7 Summary and conclusion

  We presented optical long-slit spectroscopic observations of 105 barred Markarian IRAS galaxies. These observations were mainly used to assign a spectral type (nuclear starburst, HII region, Seyfert 1 or 2) to each emission-line region along the slit, and to define a homogeneous sample of starburst nuclei and extranuclear HII regions.

Our selection criteria (UV excess, FIR emission and barred morphology) have been very efficient for selecting star-forming galaxies, since our sample of 221 emission-line regions includes 82% nuclear or extranuclear starbursts. The contamination by AGNs is low (9%), with 13 Seyfert 1 and 8 Seyfert 2 nuclei. The remainder are objects with ambiguous classification between HII and LINER.

Our sample of star-forming galaxies contains only 5 HII galaxies, characterized by a high excitation parameter, equivalent to a low metallicity. In fact, our sample is strongly biased towards energetic starbursts located in the nuclear or extranuclear regions of more massive and chemically evolved galaxies than HII galaxies. Three Wolf-Rayet galaxies (Mrk 52, 710 and 712) are also included in our sample.

We first compared the physical properties of the starburst nuclei to those of extranuclear HII regions distributed along the bar and to those of typical disk HII regions (Kennicutt et al. 1989). The amount of reddening, and hence the dust content, increases towards the nucleus of the galaxy. The same trend is also observed for the H$\alpha$ luminosity; the highest star formation rates are observed in the nuclei of the galaxies. We also found that the mean H$\alpha$ luminosity of the bar HII regions is higher than that of typical disk HII regions, probably because we are dealing with starburst galaxies. We did not observe any significant variation of the electron density in the nuclei and bar HII regions, but the measured values are higher (by a factor of about 3) than in typical disk HII regions. The excitation parameter, [OIII]/H$\beta$, generally decreases from the center outwards, reflecting the negative metallicity gradient observed in barred (Considère et al., in preparation) and ordinary spiral galaxies.

We investigated different mechanisms for explaining the excess of nitrogen emission observed in starburst nuclei, which however is low compared to that estimated in nearby HII nuclei and other SBNGs. There is no evidence for the presence of a weak hidden AGN in the nuclei of our starburst galaxies, as suggested by Kennicutt et al. (1989) for explaining the excess of nitrogen in other samples of such galaxies. The most likely explanation is a selective enrichment of nitrogen in the nuclei of galaxies, following a succession of short and intense bursts of star formation.

The properties of our sample of SBNGs are very much like those of other samples of SBNGs (Coziol et al.1997b) and starburst galaxies (Balzano 1983) located at a redshift of $\sim\! 0.01{-}0.02$ and with nearly the same H$\alpha$ and FIR luminosities. The host galaxies are distributed equally among early- and late-type giant spirals with a slight preference for Sbc/Sc types in our sample, a selection effect caused by the presence of a bar. The majority of SBNGs are isolated with no sign of gravitational interaction, contrary to the opinion that starbursts in massive galaxies are produced by gravitational interactions. This result suggests that, in the majority of spiral galaxies, bursts of star formation may depend on internal mechanisms, rather than on gravitational interactions.

SBNGs are intermediate between low-mass irregular HII galaxies (Terlevich et al.1991) or nearby HII nuclei (Ho et al. 1997a) and luminous or ultra-luminous infrared galaxies (Veilleux et al.1995). The former are closer and intrinsically less luminous in H$\alpha$ and in the FIR whereas the latter are farther and more luminous, both in H$\alpha$ and FIR, with a high proportion of interacting galaxies.

This spectrophotometric dataset on starbursts has been used to determine the age and star formation rate of the starbursts (Contini et al., in preparation) and, together with millimetric observations of the molecular gas, to establish the presence of molecular gas outflows in the nuclei of barred starburst galaxies (Contini et al.1997a). New spectrophotometric observations of a subset of this sample, with better signal-to-noise ratio and including the [OII]$\lambda$3727 emission line, have been used to derive the metallicity of the nebular gas and its gradient along the bar of the galaxies (Considère et al., in preparation), and have revealed yet another Wolf-Rayet galaxy.

The detailed spectroscopic analysis of some galaxies of the sample (Mrk 710, 712 and 799), combined with CCD imaging and observations of molecular clouds and atomic hydrogen has given rise to new results concerning the population of massive stars and the starburst properties in Wolf-Rayet galaxies (Contini et al.1997b). The analysis of the optical and CO velocity fields of Mrk 799 is in progress.

The link between the morphological and dynamical parameters of the bar (bar strength and relative length) and the starburst activity in the center of barred spiral galaxies has been investigated by Chapelon et al. (in preparation).

Finally, our data have been used together with another sample of starburst galaxies to shed new light on the formation and chemical evolution of galaxies, by showing that SBNGs are still in the process of formation because of their underabundance in oxygen with respect to "normal'' spiral galaxies (Coziol et al.1997a).

Acknowledgements

Data from the literature were obtained with the Lyon Meudon Extragalactic database (LEDA), supplied by the LEDA team at CRAL-Observatoire de Lyon (France). We thank Roger Coziol for helpful comments on the manuscript and the staff of Observatoire de Haute-Provence for assistance at the telescope.