7461( 8) - 11/14/97 15:31 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 7461 Version: 8 Check-in Date: 14-Nov-1997 15:30:12 1.Proposal Title: A Search For Old Stars in IZw18 ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 7 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: GOERAN OESTLIN ASTRONOMISKA OBSERVATORIET CoI: Nils Bergvall Astronomiska observatoriet N CoI: Jari Roennback Astronomiska observatoriet N CoI: Kjell Olofsson Astronomiska observatoriet N CoI: Ana HidalgoGamez Astronomiska observatoriet N ------------------------------------------------------------------------------------ 5. Abstract The blue compact dwarf galaxy IZw18 still stand out as one of the most interesting objects at low redshift. It's heavy element abundance is the lowest ever measured in the ionized gaseous component of a galaxy. It has therefore been considered as one of the best candidates for a genuinely young galaxy. Recent WFPC2 data has revealed a truly young stellar pupulation in the central part, with many resolved blue and red super giants. On the other hand there have been claims that its regular halo shows that it is likely to be old. The question if galaxies can form still at the present epoch is of cosmological interest, in view of theories of galaxy and structure formation. This proposal aims at determining if IZw18 is truly young or not, by investigating the presence of old stars. With deep NICMOS imaging in F110W and F160W combined with slitless grism spectroscopy (G096) it will be possible to identify carbon stars, and get age determinations. We will from the presence or absence of carbon stars be able get interesting constraints on the age of this galaxy. ------------------------------------------------------------------------------------ 7461( 8) - 11/14/97 15:31 - [ 2] Observations Description ------------------------ IZw18 is at a distance modulus of m-M=30. We want to reach down below the region were the degenreracy of old and young stars starts. This is when stars more massive than about 1.7 mix with the AGB, and this happens at about M_V=-3. Given all the uncertainties (see above) the only thing one could say from finding stars with these colors and luminosities is that there are stars at least 500 Myr old. If A luminous old or intermediate age, metal poor, red giant has an absolute V magnitude of M_V~-2.5, V-J~2.5 V-H~3.2 This means at the distance of IZw18 we expect this star to have apparent magnitude 25 in J and 24.2 in H. We want to achive a S/N of at least 10 in total for such a star in both F111W and F160W. Using the NICMOS exposure time calculator this requires 30 minutes in F110W (J) and 100 minutes in F160W (H). We will use camera2 to have an acceptable field of view and good pixel sampling. We will use option LOW\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\_SKY to lower the influence of the bright earth limb on the background. Both filters have central wavelengths below 1.6 microns, so it is not necessary to obtain background images. For this we will need 3 orbits, including overheads and taking into account that we have to use the LOW\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\_SKY option. For more luminous stars, like the expected C-stars which have M_V brighter than -3.5, the S/N will be considerably higher (>= 25). To identify C-stars we will obtain slitless grism (G096) spectra with CAMERA3 of the galaxy. C-stars stars have several strong CN and C_2 bands in the wavelength range of grism G096 (e.g. Ballick-Ramsay series 2.0, Phillips series 0.0 etc) these lines are strong enough that the band heads can be identified even at the low resolution offered by NICMOS spectroscopy. Calculations provided by Kjell Eriksson and Bengt Gustafsson (private communication) shows that these will be very pronounced even in carbon stars with low general abundances. With the proposed observations it will thus be possible to distinguish (young) red super giants from (relatively