An extended emission-line region (EELR) is a giant interstellar cloud ionized by the radiation of an active galactic nucleus (AGN) inside a galaxy or photons produced by the shocks associated with the radio jets. An EELR can appear as a resolved cloud in relative nearby galaxies and as narrow emission lines in more distant galaxies.
EELRs were first discovered in radio galaxies. Usually the clouds emitting the narrow emission lines are restricted to a few kiloparsecs within the AGN, but some galaxies have narrow emission lines that extend a few kiloparsec to over 100 kiloparsecs. These clouds where therefore called extended emission-line regions (EELRs) and usually have a large doubly ionized oxygen [O III]/Hβ ratio, as well as a strong ionized helium He II/Hβ ratio (oxygen line at 5007 Å, helium line at 4686 Å). In some cases these EELRs show highly ionized species, such as calcium [Ca V] and iron [Fe VII] [Fe X]. Another common highly ionized species is the emission of neon [Ne V] (main line at 3426 Å). This high level of ionization shows that the EELRs must be ionized by a mechanism related to the nucleus of the galaxy. HII regions do not show this high level of ionization. The first [O III] images of EELRs were around galaxies, such as 3C 79, 4C 37.43, NGC 3516 or NGC 4151.
In 2009 a large cloud was discovered that had similar spectral features as EELRs, but had no ionizing AGN nearby. It was concluded that the nearby galaxy IC 2497 hosted an AGN in the past. Today this AGN faded into inactivity. The hard ionizing radiation did however need time to travel the thousands of lightyears towards a cloud, which would later become Hanny's Voorwerp. This makes Hanny's Voorwerp the first EELR associated with a fading AGN. Previously EELRs were commonly first discovered via spectroscopy and required high-resolution imaging to further resolve the EELRs. Hanny's Voorwerp was however discovered in broad band imaging from the Sloan Digital Sky Survey. This prompted a search for more EELRs in broad-band imaging surveys by galaxy zoo volunteers, who also discovered Hanny's Voorwerp. EELRs are called "Voorwerpjes" by the galaxy zoo members. EELRs that are ionized by the AGN of a companion galaxy were also discovered. This type of ionization is called cross-ionization.
In broad-band images the [O III] emission lines usually lie at the g-band. The true color of an EELR would be dominated by the [O III] emission, which is a cyan color. In astronomical imaging slightly false-color images that transform g, r and i bands into RGB-images are more common. Sometimes the i-band is replaced with a z-band image. The EELRs appear as blue (g-band) objects in these survey images. Sometimes the H-alpha line is redshifted enough to lie at the i-band, which can make the EELR appear as a blue-violet object in these survey images. Sometimes the [O III] is redshifted enough that this line overlaps with the r-band or i-band, which makes them green or red in survey images of the Hyper Suprime-Cam. At around 0.1 < z < 0.38 the 5006.843 Å [O III] line is redshifted to the r-band and at 0.38 < z < 0.68 it is redshifted to the i-band. A team of researchers detected a resolved EELR at a high redshift of 4.54 in a galaxy protocluster with JWST.
Another related type of cloud is an extended narrow-line region (ENLR). The EELRs usually have dynamically chaotic structures and high velocities and are probably the result of mergers. ENLRs on the other hand follow the disk structure of the galaxy and have a low velocity.
Recently a connection between EELRs and Tidal Disruption Events (TDE), as well as quasi-periodic X-ray eruption (QPE) was found. This connection is explained with TDEs and QPEs favouring a gas-rich post-merger environment. It is possible that repeated TDEs could power EELRs. On the other hand, a fading AGN could result in an increase of the rate of TDEs.
List of resolved EELRs This is a list of EELRs with resolved [O III] images or other emission-lines +Name Host galaxyImage EELRDistance to nucleus (kpc)Date/ReferenceMR 2251-17830-501990Centaurus A outer and inner filaments7 & 161991IC 50632219913CR 3684519913C 35215.61992NGC 41511.161993NGC 3516419953C 27314.41996PKS 2250–4140-651997NGC 56431.81997PKS 2356−61251998NGC 525672000NGC 43883520024C 37.432020023C 17152003PKS 1932−4610020073C 792008IC 24974020093C 482009Mrk 101420093C 249.12009Ton 6162009Ton 2022009PKS 2251+11 2009Mrk 78162012SDSS J095559.88+395446.9102012SDSS J100507.88+283038.5132012IC 2637112012NGC 3758172012UGC 7342382012NGC 5252212012Mrk 273192012Mrk 463162012Teacup galaxy182012SDSS J151004.01+074037.1102012CGCG 077-117 (SDSS J152412.58+083241.2)192012NGC 5972332012Mrk 1498212012Mrk 883372012UGC 11185112012SDSS J220141.64+115124.31620123C 30520123C 381 382013NGC 725242013PGC 043234 (ASASSN-14li)102016ShaSS 073212018J023106−034513302018J083823+015012292018J090254+001116102018J091113+032604102018J092203−004443 132018J155143+434758112018J162913+441442152018J220347+020443122018J220440+00523282018J224027+004347920183C 1714.920193C 1824.820193C 3311.220193C 6338.020193C 318.1 (NGC 5920)11.720193C 32719.520193C 35317.220193C 38611.120193C 4038.320193C 42432.920193C 442 (NGC 7236)3.820193C 44518.520193C 458111.320193C 45976.02019SDSS J002944.89+001011.1 2019SDSS J005754.03+012013.82019SDSS J083902.96+470756.32019Z 180–92019NGC 33412019UGC 60812019SDSS J120149.74-015327.52019SDSS J121418.25+293146.72019Arp 239 (NGC 5278/NGC 5279)2019SDSS J135429.05+132757.22019Mrk 1172142021NGC 235262022NGC 55147520223C 98620223C 135 31.620223C 180 59.820223C 196.19.320223C 19832.620223C 22746.120223C 300 17.220222MASS J08001609+29281722023Z 119–1220232MASX J13020015+274657920232MASS J08152577+372025820232MASX J09515536+03290062023UGC 59412024Markarian 950 (iPTF16fnl)6.52024J1000+0234>8.62024GSN 0699.22024RXJ13012.82024eRO-QPE24.62024SDSS J110840.11+340552.2 (ASASSN-14ae)2.42025ESO 253-3 (ASASSN-14ko)22.92025LEDA 307968 (AT 2019ahk)6.22025
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