7506( 8) - 07/01/99 09:07 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 7506 Version: 8 Check-in Date: 01-Jul-1999 09:06:39 1.Proposal Title: NGC 1705: a Benchmark for Galaxy Evolution ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 7 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Monica Tosi Osservatorio Astronomico di Bologna CoI: Alessandra Aloisi Universita di Bologna N CoI: Mark Clampin Space Telescope Science Institute N CoI: Laura Greggio Universita di Bologna N CoI: Claus Leitherer Space Telescope Science Institute N CoI: Antonella Nota Space Telescope Science Institute N ------------------------------------------------------------------------------------ 5. Abstract We propose to perform a case study of the star formation history and chemical evolution of NGC1705, an irregular galaxy with distance modulus ~28, whose stellar content cannot be resolved from ground. For this galaxy there is direct observational evidence for the presence of a galactic wind, a phenomenon predicted by most chemical evolution models for this type of galaxy. Its star formation history will be derived by using two complementary methods: the comparison of CMD's based on WFPC2 images, with theoretical simulations for those regions of the galaxy resolved into stars, and the analysis of the UV spectra of the stellar supercluster with corresponding theoretical models. These spectra have already been taken with GHRS and are available from the archive. The derived star formation rate and IMF will be used to compute: (i) the energy input into the ISM, to be compared with the energetics of the superwinds; (ii) the chemical enrichment of the galaxy; and (iii) the mass in heavy elements lost from the galaxy. The status of NGC 1705 will be compared to the similar system NGC 1569, currently being investigated by us, and to other nearby irregulars. ------------------------------------------------------------------------------------ 7506( 8) - 07/01/99 09:07 - [ 2] Observations Description ------------------------ We require look-back times of about 500\ Myr, the evolutionary lifetime of stars which go through the AGB phase. AGB stars are important contributors to the total mass return to the ISM. The main-sequence mass corresponding to t ~ 500\ Myr is about 3\ M_\odot, which translates into M_V ~0. It is important to emphasize that the reliability of the SF rate and IMF critically depend on the accuracy of the data, so that a photometric error <= 0.2 mag is required. Ground based observations restrict the sample of galaxies resolvable with this required accuracy and limiting magnitude to systems in the Local Group, with distance moduli smaller than ~25. NGC1705 is at a distance of V_degrees + A_V - M_V ~ 28.6. Therefore, HST is necessary., We selected four WFPC2 standard passbands which are optimized for measurements of cool and hot stars: F814W, F555W, F450W, and F380W. A limiting magnitude of 28 is required for the longer wavelengths but 26 is sufficient at the shortest wavelength where the hottest (brighter) stars are observed. The choice of filters is a trade-off between exposure times (which become prohibitively long in the U and B bands), contamination by nebular emission, their suitability for constraining the stellar population (e.g. by separating age and extinction effects), and red leak. In particular, the choice of F380W and F450W were made in consultation with WFPC2 instrument scientists at STSCI for a previous program in which we observed NGC1569. These filters provide optimal throughput, and better red-leak rejection than the standard WFPC2 photometric set. For instance, F336W exhibits a significant redleak and is unsuitable for our program. , In order to demonstrate the need for the requested exposure time we will consider the case of a B1 star with a V magnitude of 28 (except in the I band observations, where we have calculated the exposure times assuming a K spectral type). All predicted exposure times have been calculated using the WWW WFPC2 exposure time calculator. The V=28, B1 star yields a signal to noise of 3.9 for a single 2500 second exposure with the F555W filter. The observation has been simulated at the RA and DEC of NGC 1705 to ensure the appropriate value for the sky background is used. A total of 6x2500 second exposures, therefore, yields an effective signal to noise of 9, at the center of a PC pixel. At the corner of a PC pixel this signal to noise degrades to 8, which is