Inside-out growth and the kiloparsec-scale star formation main sequence for low-surface-brightness disk galaxies in MaNGA
Low surface brightness galaxies (LSBGs) are very faint objects in the universe whose formation and evolution have long remained an unresolved problem. We selected a late-type, face-on, and disk-dominated galaxy sample from the MaNGA survey, composed of 38 LSBGs and 216 high surface brightness galaxies (HSBGs) by adopting a central surface brightness threshold $μ_{0}(g) = {\rm 22.0\ mag\ arcsec^{-2}}$. Based on maps derived from pipe3D products of star-forming areas in galaxies, we investigated the global and resolved star formation main sequence (SFMS and rSFMS) and radial profiles of the stellar mass density ($\mathrm{Σ_{*}}$), star formation rate density ($\mathrm{Σ_{SFR}}$), and specific star formation rate ($\mathrm{Σ_{sSFR}}$). The radial profiles of $\mathrm{Σ_{*}}$ and $\mathrm{Σ_{SFR}}$ present negative gradients, while those of $\mathrm{Σ_{sSFR}}$ present positive gradients. Compared to HSBGs, the LSBGs show similar, flatter, and steeper slopes in $\mathrm{Σ_{*}}$, $\mathrm{Σ_{SFR}}$, and $\mathrm{Σ_{sSFR}}$ gradients, respectively. The very flat $\mathrm{Σ_{SFR}}$ gradients of LSBGs, whose slopes range from -0.1 to -0.2, indicates uniform star formation throughout the whole galaxy, and the positive $\mathrm{Σ_{sSFR}}$ gradients reflects their ``inside-out'' growth. Our LSBGs and HSBGs follow the same global and resolved SFMS as the full sample, and $μ_{0}$ shows no influence on both SFMS and rSFMS. For our full sample and two subsamples (HSBGs and LSBGs), the slopes of global SFMS and rSFMS are consistent, which indicates that the SFMS still hold on a kiloparsec scale and the star formation activities are regulated by local events. The treatment of different excitation sources in galaxies affects the slope of the SFMS and rSFMS, and using star-forming areas only is more advisable.