AGN
Jason Jeong
Intro: AGN
AGN stands for active galactic nuclei.
It is the core of the galaxy that produces more
radiation than the rest of the galaxy.
Intro: AGN
It is an active galaxy because of one of the
following:
• Nuclear optical continuum emission
Nuclear infra-red emission
Broad optical emission lines
Narrow optical emission lines
Radio continuum emission
X-ray continuum emission
X-ray line emission
Intro: History
Astronomers knew early in the twentieth century that some galaxies have
emission-line nuclei. However, even the systematic study by Seyfert (1943)
was not enough to launch active galactic nuclei (AGN) as a major topic of
astronomy. The advances in radio astronomy in the 1950s revealed a new
universe of energetic phenomena, and inevitably led to the discovery of
quasars. These discoveries demanded the attention of observers and
theorists, and AGN have been a subject of intense effort ever since. Only a
year after the recognition of the redshifts of 3C 273 and 3C 48 in 1963, the
idea of energy production by accretion onto a black hole was advanced.
However, acceptance of this idea came slowly, encouraged by the
discovery of black hole X-ray sources in our Galaxy and, more recently,
supermassive black holes in the center of the Milky Way and other
galaxies. Many questions remain as to the formation and fueling of the
hole, the geometry of the central regions, the detailed emission
mechanisms, the production of jets, and other aspects. The study of AGN
will remain a vigorous part of astronomy for the foreseeable future.
Intro: types
Elliptical Galaxy: Elliptical galaxies are very common in our universe. However, they are
faint in the radio spectrum because they are mostly cool stars.
Starburst Galaxy: Some galaxies are currently forming stars at a furious rate, going
through a stellar “baby boom.” These galaxies are known as starburst galaxies.
Radio Galaxy: The term radio galaxy was coined to describe objects that look like
normal galaxies in optical images, but were found to emit enormous amounts of
radio waves.
Quasars: Quasars are the most distant and most luminous type of AGN known; and
their spectra doesn’t look like normal galaxies at all.
BL Lac objects: Most AGN have strong emission lines, but a special class of AGN are
notorious for having only very weak emission lines, if any at all.
Seyfert galaxies: are characterized by extremely bright nuclei, and spectra which
have very bright emission lines of hydrogen, helium, nitrogen, and oxygen.
Optically Violent Variable Quasar (OVV quasar):is a type of highly variable quasar.
It is a subtype of blazar that consists of a few rare, bright radio galaxies, whose
visible light output can change by 50% in a day. They are similar in appearance to
BL Lacs but generally have a stronger broad emission line, and tend to have higher
red shift components.
Need to know: Spectroscopy
Spectroscopy pertains to the dispersion of an
object's light into its component colors (i.e.
energies).
There are three types of spectra: Continuous,
Emission, and absorption lines.
Types of spectra
Continuous is all wavelengths of light produced by hot, dense objects.
EX: the sun is hot and dense, it produces the light to us which is the
visible
spectrum.
Emission line is a short wavelength produced by hot, low-density
objects.
EX: hydrogen gas, when electricity runs through it, gives off a
couple of spectra.
I think this could also be like neon lights that are filled with argon
and other noble
gases (most likely).
Absorption line is a wavelength absorbed by cool gas/gases (the light
comes from a different source). It absorbs what ever wavelength it
would usually emit at high temperatures.
EX: any gas
Applications of spectra
You can use the knowledge of spectra especially
the emission lines to identify the element or
galaxy depending on its unique spectrograph.
Identifying galaxies with spectrographs
Elliptical Galaxy: Elliptical galaxies are very common in
our universe. However, they are faint in the radio
spectrum because they are mostly cool stars. Since
these galaxies are close to us they have very small red
shifts. Also their graphs rarely ever have emission lines
because of the cool nature of the gases and can be
classified by the unique curvature of its graph. Another
feature of the elliptical galaxies are that their flux drops
about 50% or half its value around wavelengths below
the Ca II lines (also known as Ca II break) see graph
below for an example of an Elliptical Galaxy. (notes
from AGN article edited by JJ).