still sufficient to meet our required goal of a 0.2 mag photometric error. In addition to the V band (F555W) observations, we also require U (F380W), B (F450W) and I (F814W) band observations to limiting V magnitudes of 26, 28 and 28, respectively. For each filter, 3 pairs of observations will be made at 3 dither points, each with a signal to noise of ~3, plus one short exposure to obtain unsaturated images of brighter stars in the field. This strategy will allow us to: 1) remove cosmic rays from each pair of images and correct for residuals resulting from this process, using the whole dataset, 2) identify hot pixels from three pairs of images obtained at different dither points, 3) smooth local pixel to pixel variations from the images obtained at different dither points, 4) improve the sampling of the images, 5) improve background estimation. In the following table we show our estimated times to achieve the required S/N for the deep imaging observations. With the assumption of an orbit lasting 55 minutes, it is clear from this table that we require 20 orbits, including overhead, in order to meet our goal of limiting I, V and B band magnitudes of 28 and a limiting U magnitude of 26. The times is broken into 6 orbits each for the B, V and I observations and 3 orbits for the U band observations. begintable begintabular c c c c multicolumn1 cFilter & multicolumn1 c S/N (brightest pixel) & multicolumn1 c Limiting Magnitude & multicolumn1 c Exposures , U (F380W) & 8 & 26 & 6x600 , B (F450W) & 8 & 28 & 6x2500 , V (F555W) & 9 & 28 & 6x2500 , I (F814W) & 8 & 28 & 6x2500 , endtabular endtable The proposed observing strategy has been carefully designed to address knowm problems in doing photometry with WFPC2 (Whitmore 1995). We believe the results of WFPC2 deep photometric studies now available, including the observations of NGC 1569 recently made by us, have demonstrated that these limiting magnitudes can be achieved, and that the observing strategy is valid. Real Time Justification ----------------------- No special requirements are needed GHRS spectroscopy of NGC 1705 is already available from the archive, from the carry-over version of proposal 3591 which has been completed during Cycle 4. These spectra will be retrieved and used by us to determine the SF history of the NGC 1705 SSC. Calibration Justification ------------------------- Real_Time_Justification Additional Comments ------------------- ------------------------------------------------------------------------------------ 7506( 8) - 07/01/99 09:07 - [ 3] Data Distribution Paper Products: Media: DAT Blocking Factor: 10 Ship To: PI_Address Ship Via: Email: ------------------------------------------------------------------------------------ 7506( 8) - 07/01/99 09:07 - [ 4] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC1705 EXT-CLUSTER, OPEN RA=4H 54M 13.3261S +/- 0.1S, 2000 F(1500)=1.0e-13 CLUSTER DEC=-53D 21' 38.374" +/- 1.0", PLATE-ID=061F Comments: coordinates from WF/PC1 image w0ou0504t ID 2389 PI O'Connell Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 1986.971 0.0 0.0 0.0 7506( 8) - 07/01/99 09:07 - [ 5] Visit: 01 Visit Priority: Visit Requirements: ORIENT 80D to 100D GROUP 1-4 WITHIN 10D 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 NGC1705 WFPC2 IMAGE PC1 F555W CR-SPLIT=0.5,CR-TOLERANCE=0.0, 1 2700 S Dither-type=box ------------------------------------------------------------------------------------------------------------------------------------ 7506( 8) - 07/01/99 09:07 - [ 6] 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 ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC1705 WFPC2 IMAGE PC1 F814W CR-SPLIT=0.5,CR-TOLERANCE=0.0, 1 2750 S Dither-type=box ------------------------------------------------------------------------------------------------------------------------------------ 7506( 8) - 07/01/99 09:07 - [ 7] Visit: 03 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 ------------------------------------------------------------------------------------------------------------------------------------ 3 NGC1705 WFPC2 IMAGE PC1 F439W CR-SPLIT=0.5,Dither-type=box 1 5800 S ------------------------------------------------------------------------------------------------------------------------------------ 7506( 8) - 07/01/99 09:07 - [ 8] Visit: 04 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 ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC1705 WFPC2 IMAGE PC1 F380W CR-SPLIT=0.5,CR-TOLERANCE=0.0 