Understanding Lyα Nebulae at Low Redshift. I. The Sizes, Powering, and Kinematics of “Green Bean” Galaxies

Abstract

Abstract A new but rare sample of spatially extended emission line nebulae, nicknamed “Green Beans,” was discovered at z ≈ 0.3 thanks to strong [O iii] emission, and subsequently shown to be local cousins of the Lyα nebulae found at high redshift. Here we use follow-up APO/DIS spectroscopy to better understand how these low-redshift Lyα nebulae compare to other populations of strong emission line sources. Our spectroscopic data show that low-z Lyα nebulae have active galactic nucleus (AGN)-like emission line ratios, relatively narrow line widths (FWHM ≲ 1000 km s−1), and emission line kinematics resembling those of Type 2 AGN at the same redshift, confirming that they are powered by Type 2 AGN with typical ionizing continua. While low-z Lyα nebulae are larger and less concentrated than compact, star-forming Green Pea galaxies, we find that they resemble typical Type 2 AGN in terms of r-band concentration and size. Based on this pilot study, low-z Lyα nebulae appear to be a subset of Type 2 AGN with bluer optical continua and high [O iii] equivalent widths but with comparable sizes and similar [O iii] kinematics. These characteristics may simply reflect the fact that low-z Lyα nebulae are drawn from the high-luminosity end of the Type 2 AGN distribution, with higher nuclear activity driving higher [O iii] equivalent widths and more central star formation leading to bluer optical continua. Deeper spectroscopic follow-up of the full sample will shed further light on these issues and on the relationship between these low-z Lyα nebulae and the Lyα nebula population at high redshift.