2. Hubble’s law（哈勃定律）
•Hubble measured the spectroscopy of galaxies and found a galaxy's receding velocity
（退⾏速度） is directly proportional to its distance from us.
V = Hr
Hubble
constant
How long distance
galaxy is away from
us
The receding velocity
of galaxy
4. •We often use “redshift(红移)” to express distant or past（过去） universe.
Redshift
Redshift
Due to the expansion of the universe, the wavelength of the distant or past universe extends.
z =
0
0
Original wavelength
Extended wavelength
z=0: present universe
z=0.1: 1.4 Gyr ago
z=1: 7.8 Gyr ago
z=10: 13.2 Gyr ago
z=20: 13.7 Gyr ago
G:Giga, 10亿年
5. V = Hr
Top of the distance ladder
If we know the value of the Hubble constant , we can
measure the distance to galaxies from their receding velocity
by spectroscopy.
H
•So far, we have seen various methods to measure
the distance of stars and galaxies. (e.g. parallax,
variable stars, Tully-Fisher, supernova).
•Hubble’s law can be used to measure the
distance beyond 1Gpc.
•As I emphasized, the measuring distance in
astronomy is quite important!
9. •Hubble discovered there are numerous galaxies in the universe
besides the Milky Way galaxy.
•Hubble also discovered the expansion of the universe.
•His discoveries have contributed tremendously to human’s
understanding of the universe.
•Unfortunately, he died before
being awarded the Nobel Prize.
But, I believe he was worthy of the
Nobel Prize.
10. Active galactic nuclei
Galactic radiation
•Most normal galaxies have a blackbody
radiation-like energy distribution with a
peak in the visible portion of the
electromagnetic spectrum.
Because this kind of energy
distribution is contributed by stars.
•But, some active galaxies have different energy
distribution from normal galaxies.
•In the energy distribution of active galaxies,
both longer and shorter wavelengths than
visible wavelength also contributes to energy
distribution.
11. Active galactic nuclei
Galactic radiation
•Most normal galaxies have a blackbody
radiation-like energy distribution with a
peak in the visible portion of the
electromagnetic spectrum.
Because this kind of energy
distribution is contributed by stars.
•But, some active galaxies have different energy
distribution from normal galaxies.
•In the energy distribution of active galaxies,
both longer and shorter wavelengths than
visible wavelength also contributes to energy
distribution.
12. Active galactic nuclei
• These activities of “active galaxies” is related to
violent events occurring in or near the galactic
nucleus.
•Thus, we call such a system active galactic
nuclei(AGN).
Active galaxy
AGN
The example of active galaxies
•Seyfert galaxy
•Radio galaxy
•Quasar
Galactic nuclei (~1kpc) are important!
13. Seyfert galaxy
Seyfert galaxy
•spiral or irregular galaxies.
•Seyfert galaxy has extremely luminous galactic nuclei. The nucleus of the
Seyfert galaxy is roughly 10 times more energetic than the entire Milky Way.
•We can observe very bright emission lines in the spectrum of the Seyfert galaxy.
14. Radio galaxies
•Radio galaxies are active galaxies that emit a large amount of energy in the radio portion of the
electromagnetic spectrum. Many radio galaxies are elliptical galaxies.
•The energy is released from two huge extended regions called radio lobes.
Central energy source
produces high-speed
jet.
Later, I will explain what a central energy source is…
Radio galaxies
15. Quasars
Quasars
•It seems star, but not a star! “Quasi (starlike)-Stellar radio sources(Quasars)”
•Quasars are very bright and distant objects in the universe. Some quasars are discovered
at .
z > 7
Narayan and Bharat (2021)
Redshift distribution of quasars
, highest redshift quasar
z = 7.64
16. The central engine of an active galaxy
Active galaxies
•Seyfert galaxies
•Radio galaxies
•Quasars
•Active galaxies, as well as “normal” galactic nuclei, share
common energy generation mechanism
•The activity of such active galaxies are related to active galactic
nuclei (AGN)
AGN
What is the engine of an active galaxy or AGN??
17. The central engine of an active galaxy
The properties of AGN
•They have high luminosity, which is greater than normal galaxies.
•Their energy emission is mostly nonstellar. It cannot be explained as the combined
radiation of stars.
•They may exhibit jets and other signs of explosive activity.
Astronomers suggest that the central engine of an AGN
is Supermassive black holes (SMBH)
•If we assume the central engine of the AGN is SMBH,
we can successfully explain the properties of AGN
Accretion disk
Black hole
Jet
Magnetic field
18.
19.
20.
21.
22. Summary
• There are numerous galaxies in the universe.
• Hubble discovered the expansion of the universe by
measuring the velocity of galaxies.
• Some galaxies are active (Quasars, radio galaxies, and
Seyfert galaxies). The activity of active galaxies is related
to their nuclei (AGN).
• The central engine of the AGNs is supermassive black
holes.
