1. Molecules in ULIRGS, and high density tracers SSAS-FEE Lecture 6 Françoise COMBES

2. 2 Ultra-Luminous Galaxies Interacting galaxies appear to have more H 2 content or at least much more CO emission The H 2 gas is also more concentrated In average, the H 2 content is multiplied by 4-5 (Braine & Combes 1993) This can be explained by the gravitational torques of the interactions driving gas very quickly to the centers triggers a starburst The condition of starburst: accumulating gas in a time short enough that feedback mechanisms have no time to regulate

3. 3 More M(H 2 ) in proportion for disturbed=interacting More star-formation, too Would it be the conversion X? Problems, since high density X ~n 1/2 /T r Star formation efficiency SFE=L FIR /M(H 2 ) SFE too large?

4. 4 High end of the luminosity function At L > 10 11 L o infrared galaxies are the dominant population z<0.3 more abundant than QSOs Energy from starbursts at L> 10 12 L o, all major mergers In some cases, an AGN is superposed

5. 5 Sanders & Mirabel 96 Spectral Energy Distribution SED The ratio L IR /L B varies considerably and is an indication of starbursts The brightest objects are the more obscured ones The ratio F60/F100 also increases with LIR: the brightest objects are hotter (more star formation)

6. 6 Borne et al 99 HST WFPC2 ULIRGS

7. 7 Excellent correlation Radio -FIR q=log(FIR/Radio21cm) some exceptions are the radio-loud AGN Origin of the correlation starburst, SN ULIRGS have very high SF efficiency

8. 8 Molecular gas in ULIRGS These ultra-luminous galaxies have huge quantities of H 2 gas The gas is dense and hot 10 5 to 10 7 cm -3, 60-80K similar to the star forming regions in GMC Large sample observed in Solomon et al (97) Tight correlation between the CO and 100μ luminosity ==> black body emission Small sizes of the emission, example Arp200, 300pc justifies the optical thickness usually 100μ emission is thin τ ~ ν β with β ~2, but begins at 60μ

9. 9 CO on the optical HST image Downes & Solomon 98 The molecular emission is highly concentrated within 1kpc or even smaller, cf Arp220 Two disks are merging, as seen in the dispersion, and mm continuum

10. 10 Gas is concentrated in central nuclear disks or rings Stability in these central disks? Q = 2.2 for gas only, Q= 1 for gas+stars (Downes et al 98) ==> formation of giant clusters If the dispersion is larger, the Jeans mass is larger Jeans length λ J ~σ 2 / Σ g τ ff ~ σ/ Σ g instability as soon as Q ~σκ/ Σ g =1 For the same ratio σ/ Σ g, complexes of masses M ~ Σ g λ J 2 M~σ 4 / Σ g will condense on the same time-scale

11. 11 Tight correlation CO-100μ, supports Black-Body model Small sizes, N(H 2 ) > 10 24 cm -2, ==> τ ~1 at 100μ for the dust Slope 1

12. 12 Black- Body model LFIR = 4 π R 2 σ T d 4 (no term in τ ~ν β ) LCO = 4 π 2 R 2 (2k/λ 2 ) σƒ T b dν LFIR/LCO ~ T d 3 /(f v ΔV) predicted curve f v filling factor in velocity The relation departs slightly, because of T d different than T b CO and FIR not exacty the same regions (CO size larger?) filling factor not unity

13. 13 AGN or starburst? Molecular disks of 500pc, V = 300km/s, Periods 10 Myr LFIR = 10 12 L o 50 Mo/yr formation rate, in 100 Myr (or 10 rotations) half of the gas is turned in new stars, 5 10 9 M o of H 2 gas => stars M * /LFIR= 5 10 -3 Mo/Lo (L/M ~200) If 10 12 L o comes from accretion onto a black hole, at the efficiency of L = 0.1 dm/dt c 2, the accretion must be only 1 Mo/yr, and therefore only 1% of the gas would be accreted on the same time-scale The gas would remain available at 99% to form stars

14. 14 Dynamical Triggering Numerical simulations (Barnes & Hernquist 92) Mihos & Hernquist 1994 including star formation recipes Galaxy interactions produce strong non-axisymmetry and torques that drive the gas towards the center, with the help of a small rate of dissipation This depends essentially on the stability of the disk prior the interaction, therefore on the bulge-to-disk ratio Finally the role of the geometry of the interaction is secondary direct or retrograde (provided there is a merger)

15. 15 Without bulge, disk more unstable At the end, the same SFR Several burst of SF according to the pericenters Star formation can be delayed

16. 16 Mihos & Hernquist 96 Simulations of disk/halo galaxies Gas and young stars are plotted

17. 17 High Density Tracers Nuclei of Galaxies possess denser gas GMC to survive to tidal forces must be denser High-J levels of CO higher critical density to be excited (>10 5 cm -3 ) as well as HCN, HCO+, CS, CH 3 OH, H 2 CO, OCS, etc.. SiO traces shocks (for instance supershells in starbursts) Isotopic studies: primary or secondary elements can trace the age of the star formation events

18. 18 M82 Mao et al (2000) High J-levels of CO Images are roughly similar in morphology although somewhat less extended than CO(1-0) Two hot spots on either side of the nucleus Part of the molecular torus seen edge-on Ring due to the bar (or also void due to starburst?)

