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Cosmology - Galaxies

We already met our Milky Way galaxy in the Our Galaxy section. We also discussed stars which are members of galaxies. Most of the beautiful phenomenon we see in the night sky - comets, planets, star clusters, planetary nebula and diffuse nebula - are all around or within our own galaxy. Beyond our galaxy are other interesting phenomenon:

By studying these galaxies and their motion relative to us and each other we learn about how our Universe was formed. What we know about galaxies gives vital clues as to how the Universe began.

It has been recently discovered that while there are a few spiral galaxies in the early Universe, the majority of galaxies are elliptical. As for the size, they are surprisingly small compared to our own galaxy.

There is a variety of galaxies, all with different sizes and shapes. The work of Edwin Hubble allows Astronomers to categorize the variety of galaxies on a diagram called the Tuning Fork diagram, or the Hubble Tuning Fork diagram. While initially thought of as an evolutionary diagram for galaxies, we now know that most galaxies do fit in a defined category:


(Image Credit)

Visit the Galaxy Morphology page for a real world example of analyzing galaxy types.

There are basically two major types of galaxies:

  • Elliptical galaxies

  • Spiral galaxies

Elliptical galaxies are unique in that there is no organize structure of stars. There are no spiral arms and stars travel in just about any direction. Giant elliptical galaxies are very rare and can be 20 times the size of any other galaxy. More common are dwarf elliptical galaxies which are smaller than spiral galaxies and contains only a few million stars.

Spiral galaxies are more organized, containing spiral arms that appear orderly in motion. The average number of stars in a spiral galaxy is about 100 billion (billion with a b)! Spiral galaxies themselves have three major categories:

  • Normal spirals

  • Barred spirals

  • Irregular

A normal spiral galaxy have their arms connected directly to the core of the galaxy while barred spirals have their arms connected to a protrusion (called a bar) emanating from the core.

The grading scheme of the Tuning Fork is as follows:

  • E0 is a round elliptical galaxy

  • E7 is a near disk

  • S0 can look like an elliptical galaxy, has a more thinned disk

  • Sa is a normal spiral with a bright central bulge and a tight spiral structure

  • Sc is a normal spiral with a dim bulge and a loose spiral structure

  • SBa is a barred spiral with a bright central bulge and a tight spiral structure

  • SBc is a barred spiral with a dim bulge and a loose spiral structure

The numbers and letters in between (i.e. E1, E2, Sb, Sb1, SBb, SBb1, ect.) are all based on visual interpretation.

Irregular galaxies are a group recently added to the Tuning Fork diagram (though not seen above). Irregular galaxies have some hints of organized spiral structures, but are loosely scattered.

A galactic comparison:

  Absolute Blue Magnitude: Mass (Solar Mass): Disk Diameter (kiloparsecs):
Elliptical Galaxies: -8 to -23 107 to 1013 ~0.3 to 100
Spiral Galaxies: -16 to -23 109 to 1012 5 to 100
Irregular Galaxies: -13 to -20 108 to 1010 1 to 10

Here are some examples of a variety of galaxy types:

SBb - Galaxy NGC5383 E0 - Galaxy M87
Sc - Galaxy M101 Irr - Galaxy IC4182

Active Galactic Nuclei is a term that describes four types of galaxies:

Active Galactic Nuclei, or AGN, produce bright emissions of non-thermal synchrotron radiation by a common source - a supermassive black hole. It is believed that every galaxy has at its core a supermassive black hole. This includes our galaxy. AGN's exist when the black hole has material to "feed." This material can be stellar debris, molecular clouds of hydrogen, even stars. If the black hole is large enough, it can strip nearby stars of their atmospheres. More information on black holes can be found in the Stars section.

Synchrotron radiation is the source of jets and lobes that are seen with AGN's. Magnetic field lines created by the supermassive black hole traps electrons from ionized particles near the accretion disk. These electrons are sent out in jets at almost the speed of light - these are called "relativistic jets." Note: the term "relativistic" is used for any phenomenon traveling near the speed of light.


(Image credit: Brooks/Cole Thomson Learning)

One of the major differences between normal galaxies and AGN's is their measured spectra. Non-thermal emission is detected by spectrometer and is responsible for the extended spectra seen here in the example on the right.

A normal galaxy has the characteristic bump seen on the lower part of the image while the AGN's have dominating non-thermal emission primarily from synchrotron radiation.


(Image credit: Brooks/Cole Thomson Learning)

Radio galaxies are giant elliptical galaxies that look normal when viewed through a telescope, but emit very strong radio emissions. While using an optical telescope, a radio galaxy will look like any other elliptical galaxy. But when an astronomer uses a radio dish to examine a radio galaxy, its "brightness" increases to nearly 1033 watts. The majority of radio emission is in the form of radio lobes, although a halo around the core of the galaxy can also emit radio waves.

The image on the left is of elliptical galaxy M87. Using the Hubble Space Telescope and using an infra-red and near UV filters, the core of the galaxy is visible as is the emanating jet of synchrotron radiation. The jet is moving rapidly, and in many cases can travel near the speed of light. Such speeds are often referred to a "relativistic."

When using radio to examine a radio galaxy, the picture changes somewhat.

This image of Centaurus A has been overlaid with its composite radio image. The radio lobes (shown perpendicular to the dust lane colored in blue, green and red) are not visible optically but are clearly visible in the radio spectrum. It is common for these lobes to extend far out from the core. The source of these lobes are the same as the jets - synchrotron radiation.

