Transcript ref observation milky way

Swinburne Online Education Exploring Galaxies and the Cosmos
The Milky Way - Detailed Structure
NROA VLA radio image
Activity:
The Galactic Centre
‘Our Galaxy' modelled by ESO VLT Image of NGC2997
© Swinburne University of Technology
Summary
This Activity should enable you to:
• Appreciate the challenges
to observing the centre of
our Galaxy.
• Know the observing tools
which do penetrate to the
Galactic centre.
• Learn of the objects
currently observed at the
Galactic centre.
AAT 028 Absorbing material blocking
visual observation of the Galactic centre
Recent history
The story of the study of the central regions of our Milky
Way Galaxy parallels:
• the progress in observation at wavelengths other than
visual infrared
radio
X-ray
gamma ray
• the progress in telescopes using these wavelengths higher resolutions and orbiting telescopes.
• the discovery of phenomena associated with the
central regions of other galaxies
black holes
jets
Let’s briefly see some examples of these, in order to see
what features we might expect to find at the centre of our
galaxy.
High resolution of galactic centres
NOAO
Even in visible light, the Hubble Space
Telescope resolved a bright split source
at the centre of the M31 galaxy in
Andromeda.
Ground view of M31 core
2,000 light years
HST view of M31 nucleus
40 light years
HST Black Hole ‘images’
A black hole itself, by definition, cannot be imaged.
Radiation, emitted by gas and dust orbiting a massive
object at high speed, is detectable.
Estimation of the rotation
speed and the orbit radius
leads (by Kepler’s 3rd Law) to
the central mass and an upper
limit to its diameter.
The HST has imaged several
objects meeting black hole*
criteria; this dramatic image is
from an object at the centre of
galaxy NGC4261.
*Click here to find out about black holes
HST Spectrograph black hole evidence
The HST imaged a
spectrum of the core
of the M84 galaxy
(May 1997).
slit
M84
UKS 024 The Virgo cluster
HST M84 nucleus
slit
Model:
Material orbiting
a central object
- with higher
velocities
toward centre
Doppler shift of
central line
HST Imaging
Spectrograph
Finding: Orbital
speeds of 400 km/sec
within 26 light years
of the central object.
M84 central object mass
Compute the mass using the form of
Kepler’s 3rd law*:
d3/P2 = M + m
Where mass m (solar masses) orbits mass M at a
distance d (astronomical units) in a period of P (years).
Use the M84 finding of orbital speeds of 400 km/sec within
26 light years of the central object.
In the above units, d=1,641,000 AU and P=122,523 years.
Assuming m can be neglected compared with M,
d3/P2 gives M=294 million solar masses!
The escape velocity for such a massive object, of upper size
limit set by HST resolution, is greater than c, the velocity of
light. By definition, a black hole is inferred.
*Click here to revise Kepler’s 3rd Law
Jets from Galactic Nuclei
Some galaxies show jets of material emitted in opposite
directions from their nuclei.
Cygnus A radio source
VLA
This galaxy's nucleus is the small point in the centre of the image.
These jets impact material surrounding the galaxy, giving rise to
the giant "lobes" of radio emission seen in this image. The energy
required to produce these jets is believed to be due to the
influence of a black hole millions of times more massive than the
Sun.
Back to our own Galaxy
With our appetite whetted by what may lurk at our Galaxy’s
centre, what are the observational difficulties involved?
The previous Activity showed the difficulty in identifying
spiral arms in our own Galaxy, though they are clearly
evident in external galaxies.
The difficulty is worse when our target is the very nucleus
of our Galaxy - some 8Kpc away across the densest
regions of absorbing gas and dust in the Galactic plane.
Visible light is reduced by 28 magnitudes.
As with infrared night vision and for observations through
fog or dust on Earth, other wavelengths are needed for
astronomy of the Galactic centre.
Radiation reaching us from the centre
Astronomy now utilizes a wide range of the electromagnetic
spectrum.
511 keV gamma rays
detected from
Galactic nucleus
2.2mm infrared enables detection
of central old Population I K and M
giant stars (temperature ~4000oK)
< 0.6nm X-rays detected from
central region by orbiting Einstein
X-Ray Observatory
21 cm radio detects
H I regions >100pc
from centre
Instruments
A selection of ground based and satellite telescopes
Australia Telescope
Very Large Array
Einstein X-Ray satellite
NRAO 12m millimetre
telescope at Kitt Peak
COBE Satellite
Parkes 64m
Abbreviations
From this point on, certain terminology (eg ‘centre’) will
save repetition of full terms such as ‘the Galactic centre’.
