14809( 3) - 14-Jul-2016 12:58:14 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 14809 Version: 3 Check-in Time: 14-Jul-2016 12:58:14 STScI Edit Number: 0 Title An Accurate Measurement of the IGM HeII Lyman Alpha Forest toward a Newly Discovered UV-bright Quasar at z>3.5 ------------------------------------------------------------------------------------ Type Cycle GO 24 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Gabor Worseck Max-Planck-Institut fur Astronomie, Y Heidelberg CoI: Prof. Joseph Hennawi Max-Planck-Institut fur Astronomie, N Heidelberg CoI: Dr. Jason X. Prochaska University of California - Santa Cruz N CoI: Dr. Frederick Davies Max Planck Institute for Astronomy N CoI: Ilya Khrykin Max-Planck-Institut fur Astronomie, N Heidelberg CoI: Tobias O.B. Schmidt Max-Planck-Institut fur Astronomie, N Heidelberg ------------------------------------------------------------------------------------ Abstract The advent of GALEX and COS have revolutionized our view of HeII reionization, the final major phase transition of the intergalactic medium. COS spectra of the HeII Lyman alpha forest have confirmed with high confidence the high HeII transmission that signifies the completion of HeII reionization at z~2.7. However, the handful of z>3.5 quasars observed to date show a set of HeII transmission 'spikes' and larger regions with non-zero transmission that suggest HeII reionization was well underway by z=4. This is in striking conflict with predictions from state-of-the-art 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. We propose here to observe J2354-2033, an FUV-bright quasar at z=3.786 that we recently discovered in a dedicated survey for likely HeII-transmitting quasars. With this COS/G140L spectrum, we would confirm that the quasar is valuable for studies of the HeII Lyman alpha forest by identifying possible interloping low-z HI absorbers, provide accurate measurements of the IGM HeII opacity, and provide only the third z>3.5 sightline that would allow for high-resolution G130M spectroscopy before the end of HST's mission. The proposed observations would mark only the fourth observation of the HeII Lyman alpha forest at z>3.7 and the source would be the 2nd-brightest known on the sky at these redshifts. ------------------------------------------------------------------------------------ Observations Description ------------------------ We will obtain a COS FUV G140L spectrum of a newly discovered UV-bright z=3.8 quasar 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), and a sufficiently accurate GALEX NUV magnitude for reliable acquisition via ACQ/IMAGE. The target is safe to observe with COS and has precise SDSS coordinates. It is relatively faint in the NUV, resulting in a NUV acquisition exposure time of 120sec to reach the 14809( 3) - 14-Jul-2016 12:58:14 - [ 2] recommended S/N~40. The G140L grating provides the wavelength coverage to define the quasar continuum and the necessary resolution to measure the onset of patchy HeII absorption. 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>1150A) is required to maximize the coverage of the HeII Lyman alpha absorption, only provided by the G140L 1105A setup. 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. 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 in press, arXiv:1405.7405). For this calibration we require COS dark monitoring exposures taken at the same detector voltage as the science observations in a ~3 month window around the date of the science observations. Therefore, our observations should not be scheduled immediately before or after a voltage change on COS Segment A. To minimize scattered geocoronal Lyman alpha emission that contaminates the HeII absorption region (Worseck et al. 2016, ApJ in press, arXiv:1405.7405) we strongly prefer our observations being scheduled in orbits with a large fraction of the visibility period spent in orbital night. ------------------------------------------------------------------------------------ 14809( 3) - 14-Jul-2016 12:58:14 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 QSO-235452-2 EXT-MEDIUM, ABSORPTION RA=23H54M52.0000S +/- 0.1", J2000 Z = V = 19.06 +/- 0.10 GALEX 03320 LINE SYSTEM - DEC=-20D33'20.73" +/- 0.1" 3.786 FUV=21.50+/-0.25 NUV=21.29+/-0.19 EXTRAGALACTIC, IGM Reference Frame: ICRS Extended: NO ------------------------------------------------------------------------------------------------------------------------------------ 14809( 3) - 14-Jul-2016 12:58:14 - [ 4] Visit: 01 Visit Requirements: BETWEEN 2016.298 AND 2016.342; BETWEEN 2017.276 AND 2017.296; BETWEEN 2017.309 AND 2017.315 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-235452- COS/NUV ACQ/IMA PSA MIRRORA 1 120 S (120 203320 GE S) ------------------------------------------------------------------------------------------------------------------------------------ 2 QSO-235452- COS/FUV TIME-TA PSA G140L 1105 BUFFER-TIME=4995, FLASH=YES, 1 3000 S 203320 G FP-POS=1 see below (2323 S) Comments: The ETC run conservatively assumes high background conditions, but given the faint target, the buffer time is still considerably larger than the exposure time. ------------------------------------------------------------------------------------------------------------------------------------ 3 QSO-235452- COS/FUV TIME-TA PSA G140L 1105 BUFFER-TIME=4995, FLASH=YES, 1 3000 S 203320 G FP-POS=2 see below (2936 S) Comments: The ETC run conservatively assumes high background conditions, but given the faint target, the buffer time is still considerably larger than the exposure time. ------------------------------------------------------------------------------------------------------------------------------------ 4 QSO-235452- COS/FUV TIME-TA PSA G140L 1105 BUFFER-TIME=4995, FLASH=YES, 1 3000 S 203320 G FP-POS=3 see below (2936 S) Comments: The ETC run conservatively assumes high background conditions, but given the faint target, the buffer time is still considerably larger than the exposure time. ------------------------------------------------------------------------------------------------------------------------------------ 5 QSO-235452- COS/FUV TIME-TA PSA G140L 1105 BUFFER-TIME=4995, FLASH=YES, 1 3000 S 203320 G FP-POS=4 see below (2936 S) Comments: The ETC run conservatively assumes high background conditions, but given the faint target, the buffer time is still considerably larger than the exposure time. ------------------------------------------------------------------------------------------------------------------------------------ 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 | ------------------------------------------------------------------------------------------------------------------------------------ 14809( 3) - 14-Jul-2016 12:58:14 - [ 5] 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-235452-203320 1 COS/NUV ACQ/IMA PSA MIRRORA none none none none 1 N/A GE QSO-235452-203320 2 COS/FUV TIME-TA PSA G140L 1105 none none none none 1 2323.0 S G QSO-235452-203320 3 COS/FUV TIME-TA PSA G140L 1105 none none none none 2 2936.0 S G QSO-235452-203320 4 COS/FUV TIME-TA PSA G140L 1105 none none none none 3 2936.0 S G QSO-235452-203320 5 COS/FUV TIME-TA PSA G140L 1105 none none none none 4 2936.0 S G 14809( 3) - 14-Jul-2016 12:58:14 - [ 6] Summary Form for Proposal 14809 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures PSA ------------------------------------------------------------------------------------------------------------------------------------ Configurations COS/FUV, COS/NUV ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ/IMAGE, TIME-TAG ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters BUFFER-TIME=4995, FLASH=YES, FP-POS=1, FP-POS=2, FP-POS=3, FP-POS=4 ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements BETWEEN 2016.298 AND 2016.342, BETWEEN 2017.276 AND 2017.296, BETWEEN 2017.309 AND 2017.315 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements G140L, MIRRORA ------------------------------------------------------------------------------------------------------------------------------------ Target Names QSO-235452-203320 ------------------------------------------------------------------------------------------------------------------------------------