Spectroscopic, photometric, and dynamical data of the inner 3 kpc of the starburst galaxy M82 are analyzed in order to investigate the star formation history of its disk. The long-slit spectra along the major axis are dominated by Balmer absorption lines in the region outside the nuclear starburst all the way up to %3.5 scale-lengths ( B ¼ 22 mag arcsec À2 ). Single stellar population (SSP) spectra of age 0.4Y1.0 Gyr match the observed spectra in the 1Y3 kpc zone well, with a marginally higher mean age of the stellar population in the outer parts. The mass in these populations, along with that in the gas component, makes up for the inferred dynamical mass in the same annular zone for a Kroupa initial mass function, with a low-mass cutoff of m l ¼ 0:4 M . The observed ratio of the abundances of -elements with respect to Fe is also consistent with the idea that almost all the stars in the M82 disk formed in a burst of short duration (0.3 Gyr) around 0.8 Gyr ago. We find that the optical/near-infrared colors and their gradients in the disk are determined by the reddening, with visual extinction exceeding 1 mag even in the outer parts of the disk, where there is apparently no current star formation. The disk-wide starburst activity was most likely triggered by the interaction of M82 with its massive neighbor M81, around 1 Gyr ago. The properties of the disk of M82 very much resemble the properties of the disks of luminous compact blue galaxies seen at z ¼ 0:2Y1:0.

Aims.[O iii] 5007 Å on-band off-band images, obtained with the Very Large Telescope (VLT) and FORS 2 spectrograph in two zones (center and outskirts) of the spiral galaxy NGC 300, are analyzed searching for emission line objects. In particular we search for planetary nebula (PN) candidates to analyze their distribution and luminosity properties, to perform follow-up spectroscopy, and to study the planetary nebula luminosity function, PNLF. Methods. In the continuum-subtracted images, a large number of emission line objects were detected. From this sample we selected as PN candidates those objects with stellar appearance and no detectable central star. [O iii] 5007 instrumental magnitudes were measured and calibrated by using spectrophotometric data from the follow-up spectroscopy. Results. We have identified more than a hundred PN candidates and a number of compact HII regions. The PN sample is the largest one reported for this galaxy so far. For all the objects we present coordinates, instrumental [O iii] 5007 magnitudes and apparent nebular [O iii] 5007 fluxes and magnitudes. The [O iii] 5007 observed luminosity function for PNe (PNLF) was calculated for the whole sample and for the central and outskirts samples. The three PNLF are similar within uncertainties. We fit the empirical PNLF to the observed PNLF for all the samples. From our best fit for the whole sample we derived a maximum value for the apparent magnitudes of m ⋆ 5007 = 22.019 ± 0.022 and we obtained a tentative estimate of the distance modulus m 5007 −M 5007 = 26.29 +0.12 −0.22 mag, which agrees well with the recent value derived from Cepheid stars.

Aims. We aim to derive the chemical behavior of a significant sample of PNe and HII regions in the irregular galaxy NGC 6822. The selected objects are distributed in different zones of the galaxy. Our purpose is to obtain the chemical abundances of the present interstellar medium (ISM), represented by H ii regions, and the corresponding values at the time of formation of PNe. With these data the chemical homogeneity of NGC 6822 were tested and the abundance pattern given by H ii regions and PNe used as an observational constraint for computing chemical evolution models to infer the chemical history of NGC 6822.Methods. Due to the faintness of PNe and H ii regions in NGC 6822, to gather spectroscopic data with large telescopes is necessary.We obtained a well suited sample of spectra by employing VLT-FORS 2 and Gemini-GMOS spectrographs. Ionic and total abundances were calculated for the objects where electron temperatures could determined through the detection of [O iii] λ4363 or/and [N ii] λ5755 lines. A "simple" chemical evolution model was developed and the observed data were used to compute a model for NGC 6822 in order to infer a preliminary chemical history in this galaxy.Results. Confident determinations of He, O, N, Ne, S and Ar abundances were derived for a sample of 11 PNe and one H ii region. We confirm that the present ISM is chemically homogeneous, at least in the central 2 kpc of the galaxy, showing a value 12 + log O/H = 8.06 ± 0.04. From the abundance pattern of PNe, we identified two populations: a group of young PNe with abundances similar to H ii regions and a group of older objects with abundances a factor of two lower. A pair of extreme Type I PNe were found. No third dredge-up O enrichement was detected in PNe of this galaxy. The abundance determinations allow us to discuss the chemical behavior of the present and past ISM in NGC 6822. Our preliminary chemical evolution model predicts that an important gas-mass loss occurred during the first 5.3 Gyr, that no star higher than 40 M was formed, and that 1% of all 3-15 M stars became binary system progenitors of SNIa.

