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Astron. Astrophys. 361, 491-499 (2000)

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8. Molecular content of NGC 3109

Assuming that both AGN have canonical powerlaw photon indices [FORMULA] = -(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 [FORMULA] and from the 21-map of JC90 [FORMULA]. For RX J1002.5-2602 we constrain the hydrogen column density from the X-ray spectral fit to [FORMULA] and from the 21-map of JC90 to [FORMULA]. 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 [FORMULA] 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 [FORMULA] from the total hydrogen column density [FORMULA] derived from the X-ray spectral fit and the atomic hydrogen column density [FORMULA] 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 [FORMULA].

Towards RX J1003.2-2607 the amount of [FORMULA] is [FORMULA][FORMULA]. Along this line of sight the molecular mass fraction, [FORMULA]. This means [FORMULA]% 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 [FORMULA][FORMULA] (Rowan-Robinson et al. 1980). With a HI mass of [FORMULA] a molecular mass fraction of [FORMULA]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 [FORMULA] 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 [FORMULA]1 kpc above the galaxy plane while the candidate AGN RX J1002.5-2602 is seen through gas with [FORMULA]2.4 kpc.


Table 6. ROSAT PSPC X-ray catalog of the NGC 3109 [FORMULA].


Table 6. (continued).
a) Column 1 gives the catalog index, Column 2 the source number (we refer to it in the text), Column 3 the ROSAT source name, Column 4 and 5 the source position, the right ascension (RA) and declination (Dec) for the epoch J2000 with the 90% confidence positional uncertainty (Column 6). Column 7 gives the total count rate (with the [FORMULA] errors). Column 8 and 9 give the soft ([FORMULA]) and hard ([FORMULA]) hardness ratio (with [FORMULA] errors), Column 10 the likelihood ratio of existence ([FORMULA]), Column 11 the source extent in cases where the likelihood ratio for extent is greater than 20, and Column 12 the off-axis angle ([FORMULA]).
Notes on individual sources
1. - RX J0959.6-2605 is the most western source and the X-ray brightest object. It has a very soft X-ray spectrum. Spectral fitting gives a very low blackbody temperature of [FORMULA] and an absorbing hydrogen column density of [FORMULA] (68% confidence). The unabsorbed flux (0.1-2.0 keV) is [FORMULA]. An unabsorbed (0.1-2.0 keV) X-ray luminosity of [FORMULA] is derived for a distance of 350 pc. RX J0959.6-2605 is 58" distant from a source in the ROSAT all-sky survey catalog 1RXS J0959.6-2604 (Voges et al. 1999) which has a comparable count rate and is an ultrasoft X-ray source ([FORMULA]). Due to the large off-axis angle (48´) the uncertainty of the position of RX J0959.6-2605 may indeed be [FORMULA]1´. 1RXS J0959.6-2604 is optically identified with a hot DA white dwarf with [FORMULA]=16.4 by Vennes et al. (1999). Modelling of a high-dispersion spectrum with LTE atmospheres gives a low-mass (0.5 [FORMULA]) white dwarf with an effective atmospheric temperature of [FORMULA]=52,000 K and a gravity [FORMULA] g=7.66. A distance of 350 pc is derived (Vennes 1999).
67. - This source correlates within the positional ROSAT error circle with the [FORMULA]=10.0, [FORMULA]=9.6 galactic foreground star HD 87276 of spectral type F5V. We derive [FORMULA]=-4.0.
109. - This source correlates with the [FORMULA]=8.7, [FORMULA]=9.8 galactic foreground star HD 87658 of spectral type F2. We derive [FORMULA]=-4.0.

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Online publication: October 2, 2000