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

Research output: Contribution to journalArticle

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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 -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

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emitters
quasars
active galactic nuclei
x rays
luminosity
estimates
detection

Keywords

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

ASJC Scopus subject areas

  • Space and Planetary Science
  • Astronomy and Astrophysics

Cite this

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

X-ray properties of the z ∼ 4.5 Lyα emitters in the Chandra Deep Field South region. / Zheng, Z. Y.; Wang, J. X.; Finkelstein, S. L.; Malhotra, S.; Rhoads, J. E.; Finkelstein, K. D.

In: Astrophysical Journal, Vol. 718, No. 1, 20.07.2010, p. 52-59.

Research output: Contribution to journalArticle

Zheng, ZY, Wang, JX, Finkelstein, SL, Malhotra, S, Rhoads, JE & Finkelstein, KD 2010, 'X-ray properties of the z ∼ 4.5 Lyα emitters in the Chandra Deep Field South region', Astrophysical Journal, vol. 718, no. 1, pp. 52-59. https://doi.org/10.1088/0004-637X/718/1/52
Zheng ZY, Wang JX, Finkelstein SL, Malhotra S, Rhoads JE, Finkelstein KD. X-ray properties of the z ∼ 4.5 Lyα emitters in the Chandra Deep Field South region. Astrophysical Journal. 2010 Jul 20;718(1):52-59. https://doi.org/10.1088/0004-637X/718/1/52
Zheng, Z. Y. ; Wang, J. X. ; Finkelstein, S. L. ; Malhotra, S. ; Rhoads, J. E. ; Finkelstein, K. D. / X-ray properties of the z ∼ 4.5 Lyα emitters in the Chandra Deep Field South region. In: Astrophysical Journal. 2010 ; Vol. 718, No. 1. pp. 52-59.
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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 {\AA} 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 -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.",
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AU - Zheng, Z. Y.

AU - Wang, J. X.

AU - Finkelstein, S. L.

AU - Malhotra, S.

AU - Rhoads, J. E.

AU - Finkelstein, K. D.

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N2 - 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 -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.

AB - 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 -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.

KW - Galaxies: active

KW - Galaxies: high-redshift

KW - Galaxies: starburst

KW - X-rays: galaxies

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