8293( 4) - 02/23/01 08:59 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 8293 Version: 4 Check-in Date: 23-Feb-2001 08:58:58 1.Proposal Title: Spatial Structure of Super Star Clusters in NGC 1569 ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 8 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Dan Maoz Tel Aviv University - Wise Observatory CoI: Dr. Luis Ho Carnegie Observatories N CoI: Prof. Amiel Sternberg Tel-Aviv University N ------------------------------------------------------------------------------------ 5. Abstract HST and ground-based observations have revealed that a large fraction of star formation in starburst environments occurs in compact super star clusters (SSCs). SSCs appear to be gravitationally bound, implying a very high star-forming efficiency, which may be difficult to explain. We propose spatially-resolved long- slit spectroscopy of three of the nearest and brightest SSCs in NGC 1569. A fortuitous alignment allows positioning the three SSCs (A1, A2, and B) simultaneously in the slit. The HST + STIS resolution and sensitivity will allow resolving the clusters (half-light radius = 1.6 pc) into three radial bins at 1-4 pc. With our 3000--10, 000 Angstrom spectra, we will determine the dominant stellar populations at each radius, and hence the structural star-forming history of the clusters. The only SSC that has been studied in this way is R136 in the LMC, and it shows evidence for population segregation, suggesting the massive stars form last, in the already-established potential well of lower-mass stars. We will test this idea in three additional, more massive, SSCs. The measured radial stellar-population gradient, combined with Keck high-spectral-resolution data will refine the total mass estimate for the cluster and constrain the initial mass function. The mass estimate can reveal the presence or absence of low-mass stars, and thus address the open question of whether SSCs ultimately evolve into globular clusters. ------------------------------------------------------------------------------------ 8293( 4) - 02/23/01 08:59 - [ 2] Observations Description ------------------------ We will obtain spatially-resolved STIS/CCD spectra for three SSCs in NGC 1569 by orienting the slit to P.A. 140 degrees. Fig. 1 (left panel) shows a WFPC2 F555W image of the galaxy, with the proposed STIS slit position marked on it. Photometry and size measurements of the three SSCs have been published by De Marchi et al. (1997). In all three bands which they used (F380W, F439W, and F555W), the observed magnitudes of each of the clusters are similar, and are approximately 15, 16.2, and 16.2 for clusters A1, A2, and B, respectively. The FWHM of all the clusters are ~ 0.3''. We can therefore bin our spectra on each side of each cluster into three spatially-resolved bins of unequal size, each containing about a third of the total flux on that side. We illustrate this in Fig. 1 (right panel), using the actual WFPC2 profile of one of the clusters along the proposed slit direction. In the dispersion direction, the 52X0.1 slit will transmit about 40 percent of the total light in each spatial bin. Thus the transmitted flux in each spatial bin of the fainter clusters is equivalent to a V~18.5 mag source having a spectrum that is approximately flat if f_Lambda (based on the published integrated-light spectra). From the exposure-time calculator, in a 30-min exposure in the G750L grating, and 40 min in G430L, we will reach S/N~ 30 in the continuum in the fainter clusters. Such a S/N is highly desirable for the accurate measurement of the most spectral features, essential for correctly determining the mix of stellar populations. For cluster A1, we will reach S/N~55 in these exposure times. A high S/N in G750L is needed for measuring the Wolf-Rayet WC C IV Lambda5808 line (e.g. Schaerer, Contini, & Kunth 1998) and for determining the continuum level at ~ 8100 Angstrom, outside the range covered by the G750M grating, when measuring the Ca II triplet EW. This accurate continnum placement is important for using the triplet to estimate age and metallicity (Bica & Alloin 1987; Prada, Greve, & McKeith 1994; Garcia-Vargas, Molla, & Bressan 1998). Two orbits (78-min exposure) will be devoted to the G750M grating, set to the secondary tilt covering 8275--8847 Angstrom, for mapping the Ca II triplet. Here we will achieve S/N~ 19 per 1 Angstrom resolution element in each radial bin of the fainter clusters, and S/N~ 35 in cluster A1. In clusters A1 and A2, which are in close (0.2'') projection, we will disentangle the overlapping internal spatial bins by subtracting the spectra of the spatial bins on the outer sides. In any case, a Wolf-Rayet population that exists in only one of the clusters (as suggested by De Marchi et al. 1997) should be directly apparent in the spectral cut across the two clusters. We will split each orbit's exposure into six images at three integer-pixel dither positions stepped along the slit, to remove cosmic rays and hot pixels. Overheads will consist of initial acquisition (7 min), acquisition peakup (6 min), first scientific exposure (5 min), exposure splitting + dither (6 min), and CCD fringe flats (2 min) to remove fringing in the G750L and G750M spectra. With a visibility period