17198( 4) - 23-Jun-2023 02:05:00 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 17198 Version: 4 Check-in Time: 23-Jun-2023 02:05:00 STScI Edit Number: 1 Title First look at the high-ionization UV nebular emission powered by the youngest stellar populations below 5% solar metallicity: the missing link to the reionization era? ------------------------------------------------------------------------------------ Type Cycle GO 30 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Peter Senchyna Carnegie Institution of Washington Y CoI: Prof. Daniel P. Stark University of Arizona N CoI: Mr. Zuyi Chen University of Arizona N CoI: Dr. Danielle Berg University of Texas at Austin N CoI: Prof. John Chisholm University of Texas at Austin N CoI: Dr. Stephane Charlot CNRS, Institut d'Astrophysique de Paris N CoI: Dr. Jacopo Chevallard CNRS, Institut d'Astrophysique de Paris N CoI: Dr. Adele Plat University of Arizona N ------------------------------------------------------------------------------------ Abstract Despite a substantial investment of HST orbits over the last five years, the intense nebular CIV detected at z>6 remains mysteriously without precedent among star-forming galaxies in the local Universe. The persistent difficulty in interpreting this emission with the launch of JWST looming foreshadows potentially significant challenges in the coming years. Uncertain photoionization models suggest that extraordinarily metal-poor massive stellar populations at <5% solar are likely to be the most efficient engines for this high-ionization emission. However, UV and most optical observations have so far been restricted to relatively bright and unfortunately low specific star-formation rate regions at these lowest metallicities. The relatively recent unveiling of a significant population of extremely metal-deficient galaxies dominated by young stars below the detection limits of previous surveys promises to at last open a window onto the highly uncertain ionzing spectrum of massive stellar populations below 5% solar. Here we propose G160M observations to constrain CIV and HeII at gas-phase oxygen abundances below 5% and effective ages <~5 Myr for the first time, targeting the only three systems known in this regime which are sufficiently bright and low-redshift to be amenable to relatively inexpensive UV spectroscopic follow-up. These data will provide unique insight onto the high-energy radiation fields powered by very low-metallicity massive stars, immediately either resolving or significantly heightening the tension between local galaxy populations and the first systems targeted with rest-UV spectroscopy in the reionization era. ------------------------------------------------------------------------------------ Observations Description ------------------------ This program will acquire FUV spectroscopy designed to constrain high-ionization nebular emission in C IV and He II for three extremely metal-deficient nearby dwarf galaxies dominated by very young stellar populations. The targets are each covered by SDSS and GALEX imaging which we leverage in planning their observation. Coordinates and acquisiton: Each target is a compact source unresolved in 17198( 4) - 23-Jun-2023 02:05:00 - [ 2] ground-based imaging. The target coordinates are determined from the SDSS photometric outputs and confirmed to agree precisely with the peak of u-band emission for each target in the ICRS-aligned SDSS imaging. Each target is thus acquired with a standard ACQ/IMAGE exposure, with exposure time chosen to deliver S/N=50 for a point source with an NUV AB magnitude equal to the faintest of the u-band fiber and cModel magnitudes (assuming a flat continuum in F_nu, appropriate for such sources). We confirm that all three have GALEX/NUV magnitudes in-excess of this conservative estimate; and to ensure detector safely, we likewise confirm that no warnings are raised for a Mirror A+PSA observation of a point source with the brightest GALEX/NUV magnitude among the three. Based upon previous observations of similar sources (and exisiting HST optical imagery for J210455), we expect these targets to be more extended than a point source, but with FWHM at NUV wavelengths significantly smaller than 0.6''; given this, the large requested S/N provides an appropriate buffer to ensure accurate acquisition of such targets. Observing plan: Our primary objective is nebular line measurement; our observations are thus in the low-S/N regime (<5 /resel in the continuum). To maximize the exposure time on-source, we thus devote each orbit to a single spectroscopic exposure, which enables us to relegate most of the buffer dump and expensive SPLIT wavecal overheads to occultation. Thus, 2-3 FP-POS positions are observed for each object (always including FP-POS=1 and 4), as allowed under the updated Cycle 30 requirements. A full buffer dump is requested for each exposure given the uncertain FUV countrates and the unimportant impact on overheads. A warning is generated and acknowledged for the full-orbit spectroscopic exposures