The first spectral line surveys searching for signals from the dark ages
Artikel i vetenskaplig tidskrift, 2010

Aims. Our aim is to observationally investigate the cosmic Dark Ages in order to constrain star and structure formation models, as well as the chemical evolution in the early Universe. Methods. Spectral lines from atoms and molecules in primordial perturbations at high redshifts can give information about the conditions in the early universe before and during the formation of the first stars in addition to the epoch of reionisation. The lines may arise from moving primordial perturbations before the formation of the first stars (resonant scattering lines), or could be thermal absorption or emission lines at lower redshifts. The difficulties in these searches are that the source redshift and evolutionary state, as well as molecular species and transition are unknown, which implies that an observed line can fall within a wide range of frequencies. The lines are also expected to be very weak. Observations from space have the advantages of stability and the lack of atmospheric features which is important in such observations. We have therefore, as a first step in our searches, used the Odin* satellite to perform two sets of spectral line surveys towards several positions. The first survey covered the band 547-578 GHz towards two positions, and the second one covered the bands 542.0-547.5 GHz and 486.5-492.0 GHz towards six positions selected to test different sizes of the primordial clouds. Two deep searches centred at 543.250 and 543.100 GHz with 1 GHz bandwidth were also performed towards one position. The two lowest rotational transitions of H-2 will be redshifted to these frequencies from z similar to 20-30, which is the predicted epoch of the first star formation. Results. No lines are detected at an rms level of 14-90 and 5-35 mK for the two surveys, respectively, and 2-7 mK in the deep searches with a channel spacing of 1-16 MHz. The broad bandwidth covered allows a wide range of redshifts to be explored for a number of atomic and molecular species and transitions. From the theoretical side, our sensitivity analysis show that the largest possible amplitudes of the resonant lines are about 1 mK at frequencies less than or similar to 200 GHz, and a few mu K around 500-600 GHz, assuming optically thick lines and no beam-dilution. However, if existing, thermal absorption lines have the potential to be orders of magnitude stronger than the resonant lines. We make a simple estimation of the sizes and masses of the primordial perturbations at their turnaround epochs, which previously has been identified as the most favourable epoch for a detection. This work may be considered as an important pilot study for our forthcoming observations with the Herschel Space Observatory.


submillimeter: ISM

line: formation

cosmology: observations

molecular gas

early Universe

galaxy formation

primordial gas

large-scale structure of

cosmological hydrogen recombination

microwave background spectrum


odin satellite

early universe

gamma-ray burst

ISM: molecules



Carina Persson

Chalmers, Institutionen för radio- och rymdvetenskap, Radioastronomi och astrofysik

R. Maoli

Institut d 'Astrophysique de Paris

Sapienza, Università di Roma

P. Encrenaz

LERMA - Laboratoire d'Etudes du Rayonnement et de la Matiere en Astrophysique et Atmospheres

Åke Hjalmarson

Chalmers, Institutionen för radio- och rymdvetenskap, Radioastronomi och astrofysik

Michael Olberg

Chalmers, Institutionen för radio- och rymdvetenskap, Nationella anläggningen för radioastronomi

Gustaf Rydbeck

Chalmers, Institutionen för radio- och rymdvetenskap, Radioastronomi och astrofysik

M. Signore

LERMA - Laboratoire d'Etudes du Rayonnement et de la Matiere en Astrophysique et Atmospheres

U. Frisk

Swedish Space Corporation (SSC)

A. Sandqvist

Alba Nova Universitetscentrum

J. Y. Daniel

Institut d 'Astrophysique de Paris

Astronomy and Astrophysics

0004-6361 (ISSN) 1432-0746 (eISSN)

Vol. 515 8 A72


Astronomi, astrofysik och kosmologi



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