Download 21-cm radiation from galactic centre

ASTR112 The Galaxy
Lecture 7
11. The galactic nucleus and central bulge
11.1 Infrared observations (cont.)
Prof. John Hearnshaw
11.2 Radio observations
11.3 The nature of the galactic nucleus
In the 1980s the IRAS infrared astronomy satellite surveyed
the Milky Way in infrared. This image shows thermal
emission from warm dust at mid to far IR wavelengths.
Note: zodiacal light, Magellanic Clouds
Prof. John Hearnshaw
ASTR112 The Galaxy
Lecture 7
IRAS false-colour image of the central Milky Way,
showing that dust is confined to a very thin layer
Prof. John Hearnshaw
ASTR112 The Galaxy
Lecture 7
ASTR112 The Galaxy
Lecture 7
Prof. John Hearnshaw
In the early 1990s the COBE
satellite did an all sky IR
survey at near and far IR
wavelengths. One of COBE’s
instruments was DIRBE,
the Diffuse InfraRed
Background Experiment.
COBE/DIRBE near IR view of the Galaxy showing
the distribution of red stars in the disk and bulge.
Prof. John Hearnshaw
ASTR112 The Galaxy
Lecture 7
ASTR112 The Galaxy
Lecture 7
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.
2MASS – the 2 micron all-sky survey. This wide-field
image shows red stars and warm dust in the central bulge
of the Milky Way. Stars are shown blue, the cooler dust is
red in this false colour image at several near IR wavelengths.
Prof. John Hearnshaw
ASTR112 The Galaxy
Lecture 7
ASTR112 The Galaxy
Lecture 7
Four different types of radio source:
• Synchrotron radiation (high energy electrons
spiralling in a magnetic field)
• 21-cm radiation from HI (neutral H atoms)
• Molecular emission lines (from dense
molecular clouds)
• HII sources, emitting both (a) a free-free
continuous spectrum and (b) H emission lines
Prof. John Hearnshaw
Types of radio emission in astronomy
ASTR112 The Galaxy
Lecture 7
HII regions are thermal (i.e. hot) sources, and the
free-free radiation is also known by the name
‘thermal Bremsstrahlung’ which results from the
random motion of electrons in a hot ionized gas.
Prof. John Hearnshaw
Synchrotron sources are often described as
non-thermal radio emission, as the high electron
energies result from some process other than high
temperature.
The different continuous spectra from HII (thermal)
and supernova remnant (non-thermal, synchrotron)
radio sources
Prof. John Hearnshaw
ASTR112 The Galaxy
Lecture 7
ASTR112 The Galaxy
Lecture 7
• Sgr A (or Sgr A*) is a synchrotron source at very
centre of Galaxy
• Sgr A East (a non-thermal supernova remnant)
• Sgr A West (a thermal source – HII region)
• Sgr B (about 0.7º east of Sgr A on galactic equator)
and other sources nearby are thermal HII regions
Prof. John Hearnshaw
All four types of radio source are observed
from the central region (nucleus) of the Galaxy.
There is no extinction at all by dust.
ASTR112 The Galaxy
Lecture 7
21-cm radiation from galactic centre
The observations show a 3 kpc expanding and
rotating arm (VR ~ 50 km/s) of HI gas extending
~ 3 kpc from centre.
Prof. John Hearnshaw
Radio telescopes can measure the mass of H
(from intensity of emission) and its velocity
in line-of-sight, VR (from Doppler effect)
ASTR112 The Galaxy
Lecture 7
Vertical axis:
radial velocity
Horizontal axis:
gal. latitude b
Prof. John Hearnshaw
21-cm intensity
plot near the
galactic centre.
A false colour image showing 21-cm profiles near the
galactic centre. The vertical axis is latitude, b. The
horizontal axis is radial velocity. Note the 3 kpc arm to
the left of the main peak.
Prof. John Hearnshaw
ASTR112 The Galaxy
Lecture 7
Within 2 kpc of the centre there is a rapidly
rotating HI disk of gas, titlted at about 40º to the
galactic plane.
• The disk’s rotational velocity is ~360 km/s
• Mass ~107 M⊙ of HI, and ~109 to 1010 M⊙
of molecular hydrogen, H2
• Thickness of inner disk ~100 pc
• Molecular emission lines from various
molecules especially OH, H2O, NH3, CO and
H2CO are also observed from this disk.
Prof. John Hearnshaw
ASTR112 The Galaxy
Lecture 7
ASTR112 The Galaxy
Lecture 7
Radio 73.5 cm
Radio HI 21 cm
2.6 mm H2
Mid and far IR (dust)
Optical (nearby stars
and dark clouds)
X-rays
Gamma rays
The Galaxy at different wavelengths
Prof. John Hearnshaw
Near IR (stars)
ASTR112 The Galaxy
Lecture 7
Nature of the galactic nucleus
• Innermost 1.5 pc contains a very small source (Sgr A*)
• Size of central region ~140 pc
Prof. John Hearnshaw
• Even around this region there exists gas with circular
velocity of ~200 km/s indicating a small but massive
central object, M ~ 106 M⊙ . This is probably a
massive black hole.
The very centre of
the Galaxy, the nucleus,
is seen in this near IR
image from ESO
(European Southern
Observatory). A very
crowded field of red stars
is seen. The arrows show
the presumed location of
a supermassive black hole.
Stars and dust orbit this massive object.
Prof. John Hearnshaw
ASTR112 The Galaxy
Lecture 7
IR sources in the galactic nucleus. This is a mid-IR image of the
nucleus taken with the Keck II telescope. The suspected black hole is
in centre of image. Swirling warm dust is spiralling into the hole from
the Nothern Arm. Dust enshrouded supergiant stars are the IR sources
Prof. John Hearnshaw
ASTR112 The Galaxy
Lecture 7
ASTR112 The Galaxy
Lecture 7
Prof. John Hearnshaw
Far infrared and 2-cm radio
thermal emission from the
region of the galactic centre.
ASTR112 The Galaxy
Lecture 7
Prof. John Hearnshaw
Radio contour image
of the galactic centre,
showing Sgr A and
Sgr B sources.
Radio image of the galactic nucleus at 1-m wavelength
showing Sgr A and Sgr B and various other sources.
Image size ~ 4 º × 4º. VLA radio image, New Mexico
Prof. John Hearnshaw
ASTR112 The Galaxy
Lecture 7
ASTR112 The Galaxy
Lecture 7
The nucleus contains dense molecular clouds, supernova
remnants (SNR), mysterious filaments of gas, and a
massive black hole (MBH ~ 2.6 × 106 M⊙).
Within Sgr A is Sgr A*, the presumed black hole site.
Sgr A* is also a recently discovered X-ray source, which
is characteristic emission from an accretion disk that
surrounds a supermassive black hole.
Prof. John Hearnshaw
The nucleus of the Milky Way
Prof. John Hearnshaw
Galactic nucleus
in the near IR (ESO)
ASTR112 The Galaxy
Lecture 7
End of lecture 7