H 2 in External Galaxies

1. H2 in External Galaxies IRAM Summer School Lecture 3 Françoise COMBES

2. The molecular component is essential for the star formation and dynamics Morphological type CO in dwarfs CO in LSB Radial distribution, spiral structure CO in bars Fueling of nuclei CO in polar rings CO in E-galaxies, CO in shells CO in tidal dwarfs

3. 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 More recent survey by Casoli et al (1998) M(H2)/M(HI) in average = 0.2 Varies with morphological type, by a factor ~ 10

4. From Young & Scoville 91

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

6. H2 to HI mass ratios versus type

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

8. 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

9. 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) Environment? There is no CO deficiency in galaxy clusters, as there is HI (cf Virgo cluster, Kenney & Young 88) According to the FIR bias, M(H2)/M(HI) can vary from 0.2 to 1

10. 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) cf the dE Haro2 (Bravo-Alfaro et al 2003) The more recent observations by Barone et al (98, 00), Gondhalekar et al (98), Taylor et al (98) confirm a steep dependence on Z Well know examples: LMC, SMC X-factor multiplied by 10 (Rubio et al 1993) even higher below 1/10 of solar

11. Taylor et al 98

12. 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)

13. Low Surface Brightness LSB Sometimes large radii, always 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

14. Scaling properties SDSS - Kauffmann et al 2002b 120 000 galaxies Different behaviors for LSB and HSB SFE lower in LSB/dwarfs

15. Bothun et al 97 LSB are a reservoir of baryons Unevolved, since less gravitationnally Unstable S < Scrit Poor environments? High Spin?

16. LSB dwarfs low masses, small sizes Masses surface density radius Schombert et al. 2001

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

18. 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

19. UGC 1922: O’Neil & Schinnerer 2003 OVRO, 109 Mo of H2, with 1010 Mo of HI

20. 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)

21. 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

22. 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

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

24. Bima SONG (Regan et al 2001)

25. Bima-SONG, radial distributions

26. 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

27. M51 spiral + nuclear ring Tilanus & Allen 1991 Pearls on string GMC complexes More recent map From Aalto, Hüttemister et al

28. Full map of M31 with IRAM 30m CO and dust coinciding M(H2) < M(HI) Mvirial > M(CO) Müller & Guélin 2003 On the Fly map of M31 at IRAM 30m, Neininger et al (1998)

29. M31 OTF Nieten et al 2000 Müller & Guélin 2003

30. 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

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

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

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

34. 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

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

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

37. NUGA LINER LINER/H SEYF. 1 SEYF. 1 LINER SEYF. 2 LINER SEYF. 2

38. NUGA reveals m=1 instabilities • The new 0.5’’-0.7’’ maps reveal m=1 perturbations appearing as one-arm spiralsandlopsided disksin several galaxies. Perturbations appear at various scales, from severaltenstoseveral hundreds of pc. LINER LINER Seyf. 1 NGC4826 NGC1961 NGC4579  150pc AGN AGN AGN   500pc  20pc  0’’.7 (=180pc) CO(2-1) map of NGC1961 shows strong one arm spiral response 0’’.6 (=12pc) CO(2-1) map of NGC4826 shows molecular gas distribution strongly lopsided near AGN. 0’’.6 (=48pc) CO(2-1) map of NGC4579 showsm=1 instability in distribution+kinematics 

39. 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)

40. Polar Ring Galaxies (PRG) PRG are composed of an early-type host surrounded by a gas+stars perpendicular ring The polar ring is akin to late-type galaxies large amount of HI, young stars, blue colors Unique opportunity to check the shape of dark matter halo But how to relate DM of PRG to DM of spiral progenitors? Formation scenarios

41. Molecular gas in Polar Rings PRG are due either to accreted gas from a companion or are formed in a merger of two galaxies with orthogonal disk orientations (Bekki 1998, Bournaud & Combes 2003) 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)

42. Formation of Polar Rings By accretion? Schweizer et al 83 Reshetnikov et al 97 By collision? Bekki 97, 98

43. Formation of PRG by collision

44. Formation of PRG by accretion Bournaud & Combes 2003

45. Accretion Scenario Able to form inclined polar ring Gas+stars Gas only NGC 660 host galaxy contains gas Probably unstable by precession even if self-gravitating In merging scenarios, no formation NGC 660 CO rich

46. 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

47. 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

48. 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 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)

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

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