1. The matter in our Galaxy emits different kinds of radiation.
5/22/2018
The matter in our Galaxy emits different kinds of radiation.

3. The ground state in hydrogen is actually two energy levels and if it is left alone long enough…
Called the “spin flip” transition
l = 21 cm
And there’s lots of Hydrogen out there!!!

4. Radio observations help map the galactic disk
Looking for 21-cm wavelengths of light …
emitted by interstellar hydrogen
as we look along the disk of the Milky Way (from inside), we see 21-cm photons Doppler shifted varying amounts
this allows the interstellar hydrogen to be mapped

5. A Map of the Milky Way Based on 21-cm wavelength light mapping

6. Zone of Avoidance

7. Spiral Galaxy M83 observed in both visible light and radio wavelengths.

8. Don’t galaxies look like they spin?

9. We investigate the “Rotation Curve” of our Galaxy by plotting the velocity as a function of radial distance d from the center. d d

10. Orbital Velocities in the Disk
5/22/2018
Orbital Velocities in the Disk
rotation curve – a plot of rotational (orbital) speed .vs. distance from the center
speed
radius

11. Do galaxies rotate like a Merry-Go-Round ?
(solid body rotation)

12. Differential Rotation of the Galaxy
The Sun orbits at 230 km/s or about 500,000 mph

13. Do they rotate like planets in our solar system ? (Keplerian falloff)
5/22/2018
Do they rotate like planets in our solar system ?
(Keplerian falloff)

14. The Galaxy’s Rotation Curve

15. Most of the matter in the Galaxy has not yet been identified
According to Kepler’s Third Law, the farther a star is from the center, the slower it should orbit
Observations show that speed actually increases with distance from the center
This could be due to gravity from extra mass we cannot see - called DARK MATTER.

16. Orbital Velocities in the Disk
Stars in the Galactic disk should orbit according to Kepler’s Laws
Here is what we observe:
The flat rotation curve of our Galaxy implies that:
its mass in not concentrated in the center
its mass extends far out into the halo
But we do not “see” this mass
we do not detect light from most of this mass in the halo
so we refer to it as dark matter

17. Zone of Avoidance
Halo

19. The Very Large Array (VLA) in New Mexico

21. Center of the Galaxy Radio
Although dark in visual light, there are bright radio, IR, and X-ray sources at the center of the Galaxy, known as Sgr A*.
X-ray

22. Center of the Galaxy in Sagittarius
Infrared
Visual

23. Center of the Galaxy
We measure the orbits of fast-moving stars near the Galactic center.
these measurements must be made in the infrared
in particular, this star passed within 1 light-day of Sgr A*
using Kepler’s Law, we infer a mass of 2.6 million M for Sgr A*
What can be so small, yet be so massive?

24. X-ray Flare from Sgr A*
The rapid flare rise/drop time (< 10 min) implied that the emission region is only 20 times the size of the event horizon of the 2.6 million M black hole.
Observations are consistent with the existence of a supermassive black hole at the center of our Galaxy.
Energy from flare probably came from a comet-sized lump of matter…torn apart before falling beneath the event horizon!
Chandra image of Sgr A*

25. The Star–Gas–Star Cycle

26. Halo vs. Disk Stars Stars in the disk are relatively young.
fraction of heavy elements same as or greater than the Sun
plenty of high- and low-mass stars, blue and red
Stars in the halo are old.
fraction of heavy elements much less than the Sun
mostly low-mass, red stars
Stars in the halo must have formed early in the Milky Way Galaxy’s history.
they formed at a time when few heavy elements existed
there is no ISM in the halo
star formation stopped long ago in the halo when all the gas flattened into the disk

27. Stellar Orbits in the Galaxy
Stars in the disk all orbit the Galactic center:
in the same direction
in the same plane (like planets do)
they “bobble” up and down
this is due to gravitational pull from the disk
this gives the disk its thickness
Stars in the bulge and halo all orbit the Galactic center:
in different directions
at various inclinations to the disk
they have higher velocities
they are not slowed by disk as they plunge through it
nearby example: Barnard’s Star

28. Mass of the Galaxy We can use Kepler’s Third Law to estimate the mass
Sun’s distance from center: 28,000 l.y. = 1.75 x 109 AU
Sun’s orbital period: 230 million years (2.3 x 108 yr)
P2 = 42/GM a3  mass within Sun’s orbit is 1011 M
Total mass of MW Galaxy : 1012 M
Total number of stars in MW Galaxy  2 x 1011

29. Galaxies

30. Galaxies seem to take one of four different appearances
I. SPIRALS

31. SPIRALS

32. SPIRALS

33. The tightness of a spiral galaxy’s arms is correlated to the size of its nuclear bulge
Type Sa
Type Sb
Type Sc

34. Variety of Spiral Arms Flocculent spirals (fleecy)
Grand-design spirals (highly organized)

35. We easily see these spiral arms because they contain numerous bright O and B stars which illuminate dust in the arms.
However, stars in total seem to be evenly distributed throughout the disk.

36. Spiral Structure Our Galactic disk does not appear solid.
it has spiral arms, much like we see in other galaxies like M51
These arms are not fixed strings of stars which revolve like the fins of a fan.
They are caused by compression waves which propagate around the disk.
such waves increase the density of matter at their crests
we call them density waves
they revolve at a different speed than individual star orbit the Galactic center
Note how the spiral arms appear bluer compared to the bulge or the gaps between the arms.
M 51

38. Compression Wave

39. Spiral Arms
The compression caused by density waves triggers star formation.
molecular clouds are concentrated in arms…plenty of source matter for stars
short-lived O & B stars delineate the arms and make them blue & bright
long-lived low-mass stars pass through several spiral arms in their orbits around the disk

44. Galaxies seem to take one of four different appearances
II. BARRED SPIRALS

45. BARRED SPIRALS

46. BARRED SPIRALS

47. Bars of stars run through the nuclear bulges of barred spiral galaxies
Type SBa
Type SBb
Type SBc

48. Galaxies seem to take one of four different appearances
Type E0
Type E3
Type E6
III. ELLIPTICALS

49. ELLIPTICALS

50. Elliptical galaxies display a variety of sizes and masses
Giant elliptical galaxies can be 20 times larger than the Milky Way
Dwarf elliptical galaxies are extremely common and can contain as few as a million stars

51. Galaxies seem to take one of four different appearances
IV. IRREGULAR

52. Galaxies seem to take one of four different appearances
Spirals
Barred Spirals
Ellipticals
Irregulars

53. This classification scheme is known as the Hubble Tuning Fork Scheme