6536( 7) - 09/09/97 16:16 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 6536 Version: 7 Check-in Date: 09-Sep-1997 16:15:53 1.Proposal Title: I Zw 18: BUILDING A MODEL ------------------------------------------------------------------------------------ 2. Scientific Category 3. Proposal For 4. Cycle GALAXIES & CLUSTERS GO 6 ------------------------------------------------------------------------------------ 5. Investigators Contact? PI: Evan Skillman UNIVERSITY OF MINNESOTA CoI: Donald Garnett University of Minnesota N CoI: Reginald Dufour Rice University N CoI: Robert Benjamin University of Minnesota N ------------------------------------------------------------------------------------ 6. Abstract I Zw 18 is host to the lowest metallicity H II region known, and, therefore, is pivotal in studies of the chemical evolution of galaxies, the primordial He abundance, and low metallicity stellar populations. Our previous HST observations of I Zw 18 have allowed us to establish a time sequence of the recent star formation history in I Zw 18, obtain an accurate C abundance, and delineate the stellar, neutral gas, and ionized gas distributions. Here we are proposing to add WFC2 narrow - band imaging at H Beta, O II Lambda3727, and S II Lambda6717,31 to combine with our cycle 4 narrow band imaging at H Alpha\ and O III Lambda5007 in order to map the reddening and ionization structure of the ionized gas and to search for enhanced collisional excitation due to shocks. These new observations will be invaluable to our goal of producing a detailed photoionization model of the high surface brightness knots of I Zw 18 to ensure the most reliable chemical abundance determinations possible. We also propose to obtain an FOS spectrum of the SE component in order to derive a C abundance measurement. We will compare this with the C/O and C/N abundance ratios observed in the NE component. The C abundance in I Zw 18 is key to determining whether there are trends of increasing C/O and C/N with increasing O/H in dwarf star forming galaxies. ------------------------------------------------------------------------------------ 6536( 7) - 09/09/97 16:16 - [ 2] Observations Description ------------------------ The observing will be split into 5 orbits of imaging and 3 orbits of spectroscopy. The integration times are based on our previous experience with HST imaging and spectroscopy of I Zw 18. The 4 orbits of imaging will be allocated as: (1) acquisition and HBeta imaging (F487N), (2) S II imaging (F673N), (3+4) O II imaging (F375N), and (5) repositioning and HAlpha imaging in the PC (F658N). Each observation will be split into 3 integrations for cosmic-ray rejection. While the O II lines are, on average, intrinsically 5 times stronger than the S II lines, the factor of 15 greater sensitivity of the F673N filter over the F375N necessitates a longer integration for in the O II line. The small redshift of I Zw 18 places the emission lines in the highest sensitivity regions of the response curves for all four filters. We will use our cycle 4 F439W, F555W, and F675W observations to construct appropriate images for continuum subtraction. We have found this to work very well for our HAlpha and O III images. The 3 orbits of FOS spectroscopy will be allocated as: (1) acquisition and integration in G400H and G570H, (2+3) integration in G190H. We hope to detect the O III Lambda1666 line in the G190H exposure (in addition to the C III Lambda1909 line), as this allows a nearly reddening and temperature independent determination of the C/O abundance ratio (see discussion in Garnett et al.\\ 1995a). However, if the Lambda1666 line is of insufficient s/n (it is very difficult to predict exactly as the UV continuum is a dominant contributor to the line measurement uncertainty), then we can still obtain an accurate C/O ratio (uncertainty less than 0.2 dex). We do this by measuring the reddening from the HAlpha/HBeta ratio, adopting the electron temperature from the ground-based observations of SK93, and measuring the Lambda1909/Lambda5007 ratio. Real Time Justification ----------------------- None None Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ Data Distribution Media: 8MM Blocking Factor: 10 Ship To: PI_Address Ship Via: UPS Email: ------------------------------------------------------------------------------------ 6536( 7) - 09/09/97 16:16 - [ 3] 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 02.04S +/- 0.05S, J2000 V = +759 V = 23.0 MAINBODY DEC=+55D 14' 28.01" +/- 0.1" TYPE=B0V E(B-V) = 0.1 Comments: This is a dwarf irregular galaxy Brightest stars are at V=23.0 2 IZW18-SEHIIR EXT-MEDIUM, HII Region RA-OFF = -3.9019S +/- 0.005S, J2000 V = +759 F-LINE(6563)=8.9+/-1e-15 DEC-OFF = -14.346" +/- 0.05", F-LINE(5007)=6.2+/-1e-15 FROM 3 Comments: This is a bright ionized knot in the SE of the dwarf irregular galaxy 3 OFFSET-STAR STAR, G V-IV RA=09H 34M 06.220S +/- 0.005S, J2000 V = 0 V = 15.4 DEC=+55D 14' 37.08" +/- 0.05" B-V = +0.6 TYPE=G2V Comments: This is a foreground star used for offset to the position of the bright ionized knot in the SE of the dwarf irregular galaxy for FOS spectroscopy 6536( 7) - 09/09/97 16:16 - [ 4] Visit: 01 Visit Priority: Visit Requirements: On Hold Comments: Additional Comments: We are specifying an orientation which will closely match the orientation of WFC imaging obtained in cycle 4. Thus, when we combine the imaging we will not have to use a large rotation, which will introduce loss of resolution. