6710( 12) - 08/28/96 17:42 - [ 0] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 6710 Version: 12 Check-in Date: 28-Aug-1996 18:39:53 1.Proposal Title: Abundances in the neutral medium of the blue compact galaxy IZw 18 ------------------------------------------------------------------------------------ 2. Scientific Category 3. Proposal For 4. Cycle GALAXIES & CLUSTERS GO 6 ------------------------------------------------------------------------------------ 5. Investigators Contact? PI: James Lequeux Observatoire de Paris CoI: David Bowen Royal Observatory, Edinburgh Y CoI: Daniel Kunth Institut d'Astrophysique N CoI: Miguel Mas-Hesse Laboratorio de Astrofisica Espacial y N Fisica Fundamental CoI: Max Pettini Royal Greenwich Observatory N ------------------------------------------------------------------------------------ 6. Abstract In a previous observation with the GHRS we obtained a very low value (1/1000 of solar) of the abundance of oxygen in the neutral medium surrounding the ionH2 regions of the blue compact galaxy IZw 18. This low value has profound consequences on the evolution of blue compact galaxies, on the mixing processes in such objects, and on the possible existence of the Population III. Unfortunately, this abundance determination is of very low accuracy due to the saturation of the ionO1 line. We now propose using the GHRS with the G140M grating at full resolution to observe ionS2 absorption lines at 1251, 1254 and 1260 Angstrom\ arising from the gas in front of the ionizing stars of the ionH2 region. The relatively low column density of ionS2, the broad range of oscillator strengths in the three lines, and the good wavelength-resolving power of G140M, will allow us to obtain an accurate value of the column density of ionS2 and of the velocity dispersion. These measurements will enable us to directly test whether the metallicity of the absorbing gas is as low as previously deduced. The sightline toward IZw 18 also passes through a Milky Way High Velocity H I Cloud (HVC); the data we propose to obtain will enable us to determine a limit to the metallicity of the gas in this HVC, a measurement not yet made for any of these types of clouds. ------------------------------------------------------------------------------------ Observations Description ------------------------ We propose to accumulate GHRS spectra obtained with the LSA and with G140M centered at 1250 Angstrom . Exposure times are estimated from the known UV flux, F_lambda(1250)\\:~eq\\:1.3*10^-14 ergs cm^-2 s^-1 Angstrom ^-1, which gives a count rate of 22 counts diode^-1 hr^-1. For half-stepped data, assuming a background count rate of 26 counts diode^-1 hr^-1, a S/N of 7.9 is reached in 15 hrs (18 orbits), which is necessary to differentiate between gas with metallicities of more or less than 1/400 Z_\\odot. Simulated data with this S/N are presented in Fig. 1. A 2Sigma equivalent width limit of 16 mAngstrom\\ is obtained. Not only will this limit enable us to detect low metallicty gas from IZw 18, but will all also provide extremely sensitive limits to absorption from the Milky Way HVC which the sightline intercepts. Real Time Justification ----------------------- None None needed Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ Data Distribution Media: 8MM Blocking Factor: 10 Ship To: *Dr. David V. Bowen*Royal Observatory, Edinburgh (R.O.E.)*Blackford Hill*Edinburgh EH9 3HJ*United Kingdom Ship Via: UPS Email: dvb@roe.ac.uk , ------------------------------------------------------------------------------------ 6710( 12) - 08/28/96 17:42 - [ 1] TARGET LIST a) Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 IZW18 Galaxy, Dwarf Compact RA=09H 34M 2.03S +/- 0.005S, 2000.0 v=740 V = 15.7 DEC=55D 14' 27.4" +/-0.05", F-CONT(1250)=1.3e-14 PLATE-ID=0009 Comments: RA & DEC are those given in Phase 2 of Cycle 2 observations 6710( 12) - 08/28/96 17:42 - [ 2] Visit: 01 Visit Requirements: On Hold Comments: Additional Comments: FIRST visit Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 10 IZW18 HRS ACQ 2.0 MIRROR-N2 bright=return,search-size=3 1 90 S ONBOARD ACQ FOR 20-70 Comments: step-time=10s ------------------------------------------------------------------------------------------------------------------------------------ 20 WAVE HRS ACCUM SC2 G160M 1254 1 DEF Comments: Wave calib for 30 ------------------------------------------------------------------------------------------------------------------------------------ 30 IZW18 HRS ACCUM 2.0 G160M 1254 fp-split=no,step-patt=5,spybal=no 