1. Galaxies – review Lecture 12, 13, 27 Upcoming classes Galaxies
Phys 1830: Lecture 32
M82 by Remi Lacasse
Office Hours:
Monday 3pm
Galactic wind i.e. fountain
Previous Classes:
pulsars, gamma-ray bursts
Black holes
This Class
Milky Way Black Hole
Galaxies – review Lecture 12, 13, 27
Upcoming classes
Galaxies
Cosmology.
ALL NOTES COPYRIGHT JAYANNE ENGLISH

2. Types of Black Holes Type Mass Rsch Location Detection Method Lifetime
Solar Masses
yrs
Supermassive
>3*10**6
1/2 light-min
Centre of Galaxies
Doppler shift of orbiting
10**97 to 10**106
gas or stars.
Period of orbit of stars in
images.
Mid-mass
smaller than
Globular Clusters
Doppler shift
(Intermediate)
the solar radius
X-rays from accretion.
Stellar Mass
1 to 10
3 to 30 km
Throughout disk
Doppler Shift of companion.
around 10**67
of galaxies
Gravitational lenses.
Primordial
less than 1
1 cm
Throughout universe
Gamma-rays if
around 10**10
evaporating but this is
the only type of black hole that has
not been observed
even indirectly.
got to here
Come in all sizes. (Note – a googol is 10**100 and 10**googol is called googolplex.)
Primordial BH form in fluctuations at the beginning of the Big Bang. Created by compression of matter by external forces (pressure) in the early universe.
All types of BH are rare.

3. Intermediate Black Holes. Formation by mergers of stars.
Types of Black Holes:
M 13 Danny Lee Russell
Intermediate Black Holes.
Formation by mergers of stars.
Globular cluster has about a 1 million stars within several pc.
Star density high in globular clusters so more opportunity for the merger of stars.

4. Types of Black Holes: Supermassive Black Holes
Measure the motion of stars or gas (using spectra) in the centre of galaxies.
Become an astronomer and figure this out!
Supermassive Black Holes
Centres of Galaxies (including Milky Way)
In both spirals and ellipticals.
If the bulge is small and featureless then there is no evidence that there is a black hole.
Do galaxies form around black holes? Or do galaxies form and in the centre black holes accumulate? This is a question of current research.

5. Black hole in Our Milky Way
Galactic Center Close-Up
(a) An infrared image of part of the Galactic plane shows many bright stars packed into a relatively small volume surrounding the Galactic center (white box). The average density of matter in this boxed region is estimated to be about a million times that in the solar neighborhood. (b) The central portion of our Galaxy, as observed in the radio part of the spectrum. This image shows a region about 100 pc across surrounding the Galactic center (which lies within the bright blob at the bottom right). The long-wavelength radio emission cuts through the Galaxy's dust, providing a view of matter in the immediate vicinity of the Galaxy's center. (c) A recent Chandra image showing the relation of a hot supernova remnant (red) and Sgr A*, the suspected black hole at the very center of our Galaxy. (d) The spiral pattern of radio emission arising from Sagittarius A itself suggests a rotating ring of matter only a few parsecs across. All images are false-color, since they lie outside the visible spectrum. (SST; NRAO; NASA)
Strong radio continuum emission in the centre of our Milky Way.
Sgr A* (pronounced Sagittarius A star – it is NOT a star) has a rotating ring of matter only a few pc across

6. Black hole in Our Milky Way
Use IR wavelengths to see through the dust.
Measure the orbits of stars near Sgr A*

7. Black hole in Our Milky Way
Compare the semi-major axis with the Kuiper Belt which extends a few hundred AU and the Oort Cloud which starts at 50,000 AU. The closest approach distance is within the Kuiper Belt.
Orbits Near the Galactic Center
This extremely close-up map of the Galactic center (left) was obtained by infrared adaptive optics, resulting in an ultra-high-resolution image of the innermost 0.1 pc of the Milky Way. The resolution is high enough that the orbits of individual stars can be tracked with confidence around a still-unseen Sgr A* (marked with a cross). The inset shows the orbit of the innermost star in the frame, labeled S2, between 1992 and The solid line shows the best-fitting orbit for S2 around a black hole of 3.7 million solar masses, located at Sgr A*. (ESO)
The semi-major axis is 950AU and the closest approach is about 110 AU (about 15 lt-hr).
Orbital velocity of star is from the circumference of the orbit divided by the period of the orbit.
Setting F_orbit = F_ gravity  v**2 = GM/r
re-arranged for mass M = r * v**2 / G

8. Black hole in Our Milky Way
Globular cluster radii are more than several pc. (If you could magically force the stars in a globular cluster together to fit in this space, that would cause them to merge and form a massive black hole.)
 3.7 million solar masses at the position of Sgr A*
The only thing that fits within the volume of our solar system that has that amount of mass is a black hole.
The Rsch ~3km * 3.7 * 10**6
~ 10 million km (~10 times radius of sun)
0.02 AU (Mercury’s closest approach is 0.31 AU)

9. Upcoming “snack” for Milky Way’s BH
Gas+dust cloud “G2”
Ejected gas from old star or brown dwarf
According to simulations G2 will pass more than 2000 Rsch but will be tidally disrupted.
Distortions are happening currently.

10. Jet from the Black Hole in the Milky Way
Magenta: Chandra X-ray data
Blue: VLA Radio data

11. Calculate this!!!
Since our sun goes around the centre of our Milky Way Galaxy, how long does it take to orbit?
Radius of orbit is 8.5 kpc and velocity = 220 km/s.
1 pc = 3.09 * 10**13 km

12. stars near the centre of the Milky Way are disappearing.
Review:
Astronomers conclude that a black hole resides in the centre of our Milky Way because:
stars near the centre of the Milky Way are disappearing.
no stars can be seen in the vicinity of the Galactic centre.
stars near the centre of the Mily Way have been observed orbiting some unseen object.
the Galaxy rotates faster than astronomers would expect.

