Hubble has uncovered a goldmine of young dwarf galaxies that are undergoing intense bursts of star formation.
Dwarf galaxies are the most common in the universe but until now astronomers had seen few examples of distant dwarf galaxies because they are small and not very bright. Observing distant dwarf galaxies used to require training telescopes on very small patches of sky — but the CANDELS survey, which started in 2010, is able to observe much larger patches of sky in the same detail.
A CANDELS team led by Arjen van der Wel from found sixty-nine distant dwarf galaxies in near-infrared images taken with Hubble’s Wide Field Camera 3 and Advanced Camera for Surveys. The team stumbled upon the young galaxies while looking in two regions of the sky called the Great Observatories Origins Deep Survey (GOODS) and the UKIDSS Ultra Deep Survey (part of the UKIRT Infrared Deep Sky Survey). They weren’t looking for these galaxies specifically, but, once they’d spotted the galaxies’ unusual colours, could not resist taking a closer look. Their results were published in the December 1st issue of Astrophysical Journal.
This video zooms in on the GOODS South Deep field, right up to some of these newly discovered dwarf galaxies.
The galaxies are much smaller than the Milky Way but are creating stars at a massive pace, accumulating stars with a total mass of a hundred million times that of our Sun in just 30 million years. This intense activity is what produces their brilliant colours.
Now, these dwarf galaxies are nine billion light years away, which means we are seeing them as they existed nine billion years ago in the early universe. In astronomy, saying something is distant is the same as saying something existed when the universe was young. We see distant objects as they were when the light we use to see them left. We see something 9 billion light years away as it was nine billion years ago. That’s why light years are handy distance units to use when talking about astronomy.
Astronomers do think that galaxies in the early universe must have created stars at a much higher rate than they do today, but these dwarf galaxies are going fast even when this is taken into account.
One solution to this apparent problem is to say that star formation in small galaxies is not continuous. Simulations have shown that stars could form in short bursts. In between these bursts the galaxies would take a break from creating stars.
Stars form when clouds of gas collapse. The news stars could then reheat the gas and blow it away from the galaxy. In time, the gas would cool and collapse again, starting the next burst of star formation. But these dwarf galaxies appear to be going through bursts much more intense than simulations have produced.
This stop-start method of star formation does not fit with what astronomers have observed in near by dwarf galaxies. There are several dwarf galaxies orbiting the Milky Way as satellite galaxies, and astronomers have been able to look in detail at the rates of start formation in these ones before. Those surveys seemed to suggest that star formation takes place over billions of years.
Arjen van der Wel and colleagues conclude that these extremely intense bursts of star formation in the early universe may provide the majority of stars we see today in more mature dwarf galaxies.
While creating some problems of its own, this conclusion may help to solve another — one relating to that elusive substance known as dark matter. Simulations have shown that dark matter should be clustered towards the middle of galaxies, but observations show that in reality it is distributed much more evenly. This discrepancy could be explained if, during bursts of star formation, dark matter from the centre of a galaxy was pushed out towards the edges. We already know that normal matter does get pushed around in this way.
The James Webb Space Telescope, which was recently saved from the scrap heap, will be able to observe dwarf galaxies at a greater distance — meaning seeing them at an even earlier time — and will hopefully be able to offer more details surrounding their evolution.
Video credit: NASA, ESA, and G. Bacon (STScI)
van der Wel, A., Straughn, A., Rix, H., Finkelstein, S., Koekemoer, A., Weiner, B., Wuyts, S., Bell, E., Faber, S., Trump, J., Koo, D., Ferguson, H., Scarlata, C., Hathi, N., Dunlop, J., Newman, J., Dickinson, M., Jahnke, K., Salmon, B., de Mello, D., Kocevski, D., Lai, K., Grogin, N., Rodney, S., Guo, Y., McGrath, E., Lee, K., Barro, G., Huang, K., Riess, A., Ashby, M., & Willner, S. (2011). EXTREME EMISSION-LINE GALAXIES IN CANDELS: BROADBAND-SELECTED, STARBURSTING DWARF GALAXIES AT z > 1 The Astrophysical Journal, 742 (2) DOI: 10.1088/0004-637X/742/2/111