! File: 2078C.PROP ! Database: PEPDB ! Date: 17-FEB-1994:00:26:13 coverpage: title_1: A SEARCH FOR PRIMORDIAL GAS; IS IZW18 A YOUNG GALAXY ? sci_cat: GALAXIES & CLUSTERS sci_subcat: GAS & DUST proposal_for: GO pi_fname: JAMES pi_lname: LEQUEUX pi_inst: MEUDON OBSERVATORY pi_country: FRANCE pi_phone: (33) 1 45 07 06 30 keywords_1: DWARF GALAXY, ABUNDANCE, UV SPECTROSCOPY. hours_pri: 6.00 num_pri: 1 hrs: X pi_position: DOCTOR off_fname: JAMES off_lname: LEQUEUX off_inst: MEUDON OBSERVATORY off_addr_1: PLACE JANSSEN off_addr_3: 92195 MEUDON CEDEX off_city: MEUDON off_country: FRANCE off_phone: (33) 1 45 07 06 30 off_telex: (42) 270 912 F ! end of coverpage abstract: line_1: Amongst blue compact galaxies, IZw 18 has the lowest heavy-element line_2: abundances in its HII regions and is by far the best candidate for a young line_3: galaxy experiencing its first star formation. If this is the case, its line_4: HII-region heavy elements may have been produced by the present burst of star line_5: formation and the surrounding neutral gas may be primeval, without heavy line_6: elements. We aim at checking this possibility by obtaining upper limits or line_7: measuring the abundance of neutral oxygen in this gas using the strong OI line line_8: at 1302A in absorption in front of the star cluster that ionizes the HII line_9: region. ! ! end of abstract general_form_proposers: lname: LEQUEUX fname: JAMES title: P.I. inst: MEUDON OBSERVATORY country: FRANCE esa: X ! lname: SARGENT fname: W. inst: CALTECH country: USA ! lname: KUNTH fname: DANIEL inst: INSTITUT D'ASTROPHYSIQUE, PARIS country: FRANCE esa: X ! lname: VIALLEFOND fname: F. inst: PARIS OBSERVATORY country: FRANCE esa: X ! ! end of general_form_proposers block general_form_text: question: 3 section: 1 line_1: The expected lines in the 1302 A region of the spectrum of IZw 18 line_2: are: line_3: OI galactic 1302.169 A narrow EW LE 200 mA line_4: SiII galactic 1304.372 A narrow EW LE 130 mA line_5: OI IZw18 (possible) 1305.49 A width LE 500 mA EW = ? line_6: SiII IZw 18 (possible) 1307.70 A width LE 500 mA EW = ? line_8: No OI* and OI** lines at 1304.858 and 1306.029 A are expected. The widths quoted line_9: for the IZw 18 lines are generous upper limits. Assuming a width of 100 km/s line_10: based on 21-cm observations we estimate that the OI line would be as deep as line_11: half the continuum level for a OI column density of 3 E14 cm-2. With a S/N of 30 line_12: we may very safely measure N(OI) = E14 cm-2. If the OI behind the ionization line_13: front is negligible (see previous section) and if the stellar OI lines are also line_14: negligible (they should be very faint in any case) we could then easily detect line_15: O/H = 2 E-8, almost 1000 times less than O/H in the HII region and 30 000 times line_16: less than the solar abundance. If the other sources of OI absorption are not line_17: negligible we will obtain upper limits. line_19: The ionizing star cluster emits 2 E-14 erg s-1 cm-2 A-1 in the far-UV from IUE line_20: observations, almost independent on wavelength. Its dimensions are unknown line_21: but <3" thus the LSA of the GHRS (2x2") should contain most of it while line_22: probably little would go through the SSA. However observing with the LSA line_23: degades the resolution by a factor 8. This forces us to use the G160M grating ! question: 3 section: 2 line_1: at an effective resolution of 20 000/8 = 2 500 corresponding to 0.5 A, in order line_2: to secure a good separation of the lines. Binning the diodes off-line in the line_3: useful portions of the lines will increase the S/N ratio, hence an effective line_4: S/N ratio of 24. The same resolution is required to separate the Lyman alpha line_5: absorption line from the geocoronal line, but here we do not require such a line_6: high S/N ratio and an exposure time of about 160m is sufficient. ! question: 4 section: 1 line_1: I Zw 18 has been observed with IUE at low resolution. The spectrum line_2: is too noisy for the detection of even the galactic OI line and the Lyman alpha line_3: line is buried in the geocoronal line. IUE cannot do any better and it is clear line_4: that only the HST can obtain spectra of sufficient quality for our purpose. ! question: 7 section: 1 line_1: The data reduction is relatively straightforward and can be done line_2: in any of the participating institutes which have adequate facilities based line_3: on VAX computers. ! question: 8 section: 1 line_1: The target is extended but <3 arc seconds. We believe that onboard line_2: acquisition will be sufficient, however. Far-UV imaging observations of the line_3: target will perhaps be made in GTO program 1246 FOC. If these observations are line_4: scheduled well in advance of the present program we require to have access line_5: to them before our run in order to refine acquisition if necessary. ! question: 10 section: 1 line_1: Data analysis capabilities are available at all institutes with line_2: no access restrictions. ! !end of general form text general_form_address: lname: LEQUEUX fname: JAMES title: DR. category: PI inst: OBSERVATOIRE DE MEUDON addr_1: PLACE JANSSEN addr_2: 92195 MEUDON CEDEX city: MEUDON country: FRANCE phone: (33) 1 45 07 06 30 telex: (42) 270 912 F ! ! end of general_form_address records fixed_targets: targnum: 1 name_1: IZW18 descr_1: GALAXY;TYPE=BLUE COMPACT GALAXY pos_1: RA = 09H 34M 2.033S +/- 0.005S, pos_2: DEC = +55D 14' 27.42" +/- 0.05", pos_3: PLATE-ID=0009 equinox: 2000 rv_or_z: V=765 acqpr_1: EXT fluxnum_1: 1 fluxval_1: F-CONT(1300) = 2.0 E-14 fluxnum_2: 2 fluxval_2: F-CONT(1216) = 2.3 E-14 ! ! end of fixed targets ! No solar system records found ! No generic target records found exposure_logsheet: linenum: 1.000 targname: IZW18 config: HRS opmode: ACQ aperture: 2.0 sp_element: MIRROR-N2 num_exp: 1 time_per_exp: 4M s_to_n: 30 fluxnum_1: 1 priority: 1 param_1: SEARCH-SIZE=5, FAINT=300.0, param_2: MAP=DEF, param_3: LOCATE=YES req_1: ONBOARD ACQ FOR 2-3; DARK TIME; req_2: CYCLE 1 / 1-3 ! linenum: 2.000 targname: IZW18 config: HRS opmode: ACCUM aperture: 2.0 sp_element: G160M wavelength: 1304 num_exp: 8 time_per_exp: 22M s_to_n: 3.0 fluxnum_1: 1 priority: 1 param_1: FP-SPLIT=DSFOUR req_1: DARK TIME ! linenum: 3.000 targname: IZW18 config: HRS opmode: ACCUM aperture: 2.0 sp_element: G160M wavelength: 1216 num_exp: 8 time_per_exp: 19.5M s_to_n: 2.0 fluxnum_1: 2 priority: 1 param_1: FP-SPLIT=DSFOUR req_1: DARK TIME ! ! end of exposure logsheet ! No scan data records found