Suzanne Considere

Starbursts in barred spiral galaxies - III. Definition of a homogeneous sample of starburst nucleus galaxies

This paper presents optical long-slit spectro- scopic observations of 105 barred Markarian IRAS galax- ies. These observations are used to determine the spectral type (starburst or Seyfert) of emission-line regions in the nucleus and along the bar of the galaxies, in order to de- ne a homogeneous sample of Starburst Nucleus Galaxies (SBNGs). Our selection criteria (ultraviolet excess, far infrared emission and barred morphology) have been very ecient for selecting star-forming galaxies, since our sample of 221 emission-line regions includes 82% nuclear or extranuclear starbursts. The contamination by Seyferts is low (9%). The remaining galaxies (9%) are objects with ambiguous classication (Hii or LINER). The dust content and H luminosity increase towards the nuclei of the galaxies. No signicant variation of the electron density is found between nuclear and bar Hii re- gions. However, the mean H luminosity and electron den- sity in the bar are higher than in typical disk Hii regions. We investigate dierent mechanisms for explaining the excess of nitrogen emission observed in our starburst nu- clei. There is no evidence for the presence of a weak hidden active galactic nucleus in our starburst galaxies. The cause of this excess is probably a selective enrichment of nitro- gen in the nuclei of the galaxies, following a succession of short and intense bursts of star formation. Our sample of SBNGs, located at a mean redshift of 0.015, has moderate H ( 10 41 erg s 1 ) and far in- frared (10 10 L) luminosities. The types are distributed equally among early- and late-type giant spirals with a

The Role of Starbursts in the Evolution of Galaxies

We have determined, for the first time, the nitrogen-to-oxygen abundance ratio (N/O) for a large sample of starburst nucleus galaxies (SBNGs) and compared it to the values observed in normal spiral galaxies. The N/O ratios in SBNGs are ∼ 0.2 dex higher than in normal HII regions observed in the discs of late-type spirals, but are comparable to the values found in the bulges of normal early-type spirals. The variation of the N/O ratio as a function of metallicity follows a primary + secondary relation, but the increase in nitrogen does not appear as a continuous process. Assuming that nitrogen is produced by intermediate-mass stars, we show that our observations are consistent with a model in which the bulk of the nitrogen was formed during past sequences of bursts of star formation which have probably started 2 or 3 Gyr in the past. What we observe, therefore, could be the main production of nitrogen in the bulges of these galaxies.

The Role of Starbursts in the Chemical Evolution of Galaxies

In a diagram of metallicity (\~z) vs. luminosity (M

Kinematical observations of pairs of galaxies.

_B

The Massive Nearby SBNGs: Young Galaxies in Formation

), the different types of nearby (Z

The Chemical Evolution of Galaxies by Successive Starbursts

< 0.05

The Origin of the Excess of Nitrogen Emission in the SBNGs

) starburst galaxies seem to follow the same linear relationship as the normal spiral and irregular galaxies. However, for comparable luminosities the more massive starburst nucleus galaxies (SBNGs) show a slight metallic defficiency as compared to the giant spiral galaxies. Furthermore, the SBNGs do not seem to follow the same relationship between \~z and Hubble type (T) than the normal galaxies. The early-type SBNGs are metal poor as compared to normal galaxies. It may suggests that the chemical evolution of a majority of the nearby starbursts galaxies is not completely over and that the present burst represent an important phase of this process.