Question: How would you know when galaxies are spewing out energy?

Jen Gupta answered on 13 Jun 2011:

OK I hope this answer explains it well enough for you – if not please comment below and I’ll try to make it clearer!

When we use our eyes to look at things we are looking at visible light. But the light that we see is only a small part of what is called the electromagnetic spectrum. We can look at the sky using telescopes that are sensitive to the different types of light, like radio waves and x-rays, and this way we can find out things that we can’t tell just by using visible light. This might sound weird to you but if you imagine looking at a photo of a person, you can probably see that they have skin, they have eyes, a nose, a mouth etc. But you don’t know everything. You can then take an x-ray and see the bones or use a night vision camera (which is sensitive to infrared light) to see which bits are warm and which bits are cold. We do the same things with galaxies.

A good example of a galaxy that is spewing out loads of energy is one called M87. When you look at it in visible light, it just looks like a normal elliptical galaxy, although if you look closely you can sometimes see something coming out of it – this already tells us that M87 is a bit different. If we use a radio telescope to look at the radio “light” that is being emitted by the galaxy we see massive jet structures coming out of the galaxy. When we look at the x-rays coming from the galaxy we again see that there are x-rays coming from the galaxy and that they extend out much further than the galaxy we can see in visible light. This tells us that M87 isn’t a normal galaxy but instead is active. There are pictures online that show you “multiwavelength” i.e. different light, images of M87 like here: http://chandra.harvard.edu/photo/2008/m87/ where the visible light is yellow, the radio light is red and the x-ray light is blue.

Your question asks about how we know *when* galaxies are being active. One question that we still don’t know the answer to is whether all galaxies go through an active phase as part of their life or if some galaxies will never be active. The unanswered questions are what makes studying them so much fun 😀