15 435.2 S Comments: FIRST set ------------------------------------------------------------------------------------------------------------------------------------ 40 WAVE HRS ACCUM SC2 G160M 1254 1 DEF Comments: Wave calib for 50; really to set SPYBAL again ------------------------------------------------------------------------------------------------------------------------------------ 50 IZW18 HRS ACCUM 2.0 G160M 1254 fp-split=no,step-patt=5,spybal=no 15 435.2 S Comments: SECOND set ------------------------------------------------------------------------------------------------------------------------------------ 60 WAVE HRS ACCUM SC2 G160M 1254 1 DEF Comments: Wave calib for 70 ------------------------------------------------------------------------------------------------------------------------------------ 70 IZW18 HRS ACCUM 2.0 G160M 1254 fp-split=no,step-patt=5,spybal=no 19 435.2 S Comments: THIRD set ------------------------------------------------------------------------------------------------------------------------------------ 6710( 12) - 08/28/96 17:42 - [ 3] Visit: 02 Visit Requirements: On Hold Comments: Additional Comments: SECOND visit Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 80 IZW18 HRS ACQ 2.0 MIRROR-N2 bright=return,search-size=3 1 90 S ONBOARD ACQ FOR 90-140 Comments: step-time=10s ------------------------------------------------------------------------------------------------------------------------------------ 90 WAVE HRS ACCUM SC2 G160M 1254 1 DEF Comments: Wave calib for 100 ------------------------------------------------------------------------------------------------------------------------------------ 100 IZW18 HRS ACCUM 2.0 G160M 1254 fp-split=no,step-patt=5,spybal=no 15 435.2 S Comments: FIRST set ------------------------------------------------------------------------------------------------------------------------------------ 110 WAVE HRS ACCUM SC2 G160M 1254 1 DEF Comments: Wave calib for 120; really to set SPYBAL again ------------------------------------------------------------------------------------------------------------------------------------ 120 IZW18 HRS ACCUM 2.0 G160M 1254 fp-split=no,step-patt=5,spybal=no 15 435.2 S Comments: SECOND set ------------------------------------------------------------------------------------------------------------------------------------ 130 WAVE HRS ACCUM SC2 G160M 1254 1 DEF Comments: Wave calib for 140 ------------------------------------------------------------------------------------------------------------------------------------ 140 IZW18 HRS ACCUM 2.0 G160M 1254 fp-split=no,step-patt=5,spybal=no 19 435.2 S Comments: THIRD set ------------------------------------------------------------------------------------------------------------------------------------ 6710( 12) - 08/28/96 17:42 - [ 4] Visit: 03 Visit Requirements: On Hold Comments: Additional Comments: THIRD visit; would prefer this short visit to be combined with previous two if possible Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 150 IZW18 HRS ACQ 2.0 MIRROR-N2 bright=return,search-size=3 1 90 S ONBOARD ACQ FOR 160-170 Comments: step-time=10s ------------------------------------------------------------------------------------------------------------------------------------ 160 WAVE HRS ACCUM SC2 G160M 1254 1 DEF Comments: Wave calib for 170 ------------------------------------------------------------------------------------------------------------------------------------ 170 IZW18 HRS ACCUM 2.0 G160M 1254 fp-split=no,step-patt=5,spybal=no 8 435.2 S Comments: Last set ------------------------------------------------------------------------------------------------------------------------------------ 6710( 12) - 08/28/96 17:42 - [ 5] Summary Form for Proposal 6710 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations HRS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters bright=return search-size=3 spybal=no step-patt=5 fp-split=no ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Scientific Category GALAXIES & CLUSTERS ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ONBOARD ACQ FOR 20-70 ONBOARD ACQ FOR 90-140 ONBOARD ACQ FOR 160-170 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements MIRROR-N2 G160M ------------------------------------------------------------------------------------------------------------------------------------ Target Names IZW18 WAVE ------------------------------------------------------------------------------------------------------------------------------------