Detections of Lyman continuum from star-forming galaxies at z ∼ 3 through Subaru/Suprime-Cam narrow-band imaging

I. Iwata*, A. K. Inoue, Y. Matsuda, H. Furusawa, T. Hayashino, K. Kousai, M. Akiyama, T. Yamada, D. Burgarella, J. M. Deharveng

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

64 Citations (Scopus)


Knowing the amount of ionizing photons from young star-forming galaxies is of particular importance to understanding the reionization process. Here we report initial results of a Subaru/Suprime-Cam deep imaging observation of the SSA22 proto-cluster region at z = 3.09, using a special narrow-band filter to optimally trace ionizing radiation from galaxies at z ∼ 3. The unique wide field-of-view of Suprime-Cam enabled us to search for ionizing photons from 198 galaxies (73 Lyman break galaxies (LBGs) and 125 Lyα emitters (LAEs)) with spectroscopically measured redshifts z ≃ 3.1. We detected ionizing radiation from 7 LBGs, as well as from 10 LAE candidates. Some of the detected galaxies show significant spatial offsets of ionizing radiation from nonionizing UV emission. For some LBGs the observed nonionizing UV to Lyman continuum flux density ratios are smaller than values expected from population synthesis models with a standard Salpeter initial mass function (IMF) with moderate dust attenuation (which is suggested from the observed UV slopes), even if we assume very transparent intergalactic medium along the sightlines of these objects. This implies an intrinsically bluer spectral energy distribution, e.g., that produced by a top-heavy IMF, for these LBGs. The observed flux density ratios of nonionizing UV to ionizing radiation of 7 detected LBGs range from 2.4 to 23.8 and the median is 6.6. The observed flux density ratios of the detected LAEs are even smaller than LBGs, if they are truly at z ≃ 3.1. We find that the median value of the flux density ratio for the detected LBGs suggests that their escape fractions are likely to be higher than 4%, if the Lyman continuum escape is isotropic. The results imply that some of the LBGs in the proto-cluster at z ∼ 3 have escape fraction significantly higher than that of galaxies (in a general field) at z ∼ 1 studied previously.

Original languageEnglish
Pages (from-to)1287-1293
Number of pages7
JournalAstrophysical Journal
Issue number2
Publication statusPublished - 2009 Feb 20
Externally publishedYes


  • cosmology: observations
  • diffuse radiation
  • galaxies: evolution
  • galaxies: high-redshift
  • intergalactic medium

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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