old) carbon stars. The luminosity of a carbon star also correlates with its mass (Mould and Aaronson), so by the photometry from CAMERA2 it will be possible to estimate their ages. For the grism spectroscopy part we also use the exposure time calculator and assume the following: A typical carbon star has an absolute V magnitude of ~-4.5 and V-J~4.5, so we expect m_J to be ~21. To reach a S/N of 10 per wavelength element in the continuum we thus need ~ 240 minutes of observing time. this S/N would then also allow somewhat fainter C-stars to be classified. We will also include short exposures (~2 minutes per roll angle) in F110W with the same camera (3) to facilate easy object identification. To further improve the interpretation of the spectra we follow the recomendation that these observations should be performed at two or more different roll angles. This because it is an extended object and some regions in IZw18 are quite crowded. In total, this part will need 6 orbits. We will identify molecular band heads, for which the low spectral resolution is still sufficient. Given secure indentifications, and the high S/N for the CAMERA2 imaging, we expect that the main uncertainties will not be dominated by photometric errors. Uncertainties that will effect our results come from: models of late AGB phases, metallicity effects, uncertainty in the distance modulus of IZw18 and possibly by problems with field crowding. Alltogether the proposed observations would require 9 orbits. Real Time Justification ----------------------- The text below is now obsolete since the grism observations have been dropped according to the recomendations from the time allocation comitte. We request that the NICMOS grism spectra should be taken at tow or more different telescope roll angles, as recomended in the NICMOS handbook. We therefore suggest that 3 different roll angles are used to obtain the spectra. Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 7461( 8) - 11/14/97 15:31 - [ 3] Data Distribution Media: DAT Blocking Factor: 10 Ship To: PI_Address Ship Via: Email: ------------------------------------------------------------------------------------ 7461( 8) - 11/14/97 15:31 - [ 4] 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=9H 34M 2.08S +/- J2000 V=+750 SURF(B) = 20.8 0.07S,DEC=55D 14' 26.64" +/- B-V = 0.3 1.0", PLATE-ID=0009 E(B-V) = 0 Comments: SURF(B) refers to galaxy (central parts), prime targets are stars expected to have V<27.5, (V-J)>2.5 2 PARALLEL- UNIDENTIFIED, BLANK RA=9H 34M 2.08S +/- 1',DEC=55D J2000 V=+750 SURF(V) = 23.2 TARGET-1 FIELD 14' 26.64" +/- 1', PLATE- ID=0009, REGION Comments: BLANK PIECE OF SKY. FLUX REFERS TO ESTIMATED SKY BACKROUND (NICMOS INSTRUMENT HANDBOOK TABLE 3.5) 3 PARALLEL- UNIDENTIFIED, BLANK RA=9H 34M 2.08S +/- 2',DEC=55D J2000 V=+750 SURF(V) = 23.2 TARGET-2 FIELD 14' 26.64" +/- 2', PLATE- ID=0009, REGION Comments: BLANK PIECE OF SKY. FLUX REFERS TO ESTIMATED SKY BACKROUND (NICMOS INSTRUMENT HANDBOOK TABLE 3.5) 7461( 8) - 11/14/97 15:31 - [ 5] Visit: 01 Visit Priority: Visit Requirements: ORIENT 225D TO 250D 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 IZW18 NIC2 MULTIAC NIC2 F110W SAMP-SEQ=STEP64,NSAMP=10 1 DEF POS TARG +0.0375,+0.0375 CUM Comments: SHORT 'JUNK' EXPOSURE ------------------------------------------------------------------------------------------------------------------------------------ 10 IZW18 NIC2 MULTIAC NIC2 F110W SAMP-SEQ=MIF512,NSAMP=25 1 DEF POS TARG +0.0375,+0.0375 CUM Comments: HALF PIXEL OFFSET EXPOSURE, TO BE USED TO IMPROVE SAMPLING IN CROWDED REGIONS ------------------------------------------------------------------------------------------------------------------------------------ 20 IZW18 NIC2 MULTIAC NIC2 F110W SAMP-SEQ=MIF1024,NSAMP=25, 1 DEF PAR 21-22 WITH 20 CUM PATTERN=YSTRIP-DITH,NUM-POS=2, DITH-SIZE=0.75 Comments: 10 INTEGER PIXEL OFFSET BETWEEN EXPOSURES ------------------------------------------------------------------------------------------------------------------------------------ 