X-ray properties of the z ∼ 4.5 Lyα emitters in the Chandra Deep Field South region

Z. Y. Zheng, J. X. Wang, S. L. Finkelstein, S. Malhotra, J. E. Rhoads, K. D. Finkelstein

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Abstract

We report the first X-ray detection of Lyα emitters (LAEs) at redshift z ∼ 4.5. One source (J033127.2-274247) is detected in the Extended Chandra Deep Field-South (ECDF-S) X-ray data and has been spectroscopically confirmed as a z = 4.48 quasar with LX = 4.2 × 1044erg s-1. The single detection gives an Lyα quasar density of ∼ 2.7+6.2-2.2 × 10-6 Mpc-3, consistent with the X-ray luminosity function of quasars. Another 22 LAEs in the central Chandra Deep Field-South region are not detected individually, but their co-added counts yield an S/N = 2.4 (p = 99.83%) detection at soft band, with an effective exposure time of ∼36 Ms. Further analysis of the equivalent width (EW) distribution shows that all the signals come from 12 LAE candidates with EWrest < 400 Å and 2 of them contribute about half of the signal. From follow-up spectroscopic observations, we find that one of the two is a low-redshift emission-line galaxy, and the other is a Lyman break galaxy at z = 4.4 with little or no Lyα emission. Excluding these two and combined with ECDF-S data, we derive a 3σ upper limit on the average X-ray flux of F0.5-2.0keV < 1.6 × 10-18 ergcm-2s-1, which corresponds to an average luminosity of 〈L0.5-2keV〉 < 2.4 × 1042 erg s-1 for z ∼ 4.5 LAEs. If the average X-ray emission is due to star formation, it corresponds to a star formation rate (SFR) of <180-530 M⊙ yr-1. We use this SFRX as an upper limit of the unobscured SFR to constrain the escape fraction of Lyα photons and find a lower limit of fesc, Lyα > 3%-10%. However, our upper limit on the SFRX is ∼7 times larger than the upper limit on SFRX on z ∼ 3.1 LAEs in the same field and at least 30 times higher than the SFR estimated from Lyα emission. From the average X-ray-to-Lyα line ratio, we estimate that fewer than 3.2% (6.3%) of our LAEs could be high-redshift type 1 (type 2) active galactic nuclei (AGNs) and those hidden AGNs likely show low rest-frame EWs.

Original languageEnglish (US)
Pages (from-to)52-59
Number of pages8
JournalAstrophysical Journal
Volume718
Issue number1
DOIs
StatePublished - Jul 20 2010

Keywords

  • Galaxies: active
  • Galaxies: high-redshift
  • Galaxies: starburst
  • X-rays: galaxies

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

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    Zheng, Z. Y., Wang, J. X., Finkelstein, S. L., Malhotra, S., Rhoads, J. E., & Finkelstein, K. D. (2010). X-ray properties of the z ∼ 4.5 Lyα emitters in the Chandra Deep Field South region. Astrophysical Journal, 718(1), 52-59. https://doi.org/10.1088/0004-637X/718/1/52