But what else is there in the Universe ? Astronomers in the 1950's started to look at the sky in radio wavelengths. They found some radio sources seemed to be associated with rather blue objects which looked like stars (could not be resolved into fuzz as expected for a galaxy). But it was already known that normal stars produced very little energy at radio wavelengths. Taking an optical spectra of these quasi-stellar objects (or quasars for short) gave even more problems. The spectra looked NOTHING like the spectra from stars - there were strong emission lines rather than the absorption lines seen in stars and the wavelenghts of these lines were NOTHING like the wavelengths for the known atomic transitions, and the lines were very broad. Here are various types of Quasar and active galaxy spectra compared to a standard galaxy spectrum. In 1963 they realised that these emission wavelengths would line up with the known atomic transitions if the quasar was highly redshifted, implying recession velocities which were as big as those derived from the most distant galaxies then known (many galaxies with much larger redshifts are now known).
If the redshift can be interpreted as part of Hubbles law, then we can use it to get the distance, and hence the luminosity - about 1013 Lsun. i.e. the object is 100x brighter than the whole Milky Way galaxy. But it doesn't look fuzzy like a galaxy and its spectrum isn't the spectrum you expect from lots of stars! Even more startlingly, looking back on old optical photographic plates, it became clear that the optical light varied on timescales of years, months and even weeks. This sets some stringent conditions on the size of the object through light travel time arguments, leading to the conclusion that we have more optical luminosity (with a bizzare spectrum) than that from even a large galaxy, produced in a region the size of our own solar system!!! More on Quasars
Is this reasonable ? Not at first glance, certainly. A huge luminosity from a tiny region means that there must be a huge density of photons streaming out from the source. Photons interact with matter (especially electrons) and with enough of them they can blow away any matter in the region. Yet there must be matter in the region, as Einstein said E=mc2 - we have lots of energy and ultimately that must have come from matter. The only way to hold the matter in the region against the pressure of the outgoing radiation is with lots of gravity. Doing the numbers, this means that for the brightest quasars we need a mass of around 100-1000 million times bigger than the sun. The only way to get this much mass into a region the size of our solar system is in a black hole.
We saw in lecture 8 how we can get ultraviolet and X-ray emission from matter heating up as it falls into a black hole of a few solar masses. Most things scale with black hole mass - so things like effects on time and space are the same irrespective of the size of the black hole, so we can get much the same sort of emission from a supermassive black hole. So they can also produce ultraviolet and X-ray radiation from material falling into them - in fact they produce a LOT more luminosity in the X-ray and ultraviolet than in the optical. And ironically, only about 10 per cent produce much radio emission - the ones where there is a jet of material shooting out from the center at velocities close to the speed of light! Some nice pictures of quasars with jets.
For the stellar remnant black holes the source of infalling matter is a companion star. What is the source of the matter for the supermassive black holes in quasars ? If they are in the center of a galaxy, then this is a very dense environment - giant molecular clouds, star clusters etc. And it only needs something like the total mass of the sun to fall in every 10-100 years to power all the activity that we see. More on the power source for quasars.
This model (supermasssive black holes in the centers of galaxies) can also easily explain the bizzare spectrum seen - material close to the black hole will be illuminated by the intense radiation and will produce emission lines. But it will also be moving at high velocities because of the strong gravity of the black hole - so we get strongly broadened emission lines, which then get redshifted by the Hubble law (review the material in lecture 11) according to the distance the quasars are from us.
But for such a theory we want to see lots of tests - we'll trust it if keeps on passing all of the tests and there is no other obvious answer. We know that there are nearby galaxies with bright, pointlike nuclei which have broad emission line spectra - these are called active galaxies. These can fit into the same picture but with a less massive black hole (maybe only 1-10 million times the mass of the sun!). We'd expect less massive black holes to be more common than the very large ones. So the chances are that there would be some in the nearby galaxies, where we can see the galaxy as well as the nucleus. Also, for the quasars, (look at end of this section) we are now able to use the Hubble Space Telescope to take very deep images, and the fuzzy underlying galaxy can be seen. Clearly this is activity which is associated with the centers of galaxies. But is it a supermassive black hole ? We have some direct ways to estimate the mass, by looking at material very close to the center - using the excellent resolution of the Hubble Space telescope or going into the radio spectrum where we can get higher resolution than in the optical. If we can look very close to the center and measure the orbital velocities of gas and stars (by their red and blue shifts) then we can use the image to get a size and our knowledge of gravity to get a mass. Incidentally its very very hard to do this for the active galaxies and quasars as the central object is so bright that it drowns out the light from the nearby stars. So far the closest in that we can get to less active galaxies shows that there can be a mass of 10-100 million x that of the sun in a region thats just a few light years across Here is an HST image and spectrum showing the doppler shift of a gas disk in an active galaxy called M87, while here is some radio data from the center of an active galaxy NGC 4258.
So what about our own galaxy ? Whats in the middle ? Its hard to look in optical light because of the gas and dust which obscure our view. Radio is less affected by dust, so we can see into the center, and this shows a rather odd radio source (so its not stars). Instead it seems to be emission from energetic particles trapped in a magnetic field. More on the galactic centre. But we certainly wouldn't classify our galactic center as an active galaxy like the ones we looked at above - the radio emission is very weak and it doesn't have a UV and X-ray bright accretion disk. But there does seem to be something there - Infrared light is also less affected by dust so we can see a central star cluster. Take pictures every year and we can see how the stars move - these are shown in this movie. The velocities of the stars are high - which means that they must be orbiting something massive - its consistent with there being a central mass of about 2 million times the mass of the sun. But it doesn't show activity like the active galaxies and quasars. The most probable explanation is that the black hole currently doesn't have much material falling onto it.
Other apparently normal galaxies also show evidence for a mostly inactive supermassive black holes at the center - its currently thought that most galaxies have a supermassive black hole in the center and whether we see it as an active galaxy/quasar just depends on how much material is around to fuel the activity.
There are nice review articles on black holes in galaxies, both active and normal called monsters in galaxies and supermassive black holes
