8715( 2) - 02/17/00 10:21 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 8715 Version: 2 Check-in Date: 17-Feb-2000 10:20:19 1.Proposal Title: A Young Globular Cluster Surrounded by Numerous Smaller Clusters and a Giant Bubble in NGC 6946 ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 9 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Soren Larsen University of California - Santa Cruz CoI: Prof. Paul Hodge University of Washington N CoI: Dr. Tom Richtler Universidad de Concepcion N CoI: Dr. Emilio Alfaro Instituto de Astrofisica de Andalucia N CoI: Dr. Paolo Battinelli Osservatorio Astronomico di Roma N CoI: Prof. Yuri Efremov Sternberg State Astronomical Institute N CoI: Dr. Bruce Elmegreen IBM Watson Research Center N ------------------------------------------------------------------------------------ 5. Abstract Ground-based images recently lead to the discovery of a young globular cluster in the nearby spiral galaxy NGC 6946. With an absolute visual magnitude of -13, this object can be compared to the young globular clusters discovered by the HST in large numbers in recent merger galaxies like e.g. the Antennae. However, NGC 6946 is more than a factor of 3 closer than even the most nearby young merger galaxy, so observations of the young globular here could offer much more detailed information about the structure and environment of such an object. The young globular is located within a bubble-like structure with a radius of about 300 pc, in which dozens of other young clusters are seen on ground-based images. We have shown in a recent paper that the young globular could have formed by coalescence of many smaller clusters. Considering the youth of this region (15 Myr), and its relatively small distance, HST images are expected to provide detailed information about the formation and early evolution of globular clusters. This has important implications for understanding the physical conditions in the early Universe when most globular clusters and their host galaxies formed. ------------------------------------------------------------------------------------ 8715( 2) - 02/17/00 10:21 - [ 2] Observations Description ------------------------ We plan to detect and obtain ages for clusters to a limiting magnitude of M_V = -6. This requires UBV photometry accurate to 10%, which makes WFPC2 the instrument of choice, rather than STIS. Photometric calibration to the standard system can be performed using the already existing ground-based data. In the following we assume a distance modulus of 28.7 and B-band absorption of 1.6 mag. towards NGC 6946. The intrinsic colours of the clusters (U-B = -0.7, B-V = 0.0) are intermediate between those of an A0 and B0 star, the two best matches in the WFPC/2 exposure time calculator. For E(B-V) = 0.40 and a S/N of 10 at V=24, the ETC gives 7000 sec. of integration time in F336W for an A0 spectrum and 1100 sec. for a B0 spectrum. We thus adopt an intermediate value of 3000 sec. Including all overheads, we need 2 orbits to reach a S/N of 10 in UBVI for V=24. Within the 2 orbits we can push the magnitude limit to 25 (M_V = -5) by integrating for 2300 sec. in F439W, and to about 25.5 in F555W and F814W by integrating for 600 and 1500 sec. in these two filters respectively, again adopting values intermediate between those for B0 and A0 spectra (though the difference is much smaller than for F336W, e.g. 2200 vs. 2700 sec. in F439W). Deep BVI data will be useful for studying the luminosity distribution of the clusters and obtain reddening information, even though no accurate ages can be obtained for the faintest clusters. For an age of 15 Myr, individual luminous stars are also expected to show up on the deep exposures, but will in general be evolved giants, much redder than clusters. In order to avoid saturation of the young globular (V=17), we also need short integrations in each filter of 175 sec. in F336W, 140 sec. in F439W, 30 sec. in F555W and 40 sec. in F814W. To trace ongoing star formation in the region, we also apply for 300 sec. in the F656N band. There are many CVZ opportunities for NGC 6946, so we need only two orbits to complete the observations. All exposure times have been estimated using the WFPC/2 Exposure Time Calculator for Point Sources. The main noise contribution is photon shot noise from the sources themselves, so an angular extent of the clusters of a few pc will only decrease the S/N by a small amount. Background light from NGC 6946 itself does not constitute any significant contribution to the noise. By selecting a roll angle in the interval 150 to 300 degrees, we will also be able to cover the western spiral arm of NGC 6946 quite well. This arm is known to contain a few additional massive star clusters and other regions of active star formation, including a prominent dust lane. So by imposing this constraint on the roll angle we will get interesting data on the WF chips as well. Real Time Justification ----------------------- Calibration Justification ------------------------- Additional Comments ------------------- No additional comments ------------------------------------------------------------------------------------ 8715( 2) - 02/17/00 10:21 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC6946 GALAXY, STAR FORMING RA=20H 34M 31.4S +/- 0.1", J2000 V = 17.0+/-0.5 REGION DEC=60D 8' 17.0" +/- 0.1" 8715( 2) - 02/17/00 10:21 - [ 4] Visit: 01 Visit Priority: Visit Requirements: CVZ ORIENT 150D TO 300D 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 NGC6946 WFPC2 IMAGE PC1-FIX F336W 1 3000 S ------------------------------------------------------------------------------------------------------------------------------------ 20 NGC6946 WFPC2 IMAGE PC1-FIX F336W 1 175 S ------------------------------------------------------------------------------------------------------------------------------------ 30 NGC6946 WFPC2 IMAGE PC1-FIX F555W CR-SPLIT=0.5,CR-TOLERANCE=0.1 1 600 S ------------------------------------------------------------------------------------------------------------------------------------ 40 NGC6946 WFPC2 IMAGE PC1-FIX F555W 1 30 S ------------------------------------------------------------------------------------------------------------------------------------ 50 NGC6946 WFPC2 IMAGE PC1-FIX F439W 1 2300 S ------------------------------------------------------------------------------------------------------------------------------------ 60 NGC6946 WFPC2 IMAGE PC1-FIX F439W 1 140 S ------------------------------------------------------------------------------------------------------------------------------------ 70 NGC6946 WFPC2 IMAGE PC1-FIX F814W 1 1500 S ------------------------------------------------------------------------------------------------------------------------------------ 80 NGC6946 WFPC2 IMAGE PC1-FIX F814W 1 40 S ------------------------------------------------------------------------------------------------------------------------------------ 90 NGC6946 WFPC2 IMAGE PC1-FIX F656N 1 300 S ------------------------------------------------------------------------------------------------------------------------------------ 8715( 2) - 02/17/00 10:21 - [ 5] Summary Form for Proposal 8715 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures PC1-FIX ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFPC2 ------------------------------------------------------------------------------------------------------------------------------------ Opmodes IMAGE ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CR-SPLIT=0.5 CR-TOLERANCE=0.1 ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements CVZ ORIENT 150D TO 300D ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F336W F555W F439W F814W F656N ------------------------------------------------------------------------------------------------------------------------------------ Target Names NGC6946 ------------------------------------------------------------------------------------------------------------------------------------