13. Review
Black holes only form when a very massive star implodes. The core of such a star is about 1 solar mass and therefore the black hole consists of singularity with this mass surrounded by an event horizon.
True
False

14. Want to help find black holes?
Citizen science project.
Questions? Discussion?

15. Galaxies harbour black holes
HST & eVLA
1.5 * 10**6 ly long! synchrotron emission
Black holes –active galactic nucleus has a BH engine that generates jets which end in lobes (seen in radio continuum)

16. Galaxies harbour black holes
Black holes – notice the jet in M87  existence of a black hole
Measure the motion of stars or gas (using spectra) in the centre of galaxies  supermassive black holes.
Become an astronomer and figure this out!

17. Sizes range from several million stars up to 100 billion stars.
Galaxies:
Definition: A large group of stars held together by the stars’ mutual gravitational attraction.
Sizes range from several million stars up to 100 billion stars.

18. Spiral galaxies also contain significant amounts of gas and dust.

19. A major component of galaxies (90%) is dark matter. (Lecture 14.)
Gravitational Lensing
Rotation Curves of Spirals
(Dark matter review on the overhead projector.)
Using these techniques we can measure the amount of dark matter in a galaxy. That is, we compare the dynamical mass and the luminous mass.
A major component of galaxies (90%) is dark matter. (Lecture 14.)

20. The rings are from 2 galaxies behind the foreground galaxy.
Can determine that there is dark matter using GR, rather than Newton’s law (i.e. rotation curve).
The rings are from 2 galaxies behind the foreground galaxy.
Foreground galaxy has been subtracted from image on the right.
“This is an image of gravitational lens system SDSSJ as photographed by Hubble Space Telescope's Advanced Camera for Surveys. The gravitational field of an elliptical galaxy warps the light of two galaxies exactly behind it. The massive foreground galaxy is almost perfectly aligned in the sky with two background galaxies at different distances. The foreground galaxy is 3 billion light-years away, the inner ring and outer ring are comprised of multiple images of two galaxies at a distance of 6 and approximately 11 billion light-years. The odds of seeing such a special alignment are estimated to be 1 in 10,000. The right panel is a zoom onto the lens showing two concentric partial ring-like structures after subtracting the glare of the central, foreground galaxy.”

21. Gravitational Lensing in the submm!
Atacama Large Millimetre/Submillimetre Array

22. Credit: Dan Marrone
“Baby Boom”
Can also use lensing to find out other characteristics of galaxies, e.g. very young dusty, gassy galaxies. Milky Way forms 1-10 stars a year. These “starburst” galaxies form up to 10,000 stars per year.
The background galaxy (coloured pink) is about 12 billion light years away. This wavelength traces gas & dust  means that rapid star formation began a mere two billion years after the Big Bang.

23. What could dark matter be?
~10% of matter is is sufficiently luminous to be detected
Stars
X-ray gas in clusters of galaxies
Dust
HI gas
Discuss briefly with your neighbours what 90% of the rest of the matter could be.

24. What could dark matter be?
From measurements we estimate:
~20% from dim, distant objects made from normal atoms
Very cold gas
Black holes
Planets
Small stars like Brown Dwarfs
~10-20% are exotic particles called neutrinos
Recall neutrinos are created in the proton-proton chain.

25. What could dark matter be?
The rest is expected to be exotic particles. (e.g. Sudbury Neutrino Observatory is converting its apparatus to look for these.)

26. No one has determined what could be dark matter
No one has determined what could be dark matter. Planets, dwarf stars, black holes, very cold HI gas have all been eliminated as candidates.
a) True
b) False

27. Galaxies: Hubble Tuning Fork Diagram
Note at the joint of the branches there is a “spheroidal” galaxy.
Main types are
Elliptical (E)
Spiral (S or SB, if they have a bar.)
Irregular (Irr)

28. Population II and old Population I stars (i.e. old stars).
Galaxies: Elliptical
20% of observed galaxies (not including faint, distant dwarf ellipticals).
Population II and old Population I stars (i.e. old stars).
Mass: 10**5 to 10**13 solar masses
Luminosity: 3 * 10**5 to 10**11 solar L
Stars in orbit but orbits are at random orientations.
Note ** means “to the power”.
Miniscule amount of gas and dust.

29. Leo I: A low surface brightness dwarf galaxy with a spheroidal shape.
Galaxies: Elliptical
Miniscule amount of gas and dust.
Leo I: A low surface brightness dwarf galaxy with a spheroidal shape.

30. Note the dust lanes and the pink HII regions.
Galaxies: Spirals
77% of observed galaxies
Note the dust lanes and the pink HII regions.
Arms are bluish  young Pop I stars.
Nucleus and throughout disk are yellowish like an elliptical  Pop II and old Pop I stars.
AAO Malin image so this is almost “true” colour.

31. Mass: 10**9 to 4* 10**11 solar masses. Luminosity: 10**8 to 2 x 10**10
Galaxies: Spirals
Mass: 10**9 to 4* 10**11 solar masses.
Luminosity: 10**8 to 2 x 10**10
Most of the stars, gas and dust orbit in a plane.
Stars in the bulge and halo orbit in random orientations, like stars do in an elliptical.
Mass neither as small nor as large as regular E. Nor as faint or as bright as E.