliptical Galaxies · Spiral Galaxies

· Lenticular· Non-barred· Barred

· Irregular Galaxies

Elliptical Galaxies

The most massive galaxies are ellipticals, and they feature significantly in clusters of galaxies. – a smooth, featureless appearance

– little gas and dust

– reddish color; low rate of star formation; no core-collapse supernovae.

– huge range of possible masses from 106 to 1013 Solar Masses

– very elongated stellar orbits and little overall rotation.

– Classified as E0 through E7

Elliptical GalaxiesElliptical galaxies, classed by E0 to E7

The E stands for elliptical (obviously)

The number indicates how egg-shaped the ellipse is - 0 means a ball shape - 5 a bit like a football - 7 looks like a cigar

E0

E1

E5

E7

Spiral Galaxies

• Classified as S0, Sa, Sb, or Sc• Barred Spirals: SBa, SBb SBc• Have similar features

– Nucleus, Bulge, Disk and Halo

Lenticular GalaxiesS0 is the class for Lenticular galaxies–Spiral galaxies without spiral arms

S0

ngc5866

Edge-On

Face-On

Spirals

All have spiral arms, and they are grouped by how tightly those arms are wound and how large the central bulge is - the two happen to be closely related.

The name is defined by the "S" and the lower case letter after which indicates how wound up the arms are: from "a" to "c": Sa, Sb, Sc

The lower branch of the tuning fork diagram is largely a copy of the upper branch, but its occupants all have a line of stars through the center - a bar. The B stands for barred: SBa, SBb, SBc

Sa SBa

Sb SBb

Sc SBc

Spiral Galaxies• The central bulge is similar to an elliptical galaxy (i.e.

smooth and reddish in color)

• a surrounding, highly flattened disk in circular orbital motion about the spheroid

• large amounts of gas and dust in the disk where stars actively form

• spiral arms within the disk

• haloes of stars and globular clusters and dark matter in which the disk and spheroid are embedded

• masses: 1010 to 3x1011 Solar Masses• in some spirals, central bulge has a bar-shape: `barred’

spirals.

Irregular Galaxies• Some spirals are

poorly defined and merge with a set classed as `irregular'. Irregulars feature

· very large dust and gas fractions

· vigorous star formation which gives a patchy appearance.

· Masses: 106 to 1010 Solar Masses

Tuning Fork Diagram

Environmental effects

Unlike most stars, galaxies are heavily effected by their environment.

When discussing stars, collisions were barely mentioned except in the dense cores of globular clusters.

In the Solar neighborhood, an average main-sequence star (excluding binary stars) is separated by of order 107 times its size from its nearest neighbors (1 Solar Radius vs. 1 pc).

Galaxies on the other hand have sizes ranging from 1 to 100 Kpc, but are separated by of order 1 to 10 Mpc from their neighbors, only a factor of 100 to 1000. This means that almost all galaxies have probably had direct interactions, collisions and mergers with others during their lives.

For an individual star, a galaxy collision would not mean much, however, gas clouds are likely to collide and star formation affected considerably. The result may be a much higher supernova rate and the birth of a young group of stars. It is possible that the collisions of spirals disrupt their disks and lead to elliptical galaxies.

Interacting Galaxies

Galactic Populations• Population I

– Stars with heavy elements– New star formation– Found in

• Irregular galaxies• Spiral galaxy disks

• Population II– Stars with little or no heavy elements– Old stars– Found in

• Elliptical galaxies• Spiral galaxy halos and bulge

Formation of Galaxies• How do galaxies form?

– Structural differences• Spiral versus Elliptical

– Seems to be largely a matter of the original rotation

• No rotation of original gas cloud – Elliptical• Rotation – Spiral

• Elliptical (E0 thru E7)– Translation through the surrounding gas

• Leaves a ‘wake’• Irregular -- Little translation or rotation

Spiral Arms

How do the spiral arms form?

Density Waves

Spiral Rotation

If the spiral galaxies were rigid, like a wheel or a disc then we would expect to measure the speed as a function of distance from the center as:

Spiral RotationOn the other hand, if it was composed of independent stars orbiting the great mass at the center, it would follow Kepler's 3rd law and look like:

Rotation of the MilkywayEven with the uncertainties in the data, it's clear that the Milkyway in not a rigid body and is not following Kepler's 3rd law

Rotation of M31Andromeda has similar behavior

Other Galaxies RotationSeems to be a feature, not an anomaly:

Galactic Rotations• The odd speed distribution does have a

solution, but it adds to the mystery• This type of speed distribution happens

when there is a lot more mass out in the disk than toward the center.

• We can't see this mass. It is now called "Dark Matter"

• Estimates of the Dark Matter imply that the visible mass of the Universe is a very small percentage of what is really there.

Active Galaxies

Active Galaxy Zoo · Seyfert Galaxies · Radio Galaxies

The Central Engine · Energy generation efficiency of

accretion

· How big are the black-holes?

Seyfert Galaxies• Bright, point-like

nuclei– Seyfert I

• Broad emission line spectra like a quasar

• Strong X-ray• Low (compared to

quasars) luminosity– Seyfert II

• Narrow emission lines only

• Dust and Distance

Seyfert Galaxy

Depends on the View

Radio galaxies • At radio wavelengths, most sources are galaxies; stars

are feeble emitters of radio waves in general. Some galaxies are much more powerful at radio wavelengths than normal. They can exceed the Milky Way by 103 to 107 times. These are radio galaxies. When resolved many have a double-lobe appearance in which two large lobes some hundred of kiloparsecs apart emit radio waves.

• Further imaging revealed that these lobes are powered by jets emanating from the nuclei of (usually) elliptical galaxies. One can achieve remarkable resolution at radio wavelengths, and yet it is never possible to resolve the source of these jets. The jets contain material moving close to the speed of light. The lobes are formed as these jets plough into the intergalactic

medium.