which exceed 2400s; the potentially reduced wavelength calibration accuracy is unlikely to impact our primary science, and the follow-up wavecal will allow us to correct for some drift in the mechanism settings. For the 3-orbit Visit 1, we choose to observe FP-POS=3 for the short orbit 1 integration and the primary FP-POS=1+4 in the subsequent orbits; the resulting warning about inefficient FP-POS ordering is ignored as the grating shifts occur during occultation. Safety: The only health and safety warning presented is associated with J210455, which has a bright (V~16 according to the BOT) neighbor star. This exposures presents a potential danger only under the incorrect default assumption that it has an O5 spectrum. The SDSS imaging confirms that this star is located 3.77'' to the W of the target, and GAIA position and PM measurements indicate that it is moving further away to the SW; thus we are confident that it will be outside the PSA extent given typical pointing uncertainties. In addition, the SDSS photometry for this source (u, g, r, i, z = 20.49, 18.03, 17.01, 16.66, 16.47) is consistent with a ~4500K K-type star, which places the expected AB NUV magnitude negligbly faint (>26). GALEX NUV and FUV imaging, while low-resolution, confirms that there is no excess detected UV flux consistent with the position of this nearby star. Existing WFC3/F336W imaging for this source further confirms that the target star-forming region is well-isolated from both this star (located at a similar distance to that measured from SDSS) and from the remaining extent of the galaxy, and is compact relative to the PSA at blue wavelengths as expected. Thus, we proceed with a standard target acquisition for this source. ------------------------------------------------------------------------------------ 17198( 4) - 23-Jun-2023 02:05:00 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 J104609.23+4 GALAXY, DWARF COMPACT, RA=10H46M9.2300S +/- 0.1", J2000 Z = V = 20.9 04707.0 EMISSION LINE NEBULA, DEC=+40D47'7.07" +/- 0.1" 0.049 STAR FORMING REGION Reference Frame: ICRS Extended: NO ------------------------------------------------------------------------------------------------------------------------------------ 2 J135340.78+1 GALAXY, DWARF COMPACT, RA=13H53M40.7800S +/- 0.1", J2000 Z = V = 20.4 64920.9 EMISSION LINE NEBULA, DEC=+16D49'20.91" +/- 0.1" 0.084 STAR FORMING REGION Reference Frame: ICRS Extended: NO ------------------------------------------------------------------------------------------------------------------------------------ 3 J210455.39-0 GALAXY, DWARF COMPACT, RA=21H04M55.3900S +/- 0.1", J2000 Z = V = 20.6 03519.3 EMISSION LINE NEBULA, DEC=-00D35'19.30" +/- 0.1" 0.0048 STAR FORMING REGION Reference Frame: ICRS Extended: NO ------------------------------------------------------------------------------------------------------------------------------------ 17198( 4) - 23-Jun-2023 02:05:00 - [ 4] Visit: 01 Visit Requirements: 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 J104609.23+ COS/NUV ACQ/IMA PSA MIRRORA 1 180 S (180 404707.0 GE S) ------------------------------------------------------------------------------------------------------------------------------------ 2 J104609.23+ COS/FUV TIME-TA PSA G160M 1589 FP-POS=1, BUFFER-TIME=2014 1 2000 S 404707.0 G see below (2014 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 J104609.23+ COS/FUV TIME-TA PSA G160M 1589 FP-POS=2, BUFFER-TIME=1112 1 1000 S 404707.0 G see below (1222 S) ------------------------------------------------------------------------------------------------------------------------------------ 4 J104609.23+ COS/FUV TIME-TA PSA G160M 1589 FP-POS=3, BUFFER-TIME=1222 1 1000 S 404707.0 G see below (1222 S) ------------------------------------------------------------------------------------------------------------------------------------ 5 J104609.23+ COS/FUV TIME-TA PSA G160M 1589 FP-POS=4, BUFFER-TIME=2786 1 2700 S 404707.0 G see below (2786 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 | ------------------------------------------------------------------------------------------------------------------------------------ J104609.23+404707.0 1 COS/NUV ACQ/IMA PSA MIRRORA none none none none 1 N/A GE J104609.23+404707.0 2 COS/FUV TIME-TA PSA G160M 1589 none none none none 1 2014.0 S G J104609.23+404707.0 3 COS/FUV TIME-TA PSA G160M 1589 none none none none 2 1222.0 S G J104609.23+404707.0 4 COS/FUV TIME-TA PSA G160M 1589 none none none none 2 1222.0 S G J104609.23+404707.0 5 COS/FUV TIME-TA PSA G160M 1589 none none none none 3 2786.0 S G 17198( 4) - 23-Jun-2023 02:05:00 - [ 5] Visit: 02 Visit Requirements: 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 J135340.78+ COS/NUV ACQ/IMA PSA MIRRORA 1 143 S (143 164920.9 GE S) ------------------------------------------------------------------------------------------------------------------------------------ 2 J135340.78+ COS/FUV TIME-TA PSA G160M 1623 FP-POS=1, BUFFER-TIME=713 1 600 S 164920.9 G see below (823 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 