The structure of our Milky Way galaxy:a container of gas and stars arranged invarious components with various properties..

Gaseous halo?

~ 6 x 1010 solar masses of stars (~90% in disk, rest in bulge and halo)~ 9 x 109 solar masses of gas in disk (gas mass in “halo” unknown)

Course part I (interstellar matter, stars, planets) What is the physics of the interstellar medium?

--What are the phases of the interstellar medium (ISM)i.e. states identified in the (ρ, T) plane and what isthe structure of interstellar clouds?

--What is its energy balance (cooling/heating processesand transport of radiation through ISM)?

How does interstellar gas turns into stars?

What is the structure of stars?

How do stars evolve into compact objects (whitedwarfs, neutron stars, black holes)?

What is the physics of accretion disks around stars ( planet formation) and compact objects ( accretion onto black holes)?

Course part II (stellar systems, stellar dynamics) What is the physics of stellar systems (galaxies ascollections of stars)?

How do stars move in galaxies?

What are the properties of the gravitational potentialof galaxies?

Are galaxies equilibrium configurations?

What is the effect of gravitational perturbations on stellar (and gaseous) systems?

Part I and Part II interconnected.

Main Textbooks:

• The formation of stars (Wiley eds. – Stahler & Palla)

(2) Galactic Dynamics both new and old version(Princeton Univ. Press – Binney & Tremaine).

(3) Radiative processes in astrophysics (Ribicky & Lightman)

(4) Black Holes, White Dwarfs and Neutron Stars (eds. Wiley Shapiro & Teukolski)

The components of the Galaxy

BARYONS-Stars (mostly light elements (H and He), ionized or atomic phase, H and He – composition like the average composition of the Universe)

-Gas = Interstellar medium (ISM) (also mostly H and He, but also C,N,O and other elements important for the energy exchange in ISM)

-Dust grains and PAHs (heavy elements)

NEUTRINOS (e.g. produced in stars) + HIGH ENERGY PARTICLES (cosmic rays, e.g. high energy protons, neutrons, positrons and other particles produced by violent phenomena such as stellar explosions and accretion flows onto compact objects)

DARK MATTER (unknown nature, we assume it has similar behaviour as an ensamble of stars - collisionless)-

Dust grains = hard core + light mantleContain molecules with high Z elements (heavier than He) = “metals”

Cores

Olivine

Graphite

The multi-wavelength Milky Way

The best probe of galaxy structure

Interstellar dust grains (small solid particles) absorb light emitted from stars in optical/UV wavelengths and re-radiates in infrared (IR) + infrared light emitted by gas, old stars (=most of stellar mass) escapes without absorption IR emission reveals structure of the Galaxy, with its disk and bulge (both contain stars and interstellar gas)

How would our Galaxy look like if we could see it face-on?

Probably in between these two (we know there is a stellarbar but also a complex grand design spiral structure)

- Young stars and gas more abundant along spiral arms- Spiral arms from propagating density waves not stationarystructures – they are associated with transient peaks inthe density field of stars and gas

Matter: collisionless vs. collisional/dissipational

Stars in a galaxy behave as a “collisionless” component.

Disk stars moving on randomly oriented straight-line orbits- mean free pathλ=1/nσ ~ 2 x 1014 pc (n=number density=1011/((10 kpc)2 x 0.5 kpc);σ=cross section of star=πRo

2, where R0=solar radius=2.26 x 10-8 pc )For typical stars random velocity is v=50 km/s -- collision time (interval between collisions) = λ/v = 5 x 1018 yr! (>> Hubble time= age of the universe ~ 1.3 x 1010 yr)

- exchange of energy in collisions irrelevant, motion dictated only by gravitational potential of Galaxy (including stars, gas, dark matter)

Gas clouds in galaxy are collisional -- collision time << Hubble time (repeat the calculation above considering giant molecular clouds (M~106 Mo) N=n*RD

2*h = 103 and R0= 10 pc)

