Grown-up galaxy shows Universe of extremes

A galaxy spotted 10 billion light years away, when the Universe was only about one-quarter of its current age, has deepened the puzzle over how ‘fully grown’ galaxies can exist alongside seemingly ‘immature’ compact galaxies in the young Universe.
 
New infrared observations of a very distant, unusually bright and massive elliptical galaxy have shown that it is as large and equally dense as elliptical galaxies that can be found much closer to us, in our local Universe.
 
This object was selected from one of the world’s largest census of the distant Universe, the COSMOS survey, which is based on observations with the Hubble Space Telescope in combination with other space and ground-based telescopes.
 
The new finding, reported in this week’s The Astrophysical Journal Letters, in a paper led by Dr Masato Onodera (CEA/Saclay, France), follows a recent infrared observation, by a different team, of a very compact and extremely dense elliptical galaxy in the early Universe.
 
‘What our observations show is that alongside these compact galaxies were other ellipticals that were anything up to 100 times less dense and between two and five times larger – essentially ‘fully grown’ – and much more like the ellipticals we see in the local Universe around us,’ said Dr Michele Cappellari of Oxford University’s Department of Physics, an author of the report. ‘The mystery is how these two different extremes, ‘grown up’ and seemingly ‘immature’ ellipticals, co-existed so early on in the evolution of the Universe.’
 
Elliptical galaxies, which are regular in shape, can be over ten times as massive as spiral galaxies such as our own Milky Way and contain stars which formed over 10 billion years ago. One way of checking the density of such galaxies is to use the infrared spectrum they emit to measure the spread of the velocities of their stars, which has to balance the pull of gravity.
 
Measurements of a distant compact elliptical galaxy have shown that its stars were dispersing at a velocity of about 500 km per second, consistent with its size but unknown in local galaxies. The new study, using the 8.3-m Japanese Subaru telescope in Hawaii, found a ‘fully grown’ elliptical with stars dispersing at a velocity of lower than 300 km per second, much more like similar galaxies close to us.
 
‘Our next step is to use the Subaru telescope to find the relative proportion of these two extremes, fully grown and compact ellipticals, and see how they fit in with the timeline of the evolution of the young Universe,’ said Dr Michele Cappellari. ‘Hopefully this will give us new insights into solving this cosmic puzzle.’
 
For more information contact Dr Michele Cappellari on +44 (0)1865 273647 or email cappellari@astro.ox.ac.uk
 
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