Variability is one of the most conspicuous properties of AGN. The starburst model postulates that the variability observed in radio-quiet sources is produced by the supernova (SN) and compact supernova remnant (cSNR) activity resulting from the evolution of a metal-rich massive stellar cluster, product of a starburst in the nucleus of an early-type galaxy. In this context, the optical light curves of AGN are reproduced by a random sequence of SN events. The parameters that describe a given light curve are the overall rate of explosions (νSN), the energy released in each cSNR (∊51), and the density of the circumstellar medium in which the remnants evolve (n7). In the case of low-luminosity AGN (MB ≳ −22 mag) these three parameters are well constrained by the observations: νSN by the minimum of luminosity and/or by the number of peaks of the light curves; ∊51 by the amplitude and duration of typical oscillations in the light curve and/or by the equivalent width of recombination lines, such as Hβ; and n7 by the decay rate of well-isolated peaks (Aretxaga & Terlevich 1993). The physics involved in the parameters above provides two independent constraints on the distance to a low-luminosity AGN.

Firts distance indicator: SN rate versus stellar luminosity

The B-band luminosity arising from a coeval cluster at its SN II explosion phase, 10 to 60 Myr, is mainly due to Main Sequence stars and cSNR.

We present narrow-band Hα imaging and long-slit optical and near infrared spectroscopy of the giant HII region NGC 2363. We have found broad emission lines at 4686 Å and at 5810 Å attributed to WC stars at 6 arcsec to the East of the brightest core of the region. We confirm the existence of low-intensity broad components in Hα and [OIII] which extend some 500 pc. We have derived the physical conditions and chemical composition of the gas in 15 different zones in the region, and do not find significant variations in the abundances. The Paschen discontinuity has been found in emission. The Pa electron temperatures obtained are significantly smaller than those obtained from the [OIII] and [SIII] emission lines, indicating the presence of large temperature fluctuations.

Introduction

One of the targets of the GEFE programme is the giant HII region NGC 2363 located in the SW of the irregular galaxy NGC 2366. This is one of the largest extragalactic HII regions with high surface brightness. The object was observed in La Palma in narrow band Hα and long-slit spectrophotometry from [OII] λ3227 to [SIII] λ9532 at two positions, at the brighest core of the region (which we call knot A) and at 6 arcsec to the East (knot B).

Narrow-band images

The object was observed with the 1-m JKT telescope. We used a CCD with a spatial scale of 0.3 arcsec pixel−1.

There is mounting evidence that type transitions are a common property of AGN: the broad lines in at least eleven Seyfert galaxies have appeared or disappeared, leading to the reclassification of their nuclei from type 1–1.5 to type 1.8–2 or vice versa. We show that these phenomena find a natural explanation in the starburst model for AGN as transient phases without supernova activity in a 10–60 Myr old metal-rich massive stellar cluster with a low supernova rate (νSN ≲ 3 yr−1).

Type transient AGN: casuistry

Spectroscopic observations of Seyfert galaxies established early on that the broad permitted lines can experience strong variations in time scales of a few weeks–months. There is a growing number of extreme cases in which the broad components have temporarilly disappeared or become so weak that a reclassification of the objects has been allowed: from Seyfert nuclei of type 1–1.5 to type 1.8–2/LINER. Among them we can find the prototypes NGC 4151 and NGC 5548, along with NGC 1566, NGC 3516, NGC 6814, NGC 7603, Mrk 372 and 3C 390.3. In most of these cases, we know that the transitions took place while the nuclei were in deep photometric minima. Conversely, there are some narrow-line objects that have developed prominent broad components while brightening. Among them we can find Mrk 6, Mrk 993 and Mrk 1018.

We present narrow-band Hα imaging and long-slit optical and near-infrared spectroscopy of the starburst galaxy NGC 7714. We have detected WR stars in the starburst region, which indicate an age for the burst of between 3 and 5 Myr. We have obtained the physical condition of the gas in the starburst region and in three HII regions. These have moderately low abundances, while the nucleus has half solar abundance, with an overabundance of N.

Introduction

A typical starburst galaxy can be defined as a spiral galaxy with a bright nucleus bluer than expected for its morphological type, which emits strong narrow emission lines similar to low-ionization HII region spectra, as a consecuence of the photoionization by the ultraviolet radiation of hot stars, with typical Hα luminosities ranging from 1040 to 1042 erg s−1. During this intense recent burst of star formation between 107 and 1010 M⊙ of massive stars are formed within a radius of a few hundred pc about its nucleus.

NGC 7714, the prototype of the starburst (henceforth SB) galaxies (Weedman et al, 1981) and classified as a SBb peculiar, is in interaction with the irregular galaxy NGC 7715. The X-ray luminosity (6 1040 erg s−1) is explained with about 104 supernova remmants in a volume of 280 pc radius (Weedman et al. 1981). The 6-cm radio map shows a weak double radio structure separated by about 1 arcsec at p.a = 30°.

Introduction

The defining characteristic of type II SNe is the presence of very broad Hα emission with a strong P-Cygni profile. A small number of peculiar type II SNe have been found in recent years which are either very bright in the optical continuum with very strong and broad Hα emission without a P-Cygni profile, or are strong radio sources then called radio supernovae.