Aims. Images obtained with the CTIO 4-m telescope and the MOSAIC-2 wide field camera in [O iii] 5007 and Hα on-band and off-band filters are analyzed to search for emission line objects in the dwarf galaxy NGC 6822. In particular we search for planetary nebula (PN) candidates. In addition, imaging and spectroscopy of a sub-sample of objects obtained with the ESO VLT and FORS 2 spectrograph are used to calibrate the MOSAIC imaging. Methods. In the continuum-subtracted images, a large number of line emission regions were detected, for which we measured instrumental magnitudes in all the filters. The [O iii] 5007 and Hα + [N ii] magnitudes were calibrated with the spectroscopy.Results. Based upon some criteria to distinguish between PNe and compact HII regions, we have found 26 PN candidates, increasing the known sample by 8 objects. Also we detected a number of compact HII regions and about 20 stellar-like objects emitting in Hα.For all the objects we present coordinates, instrumental magnitudes and nebular [O iii] and Hα + [N ii] fluxes. The observed luminosity function for the PN [O iii] 5007 magnitudes (PNLF) and the cumulative PNLF were calculated. We confirm that the PNLF presents a dip similar to the one detected for the Small Magellanic Cloud at 2.5 mag below the maximum. The cumulative PNLF returns a value M 5007 = −3.71 +0.21−0.42 for the peak absolute magnitude of the PNLF which is faint (but within uncertainties) compared to the value expected for galaxies with a metallicity similar to that of NGC 6822. From our best fit to the observed PNLF we obtained a rough estimate of the distance modulus m − M = 23.64 +0.23−0.43 mag, which agrees within uncertainties with recent values from Cepheid stars reported in the literature. Also the number of PNe in the brightest 0.5 mag, normalized to the galactic bolometric luminosity, was estimated to be α 0.5 ∼ (3.8 +0.90 −0.71 ) E-9. This number is similar to the values derived for galaxies with recent star formation and small galaxies (M B fainter than −18 mag) and larger than the values for early-type galaxies.

Aims. The chemical behaviour of an ample sample of planetary nebulae (PNe) in NGC 6822 is analysed.Methods. Spectrophotometric data of 11 PNe and two H  regions were obtained with the OSIRIS spectrograph attached to the Gran Telescopio Canarias. Data for other 13 PNe and three H  regions were retrieved from the literature. Physical conditions and chemical abundances of O, N, Ne, Ar, and S were derived in a consistent way for 19 PNe and 4 H  regions.Results. Abundances in the PNe sample are widely distributed showing 12 + log (O/H) from 7.4 to 8.2 and 12 + log (Ar/H) from 4.97 to 5.80. Two groups of PNe can be differentiated: one old with low metallicity (12 + log (O/H) < 8.0 and 12 + log (Ar/H) < 5.7) and another younger one with metallicities similar to the values for H  regions. The old objects are distributed in a larger volume than the young ones. An important fraction of PNe (over 30%) was found to be highly N-rich (Peimbert Type I PNe). Such PNe occur at any metallicity. In addition, about 60% of the sample presents high ionization (He ++ /He ≥ 0.1), possessing a central star with effective temperature higher than 100 000 K. Possible biases in the sample are discussed. From comparison with stellar evolution models by Karakas (2010) and Fishlock et al. (2014) of the observed N/O abundance ratios, our PNe should have had initial masses that are lower than 4 M , although if the comparison is made with Ne vs. O abundances, the initial masses should have been lower than 2 M . It appears that these models of stars of 2−3 M are producing too much 22 Ne in the stellar surface at the end of the AGB. On the other hand, the comparison with another set of stellar evolution models with a different treatment of convection and on the assumptions about the overshoot of the convective core during the core H-burning phase, provided there is reasonable agreement between the observed and predicted N/O and Ne/H ratios if initial masses of more massive stars are about 4 M .