Bulge central ~3kpc diameter region of the Galaxy.
Central region - central ~300pc diameter region.
Centre ~20pc diameter centre of Galaxy.
Nucleus - ~3 parsec diameter core of Galaxy.
The region of the electromagnetic spectrum used for an
observation will appear simply as 21cm, 2.2mm, <0.6nm etc
Radio (0.4 GHz*)
Multi wavelength images
Atomic hydrogen
The website
Radio (2.7 GHz*)
http://adc.gsfc.nasa.gov/mw/
Molecular hydrogen
nicely presents multiwavelength panoramic
Infrared
views along the plane of
Near Infrared
the Milky Way, of which
just 60o either side of the
Visual
centre (l=0o) are
reproduced here.
X-Ray
The website includes
Gamma Ray
references to authors,
observations and
background material.
*Click here
Location key
to be reminded about GHz
Radio (0.4 GHz*)
Highlights
Atomic Hydrogen
Note the hopelessness Radio (2.7 GHz)
of visual observations
Molecular H2
of the Galactic centre.
Note the high central
intensities in:
• near infrared
Infrared
Near Infrared
• 0.4GHz radio
Visual
• gamma ray
X-Ray
Note the quiet centre
for atomic hydrogen.
Gamma Ray
Location key
The Galactic Bulge
As we journey to the centre of the Galaxy, we take a
quick glance at its central bulge.
The bulge, about 3kpc diameter, comprises heavy
element enriched stars, especially type M giants, Pop I
K giants, and a few metal rich RR Lyrae stars*.
IRAS 12mm shows strong sources from asymptotic giant
branch (AGB) stars in the H-R diagram*.
Apart from the very centre, the rotation curve* shows that
bulge stars rotate with similar periods (like a solid body)
with higher velocities for larger orbits about the centre.
The derived bulge mass is some 10 billion solar masses.
* Click here to revise H-R diagrams
* Click here to find out about RR Lyrae stars
*we met rotation curves in the Activity on Galactic Rotation
The inner Galactic Bulge
Enclosed solar masses
The high central stellar density affects the velocity curve.
1010
2.2mm from old Pop I K and M
21cm
giants indicates a high central
109
stellar density.
Mechanical energy exchange
108
from close stellar encounters
should lead to a close-to-flat
107
rotation curve, with enclosed
Mr  r
mass proportional to orbit radius.
106
This is confirmed, from various
0.1
1
10 100 1000
indicators, down to r = 2pc.
Radius (pc)
Closer in, velocities increase significantly, suggesting
~4x106solar masses in the inner 0.5pc.
Radio Map of Central Region
Continuous radio emission from the Galactic central region
shows a string of radio sources in the galactic plane.
The strongest source is Sagittarius A (Sgr A), followed, like
the split source in M31, by nearby Sgr B.
+15’
B2
B1
Galactic equator
C
Sagittarius B
Sagittarius A
0o00’
latitude
Centre of Galaxy
-15’
1o00’
Galactic
0o30’
longitude
0o00’
359o30’
This region is about 270x90 parsecs.
Sources of energy
As we introduce each type of source detected in the
Galactic centre region, we will consider what it might
consist of - from the point of view of energy production
or mass involved (in solar units).
For example, some of the sources in the last frame show
characteristics of HII regions.
The O and B stars necessary to keep these regions
ionized and emitting radiation, is estimated to be
equivalent to about five million Suns (close to that from
velocity measures). To a first approximation this is about
7 times the density of stars in the solar neighbourhood.
… Night skies would be rather bright!
Clues to magnetic fields
20cm radiation produced by
synchrotron radiation* reveals
filaments which stretch for 20pc, at
right angles to the galactic plane,
and then make an almost rightangle turn.
Sgr A
From the strength and polarization
of the radiation, magnetic fields would be two to four
orders of magnitude weaker than the Earth’s magnetic
field.
simulated image
Unusual filamentary features appear near Sgr A.
*Click here to find out about synchrotron radiation
High resolution radio information
We now turn to high resolution radio
mapping of the Galactic centre, for
which the Very Large Array (VLA) introduced in the next frame - has been
at the forefront.
The Very Large Array (VLA)
A high resolution radio interferometer.
