Download Lecture 6: Galactic spiral structure

ASTR112 The Galaxy
Lecture 6
10. Galactic spiral structure
11. The galactic nucleus and central bulge
Prof. John Hearnshaw
Galactic HI distribution from
21-cm radio observations
11.1 Infrared observations
ASTR112 The Galaxy
Lecture 6
• 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)
Prof. John Hearnshaw
Spiral structure from stars and gas
ASTR112 The Galaxy
Lecture 6
The pitch angle is about
25º (angle between arm
and a circle through
the arm, centred on the
galactic centre, G.C.)
Prof. John Hearnshaw
Young Popn I objects
define the location of
the galactic spiral arms
ASTR112 The Galaxy
Lecture 6
• Radio observations
can also be made of
HII clouds, and this
enables their locations to be mapped
well beyond the limit
of optical visibility
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
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.
Prof. John Hearnshaw
ASTR112 The Galaxy
Lecture 6
ASTR112 The Galaxy
Lecture 6
• 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
Prof. John Hearnshaw
Spiral arms from HI clouds
HI cloud distances
are obtained from
their radial
velocities.
But note the
ambiguity in
distance for clouds
with R < Ro
Prof. John Hearnshaw
ASTR112 The Galaxy
Lecture 6
HI 21-cm profiles in different galactic longitudes
Prof. John Hearnshaw
ASTR112 The Galaxy
Lecture 6
More 21-cm profiles in different directions.
Notice the very narrow profile in l ~ 180º
Prof. John Hearnshaw
ASTR112 The Galaxy
Lecture 6
• 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
Prof. John Hearnshaw
ASTR112 The Galaxy
Lecture 6
HI spiral arms in the outer Galaxy (and elsewhere)
Prof. John Hearnshaw
ASTR112 The Galaxy
Lecture 6
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
Prof. John Hearnshaw
ASTR112 The Galaxy
Lecture 6
ASTR112 The Galaxy
Lecture 6
• 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
Prof. John Hearnshaw
Density wave theory of spiral structure
ASTR112 The Galaxy
Lecture 6
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 therefore rotate as a solid body.
Prof. John Hearnshaw
The wind-up problem
for differentially rotating
spiral arms.
ASTR112 The Galaxy
Lecture 6
• 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.
• Lin-Shu density wave theory explains long stability
and maintenance of spiral arms, but not their origin
or formation
Prof. John Hearnshaw
• Pattern speed corresponds to one rotation of spiral
arms in 400 × 106 years
Diagram of rotation curve and pattern speed
Prof. John Hearnshaw
ASTR112 The Galaxy
Lecture 6
Arms move at circular velocity Θp = ωpR
Material (stars and ISM) move at Θ(R)=ω(R).R
ωp = constant; ω(R) > ωp
Prof. John Hearnshaw
ASTR112 The Galaxy
Lecture 6
ASTR112 The Galaxy
Lecture 6
• 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
Prof. John Hearnshaw
The galactic nucleus and central bulge
Galactic centre direction in visible light. We can only see
a few kpc in this direction, a third of the way to the centre.
Prof. John Hearnshaw
ASTR112 The Galaxy
Lecture 6
ASTR112 The Galaxy
Lecture 6
Infrared observations
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
Prof. John Hearnshaw
(a) 2.2 μm
λ = 2.2 μm: cool stars
Scale: 1 arcmin  2.5 pc
λ = 12.4 μm: warm
circumstellar dust
Prof. John Hearnshaw
ASTR112 The Galaxy
Lecture 6
ASTR112 The Galaxy
Lecture 6
The area covers about
300 × 200 pc at the
galactic centre.
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.
ASTR112 The Galaxy
Lecture 6
In general the infrared brightness of an object
depends on its temperature. Using Wien’s law
λmaxT ~ 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
Prof. John Hearnshaw
Infrared observations
ASTR112 The Galaxy
Lecture 6
The near IR shows cool stars
in the centre; the far IR shows
thermal radiation from dust
grains.
Prof. John Hearnshaw
Galactic centre in visible light
near IR (2.2 microns)
and far IR.
ASTR112 The Galaxy
Lecture 6
A near IR view of the whole
Milky Way showing the
distribution of cool stars,
including the concentration in
the galactic centre.
Both images were from the
COBE satellite, 1995.
Prof. John Hearnshaw
A far IR view of the Milky Way
showing the dust distribution.
Prof. John Hearnshaw
Spiral galaxy Messier 51
ASTR112 The Galaxy
Lecture 6
End of lecture 6