8. Molecular content of NGC 3109
Assuming that both AGN have canonical powerlaw photon indices = -(1.75 to 2.25), cf. Laor et al. (1997), we further tighten the constraints on the hydrogen column density. For RX J1003.2-2607 we find from the X-ray spectral fit and from the 21-map of JC90 . For RX J1002.5-2602 we constrain the hydrogen column density from the X-ray spectral fit to and from the 21-map of JC90 to . Both values are in agreement within the uncertainties.
The column densities of absorbing material from the X-ray data are slightly larger than those from the 21-cm line, but the uncertainty of the X-ray spectral fit leaves room for some additional hydrogen (e.g. molecular hydrogen) in the line of sight of the AGN. We assume that the photoionisation cross section is times larger for molecular hydrogen than for atomic hydrogen (cf. Cruddace et al. 1974; Yan et al. 1998). Then we determine the column density due to molecular hydrogen from the total hydrogen column density derived from the X-ray spectral fit and the atomic hydrogen column density derived from the 21-cm observations as
We so constrain the gas column due to molecular hydrogen in NGC 3109 from the X-ray spectral fit of RX J1003.2-2607 to .
Towards RX J1003.2-2607 the amount of is . Along this line of sight the molecular mass fraction, . This means % of the mass of the total gas is in molecular form. This result can be compared with CO observations of NGC 3109, from which the mass of molecular hydrogen has been determined to (Rowan-Robinson et al. 1980). With a HI mass of a molecular mass fraction of 10% is obtained.
Extraplanar absorbing clouds have been found in a high-resolution survey of a sample of 12 edge-on galaxies by Howk & Savage (1999) at distances of to 1.5 kpc from the galaxy plane. For an inclination of 80o a radius of the galaxy disk of 12 kpc and a distance to the galaxy of 1.4 Mpc similar clouds would be projected in NGC 3109 1´ to 4´ from the galaxy plane. From these clouds dust absorption has been observed but the dust should coexist with gas in the molecular and atomic phase. Similar clouds may exist in NGC 3109 and the candidate AGN RX J1003.2-2607 would be seen through gas which is at a height 1 kpc above the galaxy plane while the candidate AGN RX J1002.5-2602 is seen through gas with 2.4 kpc.
Table 6. (continued).
© European Southern Observatory (ESO) 2000
Online publication: October 2, 2000