Searching for the Lowest Metallicity Galaxies

TL;DR

This paper introduces a new method to identify low-metallicity blue compact dwarf galaxies from SDSS data and confirms their metallicity through spectroscopic observations, discovering some of the lowest-metallicity galaxies known.

Contribution

The study develops a photometric selection technique for low-metallicity galaxies and provides spectroscopic confirmation of 94 new candidates, including some with extremely low metallicity.

Findings

45 candidates have 12 + log(O/H) <= 7.65

Six systems are among the lowest-metallicity galaxies known

Confirmed some galaxies with 1/30 solar oxygen abundance

Abstract

We report a method of identifying candidate low-metallicity blue compact dwarf galaxies (BCDs) from the Sloan Digital Sky Survey (SDSS) imaging data and present 3-m Lick Observatory and 10-m W.M. Keck Observatory optical spectroscopic observations of 94 new systems that have been discovered with this method. The candidate BCDs are selected from Data Release 12 (DR12) of SDSS based on their photometric colors and morphologies. Using the Kast spectrometer on the 3-m telescope, we confirm that the candidate low-metallicity BCDs are emission-line galaxies and we make metallicity estimates using the empirical R and S calibration methods. Follow-up observations on a subset of the lowest-metallicity systems are made at Keck using the Low Resolution Imaging Spectrometer (LRIS), which allow for a direct measurement of the oxygen abundance. We determine that 45 of the reported BCDs are low-metallicity candidates with 12 + log(O/H) <= 7.65, including six systems which are either confirmed or projected to be among the lowest-metallicity galaxies known, at 1/30 of the solar oxygen abundance, or 12 + log(O/H) ~ 7.20.