New Herschel results have given us a remarkable insight into the internal dynamics of two young galaxies. Surprisingly, they have shown that just a few billion years after the big bang, some galaxies were rotating in a mature way, seemingly having completed the accumulation of their gas reservoirs. An international team, including researchers from Paris Observatory, report about the detection of the ionized carbon line, with a profile shape typical of calm rotation, in a young galaxy which has been amplified by a galaxy cluster on the same line of sight.
When galaxies form, they accumulate mass by gravitationally attracting vast, external gas clouds. As the gas clouds enter the galaxy, they fall into random orbits. These disordered paths cause turbulence in the host galaxies, which can drive star formation.
To investigate the internal conditions of forming galaxies the team has targeted two young galaxies, known as SDSS0901 and the Clone. The light from both galaxies has taken 10 billion years to reach us across space. Thus, we are seeing them when the Universe was only one 4th of its age, and the galaxies were relatively young.
The two galaxies they choose to study are average galaxies for that time in cosmic history. This means that they are about 10-20 percent the size of our Milky Way, which is considered an average galaxy in the present-day Universe.
The young galaxy SDSS090122.37+181432.3. It is distorted because of gravitational lensing. Credit:NASA/STScI; S. Allam and team; and the Master Lens Database (masterlens.org), L. A. Moustakas, K. Stewart, et al (2014).
Studying galaxies so far away is usually hampered because they appear too dim to study effectively but in this case, the researchers were helped by the amplification of a gravitational lens (see Figure above). The two galaxies both sit behind intervening groups of galaxies, whose gravity warps space. The goal is to compare the nature of the gas in these early galaxies, to the gas we see locally.
Herschel spectrum of the galaxy SDSS090122.37+181432.3. A doubled peaked emission line from carbon (red fitted line) indicates that this galaxy is rotating with little turbulence. This characteristic is not usually associated with such a young galaxy. Instead it is usually a sign of galactic maturity. Credit:ESA/Herschel/HIFI. Acknowledgments: J Rhoads and S Malhotra, Arizona State University, USA.
The researchers used HIFI to investigate the infrared light of ionized carbon, which is emitted at a wavelength of 158 micrometers. This spectral line is produced in the clouds that surround star forming regions. HIFI showed the line was broadened into a double peak (see Figure above), and this allowed the motion of the gas to be fitted with a model.
Firstly, the team fitted the overall rotation of the galaxy, and then the turbulence in the gas clouds. To their surprise they found that galaxy S0901 was extremely well behaved. Instead of turbulence, it was found to be in orderly rotation, much more akin to the majestic galaxies of today. This is unexpected, since early galaxies are known to form stars at a much higher rate than today. This star formation liberates energy and produces turbulence in the interstellar medium. This suggests first that these galaxies have finished accumulating their gas, at least for now. But it also seems that turbulence is not actually required to trigger that early, active star formation.
These two galaxies might be peculiar, and robust conclusions should wait a much larger sample. But bigger samples will not be investigated by Herschel. As predicted, the liquid helium coolant needed to keep HIFI and Herschel's other instruments working ran out in April 2013. Instead the researchers hope to continue the work pioneered by Herschel using the Atacama Large Millimetre Array (ALMA), a groundbased array of 66 radio dishes in Chile.
Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.
The HIFI instrument is a very high-resolution heterodyne spectrometer and operates in seven bands covering the wavelength range between 157 and 625 µm. HIFI has been designed and built by a consortium of institutes and university departments across Europe, Canada, and the United States under the leadership of SRON Netherlands Institute for Space Research, the Netherlands, with major contributions from Germany, France, and the USA. HIFI Consortium members are: CSA, U. Waterloo (Canada); CESR, LAB, LERMA, IRAM (France); KOSMA, MPIfR, MPS (Germany); NUI Maynooth (Ireland); ASI, IFSI-INAF, Osservatorio Astrofisico di Arcetri-INAF (Italy); SRON, TUD (Netherlands); CAMK, CBK (Poland); Observatorio Astronómico Nacional (IGN), Centro de Astrobiología (CSIC-INTA) (Spain); Chalmers University of Technology - MC2, RSS & GARD, Onsala Space Observatory, Swedish National Space Board, Stockholm University - Stockholm Observatory (Sweden); ETH Zurich, FHNW (Switzerland); Caltech, JPL, NHSC (USA).
Herschel was launched on 14 May 2009 and completed science observations on 29 April 2013.
J. Rhoads, S. Malhotra, C. Carilli, F. Combes, K Finkelstein, S. Finkelstein, B. Frye, M. Gerin, P. Guillard, N. Nesvadba, J. Rigby, M. Spaans, M. A. Strauss: "Herschel Extreme Lensing Line Observations: Dynamics of two strongly lensed star forming galaxies near redshift z = 2", 2014, The Astrophysical Journal, April 29, 2014, V785 doi:10.1088/0004-637X/787/1/8
Read the news on ESA's website