Age profiles in galaxy disks can be viewed in another way too. A series of galaxy simulations have looked at the distributions of stars of various ages in the final model. For example, Bird et al (2013) did a simulation of the Milky Way and found that older stars in the present-day disk have shorter radial scale lengths and thicker perpendicular scale heights than younger stars. Other mono-age population studies of simulated disks are in Sánchez-Blázquez et al (2009), Stinson et al (2013), Martig et al (2014), Minchev et al (2015) and Athanassoula et al (2016), giving the same result.
Bovy et al (2016) found structure related to this in the Milky Way using 14700 red clump stars. Higher metallicity populations are more centrally concentrated than lower metallicity populations (not considering the α-enhanced “thick disk” component). Each narrow metallicity range tends to have a maximum surface density of stars at a particular radius where the disk has that average metallicity. Plus, each mono-metallicity population has a perpendicular scale height that increases with radius, producing a flare.
The correspondence between metallicity and peak surface density for a population of stars suggests that star formation, feedback, halo recycling, and other processes establish an equilibrium metallicity in a region that depends primarily on local conditions, such as the local mass surface density (Bovy et al 2016). Stellar migration then broadens this distribution to produce the observed total profiles. This local equilibrium concept is consistent with the results of Rosales-Ortega et al (2012), who found for 2000 Hii regions in nearby galaxies that metallicity depends mostly on stellar mass surface density, as determined from photometry. Bresolin and Kennicutt (2015) present a similar result: that the metallicity gradients in galaxies are all the same when expressed in units of the disk scale length.