Low-ionization nuclear emission-line region

Low-ionization nuclear emission-line region

.cite journal
author= L. C. Ho, A. V. Filippenko, W. L. W. Sargent
title=A Search for "Dwarf" Seyfert Nuclei. III. Spectroscopic Parameters and Properties of the Host Galaxies
journal=Astrophysical Journal Supplement
] Credit: HST/NASA/ESA.]

A low-ionization nuclear emission-line region (LINER) is a type of galactic nucleus that is defined by its spectral line emission. The spectra typically include line emission from weakly ionized or neutral atoms, such as O, O+, N+, and S+. Conversely, the spectral line emission from strongly ionized atoms, such as O++, Ne++, and He+, is relatively weak.cite journal
author= T. M. Heckman
title=An optical and radio survey of the nuclei of bright galaxies - Activity in normal galactic nuclei
journal=Astronomy and Astrophysics
] The class of galactic nuclei was first identified by Timothy Heckman in the third of a series of papers on the spectra of galactic nuclei that were published in 1980.

Demographics of LINER galaxies

Galaxies that contain LINERs are often referred to as "LINER galaxies". LINER galaxies are very common; approximately one-third of all nearby galaxies (galaxies within approximately 20-40 Mpc) may be classified as LINER galaxies.cite journal
author= L. C. Ho, A. V. Filippenko, W. L. W. Sargent
title=A Search for "Dwarf" Seyfert Nuclei. V. Demographics of Nuclear Activity in Nearby Galaxies
journal=Astrophysical Journal
] Approximately 75% of LINER galaxies are either elliptical galaxies, lenticular galaxies, or S0/a-Sab galaxies (spiral galaxies with large bulges and tightly-wound spiral arms). LINERs are found less frequently in Sb-Scd galaxies (spiral galaxies with small bulges and loosely-wound spiral arms), and they are very rare in nearby irregular galaxies. LINERs also may be commonly found in luminous infrared galaxies (LIRGs), a class of galaxies defined by their infrared luminosities that are frequently formed when two galaxies collide with each other. Approximately one-quarter of LIRGs may contain LINERs.cite journal
author= S. Veilleux, D.-C. Kim, D. B. Sanders, J. M. Mazzarella, B. T. Soifer
title=Optical Spectroscopy of Luminous Infrared Galaxies. II. Analysis of the Nuclear and Long-Slit Data
journal=Astrophysical Journal Supplement Series

cientific debates: energy sources and ionization mechanisms

LINERs have been at the center of two major debates. First, astronomers have debated the source ofenergy that excites the ionized gas in the centers of these galaxies. Some astronomers have proposed that active galactic nuclei (AGN) with supermassive black holes are responsible for the LINER spectral emission.cite journal
author= L. C. Ho, A. V. Filippenko, W. L. W. Sargent
title=A Reevaluation of the Excitation Mechanism of LINERs
journal=Astrophysical Journal
] Other astronomers have asserted that the emission is powered by star formation regions.cite journal
author=R. Terlevich, J. Melnick
title=Warmers - The missing link between Starburst and Seyfert galaxies
journal=Monthly Notices of the Royal Astronomical Society
] cite journal
author=J. C. Shields
title=Normal O stars in dense media generate LINERs
journal=Astrophysical Journal
] The other major issue is related to how the ions are excited. Some astronomers have suggested that shock waves propagating through the gas may ionize the gas, while others have suggested that photoionization (ionization by ultraviolet light) may be responsible.

These debates are complicated by the fact that LINERs are found in a wide variety of objects with different brightnesses and morphologies. Moreover, the debate over the energy sources for LINERs is entangled with a similar debate over whether the light from star formation regions or the light from AGN produce the high infrared luminosities seen in LIRGs.

Although both the energy sources and the excitation mechanisms for LINER emission are still being studied, many LINERs are frequently referred to as AGN.

tar formation in LINERs

A number of surveys have been performed to explore the connection between star formation and LINER activity. If a connection can be found between star formation activity and LINER activity, then this strengthens the possibility that LINERs are powered by the hot stars found in star formation regions. However, if star formation cannot be found in LINERs, then this definitively excludes star formation as powering LINER emission.

tar formation in LIRGs with LINERs

Recent observations with the Spitzer Space Telescope show a clear connection between LINER emission in LIRGs and star formation activity. The mid-infrared spectra of LIRGs with LINERs have been shown to look similar to the mid-infrared spectra of starburst galaxies, which suggest that infrared-bright LINERs are powered by star formation activity. However, some mid-infrared spectral line emission from AGN have also been detected in these galaxies, indicating that star formation may not be the only energy sources in these galaxies.cite journal
author=E. Sturm, D. Rupke, A. Contursi, D.-C. Kim, D. Lutz, H. Netzer, S. Veilleux, R. Genzel, M. Lehnert, L. J. Tacconi, D. Maoz, J. Mazzarella, S. Lord, D. Sanders, A. Sternberg
title=Mid-Infrared Diagnostics of LINERS
journal=Astrophysical Journal

tar formation in normal galaxies with LINERs

Normal nearby galaxies with LINERs, however, appear to be different. A few near-infrared spectocopic surveys have identified some LINERs in normal galaxies that may be powered by star formation.cite journal
author=J. E. Larkin, L. Armus, R. A. Knop, B. T. Soifer, K. Matthews
title=A Near-Infrared Spectroscopic Survey of LINER Galaxies
journal=Astrophysical Journal Supplement Series
] However, most LINERs in nearby galaxies have low levels of star formation activity.cite journal
author=G. J. Bendo, R. D. Joseph, M. Wells, P. Gallais, M. Haas, A. M. Heras, U. Klaas, R. J. Laureijs, K. Leech, D. Lemke, L. Metcalfe, M. Rowan-Robinson, B. Schulz, C. Telesco
title=Star Formation in the Infrared Space Observatory Atlas of Bright Spiral Galaxies
journal=Astronomical Journal
] cite journal
author= G. J. Bendo, R. D. Joseph
title=Nuclear Stellar Populations in the Infrared Space Observatory Atlas of Bright Spiral Galaxies
journal=Astronomical Journal
] Moreover, the stellar populations of many LINERs appear to be very old,cite journal
author=R. Cid Fernandes, R. M. González Delgado, H. Schmitt, T. Storchi-Bergmann, Thaisa, L. P. Martins, E. Pérez, T. Heckman, C. Leitherer, D. Schaerer
title=The Stellar Populations of Low-Luminosity Active Galactic Nuclei. I. Ground-based Observations
journal=Astrophysical Journal
] cite journal
author=R. M. González Delgado, R. Cid Fernandes, E. Pérez, L. P. Martins, T. Storchi-Bergmann, H. Schmitt, T. Heckman, C. Leitherer
title=The Stellar Populations of Low-Luminosity Active Galactic Nuclei. II. Space Telescope Imaging Spectrograph Observations
journal=Astrophysical Journal
] and the mid-infrared spectra, as observed by the Spitzer Space Telescope, do not appear similar to the spectra expected from star formation. These results demonstrate that most LINER in nearby normal galaxies may not be powered by star formation, although a few exceptions clearly exist.

Notable LINER galaxies

*Messier 94
*NGC 5005
*NGC 5195
*Sombrero Galaxy

ee also

*Seyfert galaxy - "Another class of galaxies that contain AGN"


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