Extended emission-line region
Giant ionized clouds revealing past or present AGN activity.
An extended emission-line region (EELR) is a vast interstellar cloud that gets its glow from the radiation of an active galactic nucleus (AGN) within its host galaxy, or from photons generated by shocks tied to radio jets. These clouds stand out because of their unusually high ionization levels—they show strong emissions from doubly ionized oxygen ([O III]) and ionized helium, along with even more highly ionized elements like calcium ([Ca V]), iron ([Fe VII], [Fe X]), and neon ([Ne V]). This intense ionization sets them apart from ordinary HII regions, which lack such extreme signatures.
EELRs were first spotted in radio galaxies. Normally, the clouds that produce narrow emission lines are confined to a few kiloparsecs around the AGN, but some galaxies host narrow-line clouds stretching from a few kiloparsecs to over 100 kiloparsecs. These were dubbed extended emission-line regions. Early [O III] images of EELRs were captured around galaxies such as 3C 79, 4C 37.43, NGC 3516, and NGC 4151.
In 2009, a large cloud was found with spectral features matching those of EELRs, but without any active AGN nearby. It turned out that the neighboring galaxy IC 2497 once hosted an AGN that had since faded. The hard ionizing radiation took thousands of years to travel to the cloud—later named Hanny's Voorwerp—making it the first EELR linked to a fading AGN. Previously, EELRs were usually found through spectroscopy and then resolved with high-resolution imaging, but Hanny's Voorwerp was discovered in broad-band images from the Sloan Digital Sky Survey. This discovery led Galaxy Zoo volunteers to search for more EELRs in broad-band surveys. Galaxy Zoo members call these objects "Voorwerpjes." Some EELRs are ionized by the AGN of a companion galaxy, a process known as cross-ionization.
In broad-band images, the [O III] emission often falls in the g-band. An EELR’s true color is dominated by this cyan [O III] glow, but astronomical images commonly use slightly false-color composites of g, r, and i bands (sometimes replacing i with z). In such images, EELRs appear blue (from the g-band). If H-alpha is redshifted into the i-band, the EELR can look blue-violet. When [O III] shifts into the r-band (at redshifts roughly 0.1 to 0.38) or the i-band (0.38 to 0.68), the regions appear green or red in Hyper Suprime-Cam survey images.
- First discovered in
- radio galaxies
- Typical size range
- a few kiloparsecs to over 100 kiloparsecs
- Key emission lines
- [O III] at 5007 Å, He II at 4686 Å, [Ne V] at 3426 Å
- Redshift ranges for [o iii]
- 0.1 < z < 0.38 (r-band), 0.38 < z < 0.68 (i-band)
- Highest redshift resolved eelr
- 4.54
- First eelr associated with fading agn
- Hanny's Voorwerp
Lore & Background
EELRs were first discovered in radio galaxies, where narrow emission lines typically extend only a few kiloparsecs from the AGN. However, some galaxies show these lines extending from a few kiloparsecs to over 100 kiloparsecs, leading to the term extended emission-line regions. These regions exhibit a large [O III]/Hβ ratio and strong He II/Hβ ratio, along with highly ionized species such as [Ca V], [Fe VII], [Fe X], and [Ne V], indicating ionization by a mechanism related to the galaxy's nucleus rather than by stars. The first [O III] images of EELRs were obtained around galaxies like 3C 79, 4C 37.43, NGC 3516, and NGC 4151.
In 2009, a large cloud with similar spectral features was discovered but without an ionizing AGN nearby. It was concluded that the nearby galaxy IC 2497 hosted an AGN in the past that had since faded. The hard ionizing radiation took time to travel to the cloud, which became known as Hanny's Voorwerp—the first EELR associated with a fading AGN. Unlike earlier EELRs discovered via spectroscopy, Hanny's Voorwerp was found in broad-band imaging from the Sloan Digital Sky Survey, prompting a search for more such objects by Galaxy Zoo volunteers. EELRs are called 'Voorwerpjes' by Galaxy Zoo members. Some EELRs are ionized by the AGN of a companion galaxy, a process called cross-ionization.
In broad-band images, the [O III] emission lies in the g-band, giving EELRs a cyan true color. In false-color survey images, they appear blue, blue-violet, green, or red depending on the redshift of the [O III] line. A resolved EELR was detected at a high redshift of 4.54 in a galaxy protocluster with JWST. EELRs are distinguished from extended narrow-line regions (ENLRs) by their dynamically chaotic structures and high velocities, likely resulting from mergers, whereas ENLRs follow the disk structure with low velocities. A connection has been found between EELRs and tidal disruption events (TDEs) and quasi-periodic X-ray eruptions (QPEs), possibly explained by a gas-rich post-merger environment; repeated TDEs could power EELRs, and a fading AGN could increase the rate of TDEs.
Reader's Guide
The significance of EELRs lies in their role as tracers of past and present AGN activity, extending our view of nuclear ionization far beyond the immediate vicinity of the galaxy core. The discovery of Hanny's Voorwerp demonstrated that EELRs can persist after the AGN has faded, providing a fossil record of earlier high-energy output. This shifted the understanding of EELRs from being exclusively associated with active nuclei to including relics of past activity. The serendipitous detection of Hanny's Voorwerp in broad-band imaging by citizen scientists opened a new avenue for finding EELRs in large surveys, leading to the identification of many more such objects, dubbed 'Voorwerpjes' by Galaxy Zoo volunteers. The detection of a resolved EELR at redshift 4.54 with JWST extends their known range to the early universe. The connection between EELRs and TDEs and QPEs suggests that these regions may be linked to dynamic, gas-rich environments following mergers, where repeated stellar disruptions can power the ionization. This interplay between AGN variability, mergers, and tidal disruption events makes EELRs valuable probes of galaxy evolution and nuclear activity cycles.
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