astr112 the galaxy lecture 6 prof. john hearnshaw 10. galactic spiral structure 11. the galactic...

ASTR112 The GalaxyLecture 6

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10. Galactic spiral structure11. The galactic nucleus and central bulge 11.1 Infrared observations

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Spiral structure from stars and gas

• 1949 spiral structure first traced in Andromeda galaxy, M31 using OB stars and HII 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)


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Young Popn I objectsdefine the location of the galactic spiral arms

The pitch angle is about25º (angle between armand a circle throughthe arm, centred on thegalactic centre, G.C.)


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• 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 HII clouds, and this enables their locations to be mapped well beyond the limit of optical visibility


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Densities of HI and CO gas as function of distance from galactic centre. Note that HI extends out to ~16 kpc, butCO only to about 9 kpc, the distance of the Sun from centre.


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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 < Ro and b = 0º, then in direction l we have VR = Θ cosα – Θo sinl (Θo = 220 km/s) α and Θ depend on the distance from the Sun and hence VR depends on cloud distance. Measuring VR

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 HI maps


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HI cloud distancesare obtained from their radial velocities.But note theambiguity indistance for cloudswith R < Ro


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HI 21-cm profiles in different galactic longitudes


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More 21-cm profiles in different directions.Notice the very narrow profile in l ~ 180º


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


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HI spiral arms in the outer Galaxy (and elsewhere)


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A galactic plane21-cm HI map isbased on the Doppler shift of the HI clouds and the intensity of the emission from the clouds to locate the HI in the Galaxy


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Density wave theory of spiral structure

• Differential rotation means spiral arms should be wound up tight and cease to exist in a few times 108 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. ωp = constant, and at an angular velocity less than the stars and gas ωp < ω(R)• The spiral arms are a wave travelling backwards through the disk material


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The wind-up problemfor differentially rotating spiral arms.

After a few rotations, thearms should be so tightlywound that in effect theycan no longer be seen inspiral galaxies.

In practice they must there-fore rotate as a solid body.


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• 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 × 106 years

• Lin-Shu density wave theory explains long stability and maintenance of spiral arms, but not their origin or formation


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Diagram of rotation curve and pattern speed


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Arms move at circular velocity Θp = ωpRMaterial (stars and ISM) move at Θ(R)=ω(R).Rωp = constant; ω(R) > ωp


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The galactic nucleus and central bulge

• The galactic nucleus (centre) is invisible at optical wavelengths: extinction AV ~ 25 to 30 mag. (one photon in 1010 to 1012 reaches us.)• Dust extinction is much less in IR and absent in the radio region of the spectrum


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Galactic centre direction in visible light. We can only seea few kpc in this direction, a third of the way to the centre.


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Infrared observations

(a) 2.2 μm Radiation comes from millions of cool bulge stars (mainly K giants) possibly 106 stars/pc3

(b) 4–20 μm Radiation from warm dust clouds at temperatures of a few 100 K At least 4 discrete sources resolved luminosities of 106 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


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λ = 2.2 μm: cool stars λ = 12.4 μm: warm Scale: 1 arcmin 2.5 pc circumstellar dust


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Radio and infrared contourmap of the galactic centre.The elongated contour isfor 100 μm emission fromT ~ 30 K cool interstellar dust. Other warmer IRdiscrete sources and radio sources are also shown.

The area covers about 300 × 200 pc at the galactic centre.


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Infrared observations

In general the infrared brightness of an objectdepends on its temperature. Using Wien’s law λmaxT ~ 3000 μm.KThus 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


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Galactic centre in visible lightnear IR (2.2 microns)and far IR.

The near IR shows cool starsin the centre; the far IR showsthermal radiation from dustgrains.


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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 Wayshowing the dust distribution.

Both images were from theCOBE satellite, 1995.


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End of lecture 6End of lecture 6S

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astr112 the galaxy lecture 6 prof. john hearnshaw 10. galactic spiral structure 11. the galactic...

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