Strong spiral arms drive secular growth of pseudo bulges in disk
  galaxies
  Spiral-driven instabilities may drive gas inflow to enhance central star
formation in disk galaxies. We investigate this hypothesis using the Sloan
Digital Sky Survey (SDSS) in a sample of 2779 nearby unbarred star-forming
main-sequence spiral galaxies. The strength of spiral arms is quantified by
their average Fourier amplitude relative to the axisymmetric disk. The star
formation properties in the central 1--3\,kpc region were derived from the SDSS
spectra. We show that galaxies with stronger spiral arms not only tend to have
more intense central specific star formation rate (sSFR), larger Balmer
absorption line index, and lower 4000-\AA\ break strength, but also have
enhanced central sSFR relative to sSFR measured for the whole galaxy. This link
is independent of redshift, stellar mass, surface density, and concentration.
There is a lack of evidence for strong spiral arms being associated with a
significant fraction of starburst or post-starburst galaxies, implying that the
spiral-induced central star formation is likely continuous rather than bursty.
We also show that stronger spiral arms tend to have an increasing fraction of
pseudo bulges, a relatively unchanged fraction of star-forming classical
bulges, and a decreasing fraction of quenched classical bulges. Moreover, the
concentration of galaxies hosting pseudo bulges mildly increases with stronger
spiral arms, implying that spirals help pseudo bulges grow. The connection
between spirals and the bulge type is partly attributed to the suppression of
spirals by classical bulges and partly to the enhanced central star formation
driven by spirals. We explain our results in a picture where spiral arms
transport cold gas inward to trigger continuous central star formation, which
facilitates the build-up of pseudo bulges. Spiral arms thus play a role in the
secular evolution of disk galaxies.