Radio Galaxies

Centaurius A

Optical Image

Radio Image

Sagittarius A

Anatomy of a Radio Source

Why the Double Lobe?

BlazarsA Blazar is a very compact and highly variable energy source associated with a presumed supermassive black hole at the center of a host galaxy.

Blazars are among the most violent phenomena in the universeBlazars are active galactic nuclei (AGN) with a relativistic jet that is pointing in the general direction of the Earth. We observe "down" the jet, or nearly so, and this accounts for the rapid variability and compact features

Peculiar Galaxies

The Cartwheel Galaxy

IRAS Galaxies– Infrared Astronomy

• NASA's Spitzer Space Telescope has detected the building blocks of life in the distant universe.

• Training its eye on a faint object located at a distance of 3.2 billion light-years , Spitzer has observed the presence of water and organic molecules in the galaxy IRAS F00183-7111.

• With an active galactic nucleus, this is one of the most luminous galaxies in the universe, rivaling the energy output of a quasar. Because it is heavily obscured by dust, most of its luminosity is radiated at infrared wavelengths

The broad depression in the center of the spectrum denotes the presence of silicates (chemically similar to beach sand) in the galaxy. An emission peak (red) within the bottom of the trough is the chemical signature for molecular hydrogen.

The hydrocarbons (orange) are organic molecules comprised of carbon and hydrogen, two of the most common elements on Earth.

Since it has taken more than three billion years for the light from the galaxy to reach Earth, it is intriguing to note the presence of organics in a distant galaxy at a time when life is thought to have started forming on our home planet.

The Eye of the Beholder:

What we see depends on how we see it

Blazar

Quasar / Seyfert 1

Radio Galaxy / Seyfert 2

Gamma Ray Burst (GRB)• Gamma-ray bursts (GRBs) are the most luminous electromagnetic events

occurring in the universe since the Big Bang. • They are flashes of gamma rays emanating from seemingly random places in

deep space at random times. • The duration of a gamma-ray burst is typically a few seconds, but can range

from a few milliseconds to several minutes, and the initial burst is usually followed by a longer-lived "afterglow" emitting at longer wavelengths

• Most observed GRBs appear to be caused by the collapse of the core of a rapidly rotating, high-mass star into a black hole.

Magnetar• A magnetar is a neutron star with an extremely powerful magnetic field,

the decay of which powers the emission of copious amounts of high-energy electromagnetic radiation, particularly X-rays and gamma-rays.

• Magnetars are somewhere around 20 kilometers in diameter. Despite this, they are substantially more massive than our Sun. Magnetars are so compressed that a thimbleful of its material is estimated to weigh over 100 million tons.

• Most magnetars recorded rotate very rapidly, at least several times per second.

• The active life of a magnetar is short. – Their strong magnetic fields decay after about 10,000 years, after which point activity and strong

X-ray emission cease.

• Given the number of magnetars observable today, one estimate puts the number of "dead" magnetars in the Milky Way at 30 million or more.

• Quakes triggered on the surface of the magnetar cause great volatility in the star and the magnetic field which encompasses it, often leading to extremely powerful gamma ray flare emissions which have been recorded on Earth in 1979, 1998 and 2004.[

The Power Source It is now widely believed that all active galaxies

are powered by the same phenomenon: accretion onto supermassive black-holes. The various types reflect differences in viewing angle and jet activity. The evidence that suggests this model can be summarized by:

· high-velocity gas ( 10,000 Km/s) and relativistic jets imply a deep potential.

· the tiny size of the energy generation region is impossible for stable star clusters

· accreting black-holes are efficient 1014 Solar Luminosities. e.g. implies 4x1024 Kg/s at 10% conversion efficiency, or 70 solar masses per year.

· Any stellar source would use up material at 10 times the rate

Energy generation efficiency of accretion

Accretion is a source of power. In fact, other than matter/anti-matter annihilation (which does not play a significant role in astronomical energy generation), it is by some way the most

efficient source of power. – For a Neutron Star, this is about 30x more efficient

than nuclear fusion

– Black-holes are also efficient although less so than

neutron stars • This is because black-holes have no surface so much of the

energy is never released but is swallowed up by the black-hole directly and also orbits are unstable within three times the Schwarschild radius and little energy is returned inside this distance.

• These factors lead to an efficiency of about 10%

How big are the black-holes?

There is an interesting physical limit that allows us to estimate a minimum mass for the black-holes that power active galaxies, if indeed they do. It is based upon the balance of gravity with radiation pressure. – Material coming into the black-hole is hot and

ionized. Photons radiated by the black-hole interact mostly with electrons and exert an outward force on them. The electrons are electrostatically coupled to protons which are gravitationally attracted to the black-hole.

How big are the black-holes?

• If the accretion rate and corresponding luminosity are too high, the radiation pressure will exceed gravity and mass will be pushed away from the black-hole. The higher the mass of the black-hole, the larger luminosity will be required for this to take place, but in the end we conclude that · for a given black-hole there is a maximum accretion

rate and luminosity that it can sustain.

· the limiting luminosity scales linearly with the black-hole mass

· This is known as the Eddington limit after its discoverer. It applies equally to neutron stars and white dwarfs as to black-holes

How big are the black-holes?

• It is now simple to estimate what mass we need to produce 1014 L. It comes out to be 3x109 M . Such a black-hole has a Schwarschild radius of 1010 Km, comparable to the radius of Pluto's orbit around the Sun. Although large, this satisfies the restriction upon the size of the energy generation region.

• There is now good evidence that most galaxies, active or not, contain large black-holes, if not always as large as a billion solar masses.