Extreme emission-line galaxies out toz~ 1 in zCOSMOS

Abstract

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.

Characterization of star-forming dwarf galaxies at 0.1 ≲z ≲ 0.9 in VUDS: probing the low-mass end of the mass-metallicity relation

“…This sequence moves towards higher SFRs at higher z, though its slope remains almost constant (∼1) (Guo et al 2015). At lower masses, SFDGs and, in particular, those with the strongest emission lines (EELGs) are found to have increased SFRs at fixed M * (by ∼1 dex) compared to the extrapolation at low mass of the MS (Amorín et al 2015), suggesting that they are efficiently forming stars in strong bursts with stellar mass doubling times <1 Gyr, which cannot be sustained for long.…”

“…where the observed ratios are divided by the theoretical values (Hα/Hβ = 2.82 and Hγ/Hβ = 0.47) and for case B recombination with T e = 2 × 10 4 K, n e = 100 cm −3 following Amorín et al (2015); f Hα and f Hγ are the coefficients corresponding to the Cardelli et al (1989) extinction curve at the wavelength of the Hα and Hγ emission lines ( f Hα = 0.313, f Hγ = 0.157), respectively. For 111 galaxies in our sample, we only have one hydrogen line available or the Hα/Hβ and Hβ/Hγ ratios are below their theoretical values.…”

“…In this paper, we divide our galaxy sample into the four morphological classes defined by Amorín et al (2015) . HST i-band images of eight galaxies in the COSMOS field with different morphological properties.…”

“…This makes them easier to identify even beyond the local Universe in current spectroscopic surveys. Throughout this paper, we refer to this kind of faint galaxy with and have more extreme properties; for example, higher surface densities, lower starburst ages, and lower gas metallicities than the average population of star-forming dwarfs (Kniazev et al 2004;Cardamone et al 2009;Amorín et al 2010Amorín et al , 2012Amorín et al , 2014Amorín et al , 2015Atek et al 2011;van der Wel et al 2011;Maseda et al 2014). While the population of EELGs itself constitutes an ideal laboratory for studying star formation and chemical enrichment under extreme physical conditions, it also appears to contain environments that most closely resemble "typical" galaxies at very high redshifts (z > ∼ 3−4, e.g., Smit et al 2014;Stark et al 2017).…”

“…Such types of galaxies at high redshift are thought to significantly contribute to the reionization of the Universe, providing up to 20% of the total ionizing flux at z ∼ 6 (Robertson et al 2015;Dressler et al 2015). As far as the sizes are concerned, SFDGs selected by strong optical emission lines (in particular [O iii]λ5007) are typically small isolated systems with radii of the order of ∼1 kpc (Izotov et al 2016) and they show irregular morphologies (Amorín et al 2015).…”