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Science Spotlight from this week's observations

Proposal ID = 12033
Principle Investigator =  James C. Green - University of Colorado at Boulder
Title = "COS-GTO: Studies of the HeII Reionization Epoch"
Time = Mar 8, 2012 22:12:09 - Mar 9, 2012 05:51:52
Target =  Q0302-003
Instrument = COS/FUV, COS/NUV, WFC3/IR, WFC3/UVIS

Background:

Early in the history of the universe, all matter was ionized. This simply means that the electrons were not attached to the protons. As the universe cooled, the electrons "recombined" with the protons, forming the atoms we are familiar with; Hydrogen (1 electron plus 1 proton), Helium (2 electrons plus 2 protons), etc. At a later stage in the history of the universe, the atoms were again ionized, partly due to the formation of large numbers of very bright hot stars, and partly due to high energy radiation from quasars (Quasi Stellar Objects = QSOs, also known as Active Galactic Nuclei = AGN). This is called the epoch of reionization. For Hydrogen atoms, this is around a redshift of Z = 6 - 10. For Helium atoms, which are able to hold on to their electrons more tightly, the epoch of reionization is later, around Z = 3, when more very hot stars and quasars are present. In this proposal the observers are following up ground-breaking discoveries from the FUSE (Far Ultraviolet Spectroscopic Explorer) spacecraft, which operated from 1999 to 2007. In particular, they are interested in determining how patchy the radiation field is by better resolving the very dense "forest" of absorption lines in the spectrum of quasars. This proposal is part of the Guaranteed Time Observers (GTO) award to the team that built COS (Cosmic Origins Spectrograph).

Paraphrasing from the abstract:

One of the major contributions of FUSE to cosmological studies was the detection of Helium (He) II Ly-alpha (Gunn-Peterson) absorption in the spectra of two AGN at redshifts z = 2.72-2.89. The He II absorption is quite patchy between redshifts z = 2.6 and 3.2 probably because the IGM is clumpy and the reionization process is affected by source fluctuations, spectra, and radiative transfer through the IGM. Observations of the He II absorption can therefore be used as diagnostics of the ionizing sources and radiative transport over large (30-50 Mpc) distances through the IGM. The ionizing radiation field appears to be softer (higher He II/H I) in the galaxy voids. These void regions may be ionized by local soft sources (dwarf starburst galaxies), or the QSO radiation softened by escape from AGN cores and transport through denser regions in the cosmic web. Because COS has far greater throughput than either STIS or FUSE, we will be able to resolve and characterize the He II absorption lines. The region shortward of the redshifted He II (Ly-alpha) corresponds to z = 2.77-2.92, where He II exhibits patchy transmission and absorption. The ratio of He II/H I (Ly-alpha line) opacities will provide information on the ionizing radiation field (and ionizing sources) at 1 and 4 ryd.


You can find most of this information and more from the following webpage: http://www.stsci.edu/hst/ by entering "12033" in the Prop. ID box.