19. 19 LVG model N(H 2 ) ~10 23 cm -2 M(H 2 ) a few 10 8 Mo n(H 2 ) ~10 4 cm -3 close to the tidal limit Emission comes primarily from PDR photon-dominated regions quite different from the other high density tracers Two components in the molecular gas: dense cores, + intercloud A diffuse component intervenes in the CO emission, also CI/CO is high Is this representative of starburst at high z?

20. 20 Kinetic temperatures derived are 20-60K, rather low Heating: star formation, cosmic rays, turbulence Consistent with the weakness of CH 3 OH or SiO high temp tracers SiO mapped by Garcia-Burillo et al 01 SiO traces the walls of the supershells not the star forming regions Vertical filament: SiO chimney coincident with radio cm emission Gas ejected by the starburst Shock chemistry

21. 21 M82, CO3-2, 2-1, 1-0 _|¯ __ - - Isotopic ratio of about 10-15 for 12CO/13CO ==> Opticall thick gas T A * = (T ex -T bg ) (1 - e -τ ) If optically thin R(21/10) --> 4 Survey of CO(3-2) in 30 spiral galaxies (Mauersberger et al 99) R(32/10)= 0.2-0.7, predicted if T kin < 50K and n(H 2 ) < 10 3 cm -3

22. 22 High density tracers, at low temperatures CS, HCN The ratios CS/CO and HCN/CO are correlated with LFIR (1/6 in ULIRGs, 1/80 normal, as MW) Starbursts have a larger fraction of dense gas

23. 23 Downes et al 92 HCN in IC342 Same morphology than in CO 2 spiral arms winding up in a ring CO/HCN ratio from 7 to 14 going outwards The 3mm continuum is free-free, not thermal dust emission (no starburst emission) Not very high density (except dense cores, high resolution)

24. 24 Isotopic molecules 12 C/ 13 C in the MW, from 50-90 at the Sun radius towards 10-20 in the center Tracer of the astration, 13 C is secondary In the Galactic Center, also deficiency of deuterium In Starbursts and ULIRGS (Arp220 type), CO/ 13 CO larger Not due to a low optical depth, since C 18 O is normal with respect to 12 CO But 12 C is overproduced in the nucleosynthesis of a recent burst (Casoli et al 1992)

25. 25 12 C/ 13 C ratios determined in M82 and IC342 by Henkel et al 98 from CN, HCN, HCO+ observations Always 12 C/ 13 C >40 (not as low as in the Galactic Center) 16 O/ 18 O > 100, 14 N/ 15 N > 100 HC 15 N detected in LMC and N4945 (Chin et al 99) 14 N/ 15 N = 111 lower than in the Milky Way ==> 15 N is synthesized by massive stars Controversial about this formation: destruction in H-burning formation in SN-II, 14 N more secondary, and the ratio increase with time and astration

26. 26 Deuterated species LMC: DCN, DCO+ Ratios about 20 strong fractionation D/H = 2 10 -5, but the deuterated molecules have lower energies At low temperature HD +HCN --> H 2 +DCN Here temperature is 20K Chin et al 1996

27. 27 The HNC/HCN ratio Useful to disentangle abundances, excitation, density or temperature HNC (hydrogen isocyanide) is a high density tracer as well HNC is weaker than HCN, except in ULIRGS such as Arp 220 where it is > 1 Not very clear however, since in NGC 6240, it is 10 times lower Huttemeister et al 95

28. 28 Other molecules Other molecules, which trace different physical conditions OCS in NGC 253, M82 (Mauersberger et al 95) NH 3 in Maffei 2 and others (Henkel et al 00) rotational temperatures of 85K H 2 CO and CH 3 OH tracing high-density subthermally excited, clumpy structure => point out very different physical conditions and various chemistry, from one galaxy to the next (Huttemeister et al 97)

29. 29 Methanol asymmetric top A-type, E-type lines blended n(H 2 ) > 10 5 cm -3 Atomic Carbon CI fine structure line 3 P 1 - 3 P 0 at 492 GHz important tracer of non-ionising radiation In Arp 220 CI is strong, as predicted from its FIR flux, while CII emission is depleted This could be due to higher density, optical thickness of the C+ line and dust opacity

30. 30 Gerin & Phillips 2000 CI/CO = 0.2 (in Kkm/s) cooling comparable Normally smaller than C+ (except Arp220 and Mkn231!)

31. 31 Conclusions The molecular component is much more important in starbursts and ULIRGs; it is not the case for the HI component It explains the considerable enhancement in star forming efficiency Not only a problem of gas excitation, density or temperature, since all gas density tracers confirm the large H 2 abundance and density Dynamical origin of the gas flow Explains the transformation of HI --> H 2

32. 32 Only in strong starbursts is the H 2 gas dense enough to emit sufficiently high-J CO lines this has important consequence for high-z galaxies Various molecules help to constrain the physical conditions (density, temperature, excitation, clumpiness, chemical abundances) At least two components: hot dense cores where stars form + intercloud, more diffuse medium, subthermal?