5. Alternatives to the disk formation scenario
Are there other ways to explain the paucity of barred galaxies in the Markarian sample and their small disk dimensions? A majority of Markarian galaxies might not have a bar because the conditions required for the occurrence of a burst do not allow it. It is considered, for instance, that interactions can destroy the bar. But in Considère et al. (2000) we have found three clear cases of interacting galaxies where the bars seem as strong as, if not stronger than other bars in the galaxy sample. Furthermore, this hypothesis does not explain the small dimensions of the disks.
The fewer bars and the smaller disks in starburst galaxies could be due to higher dust extinction. A high level of obscuration is effectively observed in some ultraluminous infrared starburst galaxies (Mirabel et al. 1998). However, these objects are more an exception than the rule in the nearby Universe. High extinction does not generally apply to starburst galaxies (Buat & Burgarella 1998). Moreover, in this case the occurrence of smaller disks would imply that the dust opacity becomes higher in the outer disk, while the contrary is usually found: spiral galaxies are optically thin in the outer regions and moderately opaque at their center (Giovanelli et al. 1994; Moriondo et al. 1998; Xilouris et al. 1999).
There is also clear evidence that the outer regions of disks are relatively unevolved at the present epoch (Ferguson et al. 1998), which is consistent with the idea that disks are younger than bulges, as proposed in our scenario.
As a last alternative, it may be that, at the end of their evolution, the Markarian galaxies will produce mostly small disks and unbarred galaxies. A similar hypothesis was recently suggested to explain the appearance of a large number of relatively small galaxies with high luminosities at high redshifts (Lilly et al. 1998). If what we observe in nearby starburst galaxies is of the same nature, then this means that at each epoch we always see some intense star forming activity which concerns only galaxies of small dimensions. This is an intriguing hypothesis which would imply that Markarian galaxies are of a peculiar nature.
None of the above scenarios predicts that Markarian starburst galaxies should not follow the local TF relation. The dispersion observed in Fig. 4 for the Markarian starburst galaxies cannot be explained assuming only higher dust extinction. This hypothesis might work for galaxies which are above the local TF relation in Fig. 4, but not for galaxies which are below; they would need to be more luminous than normal. It is also hard to understand why dust extinction changes neither the distribution of luminosity nor the surface brightness of these galaxies (see Table 1). We need a very contrived model for dust distribution in order to explain all these observations.
In their paper on star forming galaxies at high redshifts, Simard & Pritchet (1998) found that these galaxies do not follow the local TF relation. They concluded that this could be explained by assuming that high redshift star forming galaxies are more luminous, by an average of one or two magnitudes, than normal nearby galaxies. But there is no evidence that the B luminosity of the local Markarian starburst galaxies, and of SBNGs in general, differs significantly from that of normal galaxies (see Table 1 and Coziol 1996). Furthermore, Simard & Pritchet do not know if the disks of their galaxies are smaller than those of normal galaxies, as in nearby starburst galaxies. If the Markarian starburst galaxies do not follow the local TF relation because they are more luminous than normal spiral galaxies, then, taking into account their small dimension, nearby starburst galaxies must be much more luminous than "comparable" star forming galaxies at high redshifts. This argument suggests that the reason why Markarian galaxies do not follow the local TF relation is that they are still in the process of forming their disks. This may be true also for forming galaxies at high redshifts.
© European Southern Observatory (ESO) 2000
Online publication: March 28, 2000