H 2 in External Galaxies

1. H2 in External Galaxies SAAS-FEE Lecture 5 Françoise COMBES

2. H2 content and morphological types Several surveys of CO in galaxies, Young & Knezek (1989) more than 300 galaxies in the FCRAO survey Review by Young & Scoville (1991, ARAA) The H2 mass is comparable in average to the HI mass in spiral galaxies But this could be due to the IRAS-selection for many of them Recent survey by Casoli et al (1998) M(H2)/M(HI) in average = 0.2 Varies with morphological type, by a factor ~ 10

3. From Young & Scoville 91

4. H2 content, normalised by surface or dynamical mass From Casoli et al (1998)

5. H2 to HI mass ratios versus type

6. When taken into account the mass of galaxies and not only types Mass is related to metallicity Z ~ M1/2

7. Tamura et al (2001) dwarf spheroidals line = model from Yoshii & Arimoto 87 Winds and SN ejections in small potentials Zaritsky (1993) dE, Irr, and giant spiral galaxies abundances measured at 0.4r0

8. For galaxies of high masses, there is no trend of decreasing H2 with type The dependence on type could be entirely due to metallicity The conversion factor X can vary linearly (or more) with Z Dust depleted by 20 ==> only 10% less H2 but 95% less CO (Maloney & Black 1988) There is no CO deficiency in galaxy clusters, as there is HI According to the FIR bias, M(H2)/M(HI) can vary from 0.2 to 1

9. CO in Dwarf Galaxies Difficult to detect, because of sizes, and low Z (Arnault et al 88) DE easier to detect than Dirr (Sage et al 1992) The more recent observations by Barone et al (98, 00), Gondhalekar et al (98), Taylor et al (98) confirm a steep dependence on Z even higher below 1/10 of solar Dwarfs have not deep enough potentials to retain their metals During a starbursts, large gas ejections (Lyα at large V)

10. Taylor et al 98

11. Only detections are on the right O/H is the main factor (below 7.9, galaxies are undetectable) But other factors, too; like the SFR (UV) Barone et al (2000)

12. Low Surface Brightness LSB Large characteristic radii, large gas fraction (up to fg=95% LSB dwarfs Shombert et al 2001) and dark matter dominated ==> unevolved objects Same total gas content as HSB (McGaugh & de Blok 97) Low surface density of HI, too, although large sizes Un-compact Resemble the outer parts of normal HSB galaxies No CO? (de Blok & van der Hulst 1998), but H2? Some weak bursts of SF, traveling over the galaxy

13. Bothun et al 97 LSB are a reservoir of baryons Unevolved, since less gravitationnally unstable Poor environments? High Spin?

14. LSB dwarfs compared to others low masses, small sizes Schombert et al. 2001

15. Gas fraction vs SB in LSB dwarfs compared to models by Boissier & Prantzos 00

16. CO in LSB, de Blok & van der Hulst 98 But, detection by Matthews & Gao (2001) in edge-on LSB galaxies M(H2)/M(HI) ~1-5 10-2

17. Same Tully-Fisher relation (for the same V, galaxies twice as large) M ~V2R M/L increases as surface density decreases Low efficiency of star formation (Van Zee et al 1997) Gas Σg below critical A gas rich galaxy is stable only at very low Σg (cf Malin 1, Impey & Bothun 1989) Galaxy interaction, by driving a high amount of gas ==> trigger star formation LSB have no companions (Zaritsky & Lorrimer 1993)

18. Tully-Fisher relation for gaseous galaxies works much better in adding gas mass Relation Mbaryons with Rotational V Mb ~ Vc4 McGaugh et al (2000) => Baryonic TF

19. Radial Distribution in Spirals HI versus H2 The H2 is restricted to the optical disk while the HI extends 2-4 x optical radius HI hole or depression in the centers, sometimes compensated by H2 HI CO HI CO

20. Often exponential disks similar to optical Radial distribution in NGC 6946 The HI is the only component not following star formation

21. Bima SONG (Regan et al 2001)

22. Bima-SONG, radial distributions

23. Spiral Structure • The H2 component participates even better than the HI and • stellar component to the density waves • due to its low velocity dispersion • Larger contrast than other components • streaming motions, due to the spiral density wave • excitation different in arms? Density • Star formation, heating? • Formation of GMC in arms • Formation of H2? Chemical time-scale 105 yrs • HI is formed out of photo-dissociation of H2 • CO exist also in the interarms

24. M51 spiral + nuclear ring Tilanus & Allen 1991 Pearls on string GMC complexes

25. On the Fly map of M31 at IRAM 30m, Neininger et al (1998)

26. H2 in Barred Galaxies H2 is particularly useful to map the bar and rings since HI is in general depleted in central regions Hα is often obscured Barred galaxies have more CO emission, and the H2 gas is more concentrated (Sakamoto et al 1999) This confirms dynamical theories of transport of the gas by bars More than half of the gas in the central kpc comes from outside and is too high (and recent) to have been consumed through SF Rate of 0.5-1 Mo/yr No relation, however, to the AGN activity

