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

Mischa Schirmer, Sangeeta Malhotra, Nancy A. Levenson, Hai Fu, Rebecca L. Davies, William C. Keel, Paul Torrey, Vardha N. Bennert, Anna Pancoast, James E H Turner

Research output: Contribution to journalArticlepeer-review

23 Scopus citations


We report the discovery of 14 Lyα 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 Lyα luminosities of (0.4-6.3) × 1043 erg s-1. 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 0.3. We also show that transient AGN naturally explain the ionization deficits observed in many LABs. Their Lyα and X-ray fluxes decorrelate below ≲106 years because of the delayed escape of resonantly scattering Lyα photons. High Lyα 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 three to four orders of magnitude within the last 104-5 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 Lyα escape. The fully calibrated, co-added optical FITS images are publicly available.

Original languageEnglish (US)
Pages (from-to)1554-1586
Number of pages33
JournalMonthly Notices of the Royal Astronomical Society
Issue number2
StatePublished - Dec 1 2016


  • Galaxies: active
  • Galaxies: evolution
  • Radiative transfer
  • Ultraviolet: galaxies

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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