4. Summary and conclusions
We present the first full FIR spectrum from 43 - 196.7 µm of Cen A. We detect seven fine structure lines (see Table 2), the strongest being those generated in PDRs. At the central position, the total flux in the far-infrared lines is 1% of the total FIR luminosity ( for a distance of 3.5 Mpc). The [CII] line flux is 0.4% FIR and the [OI] line flux is 0.2% FIR. These are typical values for starburst galaxies (Lord et al. 1996). The [OIII] 52 µm / [OIII] 88 µm line intensity ratio is 0.9, which corresponds to an electron density, 100 cm-3 (Rubin et al. 1994). The thermal pressure of the ionized medium in the Cen A dust lane is closer to that of starburst galaxies (ne 250 cm-3 in M82 (Colbert et al. 1999) and M83 (Stacey et al. 1999)) than that of the Milky Way (ne 3 cm-3 (Petuchowski & Bennett 1993)).
The [NIII] / [NII] line intensity ratio is 1.6, giving an abundance ratio N++/N+ 0.3, which corresponds to an effective temperature, 35 500 K (Rubin et al. 1994). Assuming a coeval starburst, then the tip of the main sequence is headed by O8.5 stars, and the starburst is 6 years old. If the burst in Cen A was triggered by the spiral-elliptical galaxy merger then its occurance was very recent. Alternatively, the merger triggered a series of bursts of star formation and we are witnessing the most recent activity.
We estimate that the N/O abundance ratio is 0.2 in the HII regions in Cen A. This value is consistent with the range of 0.2 - 0.3 found for Galactic HII regions (Rubin et al. 1988). N/O is a measure of the chemical evolution and we expect it to increase with time (c.f. the solar value of 0.12).
We estimate that 10% of the observed [CII] arises in the WIM. The CNM contributes very little (%) [CII] emission at our beam positions. The bulk of the emission is from the PDRs.
We derive the average physical conditions for the PDRs in Cen A for the first time. There is active star formation throughout the dust lane and in regions beyond the bulk of the molecular material. The FIR emission in the 70 " LWS beam at the nucleus is dominated by emission from star formation rather than AGN activity. On scales of 1 kpc the average physical properties of the PDRs are modelled with a gas density, n 103 cm-3, an incident far-UV field, G 102 and a gas temperature of 250 K.
© European Southern Observatory (ESO) 2000
Online publication: March 21, 2000