Average Metallicity and Star Formation Rate of Lya Emitters Probed by a Triple Narrow-Band Survey

Abstract: We present the average metallicity and star-formation rate of Lya emitters (LAEs) measured from our large-area survey with three narrow-band (NB) filters covering the Lya, [OII]3727, and Ha+[NII] lines of LAEs at z=2.2. We select 919 z=2.2 LAEs from Subaru/Suprime-Cam NB data in conjunction with Magellan/IMACS spectroscopy. Of these LAEs, 561 and 105 are observed with KPNO/NEWFIRM near-infrared NB filters whose central wavelengths are matched to redshifted [OII] and Ha nebular lines, respectively. By stacking the near-infrared images of the LAEs, we successfully obtain average nebular-line fluxes of LAEs, the majority of which are too faint to be identified individually by narrow-band imaging or deep spectroscopy. The stacked object has an Ha luminosity of 1.7x10^{42} erg s^{-1} corresponding to a star formation rate (SFR) of 14 M_{sun} yr^{-1}. We place, for the first time, a firm lower limit to the average metallicity of LAEs of Z>~0.09 Z_{sun} (2sigma) based on the [OII]/(Ha+[NII]) index together with photo-ionization models and empirical relations. This lower limit of metallicity rules out the hypothesis that LAEs, so far observed at z~2, are extremely metal poor (Z<2x10^{-2} Z_{sun}) young galaxies at the 4sigma level. This limit is higher than a simple extrapolation of the observed mass-metallicity relation of z~2 UV-selected galaxies toward lower masses (5x10^{8} M_{sun}), but roughly consistent with a recently proposed fundamental mass-metallicity relation when the LAEs' relatively low SFR is taken into account. The Ha and Lya luminosities of our NB-selected LAEs indicate that the escape fraction of Lya photons is ~12-30 %, much higher than the values derived for other galaxy populations at z~2.

Average Metallicity and Star Formation Rate of Ly$\alpha$ Emitters Probed by a Triple Narrow-Band Survey

The paper by Nakajima et al. presents an extensive analysis of Ly$\alpha$ emitters (LAEs) at a redshift of $z=2.2$, using a novel approach involving triple narrow-band (NB) filters. The primary focus of the study is to estimate the average metallicity and star formation rate (SFR) of these emitters, which are pivotal for understanding galaxy formation and evolution.

Key Findings

The authors conducted their survey utilizing NB filters that are finely tuned to capture specific emission lines—namely Ly$\alpha$, [O\,{\sc ii}]$\lambda 3727$, and H$\alpha$+[N\,{\sc ii}]—corresponding to the target redshift. The large sample size, involving 919 LAEs, allowed robust statistical analysis, particularly benefiting from stacking techniques that provided insights into these typically faint galaxies.

1. Star Formation Rate: The study measured an average H$\alpha$ luminosity of $1.7 \times 10^{42}$ erg s$^{-1}$, equivalent to a SFR of 14 $M_{\odot}$ yr$^{-1}$. This direct measure, derived from the nebular line luminosities, is presented as a reliable indicator of galactic star formation activities.

2. Metallicity: The analysis places a firm lower limit on the average metallicity of LAEs at $Z \gtrsim 0.09 Z_{\odot}$ ($2\sigma$). This value challenges the hypothesis of LAEs being extremely metal-poor galaxies ($Z < 0.02 Z_{\odot}$), suggesting instead that they may have metallicities that are consistent with the fundamental mass-metallicity relation, taking into account the relatively lower SFRs implied.

3. Escape Fraction of Ly$\alpha$ Photons: The paper notes an escape fraction of Ly$\alpha$ photons of about 12-30%, which is notably higher than that observed in other galaxy populations at similar redshifts. This variability is pivotal for understanding the structural properties and dust distribution within LAEs.

Implications

The findings hold significant implications for models of galaxy formation, particularly the evolution of metallicity and star formation in early galaxies. The measured metallicity offers a crucial benchmark that refines the understanding of galaxy evolution trends and supports theories which predict metallicity growth concurrent with mass.

Furthermore, the high Ly$\alpha$ escape fraction implies diverse interstellar medium (ISM) conditions, possibly hinting at clumpy or outflowing gas—leading to new avenues for research into galactic feedback processes and ISM structure.

Speculations on Future Developments

The methodological advancements showcased by Nakajima et al., particularly the use of triple NB imaging, could become a standard approach for deep-field cosmological surveys. With ongoing improvements in telescope and sensor technologies, similar methodologies can be extended to even deeper or more distant targets, potentially unraveling the mysteries of galaxy formation in the earliest epochs of the universe.

Advancements in spectroscopic techniques could also yield higher precision measurements of metallicities and other properties, enhancing the reliability of empirical models in cosmology. Ultimately, these efforts will contribute to a more nuanced understanding of the underlying principles guiding galaxy evolution, star formation, and elemental synthesis across cosmic time.