The Galaxy Zoo survey for giant AGN-ionized clouds: past and present black hole accretion events

Abstract

Some active galactic nuclei (AGN) are surrounded by extended emission‐line regions (EELRs), which trace both the illumination pattern of escaping radiation and its history over the light travel time from the AGN to the gas. From a new set of such EELRs, we present evidence that the AGN in many Seyfert galaxies undergo luminous episodes 0.2–2 years in duration. Motivated by the discovery of the spectacular nebula known as Hanny’s Voorwerp, ionized by a powerful AGN which has apparently faded dramatically within years, Galaxy Zoo volunteers have carried out both targeted and serendipitous searches for similar emission‐line clouds around low‐redshift galaxies. We present the resulting list of candidates and describe spectroscopy identifying 19 galaxies with AGN‐ionized regions at projected radii kpc. This search recovered known EELRs (such as Mrk 78, Mrk 266 and NGC 5252) and identified additional previously unknown cases, one with detected emission to kpc. One new Sy 2 was identified. At least 14/19 are in interacting or merging systems, suggesting that tidal tails are a prime source of distant gas out of the galaxy plane to be ionized by an AGN. We see a mix of one‐ and two‐sided structures, with observed cone angles from 23 to 112. We consider the energy balance in the ionized clouds, with lower and upper bounds on ionizing luminosity from recombination and ionization‐parameter arguments, and estimate the luminosity of the core from the far‐infrared data. The implied ratio of ionizing radiation seen by the clouds to that emitted by the nucleus, on the assumption of a non‐variable nuclear source, ranges from 0.02 to ; 7/19 exceed unity. Small values fit well with a heavily obscured AGN in which only a small fraction of the ionizing output escapes to be traced by surrounding gas. However, large values may require that the AGN has faded over tens of thousands of years, giving us several examples of systems in which such dramatic long‐period variation has occurred; this is the only current technique for addressing these time‐scales in AGN history. The relative numbers of faded and non‐faded objects we infer, and the projected extents of the ionized regions, give our estimate (0.2–2 years) for the length of individual bright phases.

STORM IN A “TEACUP”: A RADIO-QUIET QUASAR WITH ≈10 kpc RADIO-EMITTING BUBBLES AND EXTREME GAS KINEMATICS

“…Using a citizen science campaign to search for candidate AGN-ionized extended emission-line regions (EELRs) in SDSS images, participants of the "Galaxy Zoo" project (Lintott et al 2008) identified J1430+1339 as having an interesting EELR morphology (Keel et al 2012; later confirmed by HST imaging; Keel et al 2014). Due to the loopshaped "handle" of emission-line gas extending ≈12 kpc to the northeast of the SDSS image, Keel et al (2012) nicknamed this galaxy the "Teacup" AGN (which is the name we will now use). The HST imaging also reveals that the "Teacup" AGN resides in a bulge-dominated galaxy, with shell-like features, indicative of previous merger activity (Keel et al 2014).…”

“…In this pilot study, we combine high-resolution radio imaging and spatially resolved spectroscopy for one radio-quiet type 2 quasar, J1430+1339 (nicknamed the "Teacup AGN"; Keel et al 2012; see Section 2). This source is drawn from a larger sample of type 2 AGNs that host kiloparsec-scale ionized outflows (Harrison et al 2014).…”

Fading AGN Candidates: AGN Histories and Outflow Signatures∗

“…As given by Keel et al (2015), the sample of AGN we consider includes the eight systems from the Galaxy Zoo survey of giant ionized clouds (Keel et al 2012a) where a simple energy-budget estimate including the ionization requirements of the most distant emission line region, compared to the observed bolometric luminosity of the nuclei, showed a shortfall greater than about a factor 5. These include NGC 5252 with its well-documented set of ionization cones (Prieto & Freudling 1996, Morse et al 1998, and the large-scale double radio source hosts NGC 5972 (Véron-Cetty & Véron 2001) and Mkn 1498 (Röttgering et al 1996).…”

“…Our estimates of the bolometric luminosity of the AGN, and hence the role of obscuration on the ionizing radiation, can be updated from Keel et al (2012a) using the more sensitive and higherresolution survey data from WISE (Wright et al 2010) in place of IRAS or Akari for the midinfrared region including 22 µ. WISE data show that much of the mid-and far-IR flux attributed initially to UGC 7342 at z = 0.047 comes instead from a background starburst system at z = 0.069 (as identified in Paper I).…”

“…Starting with the unusual morphology and SDSS gri colors of Hanny's Voorwerp, a targeted search by Galaxy Zoo volunteers found 19 addi-tional EELRs, including hitherto unknown examples, of which 8 showed energy shortfalls similar to that in IC 2497 (Keel et al 2012a). This sample was selected in a homogeneous way, requiring detection of emission features showing AGN-like line ratios more than 10 kpc in projection from the nucleus.…”