10. Galactic spiral structure

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10. Galactic spiral structure 11. The galactic
nucleus and central bulge 11.1 Infrared
observations
Galactic HI distribution from 21-cm radio
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 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.)
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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 loca-
• tions 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, but CO 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 ? 1420.406 MHz for gas at
  rest)
• For R lt Ro and b 0º, then in direction l we
  have
• VR T cosa To sinl
  (To 220 km/s)
• a and T 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 distances are obtained from their
radial velocities. But note the ambiguity
in distance for clouds with R lt 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 gt 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 plane 21-cm HI map is based 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.
• ?p constant, and at an angular velocity less
  than the
• stars and gas
• ?p lt ?(R)
• The spiral arms are a wave travelling backwards
• through the disk material

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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.
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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 Tp ?pR Material
(stars and ISM) move at T(R)?(R).R ?p
constant ?(R) gt ?p
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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 see a 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) 420 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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? 2.2 µm cool stars ? 12.4
µm warm Scale 1 arcmin ? 2.5 pc
circumstellar dust
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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.
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Infrared observations In general the infrared
brightness of an object depends on its
temperature. Using Wiens law
?maxT 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
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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.
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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 Way showing the dust
distribution. Both images were from the COBE
satellite, 1995.
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End of lecture 6
Spiral galaxy Messier 51