3. The dust mass and survival of dust grains in the merger event
It is instructive to estimate the mass of dust associated with the mini-barred spiral. From our V-15 µm image, we estimate the thickness of the emission structures (bar and spiral arms) to be 12" corresponding to only 190 parsecs. From Fig. 2 we see that the spiral arms start at 1.5 kpc from the nucleus and that all structures, bar and arms are 200 pc wide. From Fig. 3 of Mirabel et al. (1999) we can see that the spiral arms subtend at least 45o. If we use a surface atomic gas mass density of 80 solar masses per square parsec (appropriate to an atomic hydrogen column density of 1022 atoms cm-2 as observed by van Gorkom et al. 1990) we derive an atomic hydrogen gas mass of 1.4 solar masses. The uncertainty is a reduction by a factor of 4: for a warped disk the line of sight can penetrate the disk several times, and the actual HI surface density will be that many times less. Eckart et al. (1990) use a line-of-sight penetration of four times. Furthermore, the integrated molecular gas mass (Eckart et al. 1990) is almost identical to that of the atomic gas mass, so we derive a total combined atomic and molecular gas mass of 2.8 solar masses. If we adopt a canonical gas-to-dust ratio of 100, we derive a dust mass of 0.7-2.8 solar masses. Such a dust mass is typical for a small spiral galaxy such as M33 or for dusty ellipticals. Typical ranges in dust masses for bright nearby ellipticals detected by IRAS are in the 104 - 106 solar mass (Goudfrooij 1996).
Our estimate for the dust mass excludes the mass of dust grains found at larger galactocentric radii which have no ISOCAM emission component. It also excludes dust grains residing in the disk of the mini-barred spiral and responsible for the diffuse emission seen in Fig. 1. This likely explains why our dust mass estimate is a factor of 10 smaller than that of Mirabel et al. (1999) using IRAS and SCUBA data. These latter instruments detect a larger fraction of the complete dust emission, not only that residing in the bar+arms struture.
Dynamical studies can be used to study the problem of dust survival in a merger event. Nicholson et al. (1992) propose that the ionized gas disk has completed more than 10 rotations (each 1 108 year at 3.0 kpc) since the merger event (Rix & Katz 1991). It is believed (Nicholson et al. 1992) that the merger and subsequent formation of the dust band occurred 109 yr ago. A dynamically evolved structure is also attested to by the well established star formation seen throughout and by the distribution of stellar shells (Malin et al. 1983) within the elliptical component of Centaurus A. Quillen et al. (1993) have constructed models wherein a warped disk evolving as a result of differential precession in a prolate potential gives an excellent fit to their infrared data. If the structure is 109 yr old, the inner 1.5 kpc disk, having completed 20 rotations, may have had time to settle into a preferred plane of the host elliptical (Steinman-Cameron & Durisen 1982). The typical lifetime of a dust grain in the absence of shock induced destructive mechanisms is of the order of 109 years and mergers would not destroy dust grains unless very strong shocks are involved. Those grains which remain in giant molecular clouds would in any case be shielded from destructive processes (Greenberg, private communication).
© European Southern Observatory (ESO) 2000
Online publication: December 8, 1999