NRAO
Photo by Dave Finley
Near Socorro, New Mexico, the VLA consists of 27 antennas arranged
in a huge Y pattern up to 36km across. Each antenna is 25 meters in
diameter. They are combined electronically to give the resolution of an
antenna 36km across, with the sensitivity of a dish 130 meters in
diameter. At its highest frequency, 43GHz, its resolution is 0.04” arc.
Internet: http://www.nrao.edu/vla/html/VLAintro.shtml
The VLA resolves Sagittarius A
This image, from 6cm and 20cm radiation, resolves detail
down to ~2” arc. It shows the following components:
Sgr East: A non-thermal
shell-like structure, usually
Sgr A East
interpreted as a supernova
remnant.
Sgr A West
Sgr West: A spiral shaped
thermal source, like an HII
region.
Dec
Sgr A*
RA
Within Sgr West is a nonthermal point source <0.1”
diameter, given the name
Sgr A* -pronounced Sadge-A-Star
-28o58’
Sagittarius A West
Here we show the
various named regions
of this complex.
1 parsec
Background
Northern
arm
2cm microwave
Eastern
arm
far infrared 40-300mm
-28o59’
Western
arc
Sgr A*
Doppler shifts, from
NeII infrared emission
at 12.8mm, reveal high
velocities in the ‘bar’
region.
Dec
Bar
Dust and
gas disk
-29o00’
N
Galactic equator
E
RA
17h42m31s
29s
27s
Diagram indicative only
25s
The Sgr A West mini-spiral and Sgr A*
Rotation velocities increase toward the site of Sgr A*.
The various ‘arms’ of the miniNorthern arm
spiral pattern are as labelled.
Western arc
The general nature of their
radial velocity (recession,
approach) is indicated (up to
~130km/sec).
Eastern arm
The next frame gives a mass
Bar
estimate from the higher
Sgr A*
velocities within the ‘bar’ region.
The Sgr A* radio luminosity is
1 parsec
~2x1027 W from within a
diameter of less than 20AU.
Mass estimate within Sgr A*
A gas cloud r=0.3pc from the centre has a measured
velocity of v=260km/sec. If this is orbiting a central
mass, calculate that mass.
Use either M=v2r/G in standard units and work through to
a result in solar masses, or
Kepler’s law, r3/P2=M, which gives M in solar masses if
we first calculate distance r in AU and period P in years.
In this case r=0.3 parsecs or ~61679 AU and P=7089 years,
leading to M = 4.7 million solar masses!
Could Sgr A* be a massive black hole?
The Schwarzschild radius (within which light cannot escape)
is Rs=2GM/c2 = 0.09AU
This is well below the current resolution limit.
X-ray emission
Time variable X-rays have been detected from the
region of Sgr A West including Sgr A*.
The speed of light limits
the diameter d of an
object from which time
fluctuations Dt of radiation
are observed: d<cDt
The upper limit for the Sgr
A West source is 0.1pc.
X-ray images of the Galactic nuclear region
One X-ray mechanism involves accretion disks around
dense stars - white dwarfs, neutron stars or black holes another hint to the nature of Sgr A*.
Gamma rays
Gamma rays at 511 keV have been observed from a
source less than 0.3pc diameter almost coincident with
the Galactic centre.
511keV, the rest mass energy of an electron, is a
signature of electron-positron annihilation.
Since it is believed black holes can produce positrons in
the space around them, this seems to support a black
hole as a candidate for Sgr A*.
However the enormous 511keV luminosity of about 5x104
times the solar luminosity implies a smaller black hole
(~500 solar masses) than that envisioned for the Galactic
centre.
VLA l=90cm - central region, wide field
Compare with the
earlier Radio Map.
Note the shell-like
structure of supernovae remnants
(SNR’s).
Note the fine
‘thread’s at high
angles to the
Galactic plane and
extending for tens of
parsecs.
Image: Kassim, LaRosa, Lazio & Hyman 1999
NRAO
Supernovae activity
Even higher energy 1.8MeV gamma rays have been
detected.
The 1.8MeV line is produced by the decay of 26Al to 26Mg.
26Al
has a half-life of 716,000 years and is only produced
in small amounts in supernovae & novae explosions
and possibly Wolf-Rayet * stars.
The detected presence of ~5 solar masses of 26Al
suggests that a large number of supernovae have
occurred in the Galactic centre over the last million years.
