# Mapping the Youngest Galaxies to Redshift One

## TL;DR

This study identifies and characterizes ultra-strong emission line galaxies at intermediate and high redshifts, revealing their role in galaxy formation and their extremely low metallicities, using narrow-band imaging and follow-up spectroscopy.

## Contribution

It presents a new method for detecting ultra-strong emission line galaxies and provides insights into their properties and significance in galaxy evolution at z<1 and beyond.

## Key findings

- 542 candidate USELs identified in a half-square-degree survey.
- USELs contribute 5-10% to the star formation density at z=0-1.
- Several USELs are extremely metal-poor, some with metallicities near the primordial level.

## Abstract

We describe results of a narrow band search for ultra-strong emission line galaxies (USELs) with EW(H beta) > 30 A. 542 candidate galaxies are found in a half square degree survey using two ~100 Angstrom 8150 A and 9140 A filters with Subaru/SuprimeCam. Followup spectroscopy for randomly selected objects in the sample with KeckII/DEIMOS shows they consist of [OIII] 5007, [OII] 3727, and H alpha selected strong-emission line galaxies at intermediate redshifts (z < 1), and Ly alpha emitting galaxies at high-redshift (z >> 5). We determine the H beta luminosity functions and the star formation density of the USELs, which is 5-10% of the value found from ultraviolet continuum objects at z=0-1, suggesting they correspond to a major epoch in galaxy formation at these redshifts. Many USELs show the temperature-sensitive [OIII] 4363 auroral lines and about a dozen have oxygen abundances characteristic of eXtremely Metal Poor Galaxies (XMPGs). These XMPGs are the most distant known today. Our high yield rate of XMPGs suggests this is a powerful method for finding such populations. The lowest metallicity measured in our sample is 12+log(O/H) = 7.06 (6.78-7.44), close to the minimum metallicity found in local galaxies. The luminosities, metallicities and star formation rates of USELs are consistent with the strong emitters being start-up intermediate mass galaxies and suggest that galaxies are still forming in relatively chemically pristine sites at z < 1.