Stellar populations in the central region of the barred spiral galaxy NGC 4900

Abstract

We present Optically Adaptive System for Imaging Spectroscopy (OASIS) observations obtained at the Canada-France-Hawaii Telescope for the SB(rs)c galaxy NGC 4900. About 800 spectra in the wavelength range 4700-5500 and 6270-7000 A have been collected with a spatial resolution of ∼50 pc. This galaxy is part of a sample to study the stellar populations and their history in the central region of galaxies. In this paper, we present our iterative technique developed to describe consistently the different stellar components seen through emission and absorption lines. In NGC 4900, we find many young bursts of star formation distributed along the galaxy large-scale bar on each side of the nucleus. They represent nearly 40 per cent of the actual stellar mass in the field of view. The age for these bursts ranges from 5.5 to 8 Myr with a metallicity near and above 2Z ⊙ . The extinction map gives E(B - V) values from 0.19 ± 0.01 near the youngest bursts to 0.62 ± 0.06 in a dusty internal bar perpendicular to the large-scale bar. The Mg 2 and Fe I absorption lines indicate the superposition of a background stellar population with an age between 100 Myr and 3 Gyr, and a subsolar metallicity on average. We propose that all these episodes of star formation are the consequence of a secular evolution. In this scenario, the galactic large-scale bar plays an important role with respect to the recent bursts and the dusty nuclear bar observed. The iterative technique allows us to improve the determination of the stellar population parameters, mainly an older age is obtained for the old component and more reliable stellar population masses are found. A composite/transition type activity in the galaxy nucleus is also revealed with this technique.