of 57 minutes, this leaves 31 min for science exposure in the first orbit (G750L), 78 min in the second and third orbits (G750M), and 40 min in the third orbit (G430L). Real Time Justification ----------------------- Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 8293( 4) - 02/23/01 08:59 - [ 3] Data Distribution Paper Products: YES Media: 8MM Blocking Factor: 10 Ship To: PI_Address Ship Via: Email: ------------------------------------------------------------------------------------ 8293( 4) - 02/23/01 08:59 - [ 4] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC1569-A1 GALAXY, Starburst, RA=67.70075D +/- 0.0003D, J2000 V=-104 V = 15 Magellanic Irregular, DEC=64.84961D +/- 0.0003D, PLATE F(4500)=3.6e-15 EXT-CLUSTER -ID=01MS 51 NGC1569-B GALAXY, STARBURST, RA=67.70411D +/- 0.0003D, J2000 V=-104 V = 16 MAGELLANIC IRREGULAR, DEC=64.84795D +/- 0.0003D, PLATE F(4500)=1.4e-15 EXT-CLUSTER -ID=01MS 8293( 4) - 02/23/01 08:59 - [ 5] Visit: 01 Visit Priority: Visit Requirements: ORIENT 183.83D TO 183.83D ORIENT 3.83D TO 3.83D 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 NGC1569-A1 STIS/C ACQ F28X50L MIRROR 1 1 S CD P ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC1569-A1 STIS/C ACQ/PEA 52X0.1 MIRROR 1 3 S CD K ------------------------------------------------------------------------------------------------------------------------------------ 3 NGC1569-A1 STIS/C ACCUM 52X0.1 G750L 7751 CR-SPLIT=2,PATTERN=ALONG-SLIT, 1 9 M CD NUM-POS=3,STEP-SIZE=0.2, PATTERN-DIRECTION=MINUS ------------------------------------------------------------------------------------------------------------------------------------ 4 CCDFLAT STIS/C ACCUM 52X0.1 G750L 7751 2 DEF CD ------------------------------------------------------------------------------------------------------------------------------------ 5 NGC1569-A1 STIS/C ACCUM 52X0.1 G750M 8561 CR-SPLIT=2,PATTERN=ALONG-SLIT, 1 26 M CD NUM-POS=3,STEP-SIZE=0.2, PATTERN-DIRECTION=PLUS ------------------------------------------------------------------------------------------------------------------------------------ 6 CCDFLAT STIS/C ACCUM 52X0.1 G750M 8561 2 DEF CD ------------------------------------------------------------------------------------------------------------------------------------ 7 NGC1569-A1 STIS/C ACCUM 52X0.1 G430L 4300 CR-SPLIT=2,PATTERN=ALONG-SLIT, 1 12 M CD NUM-POS=3,STEP-SIZE=0.2, PATTERN-DIRECTION=MINUS ------------------------------------------------------------------------------------------------------------------------------------ 8293( 4) - 02/23/01 08:59 - [ 6] Visit: 51 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 ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC1569-B STIS/C ACQ F28X50L MIRROR 1 1 S CD P ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC1569-B STIS/C ACQ/PEA 52X0.1 MIRROR 1 3 S CD K ------------------------------------------------------------------------------------------------------------------------------------ 3 NGC1569-B STIS/C ACCUM 52X0.1 G750L 7751 CR-SPLIT=2,PATTERN=ALONG-SLIT, 1 9 M CD NUM-POS=3,STEP-SIZE=0.2, PATTERN-DIRECTION=MINUS ------------------------------------------------------------------------------------------------------------------------------------ 4 CCDFLAT STIS/C ACCUM 52X0.1 G750L 7751 2 DEF CD ------------------------------------------------------------------------------------------------------------------------------------ 5 NGC1569-B STIS/C ACCUM 52X0.1 G750M 8561 CR-SPLIT=2,PATTERN=ALONG-SLIT, 1 26 M CD NUM-POS=3,STEP-SIZE=0.2, PATTERN-DIRECTION=PLUS ------------------------------------------------------------------------------------------------------------------------------------ 6 CCDFLAT STIS/C ACCUM 52X0.1 G750M 8561 2 DEF CD ------------------------------------------------------------------------------------------------------------------------------------ 7 NGC1569-B STIS/C ACCUM 52X0.1 G430L 4300 CR-SPLIT=2,PATTERN=ALONG-SLIT, 1 12 M CD NUM-POS=3,STEP-SIZE=0.2, PATTERN-DIRECTION=MINUS ------------------------------------------------------------------------------------------------------------------------------------ 8293( 4) - 02/23/01 08:59 - [ 7] Summary Form for Proposal 8293 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures F28X50LP 52X0.1 ------------------------------------------------------------------------------------------------------------------------------------ Configurations STIS/CCD ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ ACQ/PEAK ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CR-SPLIT=2 PATTERN=ALONG-SLIT NUM-POS=3 STEP-SIZE=0.2 PATTERN-DIRECTION=MINUS PATTERN-DIRECTION=PLUS ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ORIENT 183.83D TO 183.83D ORIENT 3.83D TO 3.83D ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements MIRROR G750L G750M G430L ------------------------------------------------------------------------------------------------------------------------------------ Target Names NGC1569-A1 CCDFLAT NGC1569-B ------------------------------------------------------------------------------------------------------------------------------------ Wavelengths 7751 8561 4300 ------------------------------------------------------------------------------------------------------------------------------------