1. ASTR112 The Galaxy Lecture 6 Prof. John Hearnshaw 10. Galactic spiral structure 11. The galactic nucleus and central bulge 11.1 Infrared observations Galactic HI distribution from 21-cm radio observations

2. ASTR112 The Galaxy Lecture 6 Prof. John Hearnshaw Spiral structure from stars and gas 1949 spiral structure first traced in Andromeda galaxy, M31 using OB stars and H II nebulae 1951 spiral structure first demonstarted in the Galaxy by Morgan, Osterbrock and Sharpless (Yerkes Observ.) using OB stars and young associations, and showing parts of three spiral arms The arms are: (a) the Perseus arm (~2 kpc out from Sun) (b) the Local or Orion arm (passing near Sun) (c) the Sagittarius arm (~2 kpc towards centre)

3. ASTR112 The Galaxy Lecture 6 Prof. John Hearnshaw Young Pop n I objects define the location of the galactic spiral arms The pitch angle is about 25º (angle between arm and a circle through the arm, centred on the galactic centre, G.C.)

4. ASTR112 The Galaxy Lecture 6 Prof. John Hearnshaw Radio observations of CO in dense molecular clouds provide an excellent tracer of the arms, and show an extension of the Sagaittarius arm at about l = 300º Radio observations can also be made of H II clouds, and this enables their loca- tions to be mapped well beyond the limit of optical visibility

5. ASTR112 The Galaxy Lecture 6 Prof. John Hearnshaw Densities of H I and CO gas as function of distance from galactic centre. Note that H I extends out to ~16 kpc, but CO only to about 9 kpc, the distance of the Sun from centre.

6. ASTR112 The Galaxy Lecture 6 Prof. John Hearnshaw Spiral arms from HI clouds Observations use 21-cm emission line (more precisely 21.105 cm or ν = 1420.406 MHz for gas at rest) For R < R o and b = 0º, then in direction l we have V R = Θ cosα – Θ o sinl (Θ o = 220 km/s) α and Θ depend on the distance from the Sun and hence V R depends on cloud distance. Measuring V R from Doppler effect allows distance and location of clouds to be mapped Note there are two locations for any given velocity, so there is always some ambiguity in H I maps

7. ASTR112 The Galaxy Lecture 6 Prof. John Hearnshaw H I cloud distances are obtained from their radial velocities. But note the ambiguity in distance for clouds with R < R o

8. ASTR112 The Galaxy Lecture 6 Prof. John Hearnshaw H I 21-cm profiles in different galactic longitudes

9. ASTR112 The Galaxy Lecture 6 Prof. John Hearnshaw More 21-cm profiles in different directions. Notice the very narrow profile in l ~ 180º

10. ASTR112 The Galaxy Lecture 6 Prof. John Hearnshaw For R > Ro then H I observations can still be used to map H I in the outer Galaxy, provided the rotation curve is assumed to be known, so that V R gives distances H I mapping fails within 20º of G.C. and anticentre, as here V R depends little on distance H I spiral arms are observed to have a pitch angle of about 5º, which is not in very good concordance with the value from H II regions or very young OB stars

11. ASTR112 The Galaxy Lecture 6 Prof. John Hearnshaw H I spiral arms in the outer Galaxy (and elsewhere)

12. ASTR112 The Galaxy Lecture 6 Prof. John Hearnshaw A galactic plane 21-cm H I map is based on the Doppler shift of the H I clouds and the intensity of the emission from the clouds to locate the H I in the Galaxy

13. ASTR112 The Galaxy Lecture 6 Prof. John Hearnshaw Density wave theory of spiral structure Differential rotation means spiral arms should be wound up tight and cease to exist in a few times 10 8 years This fundamental problem can be overcome by the density wave theory of Lin and Shu (1969) The spiral arm pattern rotates as a solid body, i.e. ω p = constant, and at an angular velocity less than the stars and gas ω p < ω(R) The spiral arms are a wave travelling backwards through the disk material

14. ASTR112 The Galaxy Lecture 6 Prof. John Hearnshaw The wind-up problem for differentially rotating spiral arms. After a few rotations, the arms should be so tightly wound that in effect they can no longer be seen in spiral galaxies. In practice they must there- fore rotate as a solid body.

