7880( 4) - 01/13/98 08:44 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 7880 Version: 4 Check-in Date: 13-Jan-1998 08:42:00 1.Proposal Title: A Search For Old Stars in IZw18, Continued ------------------------------------------------------------------------------------ 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 HidalgoG. Astronomiska observatoriet N ------------------------------------------------------------------------------------ 5. Abstract The blue compact dwarf galaxy IZw18 still stands 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 population in the central part, with many resolved blue and red supergiants. 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. We have been already been allocated 3 orbits for deep NICMOS imaging in F110W and F160W in Cycle7. Now we propose to extend this study with narrowband photometry, designed to detect C_2 features typical for carbon stars. Thereby we can discrimate between old carbon stars and young red supergiants. We will from the presence or absence of carbon stars be able to get interesting constraints on the age of this galaxy. ------------------------------------------------------------------------------------ 7880( 4) - 01/13/98 08:44 - [ 2] Observations Description ------------------------ IZw18 is at a distance modulus of m-M=30. A carbon star has a typical absolute magnitude in excess of M_V=-4 and an effective temperature of 3500 K or lower. This corresponds to a flux of 0.8 MuJy, assuming a black body spectral energy distribution. We want to reach a signal to noise of at least 10 for a star with flux 1.25 MuJy. Thereby a high enough S/N (20) will be guaranteed for bright C- stars. We use the latest version of the NICMOS exposure time calculator and arrive at the following integration times. In F171M we will need 7000s, in F180M 11000s and in F205W 9000s, taking into account that observations have to be split into sevral short exposures to allow sky measurments by chopping every 5 to 10 min, depending on which filter is used (frequent chopping is necessary in F205W while chopping may perhaps not be necessary in F171W). Even if background exposures are necessary, these will only be long enough to guarantee a high signal to noise for the background subtraction. In total this then require 10 orbits including overheads. To identify C-stars we will use the strength of the F180M flux relative to the F171W flux scaled with the effective temperature. The F180M filter covers the Ballick-Ramsay C_2 Delta v=0 absorption band which is strong in C-stars. 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. Near infrared observations of C-stars (Goebel et al.) show that this spectral feature is very strong, amounting to ~50\\ Since the near infrared region of late type stars typically is crowded with broad absorption features, the spectral energy distribution will deviate from a black body spectrum The effective temperature will be determined from the allocated F110W and F160W observations combined with the F205W and F171M fluxes. The stellar athmopheres group in Uppsala is currently building a new improved grid of model atmospheres for late type stars, including an extensive list of molecules (Plez, Gustafsson, Eriksson, et al. in preparation). Models for late type stars of varying luminosity, mass and chemical composition will be computed, compared and fitted to the observations. Thereby we can quantatively asses the question of separating old and young red giants i the observational data. Futhermore, as discussed above, the luminosity of a carbon star correlates with its mass (Mould and Aaronson), and thus it will be possible to estimate their ages. Given secure indentifications, and the high S/N for the luminous stars, 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. Altogether the proposed observations would require 10 orbits. Real Time Justification ----------------------- Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 7880( 4) - 01/13/98 08:44 - [ 3] Data Distribution Media: DAT Blocking Factor: 10 Ship To: PI_Address