Gas 1: Molecular clouds > 4000 known with masses ~ 10 3 to 10 5 M T ~ 10 to 25 K (cold!); number density n > 10 9 gas particles m 3 Emission bands in IR, mm, radio regions from molecules comprising H, C, O: H 2, CO, OH, NH 3, H 2 O, H 2 CO,... C 2 H 5 OH,... Dark dust clouds: star formation regions e.g. Orion & Taurus molecular clouds Mostly in spiral arms
Gas 1: Molecular clouds Seen in absorption against background stars
Gas 1: Molecular clouds Seen in reflection and absorption against bright nebular background
Molecular Clouds in the Milky Way Total intensity of CO emission Clouds mostly confined to midplane Very irregular
Molecular Clouds in the Milky Way Midplane emission vs radial velocity: spiral arms
Atomic Hydrogen Gas 2: HI clouds Well distributed through MW plane, < 200 pc thick. T ~ 100 K; Average n ~ 10 6 to 10 9 particles m 3 Detected by emission at 21 cm = 1420 MHz in radio region from HI in very low energy state: 10.15 ev n=2 Energy above ground state Lyman-α (121.5 nm) ENERGY LEVEL DIAGRAM 6x10 6 ev 0 21 cm n=1 Spins parallel Spins anti-parallel p e
The 21-cm line H atoms excited by collisions to slightly higher energy level -- once per ~10 6 years per H atom Average lifetime in excited state ~ 10 7 years......followed by spontaneous decay back to ground state, emitting radiation at λ = 21 cm But ~ 10 66 H atoms in MW plane... hence observable line emission All sky surveys mapping 21-cm emission provide: column densities of HI (no of atoms per m 2 along line of sight) Doppler shifts of moving HI clouds Get distribution of HI in the MW Galaxy, and the rotation curve -- determine circular speed as a function of distance from Galactic Centre (more later)
Neutral Hydrogen Clouds Shell surrounding supernova explosion Canadian Galactic Plane Survey
Neutral Hydrogen Clouds Mushroom cloud Canadian Galactic Plane Survey
The Milky Way in Neutral Hydrogen
Gas 3: HII regions H ionized by UV radiation from nearby hot, young stars: spectral types O & B Ionized volume ~ ionizing luminosity, L i = number of ionizing photons per second If cloud has constant density, get spherical ionized volume: Stromgren Sphere V S = 4 π R S 3 /3 ~ L i, so R S ~ L i 1/3 HII region T ~ 10 4 K O star R S Surrounding HI T ~ 100 K Spectral Type O6 B0 n ISM = 1 per cm 3 T eff (K) 45,000 25,000 R S > 100 pc 20 pc A0 10,000 1 pc BUT ISM and molecular clouds are not uniform
Size of a Stromgren Sphere L i = 4π 3 R3 n e n p α L i = ionizing luminosity: [photons per second] R = ionized radius (spherical): [m] n = number densities of electrons, protons: [m -3 ] α = recombination coefficient: [m 3 s -1 ] Calculate R if L i = 10 48 s -1, n = 1 cm -3, α = 2.6x10-19 m 3 s -1
H II Regions: Observations & Theory
Orion Nebula
Tarantula Nebula
Stromgren Sphere: Hα
Gas 3: HII regions In photoionization equilibrium: H + captures e - and recombines, then ionized, then recombines,.. Electron cascades through various energy levels, emitting radiation at radio, optical, UV wavelengths HII regions found on edges of cold molecular clouds e.g. Orion Nebula is a blister HII region Molecular cloud HII region O star Surrounding HI HII regions + CO emission from molecular clouds identify spiral arm regions: sites of star formation
Orion Nebula Face-on view of a blister HII region Stellar winds from the four Trapezium O stars, carve cavity in molecular cloud Trapezium stars ionize face of cavity Ionized gas glows in emission lines Thousands of low mass stars and circumstellar disks in cluster in addition to Trapezium stars: stellar nursery
Orion Nebula: The Movie! Walt Feimer Space Telescope Science Institute
Protoplanetary Disks in Orion
Gas 4: Coronal gas T up to 10 6 K; Average n < 10 6 particles m 3. From SN remnants; form interconnecting tunnels around cooler clouds Seen by absorption lines from ground states of highly-ionized atoms (e.g. OVI, NV) superposed on spectra of hot stars in background Continuum radio emission from synchrotron radiation. X-ray emission ISM has the morphology of Swiss cheese
Molecular clouds, T ~ 10 K, molecular emission (H 2, CO, etc) HI, T ~ 100 K, 21 cm emission line HII, T ~ 10 4 K, emission lines from H, N, O, S Hot gas from supernova explosions, T ~ 10 6 K, see absorption lines on spectrum from background stars. Also observe diffuse X-Ray emission.