What does ELG mean in UNCLASSIFIED

An Emission Line Galaxy (ELG) is a galaxy that emits strong emission lines in its spectrum. These emission lines are produced by the excitation of gas within the galaxy, typically due to star formation or the presence of an active galactic nucleus (AGN). ELGs are often used as tracers of star-forming galaxies or galaxies with ongoing AGN activity.

ELGs are identified by their strong emission lines in their spectra. These lines are typically observed in the optical or near-infrared wavelengths and include prominent features such as the H-alpha (Hα) and H-beta (Hβ) emission lines from ionized hydrogen, as well as other lines from ionized oxygen, nitrogen, and sulfur.

The emission lines in ELGs are produced by the excitation of gas within the galaxy. This excitation can be caused by several processes, including:

• Star formation: The intense radiation from young, massive stars can ionize the surrounding gas, causing it to emit strong emission lines.
• Active galactic nuclei (AGNs): The accretion of matter onto a supermassive black hole at the center of a galaxy can release a tremendous amount of energy, which can ionize the surrounding gas and produce strong emission lines.
• Shocks and outflows: Gas that is shocked or expelled from a galaxy can also emit emission lines as it interacts with the surrounding medium.

There are several types of ELGs, classified based on the characteristics of their emission lines and the underlying physical processes responsible for their excitation. Some common types include:

• HII-region ELGs: These ELGs are characterized by strong Hα emission, indicating ongoing star formation.
• LINER ELGs: These ELGs show weak emission lines, typically dominated by [NII] and [OII], and are often associated with low-luminosity AGNs.
• Seyfert ELGs: These ELGs exhibit strong, broad emission lines, indicating the presence of a luminous AGN.

ELGs are important for studying various aspects of galaxy evolution and astrophysics. They provide insights into:

• Star formation: ELGs can be used to trace the distribution and properties of star-forming galaxies, helping us understand the cosmic star formation history.
• AGN activity: ELGs with strong AGN signatures can be used to study the growth and evolution of supermassive black holes and their impact on their host galaxies.
• Galaxy interactions: ELGs can be found in pairs or groups of interacting galaxies, providing valuable information about the role of galaxy mergers and interactions in shaping galaxy properties.