21 PARALLEL-TA NIC1 MULTIAC NIC1 F110W SAMP-SEQ=SPARS256,NSAMP=6 1 DEF RGET-1 CUM ------------------------------------------------------------------------------------------------------------------------------------ 22 PARALLEL-TA NIC3 MULTIAC NIC3 F110W SAMP-SEQ=SPARS256,NSAMP=6 1 DEF RGET-2 CUM ------------------------------------------------------------------------------------------------------------------------------------ 69 IZW18 NIC2 MULTIAC NIC2 F160W SAMP-SEQ=STEP256,NSAMP=11 1 DEF CUM Comments: SHORT EXPOSURE ------------------------------------------------------------------------------------------------------------------------------------ 70 IZW18 NIC2 MULTIAC NIC2 F160W SAMP-SEQ=MIF512,NSAMP=25 1 DEF SAME POS AS 69 CUM ------------------------------------------------------------------------------------------------------------------------------------ 80 IZW18 NIC2 MULTIAC NIC2 F160W SAMP-SEQ=MIF1024,NSAMP=25 2 DEF SAME POS AS 69 PAR 81-82 CUM WITH 80 ------------------------------------------------------------------------------------------------------------------------------------ 81 PARALLEL-TA NIC1 MULTIAC NIC1 F160W SAMP-SEQ=SPARS256,NSAMP=6 2 DEF RGET-1 CUM ------------------------------------------------------------------------------------------------------------------------------------ 82 PARALLEL-TA NIC3 MULTIAC NIC3 G096 SAMP-SEQ=SPARS256,NSAMP=6 2 DEF RGET-2 CUM ------------------------------------------------------------------------------------------------------------------------------------ 89 IZW18 NIC2 MULTIAC NIC2 F160W SAMP-SEQ=STEP256,NSAMP=11 1 DEF POS TARG +0.0,+0.75 CUM Comments: 10 INTEGER PIXEL FROM EXPOSURES 70 AND 80 SHORT EXPOSURE ------------------------------------------------------------------------------------------------------------------------------------ 90 IZW18 NIC2 MULTIAC NIC2 F160W SAMP-SEQ=MIF512,NSAMP=25 1 DEF SAME POS AS 89 CUM Comments: 10 INTEGER PIXEL FROM EXPOSURES 70 AND 80 ------------------------------------------------------------------------------------------------------------------------------------ 7461( 8) - 11/14/97 15:31 - [ 6] Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 100 IZW18 NIC2 MULTIAC NIC2 F160W SAMP-SEQ=MIF1024,NSAMP=25 2 DEF SAME POS AS 89 PAR CUM 101-102 WITH 100 ------------------------------------------------------------------------------------------------------------------------------------ 101 PARALLEL-TA NIC1 MULTIAC NIC1 F160W SAMP-SEQ=SPARS256,NSAMP=6 2 DEF RGET-1 CUM ------------------------------------------------------------------------------------------------------------------------------------ 102 PARALLEL-TA NIC3 MULTIAC NIC3 G096 SAMP-SEQ=SPARS256,NSAMP=6 2 DEF RGET-2 CUM ------------------------------------------------------------------------------------------------------------------------------------ 7461( 8) - 11/14/97 15:31 - [ 7] Summary Form for Proposal 7461 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations NIC2 NIC1 NIC3 ------------------------------------------------------------------------------------------------------------------------------------ Opmodes MULTIACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters SAMP-SEQ=STEP64 NSAMP=10 NSAMP=25 DITH-SIZE=0.75 NSAMP=6 SAMP-SEQ=SPARS256 NSAMP=11 SAMP-SEQ=MIF512 SAMP-SEQ=MIF1024 SAMP-SEQ=STEP256 ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ORIENT 225D TO 250D POS TARG +0.0375,+0.0375 PAR 21-22 WITH 20 SAME POS AS 69 SAME POS AS 69 PAR 81-82 WITH 80 POS TARG +0.0,+0.75 SAME POS AS 89 SAME POS AS 89 PAR 101-102 WITH 100 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F110W F160W G096 ------------------------------------------------------------------------------------------------------------------------------------ Target Names IZW18 PARALLEL-TARGET-1 PARALLEL-TARGET-2 ------------------------------------------------------------------------------------------------------------------------------------