9473( 5) - 05/15/02 09:43 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 9473 Version: 5 Check-in Date: 15-May-2002 09:42:38 1.Proposal Title: Masses and IMF Variations in Super Star Clusters ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 11 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: William Vacca Max-Planck-Institut fur extraterrestrische Physik CoI: Dr. Matthew Lehnert Max-Planck-Institut fuer N extraterrestrische Physik CoI: Dr. Sabine Mengel University of Leiden N CoI: Dr. Niranjan Thatte Max-Planck-Institut fuer N extraterrestrische Physik ------------------------------------------------------------------------------------ 5. Abstract We are proposing to obtain high spatial resolution images of a set of super star clusters for which we have been granted observing time to measure velocity dispersions via high- resolution ground-based optical echelle and K-band spectroscopy. The images will allow us to fit the light profiles and measure the radii of the clusters, and when combined with the velocity dispersions, will enable us to estimate the cluster masses. By comparing the mass-to-light ratios with those predicted from spectral synthesis models, we will investigate possible variations in the slope and lower mass cut-offs of the initial mass functions in these clusters. Correlations of the variations of these parameters with cluster environment may provide insight into the formation mechanisms for super star clusters. By comparing the light profiles obtained in a blue and red filter, we will also search for evidence of mass segregation in the clusters. Since these clusters are too young to have experienced dynamical mass segregation, differences in the light profiles of the clusters in the two filters could be due to processes that differentiate between high and low mass stars during the birth of the clusters and would provide further constraints on theoretical models of cluster formation. Since these clusters are far too compact to be resolved by any ground-based observations, the ACS/HRC on board HST is the only instrument capable of carrying out these observations. ------------------------------------------------------------------------------------ 9473( 5) - 05/15/02 09:43 - [ 2] Observations Description ------------------------ We are proposing to obtain images in F435W (B) and F814W (I) for a sample of galaxies which comprises M82 and four galaxies that are `analogous' to the Antennae Galaxies. For M82 we also propose to obtain a high signal-to-noise F250W image of M82-F. The `Antennae-analogues' are interacting starbursts with redshifts similar to that of the Antennae. The purpose of these observations is to perform photometry and measure the light profiles of the Super Star Clusters within the galaxies. The light profiles will be combined with stellar velocity dispersion measurements obtained from ground-based high resolution spectroscopy (conducted on the VLT optical echelle spectrometer UVES for clusters which suffer relatively low extinction and with the VLT near- infrared imaging spectrometer ISAAC for clusters which lie in regions of heavier extinction) to estimate the masses of the SSCs. In addition, we will compare the light profiles in the two photometric bands (3 for the cluster M82-F) to search for evidence of mass segregation. The magnitudes will be combined with the mass estimates and ages derived from our ground- based imaging and spectroscopy to investigate possible variations in the IMF within the clusters. These observations therefore require high spatial resolution and signal-to- noise. Our choice of the ACS/HRC is driven by the fact that the ACS/HRC is the only instrument available that critically samples the HST PSF. To prevent the uncertainty in the mass estimate from being dominated by the radius error, the percentage uncertainty in the radius must be smaller than twice the uncertainty in the velocity dispersion. Based on our previous ground-based spectroscopy, we estimate that the velocity dispersion can be measured with error of only a few percent. Therefore, we need to be able to estimate the half- light radii to better than 10 percent. We have attempted to estimate the signal-to-noise (S/N) needed to achieve this accuracy, by simulating stellar clusters observed with the ACS/HRC and measuring their radii. We adopted the parameters appropriate for the clusters in the galaxies in our sample. An example of the results of these simulations is shown in Figure 4, which presents the measured radius as a function of photometric S/N for the cluster M82-F. From this figure it is clear that to achieve an accuracy of 10 percent in the radius estimate, we require a photometric S/N in the cluster image of >150. To achieve signal-to-noise ratios this high, we will integrate on M82-F for 10, 000 seconds, 1440 seconds, and 480 seconds through the F250W, F435W, and F814W filters respectively. All observations will consist of multiple dither positions. The proposed F435W and F814W observations are relatively short and care will be taken so that no image is saturated. We therefore request 2 CVZ orbits to observe M82-F with the ACS/HRC (2 out of a total of 42 orbits available for CVZ observations of M82). The SSCs in the 4 Antennae-analogues have I-band mags of roughly 19, which is the limit at which we can obtain sufficient S/N to estimate accurate velocity dispersions from our optical echelle spectroscopy. To achieve S/N>150 on these clusters will require integration times of 1440 seconds and 480 seconds through the F435W and F814W filters respectively. To obtain at least 4 dithered exposures per filter (the minimum number of exposures required to enhance the sampling of the final data set) requires only one orbit per field. The targeted clusters in NGC 7727, NGC 4105/6, and NGC 1487 are all located within a single ACS/HRC frame and can be observed in a single pointing. The SSCs in UGC128 are scattered throughout the galaxy. We will choose clusters located in 2 fields near the central region