16317( 4) - 18-Nov-2021 14:11:24 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 16317 Version: 4 Check-in Time: 18-Nov-2021 14:11:24 STScI Edit Number: 1 Title A Quantitative HeII Lyman Alpha Absorption Spectrum of the Newly Discovered Highest-Redshift UV-bright Quasar ------------------------------------------------------------------------------------ Type Cycle GO 28 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Gabor Worseck Universitat Potsdam Y CoI: Dr. Frederick Davies Max Planck Institute for Astronomy N CoI: Prof. Joseph Hennawi University of California - Santa Barbara N CoI: Dr. Ilya Khrykin University of Tokyo N CoI: Dr. Jason X. Prochaska University of California - Santa Cruz N CoI: Prof. Stefano Cristiani INAF, Osservatorio Astronomico di Trieste N CoI: Dr. Valentina D'Odorico INAF, Osservatorio Astronomico di Trieste N CoI: Dr. Andrea Grazian INAF - Osservatorio Astronomico di Padova N CoI: Dr. Konstantina Boutsia Carnegie Institution of Washington N CoI: Dr. Giorgio Calderone INAF, Osservatorio Astronomico di Trieste N CoI: Dr. Suk Sien Tie University of California - Santa Barbara N ------------------------------------------------------------------------------------ Abstract The advent of GALEX and COS have revolutionized our view of HeII reionization, the final major phase transition of the intergalactic medium. Efficient COS surveys have provided a statistical sample of strongly fluctuating HeII Lyman alpha absorption at 2.73.5 have revealed a set of HeII transmission 'spikes' and larger regions with nonzero transmission that suggest HeII reionization was well underway by z=4. This is in striking conflict with radiative transfer simulations of a HeII reionization driven by bright quasars. Explaining these measurements may require either faint quasars or more exotic sources of hard photons at z>4, with concomitant implications for HI reionization. A recent survey for high-redshift quasars in the southern hemisphere has uncovered the highest-redshift far-UV-bright quasar known (z=3.95, FUV=21.7). We propose here for COS G140L follow-up spectroscopy to (i) confirm its utility for HeII absorption studies by identifying possible interloping low-z HI absorbers, (ii) provide accurate measurements of the HeII Lyman alpha effective optical depth that are limited by systematics for much fainter already observed quasars, and (iii) constrain the lifetime of the background quasar from its ionized proximity zone. Moreover, the proposed observations would provide only the third z>3.5 sightline that would allow for high-resolution G130M spectroscopy before the end of HST's mission. ------------------------------------------------------------------------------------ Observations Description ------------------------ We will obtain a COS FUV G140L spectrum of the recently discovered highest-redshift UV-bright quasar at z=3.95 to characterize intergalactic HeII Lyman alpha absorption along its sightline. Our target has a sufficiently accurate flux in the quasar continuum just redward of the HeII break (GALEX FUV=21.7+/-0.4), and is safe to observe with COS. Its precision Gaia astrometry 16317( 4) - 18-Nov-2021 14:11:24 - [ 2] enables efficient acquisition with ACQ/IMAGE. The target is not detected in GALEX NUV, and its spectral energy distribution suggests NUV~23.9 (Fig. 5 in the proposal). With the COS ETC we estimate a NUV imaging acquisition exposure time of 400 seconds to reach the recommended S/N=20. The G140L grating provides the wavelength coverage to define the quasar continuum and the necessary resolution to measure the HeII absorption on scales of ~40 comoving Mpc. We require a S/N~3 per resolution element at 1500A, which results in a total request of 4 orbits. Continuous wavelength coverage at good sensitivity (lambda>1100A) is required to maximize the coverage of the HeII Lyman series absorption, only provided by the G140L 800A setup. In accord with our Phase I proposal and considering the significant acquisition overhead, the target will be observed in a single 4-orbit visit, using all FP-POS offsets in consecutive orbits. Spectra will be recorded in TIME-TAG mode with concurrent wavelength calibration (TAGFLASH). Time intervals spent in the Earth's shadow will be used to correct for geocoronal OI and NI emission. Given our faint source, airglow will dominate the COS count rates. However, even at the high airglow conditions conservatively assumed in our ETC calculations, the buffer time is longer than the exposure time. All exposure times have been adjusted to use the full visibility period in each orbit. In order