Background and basics
In old, non-starforming galaxies, regions with emission lines dominated by low ionization species are classified as Low-Ionization Nuclear Emission-line Regions (LINERs). The ionization mechanism behind such regions remains a lasting mystery. Active Galactic Nuclei, which have long been believed to be the source, were ruled out with spatially resolved observations. These regions are therefore renamed as Low-Ionization Emission-line Regions (LIERs). The remaining potential ionization sources need to be distinguished with in-depth analysis, and they predict different environments of the interstellar medium. Therefore, temperature measurements can provide strong constraints on this puzzle. We selected spaxels from red quiescent galaxies from the SDSS MaNGA data release 17 and attempted to measure their temperatures.
Handling the Spectrum
SDSS MaNGA is an integral field spectrograph that covers a wide spectrum range from 3600Å to 10400Å, providing data for over 10,000 galaxies with redshift values ranging from 0.01 to 0.1. In our analysis, we specifically focus on spaxels from passive, non-starforming galaxies. Each spaxel contains a spectrum, which we first correct for systematic errors and de-redshift. Subsequently, we construct a background stellar continuum to subtract the stellar emission from the gas emission. This process allows us to obtain a clean emission spectrum that originates solely from the interstellar gas.
Temperature measurement
The temperature of the interstellar medium can be determined by measuring temperature-sensitive auroral lines. Interestingly, some of these emissions are similar to the ones observed in auroras. By applying principles of atomic physics, one can establish a relationship between the strength ratio of these lines and other prominent lines, thereby indicating the temperature of the ions. The following lines have been specifically targeted for this analysis:
- [O III] λ 4363
- [N II] λ 5755
- [S II] λλ 4068, 4076
- [O II] λλ 7320, 7330