Astron. Astrophys. 325, 450-456 (1997)

3. Quantities derived from the new observations

3.1. Gas and dust mass

The mass of the interstellar medium (ISM) is a crucial quantity for modeling the infrared properties. An estimate of the gas mass can be derived from the dust emission at 1.3mm (Krügel et al., 1990) or from CO line emission; both methods give comparable results (Chini et al., 1992a).

By assuming that the dust emission at 1.3mm is optically thin and that contributions to the 1.3mm flux from sources other than dust emission are negligible the gas mass can be estimated from

where is the Planck function, S the observed bolometric flux of the dust emission, and the extinction cross section at 1.3mm. The dust extinction cross section is normalized to cm² per gram-ISM in order to derive an estimate of the gas mass.

For a mean dust temperature of = 30 K (Chini et al., 1992b) and an extinction cross section cm² per gram-ISM, derived from our standard mixture of carbon and astronomical silicate grains (Chini et al., 1986, Chini et al., 1987, Siebenmorgen & Krügel, 1992), the gas mass is about M .

In this estimate the dust extinction cross section and the assumed gas-to-dust ratio are the least certain quantities (Krügel & Siebenmorgen 1994b; Krügel & Chini 1994). For comparison, if we had assumed that all large grains are made of astronomical silicates, i.e. that no large carbon grains are present in the ISM of the galaxy, then the result would be that cm² per gram ISM and the derived dust mass would be increased by a factor of 4 accordingly.

We use a "standard" mixture of carbon and astronomical silicate grains and consequently take M as our best estimate for the gas mass in our models.

3.2. Size of the infrared nuclear source

The size has been best constrained at 3.8µm and 4.8µm where direct imaging gives a FWHM 0.7" and the speckle scans a most probable size of 0.3" or 6 pc. Around 10 µm it is 1.3" (26pc) and at 20 µm 1.5". Estimates at longer wavelengths are limited by the small sizes and hence large beams of available airborne and space telescopes. At 50 and 100 µm the best size estimate comes from IRAS CPC (Chopped Photometric Channel) observations which reveal a compact source with an estimated but uncertain FWHM 40" and little or no extended emission (Ghosh et al., 1992). Our mm-wave observations give a FWHM 23" at 1.3mm. The source size therefore increases with wavelength but remains smaller than that of the starburst ring up to wavelengths of at least 20 µm.

© European Southern Observatory (ESO) 1997

Online publication: April 28, 1998

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