of the galaxy; these observations will require 2 orbits. The total number of orbits requested for our proposed observations is only 7. We feel the scientific output of these observations is well worth the modest amount of HST time requested. beginfigureb. epsscale0.5 plotonesimrad_m82f.ps captionEstimated radius value of a SSC as a function of S/N of the measured photometric magnitude. These Monte Carlo simulations were carried out for the parameters of M82-F (V=16.3, r_hp=2.8pc at a distance of 3.6 Mpc), and assumes an ACS/HRC psf with FWHM=0.034", a plate scale of 0.027" pix^-1, and a background of 0.0055 DN s^-1. The dashed line shows the input radius (in arcsec). The dispersion in the radius values decreases as the S/N increases, and falls below about 10 percent for S/N>150. labelfigure5 endfigure Real Time Justification ----------------------- The declination of M82 places it in the CVZ. We are proposing to obtain UV images with the F250W filter, which will require approximately an exposure time of 10000 s. This observation, along with the relatively brief F435W and F814W exposures, can be carried out most efficiently through CVZ observations. None. Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 9473( 5) - 05/15/02 09:43 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 M82-F EXT-CLUSTER RA=09H 55M 47.02S +/- 0.3", J2000 V = 16.1+/-0.2 DEC=69D 40' 42.1" +/- 0.3" V-I = 1.51+/-0.10 B-V = 1.22+/-0.10 E(B-V) = 0.9+/-0.1 Coordinate Source: HST_IMAGE 2 NGC1487-CLUS EXT-CLUSTER RA=03H 55M 45.2S +/- 1", DEC=- J2000 I = 18.5+/-0.5 42D 22' 15.2" +/- 1" Coordinate Source: HST_IMAGE 3 UGCA128-CLUS EXT-STAR RA=06H 21M 5.2S +/- 1", DEC=-20D J2000 I = 18.4+/-0.5 2' 56.7" +/- 1" Coordinate Source: OTHER_SOURCE 4 NGC7727-CLUS EXT-CLUSTER RA=23H 39M 55.0S +/- 1", DEC=- J2000 I = 18.5+/-0.5 12D 17' 38.1" +/- 1" Coordinate Source: HST_IMAGE Comments: Magnitude uncertain 5 NGC4105-CLUS EXT-CLUSTER RA=12H 6M 40.1S +/- 1", DEC=-29D J2000 I = 18.5+/-0.5 45' 18.1" +/- 1" Coordinate Source: OTHER_SOURCE Comments: Magnitude uncertain 9473( 5) - 05/15/02 09:43 - [ 4] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 1 Primary_Pattern Secondary_Pattern Pattern_Type ACS-HRC-DITHER-BOX Pattern_Purpose DITHER Number_Of_Points 4 Point_Spacing 0.150 Coordinate_Frame POS-TARG Pattern_Orient 19.89 Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ 9473( 5) - 05/15/02 09:43 - [ 5] Visit: 01 Visit Priority: Visit Requirements: CVZ 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 M82-F ACS/HR ACCUM HRC F250W CR-SPLIT=2 1 2540 S PATTERN 1 1 C ------------------------------------------------------------------------------------------------------------------------------------ 9473( 5) - 05/15/02 09:43 - [ 6] Visit: 02 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 NGC1487-CLU ACS/HR ACCUM HRC F435W CR-SPLIT=NO 1 385 S PATTERN 1 1 S C ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC1487-CLU ACS/HR ACCUM HRC F814W CR-SPLIT=NO 1 160 S PATTERN 1 2 S C ------------------------------------------------------------------------------------------------------------------------------------ 9473( 5) - 05/15/02 09:43 - [ 7] Visit: 03 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 UGCA128-CLU ACS/HR ACCUM HRC F435W CR-SPLIT=NO 1 375 S PATTERN 1 1 S C ------------------------------------------------------------------------------------------------------------------------------------ 2 UGCA128-CLU ACS/HR ACCUM HRC F814W CR-SPLIT=NO 1 140 S PATTERN 1 2 S C ------------------------------------------------------------------------------------------------------------------------------------ 9473( 5) - 05/15/02 09:43 - [ 8] Visit: 04 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 NGC7727-CLU ACS/HR ACCUM HRC F435W CR-SPLIT=NO 1 375 S PATTERN 1 1 S C ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC7727-CLU ACS/HR ACCUM HRC F814W CR-SPLIT=NO 1 135 S PATTERN 1 2 S C ------------------------------------------------------------------------------------------------------------------------------------ 9473( 5) - 05/15/02 09:43 - [ 9] Visit: 05 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 NGC4105-CLU ACS/HR ACCUM HRC F435W CR-SPLIT=NO 1 375 S PATTERN 1 1 S C ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC4105-CLU ACS/HR ACCUM HRC F814W CR-SPLIT=NO 1 145 S PATTERN 1 2 S C ------------------------------------------------------------------------------------------------------------------------------------ 9473( 5) - 05/15/02 09:43 - [ 10] Visit: 06 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 M82-F ACS/HR ACCUM HRC F435W CR-SPLIT=NO 1 330 S PATTERN 1 1 C ------------------------------------------------------------------------------------------------------------------------------------ 2 M82-F ACS/HR ACCUM HRC F555W CR-SPLIT=NO 1 100 S PATTERN 1 2 C ------------------------------------------------------------------------------------------------------------------------------------ 3 M82-F ACS/HR ACCUM HRC F814W CR-SPLIT=NO 1 30 S PATTERN 1 3 C ------------------------------------------------------------------------------------------------------------------------------------ 9473( 5) - 05/15/02 09:43 - [ 11] Summary Form for Proposal 9473 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures HRC ------------------------------------------------------------------------------------------------------------------------------------ Configurations ACS/HRC ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CR-SPLIT=2 CR-SPLIT=NO ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements CVZ PATTERN 1 1 PATTERN 1 2 PATTERN 1 3 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F250W F435W F814W F555W ------------------------------------------------------------------------------------------------------------------------------------ Target Names M82-F NGC1487-CLUS UGCA128-CLUS NGC7727-CLUS NGC4105-CLUS ------------------------------------------------------------------------------------------------------------------------------------