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 IZW18-MAINB WFPC2 IMAGE WF3-FIX F487N CR-SPLIT=NO 2 800 S ODY ------------------------------------------------------------------------------------------------------------------------------------ 2 IZW18-MAINB WFPC2 IMAGE WF3-FIX F487N CR-SPLIT=NO 1 900 S SAME POS AS 1 ODY Comments: Exposures 1 and 2 will be combined for CR-rejection. ------------------------------------------------------------------------------------------------------------------------------------ 3 IZW18-MAINB WFPC2 IMAGE WF3-FIX F673N CR-SPLIT=NO 2 900 S ODY ------------------------------------------------------------------------------------------------------------------------------------ 4 IZW18-MAINB WFPC2 IMAGE WF3-FIX F673N CR-SPLIT=NO 1 800 S SAME POS AS 3 ODY Comments: Exposures 3 and 4 will be combined for CR-rejection. ------------------------------------------------------------------------------------------------------------------------------------ 5 IZW18-MAINB WFPC2 IMAGE WF3-FIX F375N CR-SPLIT=NO 2 900 S ODY ------------------------------------------------------------------------------------------------------------------------------------ 6 IZW18-MAINB WFPC2 IMAGE WF3-FIX F375N CR-SPLIT=NO 1 800 S SAME POS AS 5 ODY Comments: Exposures 5 and 6 will be combined for CR-rejection. ------------------------------------------------------------------------------------------------------------------------------------ 7 IZW18-MAINB WFPC2 IMAGE WF3-FIX F375N CR-SPLIT=NO 2 900 S POS TARG 0.2, 0.2 ODY ------------------------------------------------------------------------------------------------------------------------------------ 8 IZW18-MAINB WFPC2 IMAGE WF3-FIX F375N CR-SPLIT=NO 1 800 S SAME POS AS 7 ODY Comments: Exposures 7 and 8 will be combined for CR-rejection. Exposures 7 and 8 are offset from exposures 5 and 6 by two integer pixels on the diagonal. This will allow us to construct a map of hot pixels. ------------------------------------------------------------------------------------------------------------------------------------ 6536( 7) - 09/09/97 16:16 - [ 5] Visit: 02 Visit Priority: Visit Requirements: ORIENTation 112D TO 152D On Hold Comments: Additional Comments: We are specifying an orientation which will closely match the orientation of PC imaging obtained in cycle 4. Thus, when we combine the imaging we will not have to use a large rotation, which will introduce loss of resolution. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 IZW18-MAINB WFPC2 IMAGE PC1-FIX F658N CR-SPLIT=NO 2 800 S ODY ------------------------------------------------------------------------------------------------------------------------------------ 2 IZW18-MAINB WFPC2 IMAGE PC1-FIX F658N CR-SPLIT=NO 1 900 S SAME POS AS 1 ODY Comments: Exposures 1 and 2 will be combined for CR rejection. ------------------------------------------------------------------------------------------------------------------------------------ 6536( 7) - 09/09/97 16:16 - [ 6] Visit: 03 Visit Priority: Visit Requirements: On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 OFFSET-STAR FOS/RD ACQ/BIN 4.3 MIRROR 1 4 S ONBOARD ACQuisition FOR ARY 2-6 Comments: This offset star is about 40 arcsec from the target. It has been used successfully in cycle 4 for offsetting to the target. ------------------------------------------------------------------------------------------------------------------------------------ 2 IZW18-SEHII FOS/RD ACQ 4.3 MIRROR 1 60 S R Comments: This is a confirmation exposure to show the precise position of the target within the aperture field. ------------------------------------------------------------------------------------------------------------------------------------ 3 IZW18-SEHII FOS/RD ACCUM 1.0 G400H 1 600 S R ------------------------------------------------------------------------------------------------------------------------------------ 4 IZW18-SEHII FOS/RD ACCUM 1.0 G570H 1 600 S EXPAND R Comments: The final integration can be expanded to fill the orbit. ------------------------------------------------------------------------------------------------------------------------------------ 5 IZW18-SEHII FOS/RD ACCUM 1.0 G190H 1 2540 S EXPAND R Comments: This integration can be expanded to fill the orbit. ------------------------------------------------------------------------------------------------------------------------------------ 6 IZW18-SEHII FOS/RD ACCUM 1.0 G190H 1 2540 S EXPAND R Comments: This integration can be expanded to fill the orbit. ------------------------------------------------------------------------------------------------------------------------------------ 6536( 7) - 09/09/97 16:16 - [ 7] Summary Form for Proposal 6536 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFPC2 FOS/RD ------------------------------------------------------------------------------------------------------------------------------------ Opmodes IMAGE ACQ/BINARY ACQ ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CR-SPLIT=NO ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Scientific Category GALAXIES & CLUSTERS ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements SAME POS AS 1 SAME POS AS 3 SAME POS AS 5 POS TARG 0.2, 0.2 SAME POS AS 7 ORIENTation 112D TO 152D ONBOARD ACQuisition FOR 2-6 EXPAND ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F487N F673N F375N F658N MIRROR G400H G570H G190H ------------------------------------------------------------------------------------------------------------------------------------ Target Names IZW18-MAINBODY OFFSET-STAR IZW18-SEHIIR ------------------------------------------------------------------------------------------------------------------------------------