6507( 6) - 01/31/96 14:08 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 6507 Version: 6 Check-in Date: 31-Jan-1996 14:01:29 1.Proposal Title: NGC 3664 - a case study of shock-induced propagation of star formation ------------------------------------------------------------------------------------ 2. Scientific Category 3. Proposal For 4. Cycle STELLAR POPULATIONS GO 6 ------------------------------------------------------------------------------------ 5. Investigators Contact? PI: Sylvia M. Baggett Space Telescope Science Institute CoI: Michael Dahlem Space Telescope Science Institute Y ------------------------------------------------------------------------------------ 6. Abstract Star formation (SF) in galaxies can propagate in several ways. Spiral density waves and self-propagating SF are currently believed to be the two viable mechanisms enabling the propagation of SF in normal spiral galaxies. In the absence of density waves, in dwarf and irregular galaxies like NGC 3664, self-propagating SF appears to be the dominant mechanism. Investigations of the LMC and other Magellanic irregulars show a tendency of SF occurring preferably along the rims of giant gas shells. For NGC 3664, this is suggested by our ground- based HAlpha and R-band images which show the presence of HII regions and diffuse ionized gas on the rims of several very well -defined large-scale (>1 kpc) loops and shells. These shells might be giant old superbubbles from an earlier phase of SF. The gas is probably heated by a second, younger population of massive hot stars which formed when the shockwaves of the former phase of SF expanded into the ambient ISM. HST WFPC2 photometry will allow us to determine the ages of the stellar populations inside and on the rims of the supershells by measuring their colors and comparing them with stellar population synthesis models. From the color gradients we will derive the propagation velocity of SF, which will be compared with the expansion velocity of the gas in the shells. At the distance of NGC 3664 (24.4 Mpc) single stellar clusters are resolved by the WFPC2, which will lead to a much higher accuracy of our data than can be achieved from the ground. ------------------------------------------------------------------------------------ 6507( 6) - 01/31/96 14:08 - [ 2] Observations Description ------------------------ We propose to obtain broad-band U, B, V, and I images of NGC 3664 in order to search for gradients in the ages of stellar clusters. The images will be sensitive to stellar populations younger than 10^7 yrs (UBV) as well as older, redder stars, from the V--I color. An R band image will not be needed, because it would not provide any additional information. For the calculation of the required integration times we used the procedures described in the WFPC2 instrument handbook and the total photometric magnitudes and colors of (clusters of) early-type stars listed in the literature. Given the integral colors of NGC 3664 mentioned above, its emission appears to be dominated by hot young stars. The type reproducing these colors best is somewhere near B0. Therefore, we take B0 stars as templates for the calculation of expected apparent magnitudes of stellar clusters in the U, B, and V filters, assuming that the radiation of a cluster will be dominated by such supergiants. For an absolute V magnitude of a B0I star of --7.0 and a distance modulus of 31.94 for NGC 3664 we calculate that we must reach a V magnitude of 24.9 in order to detect such clusters at the 10Sigma level. In order to reach a sensitivity enabling us to observe fainter objects (A-type stars), we propose a limiting magnitude of 25.4 in V. Using the integral colors of NGC 3664 and taking into account the different filter throughputs we scale this value in order to calculate the required integration times in the U and B filters in a straightforward way. Here, the extremely blue color of NGC 3664 works in our favor. The estimated integration times are collected in Table 1; they can be adjusted slightly in Phase II as necessary to efficiently pack the orbits. For our search for an older population of stars we assume that the gas shells have an age of about 10^7 years, an age at which the remainders of the stellar clusters will be dominated by stars with masses of about 15 M_\\odot and absolute V magnitudes of --4.0. For a typical stellar cluster with a total of about 1000 stars one can expect to find of order 10 stars in the 10 to 15 M_\\odot range. This leads to an expected V magnitude of --6.5. The V image proposed above will be sensitive enough to detect such objects. Given a typical V--I color of 