- exchange of energy in collisions important, motion dictated by both gravity andhydrodynamical forces (pressure). Also, kinetic energy in collisions converted into thermal energy of clouds - atoms/molecules in gas clouds can radiate away additional thermal energy by emitting photons -- “dissipational” behaviourDiffuse gas behaves in same way (collisions between individual atoms/molecules)

Phases of the (dissipational) interstellar mediumISM can be in :

• Atomic (neutral) form – HI is the main component : M ~ 7 x 109 Mo

• Ionized – HII (regions of ionized hydrogen) is one exampleproduced by UV radiation from O and B stars (r ~ a few pc, MHII ~ 108 Mo) : M ~ 109 Mo

• molecular – H2 is the main component – M ~ 2 x 109 MoIn general one can identify the following phases (identified in (ρ, T) plane):

cold neutral medium (CNM), i.e. discrete clouds with ρ ~ 50 cm-3

and T= 80 K warm neutral medium (WNM), diffuse gas with ρ~ 0.5 cm-3 andT= 8000 K hot intercloud medium, diffuse gas with ρ~ 3 x 10-3 cm-3

and T= 50000 K , probably produced by supernovae heating warm ionized medium (HII regions just one component) diffuse Hα observed, but also OVI and other lines from heavy elements – both inthe in the galactic disk and around it – ρ~ 0.3 cm-3, T ~ 8000 K molecular medium ρ> 300 cm-3, T =10 K

NNGC 3079

GMCs in M33

Hot intercloud medium

HII regions

Definition: HI = neutral atomic hydrogen (emission in radio band)Note: spiral structure visible in gas, coincides with density peaks (color intensity proportional to density of HI along the line of sight)

position-velocity diagram using Doppler shift of radio emission

How do we detect the HI component?Hydrogen in ground state (rapid de-excitation through collisions ifexcitation to higher states occurs) -- how comes that it doesemit photons?

Answer: hyperfine splitting of ground state due to quantized spins of electron and proton (can be parallel or antiparallel).

Spontaneous transition between F=1 (parallel spins, collisionally excited) and F=0 (antiparallel spins) liberates a photon with wavelength = 21 cm (radio band). Wavelength long, dust grains do not absorb it!

-Transition has very low probability per atom (from quantum mechanics), 10 -7 /yr but large number of hydrogen atoms in galaxycompensates ---> 5 x 1068 H atoms!-Observed intensity proportional to column density NHI alongthe line of sight (and secondarily depends on temperature)

The Milky Way disk rotates –---> its rotation is a probe of its gravitational potential and mass distribution via Vrot = (GM/R)1/2 (the expression for Vrot is indeed more complicated because orbits of stars and gas are not exactly circular and the galaxy is not spherical).

Rotation curve reveals the presence of dark matter

The rotational velocity is obtained by measuring motion of HIcomponent via Doppler shift.

Star formation is associated with the molecular gas phasemolecular gas (mostly H2) is the densest and coldest phase of the interstellar medium (ISM)

Distribution of Molecular Gas in the Milky Way

Galactic CO-distribution

Intensity of CO lines in mm, roughly

prop. to CO density

• Most of is found in molecular clouds.

• Molecular clouds are concentrated in the galactic planethickness only 60 pc, smaller than for HI

• Total mass of molecular hydrogen: Mo.

(Dame 2000)

This is 50% of the total cold gas mass in the Galaxy (T <1000 K)

Molecular gas is mostly H2 but is detected via other molecules (CO and HCN)Reason is H2 has no electric dipole – need rotational transitions but these havevery low probability (< 10-66 yr-1)

GLOBAL RELATIONS BETWEEN GAS DENSITY AND STAR FORMATION RATE

(1)Radial distributions of phases in galaxy is different

H2 and HII have peak at same location as expected ifmolecular gas density and star formation rate are related(HII regions illuminated by massive young stars)

(2) Vertical distribution of phases is different

Let’s calculate it starting from basic physics and simple assumption (hydrostatic equilibrium)

Two important observational facts