3. The CO linewidth - starburst age correlation
The CO linewidth as a function of starburst age is plotted in Fig. 1 with the data of Table 1. When the two linewidths are comparable, we only plot the one at 115 GHz, for the sake of clarity. To correct for projection effects onto the plane of the sky, we divided the observed width by , where i is the inclination of the galaxy taken from the LEDA database. For the top diagram we used the mean of the two starburst ages determined in the previous section. We also show separate diagrams for the CMH94 and LH95 models, since they often give significantly different age estimates.
A correlation appears in the diagrams using the mean and LH95 ages. Narrow CO linewidths (less than 100 km s-1) are associated with young starbursts while older starbursts show greater linewidths (between 150 and 300 km s-1). There is one apparent exception, Mrk 306, which has an older starburst and a narrow linewidth, but the CO line profile shows several components, only one of which is taken into account in the measure of the linewidth, because it is at least twice as strong as the two others.
We have checked that the correlation between CO linewidth and starburst age does not depend on the distance or on the absolute magnitude (or, equivalently, the mass) of the galaxy; narrow linewidths are not associated with low mass objects. We have also verified that the CO linewidth is independent of the HI linewidth.
The dispersion in Fig. 1 may have several causes. The dispersion in starburst ages could be due to the contamination by younger extra-nuclear HII regions. It could also partly be due to errors on the ages, which range from 1 to 2 Myr. The dispersion in the CO linewidths could be attributed to external causes like the interaction with a companion. Let us recall that most galaxies of our sample are peculiar in one way or another (cf. Sect. 2.1). It could also be partly due to the uncertainties on the angle i.
There could also be internal causes, such as the resolving power of the radiotelescope at different frequencies. When the CO linewidth at 115 and at 230 Ghz are different, this may mean that the CO emission is resolved at the lower frequency. But different linewidth at the two frequencies may also be due to different physical conditions; this seems to be the case for Mrk 1485, for which the linewidth at the higher frequency is larger than that at the lower frequency.
Furthermore, in Mrk 306, there is a great difference between the CO(1 0) and CO(2 1) linewidths. The difference is due to the position of the most luminous HII region which is from the nucleus, inside the CO(1 0) beam but outside the CO(2 1) one; for this galaxy, we only retained the CO(1 0) data.
© European Southern Observatory (ESO) 1997
Online publication: July 8, 1998