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ASTR112 The Galaxy Lecture 9 12. The interstellar medium: gas Prof. John Hearnshaw NGC1232 12.3 HI clouds (and IS absorption lines) 12.4 Dense molecular clouds 12.5 Interstellar masers 12.6 Note on pressures in IS gas ASTR112 The Galaxy Lecture 9 • Wide distribution throughout galactic disk to R ~ 20 kpc • Greatest density of clouds for 4 kpc < R < 14 kpc • Number along a given lines of sight in glactic plane ~ 7 or 8/kpc • Typical size a few pc to a few tens of parsecs • Typical mass MHI ~ 100 M⊙ • Temperature T ~ 90 K • Radio emission λ = 21 cm, frequency f = 1420.406 MHz Prof. John Hearnshaw HI clouds and interstellar (IS) absorption lines ASTR112 The Galaxy Lecture 9 This image shows the HI emission in the face-on spiral M101 using the Westerbork radio telescope in Holland. The HI distribution is easier to determine than in the Milky Way, because we can observe this galaxy from an external vantage point. Prof. John Hearnshaw 21-cm emission in other spiral galaxies HI and CO distribution with radius R in the Galaxy Prof. John Hearnshaw ASTR112 The Galaxy Lecture 9 The Magellanic Stream and HI high velocity clouds represent weak sources of 21-cm emission located well away from the galactic plane. They are the result of tidal interation of the Galaxy on the satellite galaxies, the Magellanic Clouds. Prof. John Hearnshaw ASTR112 The Galaxy Lecture 9 ASTR112 The Galaxy Lecture 9 The 21-cm line Metastable upper energy state has e and p spins parallel, lower energy state, antiparallel. Prof. John Hearnshaw The 21-cm line of neutral atomic hydrogen is known as a hyperfine structure transition. ASTR112 The Galaxy Lecture 9 The upper energy state is populated by collisions which are relatively frequent (one such excitation per H atom occurs about every 400 yr). Three quarters of all H atoms are on average in the upper state. Prof. John Hearnshaw Lifetime of uper energy state ~ 11 million years, with spontaneous emission of a photon. HI line formation Prof. John Hearnshaw ASTR112 The Galaxy Lecture 9 ASTR112 The Galaxy Lecture 9 IS absorption lines HI cloud observer on Earth Spectrum of a distant galactic plane star contains narrow IS absorption lines produced by heavy elements in IS gas clouds. Prof. John Hearnshaw star in galactic plane IS lines in a stellar spectrum Prof. John Hearnshaw ASTR112 The Galaxy Lecture 9 ASTR112 The Galaxy Lecture 9 • IS lines due to Na, Ca, Ti, K, Fe and molecules CN, CH, CH+ are known in optical region Prof. John Hearnshaw • IS lines due to C, N, O, Mg, Si, P, S, Cl, Ar, Mn, Fe and molecules H2, HD, CO are observed in the ultraviolet ASTR112 The Galaxy Lecture 9 Right: multiple components in the IS NaD line due to clouds at different velocities in the line of sight. Prof. John Hearnshaw Above: narrow IS absorption lines in the spectrum of a distant galactic plane star differ markedly from the broader stellar line. ASTR112 The Galaxy Lecture 9 elements (e.g. Ca) are greatly depleted in IS clouds (deficient by a factor ~ 2 × 10-4), while others (e.g. C, N, O) are hardly changed relative to solar composition. • Element depletion is by heavy element accretion onto dust grains, thereby removing some refractory elements from the gas. Prof. John Hearnshaw • IS line strengths give information on chemical composition of IS HI clouds. Some heavy The depletion of heavy elements in HI clouds as deduced by the strengths of IS absorption lines. There is no correlation of depletion factor with atomic weight A, but a good correlation with the element’s condensation temperature Tc. Prof. John Hearnshaw ASTR112 The Galaxy Lecture 9 ASTR112 The Galaxy Lecture 9 Dense molecular clouds Molecule formula hydroxyl OH ammonia NH3 water H2O formaldehyde H2CO carbon monoxide CO discovery 1963 1968 1968 1969 1970 λ 1.8 cm 1.3 cm 1.3 cm 6.2 cm 2.6 mm number sources ~600 12 35 ~150 60 Prof. John