Identifying galaxies with spectrographs
Starburst Galaxy: Some galaxies are currently forming stars at a furious rate,
going through a stellar “baby boom.” These galaxies are known as
starburst galaxies. Often rapid star formation is induced in a galaxy by
gravitational interaction or collision with another galaxy. Newly-formed
massive stars in the starburst galaxy heat up gas in the interstellar medium
and create strong, narrow emission lines which are seen in addition to the
galaxy’s spectrum. Because they are newly formed and super hot, these
galaxies give off more blue light than regular galaxies. It is often difficult to
differentiate between starburst galaxies and radio galaxies because of
their similar narrow emmission lines. One difference is that the Hβ and [O
III] emission lines in starburst galaxies are usually about the same
strength. The same is true for the Hα and [N II] emission lines; however
since these two lines are so close to each other they are usually “blended”
together, as is the case in the example below. The [O II] and [S II] emission
lines are also common is starbursts, but not always present. They are rare
and are weak sources of radio waves. (notes from AGN article edited by JJ)
Identifying galaxies with spectrographs
Radio Galaxy: The term radio galaxy was coined to
describe objects that look like normal galaxies in
optical images, but were found to emit enormous
amounts of radio waves. Their optical spectra reveal
the presence of strong, narrow lines and a CaII break
strength that is <40%. For these reasons radio galaxies
are easily confused with starburst galaxies. The primary
difference is the strength of the emission lines: in radio
galaxies the forbidden lines [O II], [O III] and [N II] are
typically much stronger than Hα and Hβ (as in the
example below, where [O III] is much stronger than
Hβ). Unlike quasars, radio galaxies tend not to have
broad emission lines. (from AGN article edited by JJ).
Identifying galaxies with spectrographs
Quasars: Quasars are the most distant and most luminous
type of AGN known; and their spectra doesn’t look like
normal galaxies at all. Instead of having an optical
spectrum which looks like a galaxy (e.g., with many
absorption lines and a CaII break), quasars have a very
smooth continuum spectrum with strong and broad
emission lines. The continuum you see is not due to
starlight but synchrotron radiation from the AGN. The
quasar’s emission lines are produced by clouds of gas
within the galaxy which are heated by the AGN.
Quasars are so luminous they usually outshine their
host galaxy, often by as much as 1000 times or more.
(from AGN article edited by JJ).
Identifying galaxies with spectrographs
BL Lac objects: Most AGN have strong emission lines, but
a special class of AGN are notorious for having only
very weak emission lines, if any at all. They are known
as “BL Lacertae objects,” or BL Lacs for short. Because
they lack strong emission lines, it is often difficult to
determine redshifts for these objects. BL Lacs are most
easily differentiated from radio galaxies and quasars by
their emission lines: quasars and radio galaxies have
strong lines, BL Lacs do not. Like radio galaxies, BL Lacs
often show a Ca II break in their spectrum whereas
quasars rarely do. (from AGN article edited by JJ).
Shifts
Redshift is the shift of the light to a longer
wavelength due to the object moving away
from us the observer.
Opposite to this is the blue shift where the
object is moving towards the observer giving
us a shorter wavelength.
Calculating shifts
To find the shift you have to identify two strong
emission lines. After that you find the ratio
between the two lines and use the redshift
equation to find the shift.
Calculating other information from
shifts
Velocity, Distance and Luminosity
After you have found out what the redshift, z, is then you can find
out what the velocity, distance and the luminosity of the galaxy that
you are looking at.
To get Velocity:
If an object is pretty close (close enough that the redshift is less
than or equal to one) then you can use the equation:
v=cz where v is the velocity, c is the speed of light, and z is the
redshift.
If the redshift is greater than one then use this equation:
redshift_greater_than_one_equation.jpg
Calculating other information from
shifts
To get distance:
For near galaxies use Hubble’s Law:
Hubble's_Law.jpg
For farther galaxies we have to define a model of the universe first.
One of the simplest models of the universe we have is the "empty
Universe" model and from that we can use:
Empty_universe_model_equation.jpg
to find the distance of the galaxy we are looking at.
AGN is one of the farthest and brightest objects in the universe
because we can observe the light from its luminosity (explained
below) and the time it takes for the light to reach us is so long
(because it's so far away) that it is as if we are looking back in time
to the origins of the universe.
Calculating other information from
shifts
To get luminosity:
What we see through the telescope is the flux of the galaxy, the energy per second
of the light we see.
But, what we want is the luminosity or the total amount of light it emits per
second. Luminosity is essential because it tells us the amount of energy the galaxy
is producing.
According to the inverse-square law, flux and luminosity are related by the square
of the distance to the object. For example, if two objects have the same luminosity
but the first object is twice as far away as the second, the flux of the first object
will be one-quarter that of the second. The relationship between flux and
luminosity for very distant objects is given by:
This equation relates luminosity, L, flux, f, distance, d, and the shift, z.
We have to use the flux over the whole wavelength so we have to integrate or find
the total flux over the wavelengths (the area under the "curve". We can see a
demonstration in the picture below over a part of the wavelength.
Ca II break
CaII break: The "calcium break" is an overall
decrease in flux on the blue side (the left side)
of the spectrum beginning somewhere around
the calcium absorption doublet ~
4000Å. In other words when a "cliff" is made
in the spectra on the left side. (only cliff up as
shown in picture below).