-Preity Sukla Sahani
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Seyfert Galaxies are the nearby spiral galaxies having an active central region and are one type of Active Galactic Nuclei(AGNs). We are going to know about the classification of seyfert galaxies.
The Seyfert galaxies are broadly divided into two types based on the spectra they possess:
- Seyfert I
- Seyfert II
Seyfert I galaxies:
These are the kind of Seyfert galaxies which show both Broad and Narrow lines in their spectra.
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| The following image shows the spectral profile of a Seyfert I galaxy NGC 1275. One can clearly see both narrow and broad lines here. |
Seyfert II galaxies:
These are the kind of Seyfert galaxies which show only Narrow lines in their spectra. The broad lines are absent.
This is the spectral profile of a Seyfert II galaxy NGC 1667. One can compare the spectral profile given in the previous image and can clearly see the absence of broad lines in this image.
Why do they differ?
One evidence of this difference can be found from the following picture.
- In the following image, the supermassive blackhole can be found at the centre with an obscuring torus shaped dust cloud surrounding it. One can also see a Broad Line region (BLR) and a Narrow Line Region (NLR) being labelled. The BLR and NLR give out Broad lines and Narrow lines respectively when their spectral profile is measured.
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| Image showing the surrounding environment near a supermassive blackhole. |
- Depending on our orientation while viewing, in the case of some seyfert galaxies both the BLR and NLR can be seen and their spectral lines can be obtained. These form the group of Seyfert I galaxies.
- While in others, the BLR gets masked or hidden by the torus of dust, thus leading to only the NLR to be seen and the spectral lines being obtained. These form the Seyfert II galaxies.
- When seen through polarized light, Seyfert II galaxies show similar broad lines as in the Seyfert I galaxies. This tells us that intrinsically both Seyfert I and Seyfert II are similar objects viewed from different orientations.
References:
- https://academic.oup.com/mnras/article/350/1/140/986506
- https://skyandtelescope.org/astronomy-news/astronomers-watch-donut-rotate-around-supermassive-black-hole/
- https://ned.ipac.caltech.edu/level5/Glossary/Essay_seyfert.html#:~:text=Type%202%20Seyfert%20galaxies%20differ,2%20spectrum%20are%20shown%20below.
Written By-
Preity Sukla Sahani
