Quasars help trace ancestors of giant elliptical galaxies

By using the Hubble Space Telescope (HST) as a `time machine`, astronomers from the Universities of Edinburgh and Oxford in the UK and the Space Telescope Science Institute (STScI) in Baltimore in the USA have been able to trace back the history of massive elliptical galaxies. They have found that galaxies of this kind, which still exist today, were already luminous families of stars about 10 billion years ago when the universe was only one third its present age. Their latest observations suggest that these galaxies were still in the process of formation at that time, converting large quantities of gas into brilliant blue stars. Studying quasars, and the galaxies that host them, has been the key to this piece of cosmic archaeology. The results will be presented at the National Astronomy Meeting in Bristol on Tuesday 9 April by Dr Marek Kukula of Edinburgh University.

Quasars are amongst the most luminous objects known and very strong evidence suggests that a quasar is powered by material being sucked onto a supermassive black hole at the centre of a galaxy. Even forty years after their discovery, these objects seem extraordinary: less than one light-day across yet outshining by a thousand times or more an entire surrounding galaxy spanning thousands of light years and consisting of tens of billions of stars.

In the present-day universe quasars are extremely rare but they were much more common at earlier epochs. Their numbers peak between redshifts of 2 and 3, when the universe was roughly a third of its present age. However, astronomers now know that dormant black holes of the necessary mass to power quasars (roughly a billion times the mass of the Sun) are found in the centres of many nearby massive elliptical galaxies. Such massive black holes have been found nowhere else in the local universe and this suggests that the quasars seen at high redshifts are in the ancestors of today`s massive ellipticals. For the past 5 years, the Edinburgh/Oxford/STScI team has been using the HST to study galaxies hosting quasars at a range of distances from the local universe out to a redshift of about 2, corresponding to a distance of around 10 billion light years.

“The extraordinary luminosity of quasars poses a particular problem when we wish to observe the galaxies in which they lie,” says Marek Kukula. “The central quasar nucleus is often so bright that it completely swamps the starlight from the surrounding galaxy and for many years it proved impossible to obtain reliable information about quasar hosts. HST has changed all that, allowing us to separate the halo of starlight from the glare of the central quasar.”

The team started by observing some of the nearest quasars, at redshifts of around 0.2, and found, that with very few exceptions, their host galaxies are massive, luminous ellipticals – exactly what they expected. Then to understand how these massive elliptical galaxies formed and evolved, they turned their attention to more distant quasars. Visible light emitted by a quasar at a redshift of 2 is received as near infrared radiation, at a wavelengths of several microns. So to make a proper comparison with visible images of nearby quasars, astronomers need near-infrared images of the remote ones.

For very distant host galaxies, it is far more difficult to separate the starlight from the light of the central quasar. However, detailed analysis of the team`s high-redshift HST images shows that luminous galaxies are present around the quasars.

“This is an exciting result,” says Marek Kukula, “since it implies that today`s massive elliptical galaxies were already luminous stellar systems at a redshift of 2, when the universe was only a third of its current age. However, our infrared images alone cannot tell us is how close these high-redshift host galaxies are to becoming fully-fledged massive ellipticals like the ones we see around us in the local universe. To find out whether the hosts already contain a large mass of mature stars, or whether they are still relatively small galaxies with a high proportion of young, but very luminous stars we need to determine their colours. Young stars will be much bluer than an older, more established stellar population. Initial results from our most recent HST observations seem to indicate that the host galaxies are emitting more ultraviolet light than would be expected if they are made up purely of mature stars. This suggests that in these very distant quasars we really are seeing the massive elliptical galaxies in the process of formation.”