About AGN ionization echoes, thermal echoes, and ionization deficits in low redshift Lyman-alpha blobs

TL;DR

This paper reports the discovery of low-redshift Lyman-alpha blobs powered by fading AGN, revealing insights into AGN feedback, ionization echoes, and the evolution of LABs over billions of years.

Contribution

It presents the first detection of low-redshift LABs with AGN ionization echoes and thermal echoes, highlighting the fading AGN as the ionization source and their relation to Green Bean galaxies.

Findings

Low-redshift LABs are ionized by fading AGN.

Transient AGN explain ionization deficits in LABs.

Fading AGN leave fossil radiation across multiple wavelengths.

Abstract

We report the discovery of 14 Lyman-alpha blobs (LABs) at z~0.3, existing at least 4-7 billion years later in the Universe than all other LABs known. Their optical diameters are 20-70 kpc, and GALEX data imply Ly-alpha luminosities of (0.4-6.3)x10^43 erg/s. Contrary to high-z LABs, they live in low-density areas. They are ionized by AGN, suggesting that cold accretion streams as a power source must deplete between z=2 and z=0.3. We also show that transient AGN naturally explain the ionization deficits observed in many LABs: Their Ly-alpha and X-ray fluxes decorrelate below 10^6 years because of the delayed escape of resonantly scattering Ly-alpha photons. High Ly-alpha luminosities do not require currently powerful AGN, independent of obscuration. Chandra X-ray data reveal intrinsically weak AGN, confirming the luminous optical nebulae as impressive ionization echoes. For the first time, we also report mid-infrared thermal echoes from the dusty tori. We conclude that the AGN have faded by 3-4 orders of magnitude within the last 10^(4-5) years, leaving fossil UV, optical and thermal radiation behind. The host galaxies belong to the group of previously discovered Green Bean galaxies (GBs). Gemini optical imaging reveals smooth spheres, mergers, spectacular outflows and ionization cones. Because of their proximity and high flux densities, GBs are perfect targets to study AGN feedback, mode switching and the Ly-alpha escape. The fully calibrated, coadded optical FITS images are publicly available.