1 1000 S POS TARG 0.0, 0.0 ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC1705 WFPC2 IMAGE PC1 F380W CR-SPLIT=0.5,CR-TOLERANCE=0.0 1 1000 S POS TARG 0.25, 0.25 ------------------------------------------------------------------------------------------------------------------------------------ 3 NGC1705 WFPC2 IMAGE PC1 F380W 1 600 S POS TARG 0.0, 0.0 ------------------------------------------------------------------------------------------------------------------------------------ 4 NGC1705 WFPC2 IMAGE PC1 F380W 1 600 S POS TARG 0.25, 0.25 ------------------------------------------------------------------------------------------------------------------------------------ 5 NGC1705 WFPC2 IMAGE PC1 F555W 1 80 S POS TARG 0.0, 0.0 ------------------------------------------------------------------------------------------------------------------------------------ 6 NGC1705 WFPC2 IMAGE PC1 F439W 1 140 S ------------------------------------------------------------------------------------------------------------------------------------ 7 NGC1705 WFPC2 IMAGE PC1 F814W 1 140 S ------------------------------------------------------------------------------------------------------------------------------------ 8 NGC1705 WFPC2 IMAGE PC1 F380W 1 140 S ------------------------------------------------------------------------------------------------------------------------------------ 7506( 8) - 07/01/99 09:07 - [ 9] Visit: 53 Visit Priority: Visit Requirements: ORIENT 340D to 20D 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 ------------------------------------------------------------------------------------------------------------------------------------ 3 NGC1705 WFPC2 IMAGE PC1 F439W CR-SPLIT=0.5,Dither-type=box 1 5800 S ------------------------------------------------------------------------------------------------------------------------------------ 7506( 8) - 07/01/99 09:07 - [ 10] Visit: 54 Visit Priority: Visit Requirements: SAME ORIENT AS 53 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 NGC1705 WFPC2 IMAGE PC1 F380W CR-SPLIT=0.5,CR-TOLERANCE=0.0 1 1000 S POS TARG 0.0, 0.0 ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC1705 WFPC2 IMAGE PC1 F380W CR-SPLIT=0.5,CR-TOLERANCE=0.0 1 1000 S POS TARG 0.25, 0.25 ------------------------------------------------------------------------------------------------------------------------------------ 3 NGC1705 WFPC2 IMAGE PC1 F380W 1 600 S POS TARG 0.0, 0.0 ------------------------------------------------------------------------------------------------------------------------------------ 4 NGC1705 WFPC2 IMAGE PC1 F380W 1 600 S POS TARG 0.25, 0.25 ------------------------------------------------------------------------------------------------------------------------------------ 5 NGC1705 WFPC2 IMAGE PC1 F555W 1 80 S POS TARG 0.0, 0.0 ------------------------------------------------------------------------------------------------------------------------------------ 6 NGC1705 WFPC2 IMAGE PC1 F439W 1 140 S ------------------------------------------------------------------------------------------------------------------------------------ 7 NGC1705 WFPC2 IMAGE PC1 F814W 1 140 S ------------------------------------------------------------------------------------------------------------------------------------ 8 NGC1705 WFPC2 IMAGE PC1 F380W 1 140 S ------------------------------------------------------------------------------------------------------------------------------------ 7506( 8) - 07/01/99 09:07 - [ 11] Summary Form for Proposal 7506 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures PC1 ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFPC2 ------------------------------------------------------------------------------------------------------------------------------------ Opmodes IMAGE ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CR-SPLIT=0.5 CR-TOLERANCE=0.0 Dither-type=box ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ORIENT 80D to 100D GROUP 1-4 WITHIN 10D SAME ORIENT AS 1 POS TARG 0.0, 0.0 POS TARG 0.25, 0.25 ORIENT 340D to 20D SAME ORIENT AS 53 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F555W F814W F439W F380W ------------------------------------------------------------------------------------------------------------------------------------ Target Names NGC1705 ------------------------------------------------------------------------------------------------------------------------------------