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Astron. Astrophys. 323, 363-373 (1997)

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1. Introduction

Many clues of secular dynamical evolution in disc galaxies have recently been detected by observations or suggested by numerical simulations and theoretical approaches. The most significant progresses in the field has come from considerations of facts neglected or not well understood in the past. For instance, the role of gas, the effects of interactions not only between galaxies but also between various components of a given system, the necessity to fully take into account 3D structures, the interplay between star formation and dynamical mechanisms, etc. In fact, discs are the seat of evolutionary processes on timescales of the order of the Hubble time or less (see the reviews by Kormendy 1982; Martinet 1995; Pfenniger 1996; see also e.g. Pfenniger & Norman 1990; Friedli & Benz 1993, 1995; Courteau et al. 1996; Norman et al. 1996). In particular, bars do play a decisive role in such processes.

The presence of non-axisymmetric components seems to be a necessary condition for the onset of nuclear activity (Moles et al. 1995). However, the precise link between the presence of stellar bars and the star formation activity is not well established and even somewhat controversial (see e.g. Hawarden et al. 1996). Some authors claimed that star formation is enhanced in barred galaxies (Hawarden et al. 1986; Dressel 1988; Arsenault 1989; Huang et al. 1996), whereas others suggested that barred galaxies have star formation levels similar or lower than those in normal spirals (Pompea & Rieke 1990; Isobe & Feigelson 1992). For instance, according to Hawarden et al. (1986), more than one third of SB galaxies have 25 µm excess attributed to vigorous star formation in a circumnuclear ring located near the inner Lindblad resonance. However, this result generates many unanswered questions: Why do the other galaxies show no significant excess of star formation activity? Is the star formation enhancement dependent on the Hubble type as well? Is the 25 µm excess a reliable indicator of star formation? Where are the preferential sites of star formation located? Etc.

The aim of the present paper is threefold: 1) To quantitatively confirm through observational data from the literature that the strength and/or the length of a bar is a decisive factor for enhancing star formation activity as suggested by numerical simulations (e.g. Friedli & Benz 1993, 1995), observations (Martin 1995), as well as by our preliminary study (Friedli & Martinet 1996). 2) To re-discuss the influence of the bar strength and the star formation efficiency on the radial chemical gradient in the continuation of the work by Martin & Roy (1994). 3) To suggest steps of evolution in barred galaxies taking into account the various points previously mentioned.

This paper is structured as follows: In Sect. 2, we discuss various indicators and estimators of star formation usually considered in the literature, whereas the selection of galaxies used in this study is given in Sect. 3. Section 4 is devoted to the presentation of the various links found in our sample between observed quantities, like bar axis ratio, bar length and star formation activity as well as by considering a simple theoretical model able to link the radial abundance gradient, the bar strength, and the star formation efficiency. In Sect. 5 some other jigsaw pieces have been inserted thanks to a new set of numerical simulations. In Sect. 6, we discuss and put the previous results in the general frame of secular evolution of disc galaxies, and finally we summarize our main conclusions in Sect. 7.

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© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998