4.6. Barred Spirals
Bars are interesting because the gravitational potential of a bar is expected to induce a large-scale radial gas flow, possibly through radiative shocking and subsequent loss of angular momentum as the gas passes through the bar (Barnes 1991). The radial flow could significantly alter the metallicity distribution by mixing in gas from outer radii, thus weakening composition gradients, and is often argued to be the means to fuel the nuclei of active galaxies. The evidence so far accumulated indicates that barred spirals generally do have shallower composition gradients than weakly-/non-barred spirals (ZKH; Martin & Roy 1994). Martin & Roy (1994) have argued that the slope of the composition gradient correlates with both bar length and bar strength, defined as the ratio of bar length to width. This is illustrated in Figure 14, which shows the slope of the O/H gradient per kpc versus bar length a relative to the photometric radius (top panel) and versus bar strength EB = 10(1- b/a), where b is the bar width. Non-barred, barred, and irregular galaxies from the Martin & Roy sample are distinguished by different symbols, and the gradients have been adjusted for new distances to the galaxies. The plots show that (1) the non-barred spirals show a wide range of values for gradient slopes, although on average they are steeper than those for barred spirals; and (2) the O/H gradients for the barred spirals tend to get shallower with increasing bar length and bar strength. On the other hand, the trend depends how the gradients are scaled; if one plots the O/H gradient per unit scale length instead, the correlations disappear.
Figure 14. O/H gradient per kpc vs. bar length a relative to the photometric radius R25. Filled squares are barred spirals, unfilled squares non-barred spirals, and crosses are irregular galaxies. Bottom: O/H gradient vs. bar strength EB = 10(1 - b/a), where b/a is the ratio of bar minor and major axes. Plot adapted from Martin & Roy 1994.
Curiously, some barred spirals (e.g., NGC 1365; Roy & Walsh 1997) show an O/H gradient within the bar, with flattening only outside the bar. One way to explain this is if strong star formation occurs in the bar, building up abundances faster than the radial flow can homogenize them (Friedli, Benz, & Kennicutt 1994).