Transcript Spiral arms in the milky way galaxy and cosmic rays

Spiral arms in the
milky way
galaxy and cosmic rays
Smadar Levi
The Hebrew University of Jerusalem
International School of Cosmic
Ray Astrophysics
July 2004
Work in progress in collaboration with Nir Shaviv
A typical galaxy
~15Kpc
~0.5Kpc
The disk
The Central
Bulge
The disk is characterized with stars that move in
nearly circular orbits around the galactic center.
The Central Bulge is characterized with stars with
large random motions. Therefore its distribution is
ellipsoidal.
The star in the disk has a differential velocity,
meaning the star nearer to the center has a shorter
period of motion.
Density Waves
The Lin Shu Hypothesis
• Spiral structure is: Quasi – steady density
wave.
• The arms are a region where the star density
is the highest.
• The Star velocity is different than the arms
velocity.
• The spiral arms represent the places where
the stars spend more time than in an
equivalent area of space elsewhere in the
galaxy.
Spiral wave – the moving traffic jam
Stars=cars spiral arm=jam
t1
t2
t3
Direction of movement
Finding the arms velocity
We wish to work in a frame of reference which rotates with the arms.
And  P
We denote  as the circular velocity of the stars.
is the circular velocity of the arms, which we call: pattern speed .
birth    r    P t
 is the measured angle
At first we simplify the analysis by assuming that the orbits
are circular. Hence: rbirth  r
Analysis
• Open clusters. From Loktin
& Dias catalogs.
• Fitting the data to a
logarithmic arm with 500pc
thickness.
rarm  e
abirth b
• 3 parameters to find.
• a – the slope.
• b – the interjection point
with the r axis.
•  P - the pattern speed
Analysis
Analysis
• Fitting 500pc logarithmic arm to the data
points: Finding a minimum to a weight
function.
  1
 rarm ~ 500 pc    1  rarm  r
  d



• Bootstrap: Replacing ~37% random data
points by ~37% different random points.
• Extracting the
Bootstrap:
significant minima's
The term derives from the phrase
and finding the errors.
to pull oneself up by one's
bootstraps (Adventures of Baron
Munchausen, by Rudolph Erich
Raspe).
Looking for minimum
Two sets of arms!!
In The Milky Way ....
Two sets of arms!!
for : r  8.5kpc V  240
km
sec
km
 Pkm
 14.2  0.7
km
 P  14.2  0.7
 P sec
 30
.4  0.5
 kpc
sec kpc
sec kpc
Every ~125
million years Nir Shaviv [2003]
higher CR flux
future work:
?
Improving the position limits
Density profile
ice age