WHY DOPPLER SHIFT OCCURS IN 1420 MHz HYDROGEN OBSERVATIONS
Neutral atomic hydrogen (HI) emits its famous spectral line at:
1420.40575 MHz
However, hydrogen gas in our Galaxy is moving relative to us.
Because of the Doppler effect, this motion changes the frequency at
which we receive the hydrogen signal.
HYDROGEN MOVING TOWARDS US
Hydrogen cloud
--->
Observer
O
The hydrogen is approaching the observer.
Received frequency is HIGHER than 1420.40575 MHz.
This is called BLUE SHIFT.
HYDROGEN MOVING AWAY FROM US
Hydrogen cloud
<---
Observer
O
The hydrogen is receding from the observer.
Received frequency is LOWER than 1420.40575 MHz.
This is called RED SHIFT.
THE RELATIONSHIP
For the velocities encountered in Galactic hydrogen observations:
Delta f / f = -v / c
where:
Delta f = change in frequency
f = 1420.40575 MHz
v = radial velocity
c = speed of light
VERY USEFUL RULE AT 1420 MHz
1 km/s = approximately 4.74 kHz
Therefore:
10 km/s = 47.4 kHz
20 km/s = 94.8 kHz
50 km/s = 237.0 kHz
100 km/s = 474.0 kHz
For example:
Hydrogen approaching at 50 km/s:
1420.40575 + 0.237
=
1420.643 MHz approximately
Hydrogen receding at 50 km/s:
1420.40575 - 0.237
=
1420.169 MHz approximately
WHY DO WE SEE SEVERAL PEAKS?
When you point a radio telescope into the Milky Way, you are not
necessarily looking at just one cloud of hydrogen.
Your beam may pass through several regions of hydrogen:
EARTH
O
\
\ Cloud A
\ ***
\ \
\ Cloud B
\ ***
\ \
\ Cloud C
\ ***
\
---> Through the Milky Way
These different regions can have different radial velocities.
Therefore their hydrogen emission appears at different frequencies:
Signal
^
|
| /\
| /\ / \ /\
| / \ / \ / \
|_____/ \/ \______/ \____
|
+------------------------------------> Frequency
approaching receding
higher freq lower freq
WHY THIS IS SO IMPORTANT
The Doppler shift allows us to turn FREQUENCY into VELOCITY.
For example:
Measured frequency
|
v
Doppler shift
|
v
Radial velocity of hydrogen
|
v
Galactic rotation
|
v
Milky Way rotation curve
THERE IS ONE IMPORTANT COMPLICATION
The measured velocity is not caused only by the hydrogen.
The observer is moving as well.
Our motion includes:
1. Rotation of the Earth
2. Earth's orbit around the Sun
3. Motion of the Sun through the Galaxy
Therefore:
Observed Doppler shift
=
Motion of hydrogen
+
Motion of the observer
For accurate Galactic measurements, astronomers commonly convert the
measured velocity to the:
LSR = Local Standard of Rest
IN SUMMARY
Hydrogen rest frequency:
1420.40575 MHz
|
+------------+------------+
| |
v v
Towards us Away from us
| |
v v
Higher frequency Lower frequency
| |
v v
BLUE SHIFT RED SHIFT
This is why a 1420 MHz radio telescope can do much more than simply
detect hydrogen.
By measuring exactly WHERE the hydrogen signal occurs in frequency,
you can measure how fast the hydrogen is moving towards or away from
you - and use that information to investigate the rotation and
structure of the Milky Way.
By Admin