coronal gas (10 6 K)! high T radiates inefficiently (no ion states, only free-free)!! once gas is hot, stays hot for 10 6 yrs!

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1 Global Models of ISM! relationship between phases of ISM! phases of ISM : HII : 10 4, 10 6 K! HI : 100, 10 3 K! H 2 : 10 K!? s! 1) stationary or transient! e.g. is HI at 10 3 K, just HII cooling to 100K! 2) do phases depend on gal. location! e.g. hot phases only in halo, H 2 only in arms! coronal gas (10 6 K)! high T radiates inefficiently (no ion states, only free-free)!! once gas is hot, stays hot for 10 6 yrs!

2 two-phase model of ISM! (Field, Goldsmith & Habing 69, apj, 155,L149,! update : McKee 95, ASP, 80, 292)! balance c.r. heating against cooling (H at high T, C + at low T)! FGH : " from c.r. (2 Mev from SN)! assume # H = H ioniz. rate = 4x10-16 sec -1 H -1! get 8-20 ev per ioniz. (~35 ev primary, ~3 ev secondary)! " cr = n H # H <E> ~ 4x10-27 n H! McKee : " from photo-electric effect on small grains! $ from H & trace ions coll. excit. at high T, C + at low T!

3 Equilibrium T Pressure (nt)%! 3 phases in pressure equilibrium% F = intercloud medium, H=clouds %

4 Stability :! thermally unstable if when it heats, heating increases! if G = net heating =!-", unstable if dg dt P dg dt = #G #T + #G #$ #$ #T P = $RT % dp = RTd$+ R$dT dp $ = d$ $ + dt T % dg dt P = #G #T & $ T, for dp=0, #$ #$ = & $ T #G #$ instability if T #G #T > $ #G #T > 0 unstable if T dep. of heat fn is greater than density dep.! true for G phase where T changes rapidly%

5 schematically,! for isobaric perturbation of T, if T, n! does excess $ correct higher T! T% F% excess "% G% n% excess $% H%!phases F & H self-correct, G not self-correcting (unstable)!

6 Vertical structure perp. to gal. disk :! at high z, n is low,!phase F! going to lower z, hydrostatic equil.! n increases! at n ~ 0.2, instability! jump to phase H (clouds)! at low z, have diffuse hot phase (F) in press. equil w/ cold dense phase (H)!

7 Vertical structure perp. to gal. disk :! Equation of motion (Euler's eq.)! dv!" dt = F ( force per unit vol) = "# PdS $! dv dt = "grad P +!g " " " +!gdv = " % for v = 0 &1d, &P &z =!g " # # " ( grad P "!g) dv How to get g (grav. accel.)?%

8 Oort('65) : for stars, P * = nm * v 2!nm * v 2!"!!nm * v 2!z = nm * g z " g z = 1 n i.e. count *'s as a fn of z " g z can also get limit on total mass (*'s, ISM, & dark matter)!z from Poisson's eq., # 2 $ = %!g!z = %4&G'!g!z = 8.2x10%30 sec %1 at z = 0 " ' 0 = 8.2x10 %30 / 4&G = 9.8x10 %24 gr cm %3 (Oort limit) ' * = 4x10 %24 " ' ISM+DM = 5.7x10 %24 ( 3.6 H cm %3

9 distribution of star perpendicular to MW disk % Star Type% Dispersion (km/s)% Scale height % (pc)%% B% 6% 60% A% 9% 120% gk% 17% 270% dm% 18% 350% WD% 25% 500% g z perpendicular to MW disk from % distribution of K giant stars w/ z % -g z (10-9 cm/sec 2 )% %

10 do phases vary w/ z above gal. disk?! P max % P min % F(WNM)% H(CNM)% n% where P > P max only H (CNM)! P min < P < P max WNM & CNM in press. equilibrium! but w/o KE input to clouds, they will settle in disk!

11 problems w/ 2 phase model :! 1) c.r. ioniz rate 10x too high! determined from HD/H 2! D+H +!D + +H! D + H 2!HD+H + (reason PE heating taken up)! 2)prevasive SN cavities sweep up F phase! 3) observations of coronal gas! OVI absorption!3-7x10 5 K gas! O* winds & SN shells/cavities! soft xray background (200ev)! (must be local since & 200ev ~1 w/i 100pc)! hotter gas at 10 6 K (SiIII)! 4) high vel. opt/uv abs. lines (20-50 km/s)! not dyn. equil.!

12 SN cavities do they overlap?% SN enter mom. cons. phases at ~200km/s, R i = 20pc R f = R i / V f! 68pc when V ~ 5km / s! vol,v SNR ~ 1.3x10 6 pc 3 how long does cavity last : " rec = 1 / n e # ~ 10 7 yr@n e ~ 0.01 " dyn ~ R / c 0 ~ 68pc / 10km / s ~ 7 Myr SN rate per unit vol., S ~ 3x10 $13 yr $1 pc $3 S V SNR " SNR ~ 2.7! the SNR cavities overlap! the WIM will be swept up by SNR

13 ISM not in equil., but a steady state! 1) SNR pervasive! 2) hot SN gas conductively heats clouds! 3) clouds evaporate! Cox & Smith (74), McKee & Ostriker (77) apj 218, 148! hot SN cavities become an interconnected tunnel system! but problems with these models :! 1) SN clustered! 2) SN occur in denser than average ISM! smaller SNR! 3) GMCs (H2) entirely left out %

14 Relationship between HI & H2 % M51! conventional ISM picture% HI! 1kpc! wrong!%

15 CARMA CO(1-0)% (Koda etal 09)% resolution : 4! 160 pc % sensitivity : ~105Msun% HI map (VLA)!

16 CARMA CO(1-0)% (Koda etal 09)% Spitzer 8 micron% (SINGS)%

17 gas fraction ~80% molecular! H 2 can t be confined to arms% continuity (mass cons.) =>% M H2 /! H2 = (M HI + M HII ) /! HI-HII! M : total mass of phase w/i ring%! : lifetime of H in phase% inner disks, M H2 ~ 4 x M HI +M HII% H 2 %! H2 =! HI-HII M H2 / (M HI + M HII ) ~ 4 x! HI-HI! =>! H2 >>! HI-HII " 3 x 10 7 yrs% # typical H 2 lifetime >> 10 8 yrs!! (could be forever)% ( lifetime of H 2, not necessarily GMC )% Scoville & Hersh 79% Koda etal ( 09)%

18 equipartition of cloud KE! == > massive clouds w/ lowest ' v! requires cloud last % several GMC-GMC % collision times% & GMC-GMC > ~ 10 8 yrs!

19 Summary :% H2 GMC masses using virial analysis & CO luminosities% HI from 21-cm emission line flux % usually opt. thin : integrated emission line flux! HI mass% multiple HI phases : cold (100K) HI, warm (1000K) gas% Galactic distributions : % H2 centrally concentrated, HI mostly in outer disks% H2 much more closely correlated with SF% evidence that H2 clouds are long-lived but form stars inefficiently%

20 How does H 2 fit??! GMCs self-gravitating, not pressure equilibrium with ext. gas! but M H2 /& free-fall >> SFR! not in free-fall collapse! GMCs may last very long time and form by agglomeration! of ISM (clouds and diffuse gas)%