Joining the dots: from starburst to elliptical galaxies
Sen—Astronomers observing ancient starburst galaxies have made a connection between them and the elliptical galaxies we see today.
There are many different types of galaxies in the Universe and astronomers have long desired to join the dots and solve the puzzles of galaxy evolution. Looking at galaxies that are far, far away is also a way of looking back in time. Their light has taken billions of years to reach us, and thus we see those galaxies as they were billions of years ago. Galaxies in the ancient Universe are often very different than the host of spiral and elliptical galaxies that we are surrounded by today. For example, the extremely bright quasars are common in the distant Universe and yet none exist locally.
However, astronomers using NASA’s Spitzer Space Telescope along with ESO’s Very Large Telescope and 12 metre Atacama Pathfinder Experiment (APEX) telescope have managed to see how distant submillimetre galaxies, quasars, and modern elliptical galaxies fit together in the jigsaw of the Universe.
Submillimetre galaxies (SMGs) are situated 10 billion light years from us, and are extremely bright in the infrared region of the spectrum, specifically the submillimetre band. Because the SMGs are located so far away, the light emitted by the galaxies is shifted to much longer wavelengths. These galaxies are also starburst galaxies, meaning that for a short while there is a phenomenal rate of star formation. A supernova explosion would occur every few years and on a planet in a starburst galaxy the night sky would be almost as bright as day.
Astronomers have been able to measure the mass of the dark matter halos surrounding a group of SMGs. Dark matter is invisible and we don’t know what it is, but indirect detections tells us that galaxies are usually engulfed in it. The dark matter typically extends far beyond the edge of the visible galaxy. But measuring the mass of dark matter halos 10 billion light years away is no easy task. Ryan Hickox, lead author of the paper on the subject, explains to Sen how this was done.
“We measure how strongly the galaxies are clustered together in space, using a statistical tool called a ‘correlation function’. If the galaxies were distributed randomly, the correlation function would be equal to zero. However if they are clustered together (sort of like buildings in towns and cities) then they have a positive correlation function. We know from simulations of the Universe how halos of dark matter are clustered together, and this clustering depends strongly on the mass of the halos. Galaxies that live in these halos will be clustered the same way. So by measuring the clustering of the galaxies, we can tell how massive the typical halos that host them are.”
By knowing the mass of the halos of the SMGs, Hickox and his colleagues were able to use computer simulations to fast forward to the present day and show that these galaxies will eventually form giant elliptical galaxies in the modern Universe. However, elliptical galaxies are typically devoid of star formation. So what stopped the immense star formation in the SMGs?
It turns out that the rapid star formation rate also doomed the SMGs to a quiet later life. Starbursts typically only last around 100 million years, which is very short in astronomical time scales. It is thought that a collision between galaxies can trigger this star formation, as gas and dust slam into each other and collapses to form stars. “In a starburst many stars form almost instantaneously,” says Julie Wardlow, co-author on the paper. “The most massive of these stars will exhaust their fuel relatively quickly and will expel gas and dust from them as they die. In a 'normal' galaxy this is a minor effect because there are so few stars formed each year, but in a starburst the gas and dust expelled from stars is significant.” This extra gas and dust then goes on to feed a supermassive black hole at the centre of the galaxy. Anything that falls into a black hole will have around half of its mass converted into energy, and this is released just prior to crossing the event horizon. This in turn powers the quasar in the centre of the galaxy, causing it to light up like a beacon. The quasar emits massive amounts of energy which blows away the remaining gas in the galaxy, and thus puts an end to the star formation.
The new measurements of SMG clustering has revealed that they are grouped together in a similar style to quasars, which creates a connection between the two. The quasars themselves are very difficult to detect at this distance because they are hidden behind the massive amounts of dust in the SMGs. Quasars are also variable, and are not always “switched on,” which adds to the difficulty in detecting them.
“However, in the nearby Universe we do see quasars associated with starburst galaxies, and they are often associated with the end of the starburst phase suggesting they might shut off the star formation,” says Hickox. “Also, many theoretical models for the processes that trigger the starbursts predict that they should also trigger the quasar. So given that the starbursts and quasars are found in the same halos, it's good (but still indirect) evidence that they are part of the same systems.”
While previous attempts have been made to find out how SMGs group together, these new results are the most revealing ones so far. “There have been previous measurements of the clustering, but they have never been accurate enough to conclusively connect the distant starburst galaxies to quasars, and to the massive galaxies nearby that they will evolve into,” says Hickox. “This is why these measurements are so exciting!”