Extended emission-line region


An extended emission-line region is a giant interstellar cloud ionized by the radiation of an active galactic nucleus 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 and usually have a large doubly ionized oxygen /Hβ ratio, as well as a strong ionized helium He II/Hβ ratio. In some cases these EELRs show highly ionized species, such as calcium and iron . Another common highly ionized species is the emission of neon . 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 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 emission lines usually lie at the g-band. The true color of an EELR would be dominated by the 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 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 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 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. 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, as well as quasi-periodic X-ray eruption 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 images or other emission-lines
Name Host galaxyImage EELRDistance to nucleus Date/Reference
MR 2251-17830-501990
Centaurus A
outer and inner filaments		7 & 161991
IC 5063221991
3CR 368451991
3C 35215.61992
NGC 41511.161993
NGC 351641995
3C 27314.41996
PKS 2250–4140-651997
NGC 56431.81997
PKS 2356–61251998
NGC 525672000
NGC 4388352002
4C 37.43202002
3C 17152003
PKS 1932–461002007
3C 792008
IC 2497402009
3C 482009
Mrk 10142009
3C 249.12009
Ton 6162009
Ton 2022009
PKS 2251+112009
Mrk 78162012
SDSS J095559.88+395446.9102012
SDSS J100507.88+283038.5132012
IC 2637112012
NGC 3758172012
UGC 7342382012
NGC 5252212012
Mrk 273192012
Mrk 463162012
Teacup galaxy182012
SDSS J151004.01+074037.1102012
CGCG 077-117
192012
NGC 5972332012
Mrk 1498212012
Mrk 883372012
UGC 11185112012
SDSS J220141.64+115124.3162012
3C 3052012
3C 381382013
NGC 725242013
PGC 043234 102016
ShaSS 073212018
J023106−034513302018
J083823+015012292018
J090254+001116102018
J091113+032604102018
J092203−004443132018
J155143+434758112018
J162913+441442152018
J220347+020443122018
J220440+00523282018
J224027+00434792018
3C 1714.92019
3C 1824.82019
3C 3311.22019
3C 6338.02019
3C 318.1 11.72019
3C 32719.52019
3C 35317.22019
3C 38611.12019
3C 4038.32019
3C 42432.92019
3C 442 3.82019
3C 44518.52019
3C 458111.32019
3C 45976.02019
SDSS J002944.89+001011.12019
SDSS J005754.03+012013.82019
SDSS J083902.96+470756.32019
Z 180–92019
NGC 33412019
UGC 60812019
SDSS J120149.74-015327.52019
SDSS J121418.25+293146.72019
Arp 239 2019
SDSS J135429.05+132757.22019
Mrk 1172142021
NGC 235262022
NGC 5514752022
3C 9862022
3C 13531.62022
3C 18059.82022
3C 196.19.32022
3C 19832.62022
3C 22746.12022
3C 30017.22022
2MASS J08001609+29281722023
Z 119–122023
2MASX J13020015+27465792023
2MASS J08152577+37202582023
2MASX J09515536+03290062023
UGC 59412024
Markarian 950 6.52024
J1000+0234>8.62024
GSN 0699.22024
RXJ13012.82024
eRO-QPE24.62024
SDSS J110840.11+340552.2 2.42025
ESO 253-3 22.92025
LEDA 307968 6.22025