A clear patch in the dark age Universe? Looking for reionization sources around two bright Ly-alpha emitting galaxies at z=7

0 Assigned ID: 566 ----- Attempting Submission 1 (Fri Apr 11 16:29:59 GMT 2014) ----- Added visit 56 as copy of visit 06 lost to HST safing. 1523040897 ----- Attempting Submission 1 (Wed Jul 23 16:10:00 GMT 2014) ----- ----- Attempting Submission 2 (Fri Oct 31 13:03:48 GMT 2014) ----- ----- Attempting Submission 3 (Mon Jun 08 16:36:49 GMT 2015) ----- In house processing A clear patch in the dark age Universe? Looking for reionization sources around two bright Ly-alpha emitting galaxies at z=7 The last few years have seen a number of discoveries that provided the first glimpse of the Universe at z>7 using both space and ground-based telescopes. The spectroscopic determination of a significant decrease at z>6 of the Ly-alpha emitter fraction among Lyman break galaxies (LBGs) has been interpreted as evidence for a rapid increase of the neutral HI in the inter-galactic medium (IGM). Among the different lines of sight explored so far, the BDF field stands out as a peculiar area since it contains two bright, high EW, Ly-alpha emitting galaxies at z=7.008 and z=7.109. However, their UV luminosity is not sufficient to generate an ionized region large enough to explain the observability of their Ly-alpha lines: the close physical distance between the two objects (4.4Mpc) thus suggests that they might be included in the same, large HII bubble which is ionized thanks to contribution of a yet undetected population of fainter sources. Indeed, theoretical models predict that a factor of 4-10 overdensity of z-dropout galaxies with respect to the available, robust determinations of the z~7 UV LF, should be detected by pushing the detection limits to Y=27.5 (M(UV)=-19.5). A small investment of HST time will allow us to perform a direct test of these theoretical predictions by searching for additional ionizing sources through standard optical/near IR photometric criteria. With the proposed observations we will be able to confirm whether the BDF area is the first re-ionized "bubble'' ever detected in the early Universe. This will help us to understand the earliest phases of galaxy formation and see the process of reionization caught in the act. We will observe the area of two spectroscopically confirmed galaxies in the BDF fields through two HST pointings both in the near IR (WFC3 F105W) and ACS F606W+F814W to detected additional, fainter z~7 sources in order to assess the predicted presence of an overdensity of galaxies. We will target two regions of the BDF field each including one of the confirmed Ly-alpha emitters and centred in the area between them which is also covered by our former set of optical and near-IR data. We slightly modified the target coordinates with respect to Phase I in order to: - improve the overlap with already available images. - ensure that for any orientation the two confirmed Ly-alpha emitters will fall in the guaranteed coverage area of WFC3 and far from known defects and bad imaging regions. Each of the two regions will be observed in - 2 filters with ACS: F814W (3 orbits), F606W (2 orbits). - WFC3 F105W filter (3 orbits). For a total of 16 orbits (10 with ACS and 6 with WFC3). Apertures are always set to IR-FIX for WFC3, and WFCENTER for ACS. To maximize schedulability we divide the program into 6 visits: Visit 1: we observe the field of BDF-3299 with 3 orbits WFC3 F105W. We use a 4-point dither sequence with ~1403 sec exposure (NSAMP=15, SAMP-SEQ=SPARS100) for Orbit 1 and Orbit 2, and ~703 sec exposures for Orbit 3 (NSAMP=15, SAMP-SEQ=SPARS50). Visit 2: equivalent to Visit 1 but on target BDF-521, 3 orbits WFC3 F105W. Same sequence as Visit 1. Visit 3: we observe the field of BDF-3299 with 3 orbits ACS F814W. We use the same 4-point dither sequence already used in the COSMOS 2-Degree ACS Survey (proposal 10092, Cycle 13). This sequence allows to cover the 50 pixel gap between ACS WFC1 and WFC2, to avoid persistence of bad columns, and provide some subpixel sampling. The pattern in x,y pixel offsets from the WFC reference position is : (+1.25, -60+1.5), (+6.25, 0.0+3.0), (-1.25, 60+4.5), (3.75, 120 + 6.0), achieved by imposing CENTER=YES on the primary PATTERN. The 4 exposures maximally pack each orbit with nearly equal lengths (~560 sec in Orbit 1, ~610 sec in Orbit 2 and Orbit 3). Visit 4: we observe the field of BDF-3299 with 2 orbits ACS F606W. We adopt the same dither sequence as in Visit 2, resulting in exposures times of ~ 570-620 sec. Visit 5: we observe the field of BDF-521 with 3 orbits ACS F814W. (Strategy as in Visit 3). Visit 6: we observe the field of BDF-521 with 2 orbits ACS F606W. (Strategy as in Visit 4). COSMOLOGY 12 Regular false false false false false SUPPORTED UNIDENTIFIED BLANK FIELD J2000 J2000 UNIDENTIFIED BLANK FIELD J2000 J2000 ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F606W WFC3/IR Imaging No No F105W ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F606W WFC3/IR Imaging No No F105W POS-TARG DITHER 0.572 LINE 0.365 false false 74.653 4 18.528 WFC3-IR-DITHER-BOX-MIN 1

false

POS-TARG DITHER DITHER 2 6.10486 LINE false true 3.08553 2 91.16385 POS-TARG LINE 85.38987 4

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