0.7, we have to reach a magnitude in I of 24.7; a S/N of about 8 is achievable in 10 minutes. Although the integration times are relatively short, all exposures will be split in order to facilitate cosmic-ray corrections. Taking into account telescope and instrument overheads along with a 52 minute orbital visibility for NGC 3664, these integrations lead to a grand total of only 2 orbits. Thus, our scientific goals can be efficiently accomplished with a very small investment of HST observing time. begintable Estimated integration times for broadband observations of NGC 3664 filter band approx. int.time sensitivity (min) (mag) F336W U 25.4 15 F439W B 25.4 30 F555W V 25.4 15 F814W I 24.7 10 total 70 Real Time Justification ----------------------- None. We have obtained deep ground-based HAlpha and R-band imagery with the ESO 2.2m telescope. The HAlpha image shows, besides numerous HII regions, the presence of large amounts of diffuse gas on the rims of the shells. Concurrently, we plan HI line observations of NGC 3664 in order to map the distribution of neutral gas. If the most luminous stellar clusters in NGC 3664 are found to be bright enough, follow-up spectroscopy might be proposed at a later stage in order to gain more accurate absolute values for the ages of the cluster populations. Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 6507( 6) - 01/31/96 14:08 - [ 3] Data Distribution Media: 8MM Blocking Factor: 10 Ship To: STSCI Ship Via: UPS Email: , ------------------------------------------------------------------------------------ 6507( 6) - 01/31/96 14:08 - [ 4] TARGET LIST a) Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC3664 GALAXY,MAGELLANIC RA=11H 24M 28.0S +/- 1S, DEC=03D J2000 V = 12.81 IRREGULAR 19' 38.95" +/- 1", PLATE-ID=00ZW 6507( 6) - 01/31/96 14:08 - [ 5] Visit: 01 Visit Requirements: ORIENT 45D TO 45D; SCHED 100% 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 ------------------------------------------------------------------------------------------------------------------------------------ 11 NGC3664 WFPC2 IMAGE PC1 F439W CR-SPLIT=NO,ATD 1 800S -GAIN=7 ------------------------------------------------------------------------------------------------------------------------------------ 12 NGC3664 WFPC2 IMAGE PC1 F439W CR-SPLIT=NO,ATD 1 800S -GAIN=7 ------------------------------------------------------------------------------------------------------------------------------------ 21 NGC3664 WFPC2 IMAGE PC1 F814W CR-SPLIT=NO,ATD 1 400S EXPAND; MIN DUR; MAX DUR 125% -GAIN=7 ------------------------------------------------------------------------------------------------------------------------------------ 22 NGC3664 WFPC2 IMAGE PC1 F814W CR-SPLIT=NO,ATD 1 400S EXPAND; MIN DUR; MAX DUR 130% -GAIN=7 ------------------------------------------------------------------------------------------------------------------------------------ 31 NGC3664 WFPC2 IMAGE PC1 F555W CR-SPLIT=NO,ATD 1 300S -GAIN=7 ------------------------------------------------------------------------------------------------------------------------------------ 32 NGC3664 WFPC2 IMAGE PC1 F555W CR-SPLIT=NO,ATD 1 300S -GAIN=7 ------------------------------------------------------------------------------------------------------------------------------------ 41 NGC3664 WFPC2 IMAGE PC1 F336W CR-SPLIT=NO,ATD 1 400S EXPAND; MIN DUR; MAX DUR 125% -GAIN=7 ------------------------------------------------------------------------------------------------------------------------------------ 42 NGC3664 WFPC2 IMAGE PC1 F336W CR-SPLIT=NO,ATD 1 400S -GAIN=7 ------------------------------------------------------------------------------------------------------------------------------------ 6507( 6) - 01/31/96 14:08 - [ 6] Summary Form for Proposal 6507 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFPC2 ------------------------------------------------------------------------------------------------------------------------------------ Opmodes IMAGE ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CR-SPLIT=NO ATD-GAIN=7 ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Scientific Category STELLAR POPULATIONS ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ORIENT 45D TO 45D; SCHED 100% EXPAND; MIN DUR; MAX DUR 125% EXPAND; MIN DUR; MAX DUR 130% ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F439W F814W F555W F336W ------------------------------------------------------------------------------------------------------------------------------------ Target Names NGC3664 ------------------------------------------------------------------------------------------------------------------------------------