We have investigated the oxygen and nitrogen chemical abundances in extremely compact starforming galaxies (SFGs) with redshifts between ∼0.11 and 0.35, popularly referred to as "green peas". Direct and strong-line methods sensitive to the N/O ratio applied to their Sloan Digital Sky Survey (SDSS) spectra reveal that these systems are genuine metal-poor galaxies, with mean oxygen abundances ∼ 20% solar. At a given metallicity these galaxies display systematically large N/O ratios compared to normal galaxies, which can explain the strong difference between our metallicities measurements and previous ones. While their N/O ratios follow the relation with stellar mass of local SFGs in the SDSS, we find that the mass-metallicity relation of the "green peas" is offset 0.3 dex to lower metallicities. We argue that recent interaction-induced inflow of gas, possibly coupled with a selective metal-rich gas loss, driven by supernova winds, may explain our findings and the known galaxy properties, namely high specific star formation rates, extreme compactness, and disturbed optical morphologies. The "green pea" galaxy properties seem to be not common in the nearby universe, suggesting a short and extreme stage of their evolution. Therefore, these galaxies may allow us to study in great detail many processes, such as starburst activity and chemical enrichment, under physical conditions approaching those in galaxies at higher redshifts.

In a 6h Hα exposure of the N-W region of the cluster of galaxies A1367 we discovered a 75 kpc cometary emission of ionized gas trailing behind two Irregular galaxies. The H α trails correspond in position and length with tails of syncrotron radiation. At the galaxy side opposite to the tails the two galaxies show bright HII regions aligned along arcs, where the star formation takes place at the prodigeous rate of ∼ 1M ⊙ yr −1 . From the morphology of the galaxies and of the trailing material, we infer that the two galaxies are suffering from ram pressure due to their high velocity motion through the cluster IGM. We estimate that ∼ 10 9 M⊙ of gas, probably ionized in the giant HII regions, is swept out forming the tails. The tails cross each other at some 100 kpc from the present galaxy location, indicating that a major tidal event occurred some ∼ 5 × 10 7 yr ago. We exclude that mutual harassment produced the observed morphology and we show with numerical simulations that it could have marginally aided ram pressure stripping by loosening the potential well of the galaxies.

We demonstrate the existence of a local mass, metallicity, star formation relation using spatially resolved optical spectroscopy of H ii regions in the local universe. One of the projections of this distribution-the local mass-metallicity relation-extends over a wide range in this parameter space: three orders of magnitude in mass and a factor of eight in metallicity. We explain the new relation as the combined effect of the differential distributions of mass and metallicity in the disks of galaxies, and a selective star formation efficiency. We use this local relation to reproduce-with a noticeable agreement-the mass-metallicity relation seen in galaxies, and conclude that the latter is a scale-up integrated effect of a local relation, supporting the inside-out growth and downsizing scenarios of galaxy evolution.

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Context. The study of large and representative samples of low-metallicity star-forming galaxies at different cosmic epochs is of great interest to the detailed understanding of the assembly history and evolution of low-mass galaxies. Aims. We present a thorough characterization of a large sample of 183 extreme emission-line galaxies (EELGs) at redshift 0.11 ≤ z ≤ 0.93 selected from the 20k zCOSMOS bright survey because of their unusually large emission line equivalent widths. Methods. We use multiwavelength COSMOS photometry, HST-ACS I-band imaging, and optical zCOSMOS spectroscopy to derive the main global properties of star-forming EELGs, such as sizes, stellar masses, star formation rates (SFR), and reliable oxygen abundances using both "direct" and "strong-line" methods. Results. The EELGs are extremely compact (r 50 ∼ 1.3 kpc), low-mass (M * ∼ 10 7 −10 10 M ) galaxies forming stars at unusually high specific star formation rates (sSFR ≡ SFR/M up to 10 −7 yr −1 ) compared to main sequence star-forming galaxies of the same stellar mass and redshift. At rest-frame UV wavelengths, the EELGs are luminous and show high surface brightness and include strong Lyα emitters, as revealed by GALEX spectroscopy. We show that zCOSMOS EELGs are high-ionization, low-metallicity systems, with median 12+ log(O/H) = 8.16 ± 0.21 (0.2 Z ) including a handful of extremely metal-deficient (<0.1 Z ) EELGs. While ∼80% of the EELGs show non-axisymmetric morphologies, including clumpy and cometary or tadpole galaxies, we find that ∼29% of them show additional low-surface-brightness features, which strongly suggests recent or ongoing interactions. As star-forming dwarfs in the local Universe, EELGs are most often found in relative isolation. While only very few EELGs belong to compact groups, almost one third of them are found in spectroscopically confirmed loose pairs or triplets. Conclusions. The zCOSMOS EELGs are galaxies caught in a transient and probably early period of their evolution, where they are efficiently building up a significant fraction of their present-day stellar mass in an ongoing, galaxy-wide starburst. Therefore, the EELGs constitute an ideal benchmark for comparison studies between low-and high-redshift low-mass star-forming galaxies.

Context. The study of the radial variations of metallicity across the Galactic disc is a powerful method for understanding the history of star formation and chemical evolution of the Milky Way. Although several studies about gradients have been performed so far, the knowledge of the Galactic antincentre is still poor. Aims. This work aims to determine accurately the physical and chemical properties of a sample of H ii regions located at R G >11 kpc and to study the radial distribution of abundances in the outermost part of the Galaxy disc. Methods. We carried out new optical spectroscopic observations of nine H ii regions with the William Herschel Telescope covering the spectral range from 3500 Å to 10100Å. In addition, we increased the sample by searching the literature for optical observations of regions towards the Galactic anticentre, re-analysing them to obtain a single sample of 23 objects to be processed in a homogeneous and consistent manner. The total sample distribution covers the Galactocentric radius from 11 kpc to 18 kpc. Results. Emission line ratios were used to determine accurate electron densities and temperatures of several ionic species in 13 H ii regions. These physical parameters were applied to the spectra to determine direct total chemical abundances. For those regions without direct estimations of temperature, chemical abundances were derived by performing tailor-made photoionisation models and/or by using an empirical relation obtained from radio recombination and optical temperatures. We performed weighted least-squares fits to the distribution of the derived abundances along the Galactocentric distances to study the radial gradients of metallicity across the outermost part of the MW. The distributions O/H, N/H, S/H, and Ar/H towards the anticentre can be represented by decreasing linear radial gradients, while in the case of N/O abundances the radial distribution is better fitted with a two-zone model. The He/H radial gradient is presented here for the first time; we find a slope that is not significantly different from zero. The derived gradient for oxygen shows a clear decrease with distance with a slope of -0.053±0.009 dex kpc −1 . Although a shallower slope at large Galactocentric distances is suggested by our data, the flattening of the distribution cannot be confirmed and more objects towards the anticentre need to be studied in order to establish the true form of the metallicity gradient.

Spectroscopic observations obtained with the VLT of one planetary nebula (PN) in Sextans12 + log (O/H) = 7.6 ± 0.2 in Sextans A, and 7.8 ± 0.2 in Sextans B. For the five PNe of Sextans B, we find that 12 + log (O/H) = 8.0 ± 0.3, with a mean abundance consistent with that of H  regions. The only PN known in Sextans A appears to have been produced by a quite massive progenitor, and has a significant nitrogen overabundance. In addition, its oxygen abundance is 0.4 dex larger than the mean abundance of H  regions, possibly indicating an efficient third dredge-up for massive, low-metallicity PN progenitors. The metal enrichment of both galaxies is analyzed using these new data.

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