September 14, 2011

The birth of stars is not a matter of cosmic billiard

The observations performed with the ESA's Herschel space telescope showed that collisions between galaxies only play a secondary role in the birth process of the stars.

When the majority of stars formed, the gas quantity in the galaxies was large enough to "spontaneously" lead to a sustained production of stars. Those results were announced on September 13th in the online Astronomy&Astrophysics magazine.

It has been well-known for many years that the stars are formed upon gravitational collapse of a dense and massive enough gas cloud. It is also known that this phenomenon was much more frequent a long time ago, some 10 billion years ago. Some galaxies were yet very prolific and could have given birth the ten or a hundred times more stars than what is currently observed in the nearby Universe.

To obtain such results, the galaxy's infrared luminosity is measured.

In general, young stars' light, the majority of which is concentrated in the visible and ultraviolet, cannot be directly observed because of the intergalactic gas (in which they form) absorbing this radiation. The gas becomes hot and produces its own radiation, this time in the infrared. This wavelength range extends from 1 micron to almost 1 millimetre. We speak about near, medium and far infrared.

Two preferred scenarios exist to explain the intense stellar formation phases in the galaxies. In the first one, under the influence of a collision between two galaxies, some regions were so crowded that the gas they contained reached sufficient densities to produce stars. This burst of stellar formation, resulting from a fusion of galaxies, is called "starbust". In the second one, no catastrophic event. Simply a dense and massive gas cloud in which many stars were form.

The space and spectral signatures of those two scenarios are not the same. When a collision between two galaxies occurs, the phenomenon's violence leads to a sudden massive and spatially concentrated star production, resulting in the destruction of some weaker molecules and a diminution of the medium-infrared radiation emitted by the galaxy compared with its emissions in the far-infrared. In the case of a "normal" stellar formation, it takes place in a homogeneous way in the whole galaxy and there is no such drop in emissions.

Until now, if the nearby galaxies had been being observed in the whole infrared range, the then remote universe containing the majority of stellar formations had never been observed in the far-infrared, in the absence of a proper instrument. The investigators had hypothesis by extrapolating the observed phenomena in the nearby Universe, where only the galaxies which experienced a fusion process between two galaxies are able to produce as much stars. In the remote universe, since the galaxies were very bright, they gave priority to the collision scenario between two galaxies. When speaking about less bright objects, they preferred the normal scenario.

Herschel provides the first observations of the early galaxies in the far-infrared

The images received in the far-infrared using the Herschel space telescope contradict this double scenario. Within the frame of the GOODS-HERSCHEL international programme, leaded by David Elbaz of the CEA-Irfu, the astrophysicists directed the Herschel space telescope towards two regions of the sky (GOODS North and South) and then took the deepest pictures of the sky ever taken in the infrared range. They could thus observed 2,000 galaxies and cover 80 % of the cosmos' age. By comparing the quantity of infrared light emitted in different wavelengths by those galaxies, the team showed that galaxies with high rates of star formation did not necessarily experience any collision.

To enhance this statement, the investigators appealed to radio (for nearby galaxies) and UV observations via the Hubble Space Telescope (for remote galaxies) in order to review the space distribution of stellar formation areas within the very bright galaxies. They discovered that the "starbusts" were a very small minority compared to the "normal" galaxies.

These observations allow scientists to obtain an universal stellar formation mode and a finally very simple relation: the more gas in a galaxy, the more it gives birth to stars. “Collisions are necessary to a high rate star formation only in the galaxies which contain not enough gas”, David Elbaz explained. This applies to the current galaxies which already used the major part of the gaseous row material after forming stars during more than 10 billion years. So the galaxy collisions played a quite anecdotal role in the stellar formation process at the scale of the Universe's history.

Recent numerical simulations confirmed the hypothesis that most of the galaxies are linked by filaments of intergalactic matter which are likely to provide them gas like a cosmic catheter.

Article references

« GOODS-Herschel: an infrared main sequence for star-forming galaxies », D. Elbaz, M.Dickinson, H.S.Hwang, T.Diaz-Santos, G.Magdis, B.Magnelli, D.Le Borgne, F.Galliano, M.Pannella, P.Chanial, L.Armus, V.Charmandaris, E.Daddi, H.Aussel, P.Popesso, J.Kartaltepe, B.Altieri, I.Valtchanov, D.Coia, H.Dannerbauer, K.Dasyra, R.Leiton, J.Mazzarella, V.Buat, D.Burgarella, R.-R.Chary, R.Gilli, R.J.Ivison, S.Juneau, E.LeFloc'h, D.Lutz, G.E.Morrison, J.Mullaney, E.Murphy, A.Pope, D.Scott, D.Alexander, M.Brodwin, D.Calzetti, C.Cesarsky, S.Charlot, H.Dole, P.Eisenhardt, H.C.Ferguson, N.Foerster-Schreiber, D.Frayer, M.Giavalisco, M.Huynh, A.M.Koekemoer, C.Papovich, N.Reddy, C.Surace, H.Teplitz, M.S.Yun, G.Wilson, Astronomy & Astrophysics, september 2011, Volume 533.

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