Seyfert galaxies are considered radio quiet AGN's. They generally do not emit strong radio waves although few have been seen with very small radio lobes. Seyfert galaxies are characterized by their extended non-thermal emission lines as well as a predominantly bright nucleus.

This image of NGC7742 is a typical Seyfert galaxy. The nucleus - or center of the galaxy - is often very bright, sometimes 10 times that of a normal galaxy.

In addition to this, Seyferts also emit strong X-ray and IR emissions as well. This indicates the engine of a Seyfert being a supermassive black hole.

Seyfert galaxies themselves are in two categories:

  • Seyfert 1 - both broad and narrow emission lines, strong UV and X-ray emission
  • Seyfert 2 - narrow emission lines and weak UV and X-ray emissions but strong IR emission

The main reason behind Seyfert galaxies lack of radio emission is though to be due to a smaller "engine" (or a supermassive black hole that is not as big as the ones inhabiting radio galaxies) and large clouds of dust near the nucleus that absorb higher energy photons and re-emit them at longer, IR (InfraRed) wavelengths.

Both radio and Seyfert galaxies appear to bridge the gap between normal galaxies and quasars. This has the suggestion that all galaxies go through and evolutionary process that start out as quasars and end up as a normal galaxy.

BL Lac objects are named after the star BL Lacertae. Discovered in 1929, this star was believed to be a variable star (since the cores of AGN's do fluctuate in brightness), but measurements of redshift place this object at a distance equal to known distant galaxies.

BL Lac objects emit very strong synchrotron radiation that suggests that the orientation of this particular AGN is such that the radio lobes and jets are aimed toward us. As a result, the intensity of the energy generated by the core is such that any spiral structures are difficult to discern. A characteristic of these objects are no emission lines (or very weak ones) but are very strong emitters of X-ray and IR wavelengths. As such, a BL Lac object can look like a bright variable star.

While quasars (covered below) are radio loud, blazars - which have the same characteristics as a quasar - are optically bright. A blazer is basically a BL Lac object that has a high redshift (meaning its far away).

Quasars were discovered in 1963. These objects were believed to be stars that emitted strong radio waves - hence the name QUAsiStellAR objects.

When viewed optically, a quasar looks like a distant red star. The image on the right indicates a quasar with a redshift of 4.75 (which is about 1.7 x 1010 light-years away).

One of the characteristics of a quasar is an unusually strong UV emission. While it is believed that the major characteristic of a quasar is its strong emission of radio waves, in reality only 10% of all known quasars do emit strong radio. It is not clear as to why this is. It is suggested that "radio quiet" quasars lack the jets emanating from the core.

Instead, all quasars have in common strong emissions of near-UV and near-IR wavelengths - the result of which is primarily blue emission.

So if quasars are mostly blue, why do they look red? The perceived red color of a quasar is a result of its rapid redshift. Quasars are being accelerated by the expanding Universe and a very high rate of speed, lengthen the wavelength as it travels back towards our telescopes. In addition, any interstellar dust will absorb all wavelengths and re-emit them in red - this is called interstellar reddening.

The Unified AGN Model:

As mentioned earlier, the primary engine for AGN's is a supermassive black hole. The variability in orientation of the back hole's accretion disk and radio lobes will indicate the type of AGN. For example, a BL Lac object is an AGN with its lobes pointed directly towards Earth.


(Image credit: Brooks/Cole Thomson Learning)

Probably the most amazing thing about the AGN is its size. The image above illustrates the AGN portion of a galaxy, and the diameter from dusty torus to dusty torus is less than the diameter of our Solar System! The types of radiation seen depends on the orientation of the AGN. The emitted particles vary depending on the source location and path of emission:


(Image credit: Brooks/Cole Thomson Learning)

So what do we see based on orientation?


(Image credit: Brooks/Cole Thomson Learning)

It is important to realize that the entire premise of AGN's lie in our collected evidence of black holes. While no black hole has has ever been directly observed, what we have seen so far with AGN's is that the model fits observed data and that strongly suggests that supermassive black holes exist.


(Image credit: Brooks/Cole Thomson Learning)

The image above is one such piece of evidence. Strong red and blue shifts of the bright core of M87 suggests a rotating supermassive black hole - this visual evidence supports the AGN model of the accretion disk with the expulsion of synchrotron radiation in the form of a jet.

Summary of AGN's:

Property: Quasars: Seyfert Galaxies: Radio Galaxies: BL Lac Objects:
Galaxy Type: Spiral and Elliptical Spiral Giant Elliptical Elliptical
Appearance: Compact, Blue Compact Bright Nucleus Elliptical Bright, Star-like
Maximum Luminosity: 100-1000x Milky Way Comparable to Bright Spirals Strong Radio 10,000x Milky Way
Continuum Spectrum: Non-thermal Non-thermal Non-thermal Non-thermal
Emission Spectrum: Broad and Narrow Broad and Narrow Rare, Broad and Narrow Very Weak
Absorption Lines: Yes None Yes None
Variability: Days to Weeks Days to Weeks Days Hours
Emits Radio: Some Weak Strong Weak
Redshifts: z>0.5 z~0.05 z<0.05 z~0.1
Jets: Some Some Often Possible

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