No 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 Å 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 2 Z⊙. 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 Mg2 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.

In order to better understand the impact of the bar on the evolution of spiral galaxies, we measure the properties of giant H ii regions and the bar in the SB(s)b galaxy NGC 5430. We use two complementary data sets, both obtained at the Observatoire du Mont‐Mégantic: a hyperspectral data cube from the imaging Fourier transform spectrograph SpIOMM (Spectromètre‐Imageur à transformée de Fourier de l‐Observatoire du Mont‐Mégantic) and high‐resolution spectra across the bar from a long‐slit spectrograph. We flux‐calibrate SpIOMM spectra for the first time, and produce Hα and [N ii]λ6584 Å intensity maps from which we identify 51 giant H ii regions in the spiral arms and bar. We evaluate the type of activity, the oxygen abundance and the age of the young populations contained in these giant H ii regions and in the bar. Thus, we confirm that NGC 5430 does not harbour a strong active galactic nucleus, and that its Wolf–Rayet knot shows a pure H ii region nature. We find no variation in abundance or age between the bar and spiral arms, nor as a function of galactocentric radius. These results are consistent with the hypothesis that a chemical mixing mechanism is at work in the galaxy's disc to flatten the oxygen abundance gradient. Using the starburst99 model, we estimate the ages of the young populations, and again find no variations in age between the bar and the arms or as a function of radius. Instead, we find evidence for two galaxy‐wide waves of star formation, about 7.1 and 10.5 Myr ago. While the bar in NGC 5430 is an obvious candidate to trigger these two episodes, it is not clear how the bar could induce widespread star formation on such a short time‐scale.