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.
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.
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