A Population of Metal-Poor Galaxies with ~L* Luminosities at Intermediate Redshifts

TL;DR

This study identifies a population of metal-poor, luminous star-forming galaxies at intermediate redshifts, challenging existing galaxy formation models and suggesting they are in a unique evolutionary stage.

Contribution

It provides spectroscopic metallicity estimates for intermediate-redshift galaxies, revealing a population with extreme properties not explained by current models.

Findings

Galaxies follow a luminosity-metallicity relation offset by over ten times in metallicity.

These galaxies are in a very special evolutionary stage, possibly late-forming massive systems.

They may be undergoing intense starbursts due to mergers or gas infall.

Abstract

We present new spectroscopy and metallicity estimates for a sample of 15 star-forming galaxies with redshifts in the range 0.29 - 0.42. These objects were selected in the KPNO International Spectroscopic Survey via their strong emission lines seen in red objective-prism spectra. Originally thought to be intermediate-redshift Seyfert 2 galaxies, our new spectroscopy in the far red has revealed these objects to be metal-poor star-forming galaxies. These galaxies follow a luminosity-metallicity (L-Z) relation that parallels the one defined by low-redshift galaxies, but is offset by a factor of more than ten to lower abundances. The amount of chemical and/or luminosity evolution required to place these galaxies on the local L-Z relation is extreme, suggesting that these galaxies are in a very special stage of their evolution. They may be late-forming massive systems, which would challenge the current paradigm of galaxy formation. Alternatively, they may represent intense starbursts in dwarf-dwarf mergers or a major infall episode of pristine gas into a pre-existing galaxy. In any case, these objects represent an extreme stage of galaxy evolution taking place at relatively low redshift.