THE EVOLUTION OF THE HELIUM-IONIZING BACKGROUND AT z approx 2-3

Journal Article · Tue Dec 01 04:00:00 EST 2009 · Astrophysical Journal

DOI:https://doi.org/10.1088/0004-637X/706/2/970· OSTI ID:21378124

Dixon, Keri L

Recent observations suggest that helium became fully ionized around redshift z approx 3. The He II optical depth derived from the Lyalpha forest decreases substantially from this period to z approx 2; moreover, it fluctuates strongly near z approx 3 and then evolves smoothly at lower redshifts. From these opacities, we compute, using a semi-analytic model, the evolution of the mean photoionization rate and the attenuation length for helium over the redshift range 2.0 approx< z approx< 3.2. This model includes an inhomogeneous metagalactic radiation background, which is expected during and after helium reionization. We find that assuming a uniform background underestimates the required photoionization rate by up to a factor of approx2. When averaged over the (few) available lines of sight, the effective optical depth exhibits a discontinuity near z approx 2.8, but the measurement uncertainties are sizable. This feature translates into a jump in the photoionization rate and, provided the quasar emissivity evolves smoothly, in the effective attenuation length, perhaps signaling the helium reionization era. We then compute the evolution of the effective optical depth for a variety of simple helium reionization models, in which the measured quasar luminosity function and the attenuation length, as well as the evolving He III fraction, are inputs. A model with reionization ending around redshift z approx 2.7 is most consistent with the data, although the constraints are not strong thanks to the sparseness of the data.

• OSTI ID: 21378124
• Journal Information: Astrophysical Journal, Journal Name: Astrophysical Journal Journal Issue: 2 Vol. 706; ISSN ASJOAB; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
ABSORPTION
ATTENUATION
COSMIC RADIO SOURCES
DIMENSIONS
ELEMENTS
EMISSIVITY
EVOLUTION
FLUIDS
GASES
HELIUM
IONIZATION
LENGTH
LUMINOSITY
NONMETALS
OPTICAL PROPERTIES
PHOTOIONIZATION
PHYSICAL PROPERTIES
QUASARS
RARE GASES
RED SHIFT
SORPTION
SURFACE PROPERTIES