15. ASTR112 The Galaxy Lecture 6 Prof. John Hearnshaw In the density-wave theory, gas and dust fall into the trailing edges of the arms, giving a high density there of gas and dust and dense molecular clouds, where star formation takes place. Young stellar objects, including OB stars emerge from the leading edges of the arms. This model is confirmed by observation. Pattern speed corresponds to one rotation of spiral arms in 400 × 10 6 years Lin-Shu density wave theory explains long stability and maintenance of spiral arms, but not their origin or formation

16. ASTR112 The Galaxy Lecture 6 Prof. John Hearnshaw Diagram of rotation curve and pattern speed

17. ASTR112 The Galaxy Lecture 6 Prof. John Hearnshaw Arms move at circular velocity Θ p = ω p R Material (stars and ISM) move at Θ(R)=ω(R).R ω p = constant; ω(R) > ω p

18. ASTR112 The Galaxy Lecture 6 Prof. John Hearnshaw The galactic nucleus and central bulge The galactic nucleus (centre) is invisible at optical wavelengths: extinction A V ~ 25 to 30 mag. (one photon in 10 10 to 10 12 reaches us.) Dust extinction is much less in IR and absent in the radio region of the spectrum

19. ASTR112 The Galaxy Lecture 6 Prof. John Hearnshaw Galactic centre direction in visible light. We can only see a few kpc in this direction, a third of the way to the centre.

20. ASTR112 The Galaxy Lecture 6 Prof. John Hearnshaw Infrared observations (a) 2.2 μm Radiation comes from millions of cool bulge stars (mainly K giants) possibly 10 6 stars/pc 3 (b) 4–20 μm Radiation from warm dust clouds at temperatures of a few 100 K At least 4 discrete sources resolved luminosities of 10 6 L ⊙ (Rieke, Low) (c) 100 μm Large extended very bright source, about 1½º long, aligned with the galactic equator. It is probably cool IS dust heated to about 30 K by stars in general

21. ASTR112 The Galaxy Lecture 6 Prof. John Hearnshaw λ = 2.2 μm: cool stars λ = 12.4 μm: warm Scale: 1 arcmin  2.5 pc circumstellar dust

22. ASTR112 The Galaxy Lecture 6 Prof. John Hearnshaw Radio and infrared contour map of the galactic centre. The elongated contour is for 100 μm emission from T ~ 30 K cool interstellar dust. Other warmer IR discrete sources and radio sources are also shown. The area covers about 300 × 200 pc at the galactic centre.

23. ASTR112 The Galaxy Lecture 6 Prof. John Hearnshaw Infrared observations In general the infrared brightness of an object depends on its temperature. Using Wien’s law λ max T ~ 3000 μm.K Thus 2.2 μm → T ~ 1500 K (cool stars) * 10 μm → T ~ 300 K (warm dust) 100 μm → T ~ 30 K (cool diffuse dust layer) *Actually the coolest stars are about 3000 K and would radiate strongly at 1 μm

24. ASTR112 The Galaxy Lecture 6 Prof. John Hearnshaw Galactic centre in visible light near IR (2.2 microns) and far IR. The near IR shows cool stars in the centre; the far IR shows thermal radiation from dust grains.

25. ASTR112 The Galaxy Lecture 6 Prof. John Hearnshaw A near IR view of the whole Milky Way showing the distribution of cool stars, including the concentration in the galactic centre. A far IR view of the Milky Way showing the dust distribution. Both images were from the COBE satellite, 1995.

26. ASTR112 The Galaxy Lecture 6 Prof. John Hearnshaw End of lecture 6 Spiral galaxy Messier 51