1. der Paul van der Werf Leiden Observatory H 2 emission as a diagnostic of physical processes in star forming galaxies Paris October 1, 1999

2. Molecular hydrogen emission in star forming galaxies2 H 2 emission as a diagnostic ä H 2 rotational lines: ä H 2 vibrational lines:  cf. CO J=1 - 0: T: few hundred K n > 10 2 cm -3 T: few thousand K n > 10 4-5 cm -3 T: few tens of K n > 10 2 cm -3

3. Molecular hydrogen emission in star forming galaxies3 H 2 emission lines trace excitation ä General problem: what is the excitation mechanism? ä Shocks ä UV-pumping ä X-ray excitation ä …. H 2 2.12  m in NGC253 (Van der Werf & Moorwood, in preparation) 15” = 180 pc

4. Molecular hydrogen emission in star forming galaxies4 Two case studies ä Rotational H 2 lines in the disk of NGC891: ä Vibrational H 2 lines in the nuclear region of NGC6240: A new view of the warm molecular medium (Valentijn & Van der Werf 1999, ApJ 522, L29) A new diagnostic of the nuclei of gas-rich mergers (Van der Werf, Moorwood & Israel, in preparation)

5. Molecular hydrogen emission in star forming galaxies5 Dust in NGC891 SCUBA 850  m Sky survey Israel, Van der Werf & Tilanus 1999 A&A 344, L83

6. Molecular hydrogen emission in star forming galaxies6 H 2 emission in NGC891 ä ISO-SWS survey to 11 kpc from centre  28  m and 17  m detected at all positions ä 0th-order analysis: High-density limit Isothermal Ortho/para = 3 All three assumptions fail!

7. Molecular hydrogen emission in star forming galaxies7 Simple analysis of position 8kpc N Solution: abandon high-density assumption 0th order analysis gives: T=76K N(H 2 )=2.7  10 22 cm -2 CO gives: T~20 K N(H 2 )=1.0. 10 22 cm -2

8. Molecular hydrogen emission in star forming galaxies8 Low-density solution Adopt density from CO: n(H 2 )~200 cm -3 T~130 K N(H 2 )=3.6. 10 21 cm -2  H 2 J=3 (and CO J=2) subthermally populated subthermally populated Low-density PDRs  cf. [CII] Low-density PDRs  cf. [CII] Good throughout inner disk Good throughout inner disk In outer disk: 2 components? In outer disk: 2 components?

9. Molecular hydrogen emission in star forming galaxies9 Two-component solution Solution: ortho-para ratio ~ 1 Warm H 2 dominates S(1)Warm H 2 dominates S(1) Cool H 2 dominates S(0)Cool H 2 dominates S(0) T > 130 K T < 90 K N(H 2 ) > 10 23 cm -2 M(H 2 ) > 10. M(HI) Problem: temperature balance; [CII] indicates 15  lower mass [CII] indicates 15  lower mass

10. Molecular hydrogen emission in star forming galaxies10 Vibrational H 2 emission in ULIGs Sub-arcsecond resolution near-IR H 2 imaging shows gas components between the nuclei of merging galaxies, e.g., NGC6240 ( Van der Werf et al. 1993 ApJ 405, 522) ( Van der Werf et al. 1993 ApJ 405, 522) Origin: dissipative components merge first ?Origin: dissipative components merge first ? Fate: nuclear starburst ? AGN ?Fate: nuclear starburst ? AGN ? Properties: M gas ~ M dyn ?Properties: M gas ~ M dyn ? Role of H 2 emission ?Role of H 2 emission ? Arp220 H 2 1-0 S(1) (Van der Werf & Israel, in preparation) in preparation)

11. Molecular hydrogen emission in star forming galaxies11 H 2 2.1  m emission in NGC6240 H 2 emission peaks between the nuclei. between the nuclei. MorphologyMorphology SpectraSpectra } H 2 emission from cloud-cloud shocks cloud-cloud shocks  H 2 emission powered by dissipation of mechanical energy in nuclear medium  H 2 emission measures inflow rate of molecular gas to centre of potential well K H 2 1-0 S(1) 10” = 5 kpc

12. Molecular hydrogen emission in star forming galaxies12 H 2 vibrational spectrum in NGC6240 Vibrational lines: T~1900 KVibrational lines: T~1900 K Rotational lines: T~400 KRotational lines: T~400 K  Energy radiated by shocks: L rad ~ 1.6. 10 9 L  L rad ~ 1.6. 10 9 L  Energy dissipated by shocks: L dis ~ ½ M H 2 v 2 L dis ~ ½ M H 2 v 2. L rad = L dis v~280 km s -1    M H 2 ~ 260 M  yr -1.

13. Molecular hydrogen emission in star forming galaxies13 Fate of the infalling gas Southern nucleus: M H 2 ~ 50 M  yr -1Southern nucleus: M H 2 ~ 50 M  yr -1 Northern nucleus: M H 2 ~ 20 M  yr -1Northern nucleus: M H 2 ~ 20 M  yr -1 Central component: M H 2 ~ 190 M  yr -1Central component: M H 2 ~ 190 M  yr -1 Nuclei: total mass inflow rate 70 M  yr -1 total star formation rate 45 M  yr -1 total star formation rate 45 M  yr -1... Central component: M H 2 ~2.2. 10 9 M  ~ 30% of M dyn M H 2 ~2.2. 10 9 M  ~ 30% of M dyn t acc ~ 10 7 yr t acc ~ 10 7 yr large shear prevents star formation large shear prevents star formation explosive nuclear starburst will result explosive nuclear starburst will result K H 2 1-0 S(1) N

14. Molecular hydrogen emission in star forming galaxies14 A merger sequence? (counterrotating disks) Antennae NGC6240  Arp220 Arp220 SCUBA 850  m (Van der Werf & Moorwood, in preparation)

15. Molecular hydrogen emission in star forming galaxies15 Conclusions In disks of late-type galaxies, H 2 rotational lines traceIn disks of late-type galaxies, H 2 rotational lines trace extended low-density PDRs extended low-density PDRs In mergers, H 2 lines trace dissipation of mechanical energyIn mergers, H 2 lines trace dissipation of mechanical energy in the merging gas components in the merging gas components In both cases, H 2 lines provide unique diagnostics onceIn both cases, H 2 lines provide unique diagnostics once the excitation mechanism is understood the excitation mechanism is understood