SIMBAD references

This thesis is made of three main parts. In the first one a sample of 55 galaxies with an active nucleus (Seyfert 1, Seyfert2 and LINERs) is analysed; these were observed with the 4.2m WHT and 1m JKT in CCD narrow band H-alpha +[NII] and [OIII] to map the distribution of HII regions and the morphology of the circumnuclear extended emission associated with the active nucleus. The analysis of the extended emission and HII regions is carried out, as a function of the level of activity and of the Hubble type. One third of the sample shows circumnuclear HII regions, but only 9% of these are S1. The number surface density of the star forming sites and the location of the brightest HII region, indicates that in S2 the star formation is more important in the inner disk; however, in S1 the distribution of the star forming sites is more uniform with distance, and the brighest HII regions are farther away from the nucleus than in S2. The luminosity function, size distribution, the relationship between the Ha flux and the size, the emission measure, and the radial distribution of the HII regions in 27 out of the 55 galaxies of the sample are studied. This comprises a statistical analysis of more than 2000 HII regions.

In the second part of this thesis the giant extragalactic HII region NGC 2363 and the starburst galaxy NGC 7714 are studied; they were observed in narrow band CCD H-alpha image with the JKT and spectroscopically from 3700 to 9600 A with the WHT. Both objects are experiencing intense star formation activity. Evidence of this comes from the detection of WC and WN emission features in NGC 2363 and in NGC 7714 respectively; this suggests an age of the present burst between 3 and 5 Myr. However, evidence for the existence of a previous burst in NGC 7714 comes from the detection of the infrared CaII triplet in absorption. The physical conditions and chemical composition of the gas are derived. In both cases, the metallicity is low (12+log O/H=7.89 for NGC 2363) and moderately low (12+log O/H=8.5 for the nucleus and 8.2 for the HII regions in NGC 7714). In NGC 2363 the electron temperature deduced from the Paschen discontinuity is obtained, and it is significantly smaller than the values derived from the collisionally excited emission line diagnostics, suggesting the presence of large temperature fluctuations. One effect of these temperature fluctuations is an increase of the metal abundance by a factor of two.

In the third part of the thesis the circumnuclear region of two LINERs (NGC 4579 and NGC 6500), two S2 (NGC 5347 and NGC 5953) and two S1 (NGC 3227 and NGC 4253) is studied; the galaxies were observed spectroscopically from 3400 to 9600 A. The kinematic information in the form of the gas velocity curve and velocity dispersion for the low and high excitation emission lines is obtained. The stellar velocity dispersion through the absorption CaII triplet is also derived. The line ratios [OIII]/H-beta, [OI]/H-alpha, [NII]/H-alpha, [SII]/H-alpha, [OII]l7325/H-alpha, and [SIII]/H-alpha are used to study the excitation mechanism in the circumnuclear region. A grid of photoionization models are generated using the code CLOUDY, with a power law (n=-1 to -2), black body (Tbb=10000 to 22000 K) and stellar atmosphere (Teff=35000 to 55000 K) ionizing continuum, for a range of ionization parameter values (log U=-1 to -5) and gas metallicity one to three times solar.

The circumnuclear emission in the LINERs is kinematically perturbed, with radial motions which are compatible with an outflow wind interacting with the higher density gas in the plane of the galaxy. The line ratios, in particular [SIII]/[SII] and [OII]l7325/H-alpha, indicate that the main excitation mechanism is shock heating.

The line ratio [SIII]/[SII]/[OIII]/[OII] is used to derive a metallicity of three times solar in the circumnuclear starburst of NGC 5953, which is also characterized by an effective temperature of 42000 K. To the north-east, a region of high excitation gas, kinematically perturbed that is ionized by the active nucleus is also observed. In NGC 5347 a high excitation emission knot is detected at 3 arcsec from the nucleus and perpendicular to the stellar bar just inside the inner Lindblad resonance. The CaII triplet in the knot has an equivalent width compatible with the presence of red supergiants; however it seems to be phoionized by the active nucleus.

The circumnuclear spectrum of NGC 4253 is fitted by a HII region model with an effective temperature of 50000 K. The nucleus is a strong FeII emitter; this emission and the CaII triplet observed are discussed in terms of the Starburst model. In NGC 3227 an excess of the continuum emission with respect to a bulge+disk model is detected to the north, aligned with the companion galaxy; this is identified with a stellar bar probably induced by the interaction. We also find a circumnuclear starburst and extended emission photoionized by the active nucleus and closely aligned with the bar. All this emission is located just inside the inner Lindblad resonance. The gravitational torques induced by the bar are suggested as the mechanism responsible for the transport of the gas to the ILR where it fuels the circumnuclear region.