X-RAY PROPERTIES OF THE z similar to 4.5 Ly alpha EMITTERS IN THE CHANDRA DEEP FIELD SOUTH REGION

作者：Zheng Z Y^*; Wang J X; Finkelstein S L; Malhotra S; Rhoads J E; Finkelstein K D

来源：Astrophysical Journal, 2010, 718(1): 52-59.

DOI：10.1088/0004-637X/718/1/52

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

We report the first X-ray detection of Ly alpha emitters (LAEs) at redshift z similar to 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 L(X) = 4.2 x 10(44) erg s(-1). The single detection gives an Lya quasar density of similar to 2.7(-2.2)(+6.2) x 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 similar to 36 Ms. Further analysis of the equivalent width (EW) distribution shows that all the signals come from 12 LAE candidates with EW(rest) < 400 angstrom 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 Lya emission. Excluding these two and combined with ECDF-S data, we derive a 3 sigma upper limit on the average X-ray flux of F(0.5-2.0) keV < 1.6 x 10(-18) erg cm(-2) s(-1), which corresponds to an average luminosity of < L(0.5-2) keV > < 2.4 x 10(42) erg s(-1) for z similar to 4.5 LAEs. If the average X-ray emission is due to star formation, it corresponds to a star formation rate (SFR) of < 180-530M(circle dot) yr(-1). We use this SFR(X) as an upper limit of the unobscured SFR to constrain the escape fraction of Lya photons and find a lower limit of f(esc,Ly alpha) > 3%-10%. However, our upper limit on the SFR X is similar to 7 times larger than the upper limit on SFR X on z similar to 3.1 LAEs in the same field and at least 30 times higher than the SFR estimated from Lya emission. From the average X-ray-to-Ly alpha 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.

• 出版日期2010-7-20
• 单位中国科学技术大学