1. Introduction Flashcards
Ions and electrons with kinetic energies far greater than thermal, often extremely relativistic – energies as high as 10^21 eV have been detected.
Cosmic rays
1 Gauss
10^-4 Tesla
Thickness of the Milky Way
500 pc
Distance between sun and galactic center
8.5 kpc
Mass of Milky Way
10^11 solar masses
Half thickness
Distance above (or below) midplane where density has dropped by 50%
Most common type of gas in the Milky Way galaxy
HI (60%)
Hot Ionized Medium (HIM) in another name
Coronal gas
Coronal gas temperature
500 000 K
HII region
Ionized hydrogen in a dense volume near a dense cloud
HII gas
Gas where the hydrogen has been photoionized by ultraviolet photons from hot stars
Diffuse HII
Intercloud medium of HII gas with lower density
Warm ionized medium (WIM)
Extended low density diffuse HII
Type of gas in a planetary nebula
HII gas
Mass of stars creating planetary nebulae
0.8 - 6 solar masses
Lifetime of planetary nebulae
10 000 yr
Warm neutral medium (WNM) synonym
Warm HI
Temperature of warm HI
5000 K
Warm HI fills how much of the volume of the disk
About 40%
Cold neutral medium (CNM) synonym
Cool HI
Temperature of cool HI
100 K
Which is more dense, warm or cool HI?
Cool (30 cm^-3 vs 0.6 cm^-3)
Diffuse molecular gas
Dense HI gas
Dense molecular gas
Gravitationally bound gas (10^3 cm^-3)
Cosmic microwave background temperature
2.725 K
Is the ISM in thermal equilibrium?
No, far from it!
Why are short wavelength observations usually done on high altitude or in space whilst radio astronomy isn’t?
Because of atmospheric interference for higher frequencies
What are the four phases (or astronomical classifications) of a baryon?
IGM, ISM, Star, Stellar remnant
Largest part of stellar ejecta
Planetary nebulae (0.3-1 solar mass per year of a total of 1 solar mass per year ejecta in the Milky Way)
Can ISM become IGM?
Yes, but only through galactic wind or strong emissions from quasars and similarly which is very unlikely
How great is the infall (IGM –> ISM) in the Milky Way?
Less than one solar mass per year
Where do ISM energy come from?
Stars (photon and ejecta), Extragalactic background photons and Self-gravity (contraction)
What happens to the ISM energy?
It leaves the galaxy to the cold sky through radiative cooling
How is the coronal gas ionized?
Collisionally
Energy of a photoionization photon
13.6 eV (912Å)
Energy density CMB
0.25 eV cm^-3
Three energy densities are hydrodynamically coupled, which?
Thermal (3P/2), Hydro (rhov^2/2), Magnetic (B^2/8pi)
Energy density of starlight in local ISM
0.5 eV cm^-3
Is CMB energy density coupled to any other energy density?
No
In what process do Coronal Gas transform into HII?
In Radiative Cooling
Through what process do HII transform into HI?
Radiative Recombination
Is HI in pressure equilibrium with ISM?
Yes, it is
Is H2 in pressure equilibrium with ISM?
Yes, it is in pressure equilibrium with ISM
Temperature of diffuse H_2
60 K
What is the common cooling procedure for HI, Diffuse H_2, Dense H_2 and Cool stellar outflows/winds?
FIR-emission (Far InfraRed)
Thermal energy in terms of pressure
U_thermal = 3*p/2
In the study of collisional processes, what is capital Lambda?
It is the Debye length L_D divided by the minimum impact parameter b_min