to limit detector gain sag and contamination of the HeII absorption region by scattered geocoronal Lyman alpha emission, the observations should not be scheduled in visibility periods with large daytime portions. We have selected suitable time windows from the target viewing time report using BETWEEN constraints and setting the schedulability to 100%. Gain sag limits the accuracy of the COS dark current subtraction, which is critical to reach our science goals. We have developed a custom pipeline to determine the dark current in the COS PSA by post-processing COS dark monitoring exposures (Worseck et al. 2016, ApJ, 825, 144). For this calibration we require COS dark monitoring exposures taken at the same detector voltage as the science observations. Fortunately, up to now G140L science exposures are being taken at the same detector voltage as the dark monitoring exposures, such that special calibrations are unlikely to be required (see our program GO-15356). Reduced gyro operations will not affect our program, because we do not have orientation constraints (other than the BETWEEN constraints above). APT indicates a sufficient number of scheduling windows for our 4-orbit visit. ------------------------------------------------------------------------------------ 16317( 4) - 18-Nov-2021 14:11:24 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 QSO-045011-4 EXT-MEDIUM, ABSORPTION RA=04H50M11.3700S +/- 0.1", J2000 Z = 3.95 V = 17.8 +/- 0.1 GALEX FUV=21.73+/-0.40 32429 LINE SYSTEM, IGM DEC=-43D24'29.75" +/- 0.1" NUV~23.9 Reference Frame: ICRS Extended: NO ------------------------------------------------------------------------------------------------------------------------------------ 16317( 4) - 18-Nov-2021 14:11:24 - [ 4] Visit: 01 Visit Requirements: SCHED 100%; BETWEEN 30-OCT-2020:00:00:00 AND 05-FEB-2021:00:00:00; BETWEEN 01-OCT-2021:00:00:00 AND 20-OCT-2021:00:00:00; BETWEEN 12-NOV-2021:00:00:00 AND 17-FEB-2022:00:00:00; BETWEEN 12-MAR-2021:00:00:00 AND 20-MAR-2021:00:00:00 On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 QSO-045011- COS/NUV ACQ/IMA PSA MIRRORA 1 400 S (400 432429 GE S) ------------------------------------------------------------------------------------------------------------------------------------ 2 QSO-045011- COS/FUV TIME-TA PSA G140L 800 BUFFER-TIME=3000, FP-POS=1 1 2700 S 432429 G see below (1393 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 QSO-045011- COS/FUV TIME-TA PSA G140L 800 BUFFER-TIME=3000, FP-POS=2 1 2700 S 432429 G see below (2473 S) ------------------------------------------------------------------------------------------------------------------------------------ 4 QSO-045011- COS/FUV TIME-TA PSA G140L 800 BUFFER-TIME=3000, FP-POS=3 1 2700 S 432429 G see below (2473 S) ------------------------------------------------------------------------------------------------------------------------------------ 5 QSO-045011- COS/FUV TIME-TA PSA G140L 800 BUFFER-TIME=3000, FP-POS=4 1 2700 S 432429 G see below (2473 S) ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ QSO-045011-432429 1 COS/NUV ACQ/IMA PSA MIRRORA none none none none 1 N/A GE QSO-045011-432429 2 COS/FUV TIME-TA PSA G140L 800 none none none none 1 1393.0 S G QSO-045011-432429 3 COS/FUV TIME-TA PSA G140L 800 none none none none 2 2473.0 S G QSO-045011-432429 4 COS/FUV TIME-TA PSA G140L 800 none none none none 3 2473.0 S G QSO-045011-432429 5 COS/FUV TIME-TA PSA G140L 800 none none none none 4 2473.0 S G 16317( 4) - 18-Nov-2021 14:11:24 - [ 5] Summary Form for Proposal 16317 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures PSA ------------------------------------------------------------------------------------------------------------------------------------ Configurations COS/FUV, COS/NUV ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ/IMAGE, TIME-TAG ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters BUFFER-TIME=3000, FP-POS=1, FP-POS=2, FP-POS=3, FP-POS=4 ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements BETWEEN 01-OCT-2021:00:00:00 AND 20-OCT-2021:00:00:00, BETWEEN 12-MAR-2021:00:00:00 AND 20-MAR-2021:00:00:00, BETWEEN 12-NOV-2021:00:00:00 AND 17-FEB-2022:00:00:00, BETWEEN 30-OCT-2020:00:00:00 AND 05-FEB-2021:00:00:00, SCHED 100% ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements G140L, MIRRORA ------------------------------------------------------------------------------------------------------------------------------------ Target Names QSO-045011-432429 ------------------------------------------------------------------------------------------------------------------------------------