Hearnshaw The most common molecules are H2, CO, CN, OH, H2CO. Most molecules (but not H2) give characteristic radio emission lines, which allow them to be identified. Over 50 have been detected. Absorption lines are usually seen for OH, always for H2CO. Some interstellar molecules observed in the IS medium. The first 5 are found as optical/UV IS absorption lines in stellar spectra; the second set are seen as radio emission lines in dense molecular clouds, (or as radio absorption lines when distant sources are seen through dense molecular clouds). Prof. John Hearnshaw ASTR112 The Galaxy Lecture 9 Microwave spectrum of emission lines from a dense molecular cloud Prof. John Hearnshaw ASTR112 The Galaxy Lecture 9 ASTR112 The Galaxy Lecture 9 • Temperature T ~ 10 to 30 K • Number densities n ~ 108 – 1012 molecules m-3; mass density ρ ~ 10-15 kg.m-3 • Cloud mass may be ~ 103 M⊙ • Cloud size ~ 10 pc • Dense molecular clouds are often very dusty Prof. John Hearnshaw Properties of dense molecular clouds ASTR112 The Galaxy Lecture 9 • Also dust surfaces provide a site for the formation of the H2 molecule. Other molecules can form from gas phase reactions. Prof. John Hearnshaw • Note that dust shields molecules from UV radiation from stars, which would dissociate most molecules. ASTR112 The Galaxy Lecture 9 • Dense molecular clouds are under • They are consequently sites of star formation Prof. John Hearnshaw gravitational collapse because there is enough mass for self gravity to pull them together. Above: galactic distribution of CO in molecular clouds Below: CO cloud radial velocity vs galactic longitude Prof. John Hearnshaw ASTR112 The Galaxy Lecture 9 ASTR112 The Galaxy Lecture 9 1. η Car 2. M20 Trifid nebula 3. Orion nebula 4. M16 Eagle nebula Prof. John Hearnshaw Some HII nebulae which are also associated with dense molecular clouds ASTR112 The Galaxy Lecture 9 Note on pressures in the IS gas P = nkT Phase HII HI dense molecular hot HI coronal gas n (m-3) 108 107 109 to 1012 3 × 105 103 T (K) 9000 90 10 – 30 5000 106 P (Pa) 10-11 10-14 10-13 – 10-10 10-14 10-14 Prof. John Hearnshaw P pressure (Pa); n number density (m-3); T absolute temp. (K); k Boltzmann’s constant (J.K-1) ASTR112 The Galaxy Lecture 9 • The pressure of HII clouds is much higher than the surrounding medium (normally HI) and they therefore expand supersonically (~ 10 km/s) into the surrounding gas. Prof. John Hearnshaw • HI clouds are in pressure equilibrium with the hot HI and coronal gas intercloud medium ASTR112 The Galaxy Lecture 9 • Note IS gas pressures are always very low. On Earth 1 atmosphere ≈ 105 Pa, much higher than in ISM Prof. John Hearnshaw • Dense molecular clouds also have much higher pressures, but this is the result of their high masses, causing them to collapse and be compressed under their self gravity (they are the only phase of the ISM where self-gravity dominates over gas pressure) ASTR112 The Galaxy Lecture 9 Interstellar masers MASER: Microwave Amplification by Stimulated Emission of Radiation IR pumping from thermal IR from dust can cause a population inversion of OH in gas in a metastable upper level – this is a condition for maser action. Prof. John Hearnshaw Observed in OH lines (λ ~ 18 cm) and sometimes in lines of H2O (1.35 mm) and SiO (6.95 mm, 3.47 mm) ASTR112 The Galaxy Lecture 9 Stimulated emission can occur, resulting in a very intense emission line from a small region of space (generally a few tens of A.U. across). There are several OH and H2O maser sources in the dense molecular cloud associated with the Orion nebula – possibly where new-born stars are still enshrouded in a coccoon of circumstellar dust grains. Prof. John Hearnshaw Maser sources are compact and probably occur in dusty regions associated with star formation or in circumstellar dust shells around M-type stars. ASTR112 The Galaxy Lecture 9 Prof. John Hearnshaw END OF LECTURE 9