Abstract
ABSTRACT We analyse two-dimensional maps and radial profiles of EW(Hα), EW(HδA), and Dn(4000) of low-redshift galaxies using integral field spectroscopy from the MaNGA survey. Out of ≈1400 nearly face-on late-type galaxies with a redshift z < 0.05, we identify 121 “turnover” galaxies that each have a central upturn in EW(Hα), EW(HδA), and/or a central drop in Dn(4000), indicative of ongoing/recent star formation. The turnover features are found mostly in galaxies with a stellar mass above ∼1010 M⊙ and NUV – r colour less than ≈5. The majority of the turnover galaxies are barred, with a bar fraction of 89 ± 3 per cent. Furthermore, for barred galaxies, the radius of the central turnover region is found to tightly correlate with one-third of the bar length. Comparing the observed and the inward extrapolated star formation rate surface density, we estimate that the central SFR have been enhanced by an order of magnitude. Conversely, only half of the barred galaxies in our sample have a central turnover feature, implying that the presence of a bar is not sufficient to lead to a central SF enhancement. We further examined the SF enhancement in paired galaxies, as well as the local environment, finding no relation. This implies that the environment is not a driving factor for central SF enhancement in our sample. Our results reinforce both previous findings and theoretical expectation that galactic bars play a crucial role in the secular evolution of galaxies by driving gas inflow and enhancing the star formation and bulge growth in the centre.
Highlights
The bulges of disc-dominated galaxies are more complicated than previously thought
Following Lin et al (2017), we identify the bar structure according to the presence of an abrupt decrease of the ellipticity profile and an associated change of position angle (PA) – a galaxy is classified as having a bar if the ellipticity increases above 0.25 as the radius increase from the galactic centre outwards, before decreasing by at least 0.1 at some radius
We visually examine the Sloan Digital Sky Survey (SDSS) gri-image, twodimensional maps, and the radial profiles of each galaxy, and decide whether or not the turnover feature occurs in the inner regions
Summary
The bulges of disc-dominated galaxies are more complicated than previously thought. Initially believed to be similar to elliptical galaxies, and formed via violent-merging events and dominated by old stellar populations (e.g. Whitford 1978; Renzini 1999), a large fraction of pseudobulges were later identified, built up from discs via bar- or spiral arm-driven processes (e.g. Peletier & Balcells 1996; Helfer et al 2003; Kormendy & Kennicutt 2004). Direct observations of high-redshift galaxies suggest that bulges are formed through the inward migration and coalescence of clumps. These massive gas-rich clumps can sustain the rejuvenation of bulges, and act together with stellar migrations, minor mergers, and other dynamical effects to build up bulges in the following several billion years Any mechanism that causes disc instabilities and gas inflows may trigger central star formation, contributing to bulge growth. Galaxy evolution is governed by long-term and secular processes Internal mechanisms such as bars and spiral arms (and associated resonances) have been proposed to transfer angular momentum to the outer parts of discs, driving gas inflows towards the central regions
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