J135340.78+ COS/FUV TIME-TA PSA G160M 1623 FP-POS=2, BUFFER-TIME=822 1 600 S 164920.9 G see below (822 S) ------------------------------------------------------------------------------------------------------------------------------------ 4 J135340.78+ COS/FUV TIME-TA PSA G160M 1623 FP-POS=3, BUFFER-TIME=1071 1 1000 S 164920.9 G see below (1181 S) ------------------------------------------------------------------------------------------------------------------------------------ 5 J135340.78+ COS/FUV TIME-TA PSA G160M 1623 FP-POS=4, BUFFER-TIME=1181 1 1000 S 164920.9 G see below (1181 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 | ------------------------------------------------------------------------------------------------------------------------------------ J135340.78+164920.9 1 COS/NUV ACQ/IMA PSA MIRRORA none none none none 1 N/A GE J135340.78+164920.9 2 COS/FUV TIME-TA PSA G160M 1623 none none none none 1 823.0 S G J135340.78+164920.9 3 COS/FUV TIME-TA PSA G160M 1623 none none none none 1 822.0 S G J135340.78+164920.9 4 COS/FUV TIME-TA PSA G160M 1623 none none none none 2 1181.0 S G J135340.78+164920.9 5 COS/FUV TIME-TA PSA G160M 1623 none none none none 2 1181.0 S G 17198( 4) - 23-Jun-2023 02:05:00 - [ 6] Visit: 03 Visit Requirements: 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 J210455.39- COS/NUV ACQ/IMA PSA MIRRORA 1 148 S (148 003519.3 GE S) ------------------------------------------------------------------------------------------------------------------------------------ 2 J210455.39- COS/FUV TIME-TA PSA G160M 1533 FP-POS=1, BUFFER-TIME=697 1 600 S 003519.3 G see below (807 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 J210455.39- COS/FUV TIME-TA PSA G160M 1533 FP-POS=2, BUFFER-TIME=808 1 600 S 003519.3 G see below (808 S) ------------------------------------------------------------------------------------------------------------------------------------ 4 J210455.39- COS/FUV TIME-TA PSA G160M 1533 FP-POS=3, BUFFER-TIME=1064 1 600 S 003519.3 G see below (1174 S) ------------------------------------------------------------------------------------------------------------------------------------ 5 J210455.39- COS/FUV TIME-TA PSA G160M 1533 FP-POS=4, BUFFER-TIME=1174 1 600 S 003519.3 G see below (1174 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 | ------------------------------------------------------------------------------------------------------------------------------------ J210455.39-003519.3 1 COS/NUV ACQ/IMA PSA MIRRORA none none none none 1 N/A GE J210455.39-003519.3 2 COS/FUV TIME-TA PSA G160M 1533 none none none none 1 807.0 S G J210455.39-003519.3 3 COS/FUV TIME-TA PSA G160M 1533 none none none none 1 808.0 S G J210455.39-003519.3 4 COS/FUV TIME-TA PSA G160M 1533 none none none none 2 1174.0 S G J210455.39-003519.3 5 COS/FUV TIME-TA PSA G160M 1533 none none none none 2 1174.0 S G 17198( 4) - 23-Jun-2023 02:05:00 - [ 7] Visit: 52 Visit Requirements: 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 J135340.78+ COS/NUV ACQ/IMA PSA MIRRORA 1 143 S (143 164920.9 GE S) ------------------------------------------------------------------------------------------------------------------------------------ 2 J135340.78+ COS/FUV TIME-TA PSA G160M 1623 FP-POS=3, BUFFER-TIME=713 1 823 S (823 164920.9 G S) ------------------------------------------------------------------------------------------------------------------------------------ 3 J135340.78+ COS/FUV TIME-TA PSA G160M 1623 FP-POS=4, BUFFER-TIME=822 1 822 S (822 164920.9 G 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 | ------------------------------------------------------------------------------------------------------------------------------------ J135340.78+164920.9 1 COS/NUV ACQ/IMA PSA MIRRORA none none none none 1 N/A GE J135340.78+164920.9 2 COS/FUV TIME-TA PSA G160M 1623 none none none none 1 N/A G J135340.78+164920.9 3 COS/FUV TIME-TA PSA G160M 1623 none none none none 1 N/A G 17198( 4) - 23-Jun-2023 02:05:00 - [ 8] Summary Form for Proposal 17198 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures PSA ------------------------------------------------------------------------------------------------------------------------------------ Configurations COS/FUV, COS/NUV ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ/IMAGE, TIME-TAG ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters BUFFER-TIME=1064, BUFFER-TIME=1071, BUFFER-TIME=1112, BUFFER-TIME=1174, BUFFER-TIME=1181, BUFFER-TIME=1222, BUFFER-TIME=2014, BUFFER-TIME=2786, BUFFER-TIME=697, BUFFER-TIME=713, BUFFER-TIME=808, BUFFER-TIME=822, FP-POS=1, FP-POS=2, FP-POS=3, FP-POS=4 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements G160M, MIRRORA ------------------------------------------------------------------------------------------------------------------------------------ Target Names J104609.23+404707.0, J135340.78+164920.9, J210455.39-003519.3 ------------------------------------------------------------------------------------------------------------------------------------