The lecture today and tomorrow will be about active galactic nuclei.

And as I heard you already started the last lecture with the beginning of this topic,

I will just very briefly go over the history again and then we try to sort out what is going on.

And in any case, I encourage you very much to ask questions all the time if something is unclear,

especially because I am not 100% aware what J.N. Wilms already told you in all specific details,

so I might not explain everything you might need to know in order to understand it.

So just feel free to guess and take some handouts.

Okay, so I will very briefly skip over the history because you heard this last week.

I understand.

Very important part, I think, is that over 100 years ago,

the first evidence was an optical spectra which were thought to be nebulae,

but they had a very strange spectrum, so they had a very high degree of ionization in this nebula,

and the line widths were very large, which is shown here in these images.

And also some kind of curious straight rays or some kind of jet, as we know now, was also discovered,

but it was still very puzzling.

And besides this, also radio emission was attributed to normal galaxies,

so this was also strange at this time,

so this should maybe underline the multi-wavelength nature of these objects,

so they were kind of discovered in different wavelengths before they could be made sense of them.

So I assume you have seen these slides, so this is why I'm skipping so fast, just to bring us on the same page here.

And it was not then until 1963, Martin Schmidt discovered that these wavelengths,

if you shift them in energy, so in redshift, so in distance, in the universe, they actually match lines we know,

and so from there on, one knew that these objects are not nebulae,

which are maybe a little bit further out of our own galaxies, but actually very far away.

And I think this kind of sets a little bit the scene of how this kind of,

what sometimes is called the zoo of this active galactic nuclei in order to make sense of it.

So you see, you observe things in the optical, you observe things in the radio,

and everything seems to be a little bit extreme, if not to say it doesn't fit in the standard picture,

especially at these times, and we're trying to go through a few of these characteristics,

and we're trying to make a general sense of it.

First start with what was historically discovered, the first active galactic nuclei were the Seaford galaxies.

So those are the point-like sources in the center of galaxies, which are really bright.

So you see the galaxy around them, you can see those in these different images,

the galaxies do look kind of different, and always in the center of these galaxies you have a very bright point.

And as we know now there are two different types of these kind of Seaford galaxies.

So Seaford one galaxies, so up there are optical spectra shown,

and the most characteristic part of the Seaford galaxies as they were discovered is

that they have very broad optical lines, and broad lines means in velocity broadening,

so there is some kind of component which has a large speed to like 10,000 kilometers per second moving,

this is the only way to achieve this broadening, which kind of was puzzling how to achieve these large speeds,

and as we know those correspond to some medium and high density,

which create those broad, what we call allowed lines, and there are also in the Seaford one galaxies a few forbidden lines,

or there are forbidden lines which are rather thin compared to these broad lines as they are,

and forbidden lines in this sense is maybe this has been talked about in this lecture before,

doesn't really mean it's totally forbidden, it just means it's very unprobable,

so usually it's not measured in the lab, because the particle would maybe hit the boundary of your lab experiment

before this state of the atom can de-excite and emit a photon like this,

but if you are in space and you have very low densities, your chance to interact with other particles and de-excite is like this,

by collision is very low, so this means also very unprobable transition stage in these atoms can be seen as fluorescent lines in the spectra,

so the electron de-excites by emitting a photon, and these are what are called in astrophysics the forbidden lines,