Are irregular galaxies old enough to have heavy elements like the disks of spiral galaxies?
Gas-rich dwarf irregular galaxies like the Magellanic Clouds have lower heavy element abundances than the disk of the Milky Way by about a factor of ten. Roughly speaking, if you compare the total abundance of all elements heavier than helium ( Z ) with hydrogen (X) and helium (Y), you find that for the very young ‘Population I’ stars X = 0.75, Y=0.24 and Z=0.01. For the old Population II stars, Z = 0.001 to 0.0001. In the Milky Way, there is a so-called Intermediate Disk Population II which has ‘metalicities’ from Z= 0.01 to 0.001. Irregular galaxies have Z’s near 0.001 making them similar to the Intermediate DIsk Population II stars in our Milky Way, which means they have not been enriched by as much as the Sun has. The material out of which they formed has not been contaminated with as many generations of supernovae. In fact, irregular galaxies seem to go through episodes of star forming activity every billion years or so, rather than produce stars continuously as the Milky Way does
