Extragalactic molecular hydrogenV. Lebouteiller, D. Kunth, J. Lequeux, A. Aloisi, J.-M. Désert, G.

Hébrard, A. Lecavelier des Étangs, A. Vidal-Madjar, in preparation

• Absorption studies give the best way to determine molecular abundances in the ISM. This was first done in the Galaxy in the 1940’s for CH, CH+ and CN in the near-UV, then in the far-UV for H2 (Carruthers, Spitzer …), and finally in radio for OH, H2CO, HCO+, HCN (Encrenaz et al. 1980 A&A 88, L1), CO and many other molecules (H. Liszt, R. Lucas, B. Turner …)

• In external galaxies data are much scarcer. Here I will concentrate on H2.

FUSE observations of extragalactic H2• Apart for a few observations of redshifted H2 in Broad Absorption Line systems in front of quasars,

all that we know on extragalactic H2 comes from FUSE.• FUSE observations of LMC/SMC stars (Tumlinson et al. 2002 ApJ 566, 857); they show that the

H2/Htot ratio is smaller than in our Galaxy (in the diffuse ISM, respectively 2% and 0.5% against perhaps 10%).

Our FUSE observations

• We are conducting a program whose principal aim is to see if the abundances of heavy elements differ in extragalactic HII regions and in the neutral medium.

• We observed first several blue compact galaxies (IZw 18, Mrk 59, IZw 36; others observed SBS 0335-052 and NGC 625): absorption lines are seen in front of the hot stars of the ionizing cluster. No H2 has been detected in any of these galaxies.

• The best results have been observed for NGC 604, the brightest HII region in M 33. The spectrum of the whole cluster from 92 to 118 nm (exposure 20 000 seconds in total) is displayed to the right.

H2 in NGC 604

• H2 is detected in NGC 604, with a column density of logN(H2) = 16.86±0.30 mol. cm-2.

• This is only a tiny fraction (3 10-4) of the total hydrogen since logN(HI) = 20.75±0.30 at. cm-2.

• The excitation of H2 shows as usual a thermal part with 112±10 K for J=1 and 2 and a higher excitation for J=3 and 4.

Why so little H2 in extragalactic HII regions?

• In IZw18 no dust extinction has been detected even in the far-UV and no CO emission has been seen. In this case the lack of H2 might be real (Vidal-Madjar et al. 2000 ApJ 538, L77): formation of H2 is difficult and destruction by UV photons efficient.

• Around NGC 604 there is dust and CO. Thus the low abundance of H2 is probably spurious, due to a selection effect: if H2 resides in clouds thick in the far-UV, it cannot be seen because the stars behind are extinguished in the far-UV.

A more interesting but unexpected result!

• For all our targets we can compare the abundances in the neutral medium in front of the HII region with the abundances inside. They are generally smaller in the neutral medium.

• The best results are obtained for NGC 604, where the abundances of N, O and Ar are smaller by an order of magnitude in the neutral medium (strangely, Fe is the same). This is much larger than the errors and N and O at least are known to be undepleted on grains.

• The explanation is not obvious. Self-enrichment of HII regions by evolved stars, or dilution of the neutral medium by ‘hidden’ gas (Pfenniger, Combes, Martinet 1994 A&A 285, 79) ‘revealed’ by the UV field?