Constraining the formation of inner bars. Photometry, kinematics and
  stellar populations in NGC 357
  Double-barred galaxies are common in the local Universe, with approximately
one third of barred spirals hosting an smaller, inner bar. Nested bars have
been proposed as a mechanism for transporting gas to the very central regions
of the galaxy, trigger star formation and contribute to the growth of the
bulge. To test this idea, we perform for the first time a detailed analysis of
the photometry, kinematics and stellar populations of a double-barred galaxy:
NGC 357. We find that this galaxy is either hosting a pseudobulge or a
classical bulge together with an inner disc. We compare the relative mean
luminosity-weighted age, metallicity and alpha-enhancement between the
(pseudo)bulge, inner bar and outer bar, finding that the three structures are
nearly coeval and old. Moreover, the bulge and inner bar present the same
metallicity and overabundance, whereas the outer bar tends to be less
metal-rich and more alpha-enhanced. These results point out that, rather than
the classical secular scenario in which gas and star formation play a major
role, the redistribution of the existing stars is driving the formation of the
inner structures.
