Extended emission-line region

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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. [1] An EELR can appear as a resolved cloud in relative nearby galaxies and as narrow emission lines in more distant galaxies.

Contents

In this unified model of AGNs the EELRs should be thought as an extended version of the blue clouds labelled as "narrow line region". Emmaalexander unified agn.png
In this unified model of AGNs the EELRs should be thought as an extended version of the blue clouds labelled as "narrow line region".

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]. [2] Another common highly ionized species is the emission of neon [Ne V] (main line at 3426 Å). [3] [4] 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. [2] The first [O III] images of EELRs were around galaxies, such as 3C 79, [5] 4C 37.43, [6] NGC 3516 [7] or NGC 4151. [8]

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. [3] 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. [4] EELRs that are ionized by the AGN of a companion galaxy were also discovered. This type of ionization is called cross-ionization. [9]

A composite showing five candidate EELR taken from the Rubin Virgo Field 'first look' release in June 2025. Candidate AGN Extended Emission-line Regions in the Rubin Virgo Field.jpg
A composite showing five candidate EELR taken from the Rubin Virgo Field 'first look' release in June 2025.

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. [4] 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. [11] 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. [note 1] A team of researchers detected a resolved EELR at a high redshift of 4.54 in a galaxy protocluster with JWST. [13]

Another related type of cloud is a 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. [11]

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. [14] [15]

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/Reference
MR 2251-178 MR 2251-178 MUSE.jpg 30-501990 [16]
Centaurus A

outer and inner filaments

Centaurus A filaments (EELRs).jpg 7 & 161991 [17]
IC 5063 IC 5063 MUSE.jpg 221991 [18]
3CR 368451991 [19]
3C 35215.61992 [20]
NGC 4151 Seyfert galaxy NGC4151 (GL-2002-001035).jpg 1.161993 [8]
NGC 3516 NGC 3516.png 41995 [7]
3C 273 3C 273 MUSE.jpg 14.41996 [21]
PKS 2250–41 PKS 2250-41 legacy surveys.jpg 40-651997 [22]
NGC 5643 Center of galaxy NGC 5643.tif 1.81997 [23]
PKS 2356–61251998 [24]
NGC 5256 An ongoing cosmic collision.jpg 72000 [25]
NGC 4388 Eso0304c.jpg 352002 [26]
4C 37.43202002 [27]
3C 171 3C 171 cutout HST 05476 2l wfpc2 f702w pc sci.jpg 52003 [28]
PKS 1932–461002007 [29]
3C 79 3C 79 MUSE.jpg 2008 [5]
IC 2497 Hannys voorwerp.jpg 40 [4] 2009 [3]
3C 48 2009 [1]
Mrk 1014 Mrk 1014 MUSE.jpg 2009 [1]
3C 249.1 2009 [1]
Ton 6162009 [1]
Ton 2022009 [1]
PKS 2251+112009 [1]
Mrk 78 Mrk 78 (50838249333).png 162012 [4]
SDSS J095559.88+395446.9 SDSS J0955+3954.jpg 102012 [4]
SDSS J100507.88+283038.5 SDSS J1005+2830 legacy surveys.jpg 132012 [4]
IC 2637 112012 [4]
NGC 3758 NGC 3758 MUSE 2.jpg 172012 [4]
UGC 7342 Heic1507e.jpg 382012 [4]
NGC 5252 Heic1507f.jpg 212012 [4]
Mrk 273 Mrk 273 Hubble.jpg 192012 [4]
Mrk 463 Mrk 463 MUSE.jpg 162012 [4]
Teacup galaxy The Teacup Galaxy SDSS 1430.jpg 182012 [4]
SDSS J151004.01+074037.1 Hs-2015-13-d-large web.jpg 102012 [4]
CGCG 077-117

(SDSS J152412.58+083241.2)

CGCG 077-117 MUSE.jpg 192012 [4]
NGC 5972 NGC 5972.jpg 332012 [4]
Mrk 1498 Hs-2015-13-g-large web.jpg 212012 [4]
Mrk 883 Mrk 883 legacy surveys.jpg 372012 [4]
UGC 11185 Heic1507h.jpg 112012 [4]
SDSS J220141.64+115124.3 Hs-2015-13-i-large web.jpg 162012 [4]
3C 305 IC 1065 (3C 305) Chandra.jpg 2012 [30]
3C 381382013 [31]
NGC 7252 42013 [32]
PGC 043234102016 [33]
ShaSS 073 An echo of light ShaSS 073, ShaSS 622, ShaSS 622-073 (cropped).jpg 212018 [34]
J023106−034513302018 [11]
J083823+015012292018 [11]
J090254+001116102018 [11]
J091113+032604102018 [11]
J092203−004443132018 [11]
J155143+434758112018 [11]
J162913+441442152018 [11]
J220347+020443122018 [11]
J220440+00523282018 [11]
J224027+00434792018 [11]
3C 1714.92019 [35]
3C 1824.82019 [35]
3C 3311.22019 [35]
3C 6338.02019 [35]
3C 318.1 (NGC 5920)11.72019 [35]
3C 32719.52019 [35]
3C 35317.22019 [35]
3C 38611.12019 [35]
3C 4038.32019 [35]
3C 42432.92019 [35]
3C 442 (NGC 7236)3.82019 [35]
3C 44518.52019 [35]
3C 458 3C 458 MUSE.jpg 111.32019 [35]
3C 459 76.02019 [35]
SDSS J002944.89+001011.12019 [9]
SDSS J005754.03+012013.8 SDSS J0057+0120.jpg 2019 [9]
SDSS J083902.96+470756.32019 [9]
Z 180–92019 [9]
NGC 3341 NGC 3341 MUSE.jpg 2019 [9]
UGC 6081 UGC 6081 legacy surveys.jpg 2019 [9]
SDSS J120149.74-015327.52019 [9]
SDSS J121418.25+293146.7 SDSS J1214+2931 Hubble.jpg 2019 [9]
Arp 239 (NGC 5278/NGC 5279) NGC 5278 79 Arp239 SDSS2.jpg 2019 [9]
SDSS J135429.05+132757.22019 [9]
Mrk 1172142021 [36]
NGC 235 262022 [37]
NGC 5514 752022 [37]
3C 9862022 [38]
3C 13531.62022 [38]
3C 18059.82022 [38]
3C 196.1 9.32022 [38]
3C 19832.62022 [38]
3C 227 3C 227 MUSE.jpg 46.12022 [38]
3C 30017.22022 [38]
2MASS J08001609+29281722023 [39]
Z 119–122023 [39]
2MASX J13020015+27465792023 [39]
2MASS J08152577+37202582023 [39]
2MASX J09515536+03290062023 [39]
UGC 5941 2024 [40]
Markarian 9506.52024 [14]
J1000+0234 J1000+0234 EELR.jpg >8.62024 [13]
GSN 069 GSN 069.jpg 9.22024 [15]
RXJ1301 RXJ1301.jpg 2.82024 [15]
eRO-QPE2 ERO-QPE2.jpg 4.62024 [15]

Notes

  1. Calculated with the formula z=(λobserveemit)/λemit and the transition of SDSS filters from Figure 2 in Smith et al. 2002 [12]

References

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