Ay 20 - Fall 2004Interstellar Medium (ISM)Interstellar ExtinctionStar FormationInterstellar Medium (ISM):A Global Picture• “The stuff between the stars”• Initially all of the baryonic content of theuniverse is a gas; and the baryonic darkmatter probably still is• Stars are formed out of the ISM, and returnenriched gas to it via stellar winds, PNe,SNe - a cosmic ecology• A complex physical system with manycomponents and structuresBasic ISM Phenomenology• Interstellar gas and dust,usually mixed• Generally concentratedin the disk of the Galaxy• Distinct clouds withinthe ISM are callednebulae:– Dark (opaque)– Emission line (H IIregions, powered by theUV emission fromyoung stars)– Reflection (from thedust grains)A Basic Tool: Spin-Flip (21 cm) Line of H IIn emission generally originatesfrom warm (T ~ 100 - 6000 K)ISM, which accounts for ~ 30 -65% of the total ISM volume inthe Galactic disk. In absorption,it probes a cooler ISM (can bealso self-absorbed).A major advantage: it is notaffected by the dust absorption!Typical line profile ŸGlobal Distribution of H I in the Milky WayConcentrated in theGalactic Plane, but high-latitude features exist. These are believedto be remnants of SN and star formation driven shells and bubbles.L-V DiagramsA radio telescope measuring a line (e.g., 21 cm line of H I, or amolecular line such as the CO) provides intensity as a function ofvelocity at any given pointing. If the resulting map is compressedin one spatial direction (e.g., latitude), the result is an “L-Vdiagram”, which shows the line intensity in a plane of velocity andthe other coordinate (typically the Galactic longitude)If one has a rotation curve of the Galaxy, this can then be translatedinto a 3-D distribution of gas and its kinematicsMulti-Phase ISMThe ISM has a complex structure with 3 major components:1. Cold (T ~ 30 - 100 K), dense (n H I > 10 cm-3) atomic(H I) and molecular (H2, CO, …) gas and dust cloudsP Only ~ 1 - 5 % of the total volume, but most of the massP Confined to the thin diskP Low ionization fraction (x H II < 10-3)P Stars are born in cold, dense clouds2. Warm (T~103-104 K) neutral & ionized gas, n ~ 1 cm-3P Energized mainly by UV starlightP Most of the total ISM volume in the disk3. Hot (T ~ 105 - 106 K), low density (n ~ 10-3 cm-3) gasP Galactic coronaP Almost fully ionized, energized mainly by SN shocksActually, it is a bit more complicated … There are fivethermal phases of the ISM:• Molecular clouds: T ~ 10-20 K, n > 103 cm-3, < 1% of thevolume, ~ 30 - 60 % of the mass, gravitationally bound(protostellar) clouds with M ~ 103 - 106 Mù• Cold neutral medium (CNM): H I absorption, T ~ 100 K,n ~ 20-60 cm-3, ~ 1 - 5 % of the volume, in pressure equilibrium• Warm neutral medium (WNM): H I emission, T ~ 6000K, n ~ 0.3 cm-3, ~ 30 - 60 % of the volume• Warm ionized medium (WIM): H II emission, T ~ 6000 -12000 K, n ~ 0.1 cm-3, only ~ 10 % is in bright H II regions• Hot ionized medium (HIM): X-ray and far-UV absorption,soft X-ray emission , T ~ 105 - 106 K, n < 0.01 cm-3• There is a constant and complex exchange of mass,energy, and momentum among these phases, with aninput from stars as well (and AGN, if present)(From P. Armitage)Absorption of Light (In General)If the radiation travels through a medium which absorbs (orscatters) radiation, the energy in the beam will be reduced:InIn+dIndAdSNumber density of absorbers (particles per unit volume) = nEach absorber has cross-sectional area = sn (units cm2)If beam travels through ds, total area of absorbers is:† number of absorbers ¥ cross - section = ndAds ¥snFraction of radiation absorbed = fraction of area blocked:† dInIn= -ndAdssndA= -nsndsdIn= -nsnInds ≡ -anIndsabsorption coefficient (units cm-1)Can also write this in terms of mass:† an≡rknkn is called the mass absorption coefficient or the opacity.Opacity has units of cm2 g-1 (i.e. the cross section of a gramof gas).(From P. Armitage)Equation of radiative transfer for pure absorption:Rearrange previous equation:† dInds= -anInDifferent from emission because depends on how much radiation we already have.s=s0sIntegrate to find how radiation changes along path:† dInIns0sÚ= -an(¢ s )d¢ s s0sÚln In[ ]s0s= -an(¢ s )d¢ s s0sÚIn(s) = In(s0)e-an(¢ s )d¢ s s0sÚ(From P. Armitage)e.g. if the absorption coefficient is a constant (example, a uniform density gas of ionized hydrogen):† In(Ds) = I0e-anDsSpecific intensity afterdistance DsInitialintensityRadiation exponentiallyabsorbed with distanceRadiative transfer equation with both absorption and emission:† dInds= -anIn+ jnabsorptionemission(From P. Armitage)Optical depthLook again at general solution for pure absorption:† In(s) = In(s0)e-an( ¢ s )d ¢ s s0sÚImagine radiation traveling into a cloud of absorbing gas,exponential defines a scale over which radiation is attenuated.s=s0When:† an(¢ s )d¢ s = 1s0sÚ…intensity will be reduced to 1/e of its original value.(From P. Armitage)Define optical depth t as:† tn(s) =an(¢ s )d¢ s s0sÚdtn=andsor equivalentlyA medium is optically thick at a frequencyn if the optical depth for a typical path throughthe medium satisfies:† tn≥ 1Medium is said to be optically thin if instead:† tn< 1Interpretation: an optically thin medium is one which a typical photon of frequency n can pass through withoutbeing absorbed.(From P. Armitage)Interstellar Dust GrainsProbability of interaction with a photon increases for photonswhose wavelength is comparable to or smaller than the grainsize; longer wavelength photons pass through. Thus interstellarextinction = f(l). (Note: this breaks down for high-energy photons)Interstellar Extinction CurveNote: this is alog of thedecrement!(i.e., just likemagnitudes)UVIRThe bump at l ~ 2200 Å is due to silicates in dust grains. This istrue for most Milky Way lines of sight, but not so in some othergalaxies, e.g., the SMC.Interstellar Extinction in StandardPhotometric BandpassesNote:This is theratio ofextinction inmagnitudes!Dramatically lower extinction in IR! Which is why weuse IR imaging to see through the dust…Stars form out of dense, cold, often dusty, molecular gas.In spiral galaxies, star formation is concentrated alongspiral arms, where gas is compressed(From P. Armitage)Part of Orion molecularcloud complexSame is true on smallerscales. Observe:Giant molecular cloudsM ~ 106 Msun, R ~ 10 pcstar clusters…on large scales, down to:Molecular cloud coresM ~ few Solar