Ship Via: Email: ------------------------------------------------------------------------------------ 7880( 4) - 01/13/98 08:44 - [ 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<26, (V-J)>2.5 7880( 4) - 01/13/98 08:44 - [ 5] TARGET LIST c)Generic Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Selection Criteria | Flux data No | Name | Description | | ------------------------------------------------------------------------------------------------------------------------------------ PARALLEL- UNIDENTIFIED, PARALLEL RA=9H 34M 2.08S +/- 1H,DEC=55D 14' SURF(V) = 23.2 TARGET-1 FIELD 26.64" +/- 1D Comments: R <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<< ANDOM BLANK FIELD, FLUX REFERS TO ESTIMATED SKY BACKROUND (NICMOS INSTRUMENT HANDBOOK TABLE 3.5) ------------------------------------------------------------------------------------------------------------------------------------ PARALLEL- UNIDENTIFIED, PARALLEL RA=9H 34M 2.08S +/- 1H,DEC=55D 14' SURF(V) = 23.2 TARGET-2 FIELD 26.64" +/- 1D Comments: R <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<< ANDOM BLANK FIELD, FLUX REFERS TO ESTIMATED SKY BACKROUND (NICMOS INSTRUMENT HANDBOOK TABLE 3.5) ------------------------------------------------------------------------------------------------------------------------------------ 7880( 4) - 01/13/98 08:44 - [ 6] Visit: 01 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 ------------------------------------------------------------------------------------------------------------------------------------ 10 IZW18 NIC2 ACCUM NIC2 F110W NREAD=9 1 131.826 ------------------------------------------------------------------------------------------------------------------------------------ 20 IZW18 NIC2 MULTIAC NIC2 F171M SAMP-SEQ=MIF512,NSAMP=25, 1 DEF PAR 21-22 WITH 20 CUM PATTERN=SPIRAL-DITH-CHOP, NUM-POS=5,DITH-SIZE=1.50, CHOP-SIZE=46.0,PATTERN-ORIENT=292 ------------------------------------------------------------------------------------------------------------------------------------ 21 PARALLEL-TA NIC1 MULTIAC NIC1 F110W SAMP-SEQ=SPARS256,NSAMP=4 1 DEF RGET-1 CUM ------------------------------------------------------------------------------------------------------------------------------------ 22 PARALLEL-TA NIC3 MULTIAC NIC3 F110W SAMP-SEQ=SPARS256,NSAMP=4 1 DEF RGET-2 CUM ------------------------------------------------------------------------------------------------------------------------------------ 30 IZW18 NIC2 ACCUM NIC2 F160W NREAD=9 1 190.546 ------------------------------------------------------------------------------------------------------------------------------------ 40 IZW18 NIC2 MULTIAC NIC2 F171M SAMP-SEQ=MIF512,NSAMP=25, 1 DEF PAR 41-42 WITH 40 CUM PATTERN=SPIRAL-DITH-CHOP, NUM-POS=5,DITH-SIZE=1.50, CHOP-SIZE=46.0,PATTERN-ORIENT=292 ------------------------------------------------------------------------------------------------------------------------------------ 41 PARALLEL-TA NIC1 MULTIAC NIC1 F110W SAMP-SEQ=SPARS256,NSAMP=4 1 DEF RGET-1 CUM ------------------------------------------------------------------------------------------------------------------------------------ 42 PARALLEL-TA NIC3 MULTIAC NIC3 F110W SAMP-SEQ=SPARS256,NSAMP=4 1 DEF RGET-2 CUM ------------------------------------------------------------------------------------------------------------------------------------ 50 IZW18 NIC2 ACCUM NIC2 F160W NREAD=9 1 190.546 ------------------------------------------------------------------------------------------------------------------------------------ 60 IZW18 NIC2 MULTIAC NIC2 F171M SAMP-SEQ=MIF512,NSAMP=25, 1 DEF PAR 61-62 WITH 60 CUM PATTERN=SPIRAL-DITH-CHOP, NUM-POS=5,DITH-SIZE=1.50, CHOP-SIZE=46.0,PATTERN-ORIENT=292 ------------------------------------------------------------------------------------------------------------------------------------ 61 PARALLEL-TA NIC1 MULTIAC NIC1 F160W SAMP-SEQ=SPARS256,NSAMP=4 1 DEF RGET-1 CUM ------------------------------------------------------------------------------------------------------------------------------------ 