It certainly appears to be an active environment!
*Click here to find out about Wolf-Rayet stars
The importance of Galactic centre studies
Other galaxies also appear to have black holes at their
centres. Some are relatively quiet while others have
extremely active nuclei.
Back to our own Galaxy, the rotational dynamics, mass
distribution and energy processes of the overall Galaxy
may lead to the production of high mass density (including
black hole(s) at the centre, or
if supermassive black holes were produced at the galaxy’s
embrionic stage they may in some way power other
features of the Galaxy - even spiral arms.
Summary i)
Some of the wavelengths and sources we’ve visited.
Wavelength
radio
Telescope Region Source
nucleus Sgr A and a string of sources; HII
and SNR characteristics
IR/Radio
Nucleus metal-rich giants, low mass dwarfs
40-300mm
Dust at Sgr A heated by OB stars
12-20mM
centre Dust heated by PopI and O stars
12.8mm
nucleus NeII emission; Doppler shift
200km/sec within 1.5pc of centre
12mm
IRAS
Bulge
AGB stars
2.2mm
centre PopI K giants
<0.6 nm X-ray Einstein
<100pc weak sources in weaker halo
~10-3 nm g-ray
nucleus <0.3pc size, at or near nucleus
Summary ii)
All the observations point to massive objects within a
very small radius of the Galactic centre.
They may take the form of:
a) a massive black hole of ~4x106 solar masses,
b) a very dense star cluster of ~106 solar masses
within 2pc of the centre.
Additional support for the black hole scenario
comes from similar evidence in other galaxies.
Image Credits
AAT images © David Malin (used with permission):
http://www.aao.gov.au/local/www/dfm
Individual Malin images (© David Malin (used with permission)), shown with
a 6 character code - such as AAT028, - are found at the website ending with
that code; eg:
http://www.aao.gov.au/local/www/dfm/aat028.html
Multiwave galactic plane images
http://adc.gsfc.nasa.gov/mw/
Galactic Centre X-ray image
http://antwrp.gsfc.nasa.gov/apod/image/9807/galcen_sigma2.gif
Australia Telescope Compact Array and Parke Telescopes
http://www.atnf.csiro.au/overview/telescope.html
Image Credits
Hubble Space Telescope images indexed by subject:
http://oposite.stsci.edu/pubinfo/subject.html
ESO (European Southern Observatory) VLT images:
http://www.eso.org/outreach/info-events/ut1fl/astroimages.html
NRAO VLA 90cm radio image of Galactic centre region
http://www.nrao.edu/intro/galsrc.html
NRAO VLA site images
http://info.aoc.nrao.edu/
VLA: Cygnus A
http://www.nrao.edu/vla/html/VLA-images.shtml
COBE and Einstein satellite pictures:
http://www.gsfc.nasa.gov/astro/cobe/slide_captions.html
http://asca.gsfc.nasa.gov/docs/einstein/heao2.html
VLA
The origin of the Milky Way is the subject of the next
Activities.
Hit the Esc key (escape)
to return to the Index Page
background
Frequencies and wavelength
The previous frame showed radio observations
expressed in GHz.
(GigaHertz or 109 cycles per second)
Since the days of tinkering with valves, radio astronomers
often refer to frequencies (f) rather than wavelengths (l).
The frequency of passing wavecrests = speed of wave / wavelength
Thus: f=c/l or l=c/f
c=3x105 km/sec
What wavelengths would 2.7Ghz and 0.4 GHz be?
2.7GHz: l = 3x105/(2.7x109) = 1.11x10-4 km = 11.1 cm
0.4GHz: l = 3x105/(0.4x109) = 7.5 x10-4 km = 75 cm
What frequency is the 21cm hydrogen line?
f = 3x105/(.21x10-3) = 1.428x109 = ~1.4 GHz
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Introduction to Synchrotron Radiation
The following section is a brief introduction to thermal
and non-thermal processes, and in particular,
synchrotron radiation.
background
Thermal Radiation
Conventionally, thermal radiation refers to black body
radiation at a given temperature.
In astrophysics the term thermal includes absorption,
emission and scattering processes arising from any
interactions between electrons and atoms or molecules in a
hot medium - including:
• excitation/de-excitation1 within the atom
• ionization/recombination2 to/from free electrons
• free-free processes3 between electrons, photons and ions.
1
2
3
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Non-thermal radiation
Involving physical processes not dependent on
temperature. (Including the MASER process, not covered here.)
