The shape of the ionised gas abundance distribution in spiral galaxies

Abstract

The study of the gas-phase chemical composition of spiral galaxies has proven to be a powerful tool to improve our knowledge on the evolution of these complex systems. In particular, the analysis of HII regions (regions of ionised gas associated with star formation) is of great importance, as it is through the birth and death of stars that the galaxies chemically evolve.

In this thesis we use two sets of high-quality integral field spectroscopic (IFS) data from two different surveys, CALIFA and AMUSING, to characterise the oxygen abundance distribution of the ionised gas in star-forming (SF) regions of spiral galaxies. The first survey provides a sample of 122 disc galaxies extracted from a well-defined, statistically significant mother sample, representative of galaxies in the Local Universe. The latter provides a sample of 102 galaxies that allows us to complement the study based on CALIFA data using a higher spatial resolution dataset.

The abundance distribution of the analysed galaxies is determined based on the O3N2 strong-line indicator (although others are also tested). To measure the emission lines involved we apply FIT3D, an extensively tested code designed to deal with spatially resolved IFS data.

The study of the 2-dimensional (2D) ionised gas abundance distribution is addressed by analysing separately the radial and azimuthal trends. The large number of SF regions provided by both analysed samples, together with the good coverage of the galaxy discs with high spatial resolution, allow us to undertake this study as never done before.

The radial abundance distribution is mainly characterised by a negative gradient, that seems to present a statistically characteristic slope similar for all spiral galaxies in the sample (independent of galaxy properties such as the presence of bars). Besides this gradient, a significant number of galaxies also display a drop in the abundances towards the inner parts of the discs and a flattening in the outermost regions. The existence of these features in the radial profiles is very common, revealing that the widely accepted scenario in which the oxygen abundance distribution of spiral galaxies is well described by a single radial negative gradient might be incomplete. Deviations from this single behaviour are needed to be considered for a proper characterisation of the distribution.

As a first approach to the analysis of the azimuthal abundance distribution we have performed a study based on CALIFA data for a subsample of 63 galaxies, comparing the radial gradients displayed by the spiral arms and the area outside them (interarm region). Connecting this study with theories on the nature of the spiral structure, we distinguish between flocculent and grand design galaxies according to the symmetry, continuity, and strength of the spiral arms. We also differentiate between barred and unbarred galaxies to assess the effect of the presence of bars (considered as drivers of radial redistribution of material in these galaxies) in the abundance distribution. We find that flocculent and barred galaxies (separately) present subtle differences in the arm and interarm abundance distributions, not observed in grand design and unbarred systems. We interpret these results as flocculent arms being associated with transient local density instabilities, whereas grand design arms are linked to quasi-stationary density waves. Moreover, this suggests that bars may have a direct effect on the abundance distribution (although not reflected in the overall radial profiles).

Finally, we carry out a direct analysis of the non-radial abundance distribution based on AMUSING data. First, we perform a statistical analysis for a subsample of 24 galaxies to study the presence of local abundance variations. We find differences in the residual abundances (derived by removing the radial abundance distribution to the observed one) spatially coincident with the brightest regions of the disc (that seem to correspond to the spiral arms in most cases). These differences present a significant dependence with the morphology of the spiral arms (flocculent vs. grand design arms) and the galaxy mass, suggesting that these properties might be the main drivers of local abundance variations in spiral galaxies. In addition, we develop a methodology to analyse the azimuthal distribution of the abundance residuals and we apply it to one galaxy of the sample, NGC 6754, which present some of the highest local abundance variations. In this case, we also study the gas kinematics to assess the effect of radial migration predicted by simulations in the formation of azimuthal abundance variations. We present, for the first time, clear signatures of ongoing gas radial migration affecting the abundance distribution consistent with simulations showing transient spiral arms whose pattern speeds decrease with radius.

To sum up, this thesis comprises the most complete 2D characterisation of the oxygen abundance distribution of the ionised gas in a large and statistically significant sample of spiral galaxies up to date. We show that this distribution displays a wide range of features such as inner drops, outer flattenings, and azimuthal variations, as opposed to the simplistic view of a single radial decline. These features display clear trends with galaxy properties such as spiral structure, mass, or bar presence. These results provide strong constraints to chemical evolution models aimed at explaining the formation and evolution of spiral galaxies, trying to do our bit in the comprehension of the Universe around us.