62 PARALLEL-TA NIC3 MULTIAC NIC3 F160W SAMP-SEQ=SPARS256,NSAMP=4 1 DEF RGET-2 CUM ------------------------------------------------------------------------------------------------------------------------------------ 100 IZW18 NIC2 ACCUM NIC2 F160W NREAD=9 1 190.546 ------------------------------------------------------------------------------------------------------------------------------------ 110 IZW18 NIC2 MULTIAC NIC2 F180M SAMP-SEQ=MIF512,NSAMP=25, 1 DEF PAR 111-112 WITH 110 CUM PATTERN=SPIRAL-DITH-CHOP, NUM-POS=5,DITH-SIZE=1.50, CHOP-SIZE=46.0,PATTERN-ORIENT=292 ------------------------------------------------------------------------------------------------------------------------------------ 7880( 4) - 01/13/98 08:44 - [ 7] Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 111 PARALLEL-TA NIC1 MULTIAC NIC1 F160W SAMP-SEQ=SPARS256,NSAMP=4 1 DEF RGET-1 CUM ------------------------------------------------------------------------------------------------------------------------------------ 112 PARALLEL-TA NIC3 MULTIAC NIC3 F160W SAMP-SEQ=SPARS256,NSAMP=4 1 DEF RGET-2 CUM ------------------------------------------------------------------------------------------------------------------------------------ 120 IZW18 NIC2 ACCUM NIC2 F212N NREAD=9 1 190.546 ------------------------------------------------------------------------------------------------------------------------------------ 130 IZW18 NIC2 MULTIAC NIC2 F180M SAMP-SEQ=MIF512,NSAMP=25, 1 DEF PAR 131-132 WITH 130 CUM PATTERN=SPIRAL-DITH-CHOP, NUM-POS=5,DITH-SIZE=1.50, CHOP-SIZE=46.0,PATTERN-ORIENT=292 ------------------------------------------------------------------------------------------------------------------------------------ 131 PARALLEL-TA NIC1 MULTIAC NIC1 F164N SAMP-SEQ=SPARS256,NSAMP=4 1 DEF RGET-1 CUM ------------------------------------------------------------------------------------------------------------------------------------ 132 PARALLEL-TA NIC3 MULTIAC NIC3 F164N SAMP-SEQ=SPARS256,NSAMP=4 1 DEF RGET-2 CUM ------------------------------------------------------------------------------------------------------------------------------------ 140 IZW18 NIC2 ACCUM NIC2 F212N NREAD=9 1 190.549 ------------------------------------------------------------------------------------------------------------------------------------ 150 IZW18 NIC2 MULTIAC NIC2 F180M SAMP-SEQ=MIF512,NSAMP=25, 1 DEF PAR 151-152 WITH 150 CUM PATTERN=SPIRAL-DITH-CHOP, NUM-POS=5,DITH-SIZE=1.50, CHOP-SIZE=46.0,PATTERN-ORIENT=292 ------------------------------------------------------------------------------------------------------------------------------------ 151 PARALLEL-TA NIC1 MULTIAC NIC1 F166N SAMP-SEQ=SPARS256,NSAMP=4 1 DEF RGET-1 CUM ------------------------------------------------------------------------------------------------------------------------------------ 152 PARALLEL-TA NIC3 MULTIAC NIC3 F166N SAMP-SEQ=SPARS256,NSAMP=4 1 DEF RGET-2 CUM ------------------------------------------------------------------------------------------------------------------------------------ 160 IZW18 NIC2 ACCUM NIC2 F212N NREAD=9 1 190.546 ------------------------------------------------------------------------------------------------------------------------------------ 170 IZW18 NIC2 MULTIAC NIC2 F180M SAMP-SEQ=MIF512,NSAMP=25, 1 DEF PAR 171-172 WITH 170 CUM PATTERN=SPIRAL-DITH-CHOP, NUM-POS=5,DITH-SIZE=1.50, CHOP-SIZE=46.0,PATTERN-ORIENT=292 ------------------------------------------------------------------------------------------------------------------------------------ 171 PARALLEL-TA NIC1 MULTIAC NIC1 F187N SAMP-SEQ=SPARS256,NSAMP=4 1 DEF RGET-1 CUM ------------------------------------------------------------------------------------------------------------------------------------ 172 PARALLEL-TA NIC3 MULTIAC NIC3 F187N SAMP-SEQ=SPARS256,NSAMP=4 1 DEF RGET-2 CUM ------------------------------------------------------------------------------------------------------------------------------------ 7880( 4) - 01/13/98 08:44 - [ 8] Visit: 02 Visit Priority: Visit Requirements: SAME ORIENT AS 1 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 ------------------------------------------------------------------------------------------------------------------------------------ 200 IZW18 NIC2 ACCUM NIC2 F110W NREAD=9 1 131.826 ------------------------------------------------------------------------------------------------------------------------------------ 210 IZW18 NIC2 MULTIAC NIC2 F205W SAMP-SEQ=MIF512,NSAMP=25, 1 DEF PAR 211-212 WITH 210 CUM PATTERN=SPIRAL-DITH-CHOP, NUM-POS=5,DITH-SIZE=1.50, CHOP-SIZE=46.0,PATTERN-ORIENT=292 ------------------------------------------------------------------------------------------------------------------------------------ 211 PARALLEL-TA NIC1 MULTIAC NIC1 F110W SAMP-SEQ=SPARS256,NSAMP=4 1 DEF RGET-1 CUM ------------------------------------------------------------------------------------------------------------------------------------ 212 PARALLEL-TA NIC3 MULTIAC NIC3 F110W SAMP-SEQ=SPARS256,NSAMP=4 1 DEF RGET-2 CUM ------------------------------------------------------------------------------------------------------------------------------------ 220 IZW18 NIC2 ACCUM NIC2 F187N NREAD=9 1 190.546 ------------------------------------------------------------------------------------------------------------------------------------ 230 IZW18 NIC2 MULTIAC NIC2 F205W SAMP-SEQ=MIF512,NSAMP=25, 1 DEF PAR 231-232 WITH 230 CUM PATTERN=SPIRAL-DITH-CHOP, NUM-POS=5,DITH-SIZE=1.50, CHOP-SIZE=46.0,PATTERN-ORIENT=292 ------------------------------------------------------------------------------------------------------------------------------------ 231 PARALLEL-TA NIC1 MULTIAC NIC1 F160W SAMP-SEQ=SPARS256,NSAMP=4 1 DEF RGET-1 CUM ------------------------------------------------------------------------------------------------------------------------------------ 232 PARALLEL-TA NIC3 MULTIAC NIC3 F160W SAMP-SEQ=SPARS256,NSAMP=4 1 DEF RGET-2 CUM ------------------------------------------------------------------------------------------------------------------------------------ 240 IZW18 NIC2 ACCUM NIC2 F187N NREAD=9 1 190.546 ------------------------------------------------------------------------------------------------------------------------------------ 250 IZW18 NIC2 MULTIAC NIC2 F205W SAMP-SEQ=MIF512,NSAMP=25, 1 DEF PAR 251-252 WITH 250 CUM PATTERN=SPIRAL-DITH-CHOP, NUM-POS=5,DITH-SIZE=1.50, CHOP-SIZE=46.0,PATTERN-ORIENT=292 ------------------------------------------------------------------------------------------------------------------------------------ 251 PARALLEL-TA NIC1 MULTIAC NIC1 F160W SAMP-SEQ=SPARS256,NSAMP=4 1 DEF RGET-1 CUM ------------------------------------------------------------------------------------------------------------------------------------ 252 PARALLEL-TA NIC3 MULTIAC NIC3 F160W SAMP-SEQ=SPARS256,NSAMP=4 1 DEF RGET-2 CUM ------------------------------------------------------------------------------------------------------------------------------------ 7880( 4) - 01/13/98 08:44 - [ 9] Summary Form for Proposal 7880 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations NIC2 NIC1 NIC3 ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM MULTIACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters NREAD=9 PATTERN-ORIENT=292 NSAMP=4 SAMP-SEQ=SPARS256 CHOP-SIZE=46.0 DITH-SIZE=1.50 NUM-POS=5 PATTERN=SPIRAL-DITH-CHOP NSAMP=25 SAMP-SEQ=MIF512 ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements PAR 21-22 WITH 20 PAR 41-42 WITH 40 PAR 61-62 WITH 60 PAR 111-112 WITH 110 PAR 131-132 WITH 130 PAR 151-152 WITH 150 PAR 171-172 WITH 170 SAME ORIENT AS 1 PAR 211-212 WITH 210 PAR 231-232 WITH 230 PAR 251-252 WITH 250 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F110W F171M F160W F180M F212N F164N F166N F187N F205W ------------------------------------------------------------------------------------------------------------------------------------ Target Names IZW18 PARALLEL-TARGET-1 PARALLEL-TARGET-2 ------------------------------------------------------------------------------------------------------------------------------------