Non-thermal processes include synchrotron radiation from
electrons, moving at near light speeds, and spiralling along
magnetic flux lines. The radiation is polarized and the
process relativistic.
Electrons (mass m, charge q) spiral, in magnetic field B, at
angular frequency w=qB/(gmc)
c = speed of light
v = electron velocity
g = 1 / 1 - v2 / c2
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About Wolf-Rayet Stars
Although we feel that we know a lot about stellar evolution,
(even if only through indirect evidence), there are still
some fascinating stellar objects which are hard to explain.
Wolf-Rayet stars are very hot (T~30,000 K), massive
(perhaps 10 to 40 M) stars which are often found in
binary systems (which we use to estimate their mass), are
losing mass at very high rates, and exhibit strong, wide
emission lines of nitrogen, oxygen and carbon and weak or
nonexistent hydrogen lines.
It is believed that the high rate of mass loss in these
(probably) post-main-sequence stars has stripped them of
most of their hydrogen envelopes, exposing nuclear
processed material in inner layers near their cores.
background
If indeed these stars turn out to be typically in binary
systems, they may turn out to be the more massive and
faster evolving partners. Theoretical models suggest that
a Wolf-Rayet star in a binary system is just past its red
supergiant stage, where much of its envelope has swollen
up and spilled over onto its companion,
and just before it undergoes a supernova explosion!
However without more evidence, we can’t be sure exactly
what these intriguing stars are.
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RR Lyrae Variables are stars on a region of the H-R diagram
called the “helium burning Horizontal Branch”
The
HORIZONTAL
BRANCH
Giants
Luminosity L/L
Absolute Magnitude
background
RR Lyrae Stars
Temperature (K)
…that also happen to fall within the “Instability Strip”, the region
of the H-R diagram which contains variable stars.
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The great thing about RR Lyrae Variables is that they are
bright and all at about the same absolute magnitude
This is because RR Lyrae Variables all have about the
same mass and are all at the same phase in their
evolution.
As long as we can find such stars their brightness
immediately tells us their distance, and therefore the
distance to the cluster they are in.
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In 1905 the Danish astronomer Ejnar
Hertzsprung noticed that a graph of the
absolute magnitudes of stars versus their
colour showed a few very regular
groupings.
A bit later on, Henry Russell in America
noticed the same thing, although he used
spectral type rather than colour.
That’s why the diagrams you are
about to study are called
Hertzsprung-Russell Diagrams
(H-R for short).
-10
bright
Absolute magnitude
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H-R Diagrams
+15
faint
blue
yellow
O5 Spectral
type red
M8
Later on, when the link between
spectral type and temperature was
realised, H-R diagrams began to
appear with temperature along the
horizontal axis instead.
Boring! Why have you
suddenly gone all historical?
Because we have to explain why
temperature goes down along the
horizontal axis of an H-R diagram: a
long time ago, astronomers listed stars
by colour, from blue (hot) to red
(cool).
Ahhhh.
-10
bright
Absolute magnitude
background
Temperature versus Type
+15
faint
40000
temperature
O5 Spectral
type 2500
M8
blue
 Colour 
red
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H-R diagrams and spectral classes
We’ll use this
version of an H-R
diagram to show
how spectral
classes appear in
that format.
Supergiants
Giants
Main
sequence
White
dwarfs
Red
dwarfs
O
B
A
F
G
K
M
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Looking for
patterns
L increasing
Huge, cool stars
appear in the top
right, and small, hot
stars tend to gather in
the bottom left.
But the rest of the
stars lie somewhere
along the main
sequence.
high
temperature
low
T increasing
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Johannes Kepler’s third law for planets: There is a
fixed relationship between the cube of the radius (d) of
a planet’s orbit and the square of its period (P) of orbit.
M
radius
d
m
period
P
GMm m.4p 2 d
F

d2
P2
d 3 GM
 2 
P
4p 2
G and 4p2 are
constants
M is the mass of
the Sun
In other situations where objects are in orbit the law still
applies, but if the mass m is not tiny compared to M then
the formula becomes
d3/P2 = M + m
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Black Holes
When a star more massive than 8 M reaches the end
of its life, the star’s gravity is so strong that it collapses
into an object of zero radius and infinite density - a
black hole.
The gravitational field of a black hole is so strong that
even light cannot escape. For this reason, black holes
are not directly observable.
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