27. Sakamoto et al (99) CO Survey of barred galaxies All kinds of morphologies Rings, bars, spiral structure Twin Peaks (leading offset dust lanes)

28. Barred galaxies have more concentrated CO emission and more gas in the central parts

29. Reynaud & Downes 1997 CO trace the dust lanes NGC 1530

30. In barred galaxies, star formation is influenced by the gas flow the resonances, the accumulation in rings Offset of 320 pc in average between Hα arms and CO arms (Sheth 01) The gas flows can inhibit star formation, when too fast (Reynaud & Downes, NGC 1530, 1999) Favors SF when accumulation in rings, nuclear bars Two patterns are sometimes required by observations of morphology and dynamics (cf M100, method of gas response in the potential derived from the NIR images) Counter-rotating gas (NGC 3593), or gas outside the plane in galaxy interactions

31. Simulations of 2 bars in M100 (Garcia-Burillo et al 1998) Ω = 23 and 160km/s 1 pattern 2 patterns

32. Molecular gas in counter rotation with respect to the stellar component Simulations explain the m=1 leading arm Garcia-Burillo et al 00)

33. NGC4631/56 interacting HI Rand & van der Hulst 93 dust IRAM Neininger 00 Molecular gas out of the planes also NGC4438 in Virgo (Combes et al 1988)

34. Molecular gas in Polar Rings PRG are due either to accreted gas from a companion or are formed in a merger of tho galaxies with orthogonal disk orientations (Bekki 1998, Bournaud & Combes 2002) The polar ring is due to gas resettling in the polar plane, but stars are dispersed The ages of the stars in the polar ring date the event CO detected in polar rings (Taniguchi et al 90, Combes et al 92, Watson et al 1994) can give insight in the event: metallicity formed from the recent star formation, or the polar galaxy non destroyed? Used to determine the flattening of the dark matter (3D potential probed)

35. NGC4650a, PRG sen with HST Simulations by Bekki (1998) Self-gravity of the PR? (more mass in the PR system)

36. Molecular gas in Ellipticals Most E-galaxies possess accreted gas, already detected in HI (Knapp et al 1979, van Gorkom et al 1997) Either the remnant of the merger event at their birth, or accretion of small gas-rich companions Dust through IRAS (Knapp et al 1989), et CO molecules are also detected (Lees et al 1991, Wiklind et al 1995) M(H2)/M(HI) 2-5 times lower than in spiral galaxies more in field ellipticals Low excitation temperature Small gas-to-dust ratio (but correlated to low Tdust) No correlation with the stellar component ==> accretion

37. D> 30Mpc Comparison between H2 mass obtained from CO and FIR (dust) Wiklind et al (1995) Lines are for g/d = 700 Dash g/d = 50

38. Shells around ellipticals The merging events giving birth to ellipticals are also forming shells Stellar shells known from Malin & Carter (1983) unsharp masking Schweizer (1983) remark that they accompany mergers and interactions Simulations confirm the scenario (Quinn 84, Dupraz & Combes 87) The stars of the small companion, disrupted in the interaction phase wrap in the E-galaxy potential Recently, HI gas detected in shells (Schiminovich, 1994, 95) Normally, the diffuse gas condenses to the center in the phase-wrap process But CO is now also detected in shells (Charmandaris et al 2000)

39. Phase wrapping Formation mechanism for shells Period increasing function of radius Accumulation at apocenter

40. Star shells in yellow HI white contours CO points in red Radio jets in blue Charmandaris, Combes, van der Hulst 2000

42. CO dragged outside galaxies Interactions of galaxies, formation of tidal dwarfs CO detected in these small dwarfs, supposed to be formed in the interaction Braine et al (2000, 01) Is the molecular gas dragged with the tidal tail gas and reclump in the tidal dwarf, or the molecular gas re-formed in the collapse? Trigger some star formation, but in general insufficient to have solar metallicity More likely that the gas and metals come from the main galaxies Fate of these tidal dwarfs? In general, they are re-accreted and merge

43. Braine et al 00, 01

44. Conclusions • The CO emission depends on type, relative to HI, • but could be only a metallicity effect • Galaxies can have large gas mass fractions, when they have low • surface brightness, and are therefore stable LSB • Unevolved, extended, un-concentrated systems, may contain H2 • but have low CO emission • No companion, large spin • CO is a good tracer of density waves, spirals, bars, rings • Radial distribution overall exponential, following the optical • But large departures, contrary to stars

45. Elliptical galaxies contain H2, with lower M(H2)/M(HI) • either due to excitation? Different conversion? • Gas coming from accretion • No correlation with stellar component • CO emission very useful to trace density, star formation • perturbations like warps, polar rings, gas dragged out of the • spiral planes