The Green Chile Project. I. A New Catalog of Low-redshift Extended Emission-line Nebulae

Abstract Extended emission-line nebulae (EELN) around galaxies allow us to study their host galaxies and their surrounding environments. EELN can provide insights into the physical properties of the extended gas and, when powered by active galactic nuclei (AGN), they can constrain the ionization history of the central engine as well. Of interest is a sample of low-redshift EELN powered by AGN at z ∼ 0.3 that were discovered in the Sloan Digital Sky Survey (SDSS) through color selection. Colloquially called “Green Beans,” due to strong, spatially extended [O iii ] λ 5007 emission dominating the r -band flux, they have been shown to be low-redshift counterparts to higher-redshift Ly α nebulae. However, only 17 Green Bean EELN are known to exist at z ∼ 0.3, and the original color selection was subsequently found to be biased in terms of host galaxy properties. In order to mitigate these biases and expand the known sample, we have developed a new, physically motivated selection approach, based on simulated EELN at z ∼ 0.3, with well-defined physical properties ([O iii ] λ 5007 rest-frame equivalent width and continuum spectral slope). With this improved selection approach, we have generated a new sample of 165 EELN candidates: the “Green Chile” catalog. Of the 13 candidates with existing SDSS/Baryon Oscillation Spectroscopic Survey spectra, we confirm that five are EELN with [O iii ] λ 5007 equivalent widths >200 Å and emission-line ratios indicative of AGN powering.