6738( 4) - 02/15/96 11:03 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 6738 Version: 4 Check-in Date: 15-Feb-1996 10:58:43 1.Proposal Title: Super Star Clusters and H II Regions in Nuclear Rings ------------------------------------------------------------------------------------ 2. Scientific Category 3. Proposal For 4. Cycle GALAXIES & CLUSTERS GO 6 ------------------------------------------------------------------------------------ 5. Investigators Contact? PI: Alex Filippenko University of California, Berkeley CoI: Dan Maoz Tel-Aviv University N CoI: Amiel Sternberg Tel-Aviv University N CoI: Aaron Barth University of California, Berkeley Y CoI: Luis Ho Center for Astrophysics N ------------------------------------------------------------------------------------ 6. Abstract We propose to obtain WFPC2 optical broad-band (F547M and F814W) and narrow-band Halpha+\ionN2 (F658N) images of nuclear starburst rings in four nearby galaxies for which we already have ultraviolet (F220W) FOC data. Nuclear rings (or ``hot- spot'' regions) in barred spirals are some of the nearest and least obscured starburst regions, and HST images of nuclear rings in several galaxies show that the rings contain large populations of super star clusters similar to those recently discovered in other types of starburst systems. These compact clusters, many having luminosities exceeding that of the R136 cluster in 30 Doradus, represent a violent mode of star formation distinct from that seen in ordinary disk ionH2 regions, and the nuclear rings present us with an opportunity to study large numbers of these extreme clusters in relatively unobscured starburst environments. It has been suggested that super star clusters are present-day versions of young globular clusters. To evaluate this hypothesis, it is important to understand the physical properties and stellar contents of the clusters, but previous HST studies of nuclear ring galaxies have only used single-filter observations. Together with our UV data, new WFPC2 images will enable us to determine the H II region and cluster luminosity functions within nuclear rings, measure cluster radii, derive age and mass estimates for the clusters by comparison with evolutionary synthesis models, and study the structure and evolution of nuclear rings. ------------------------------------------------------------------------------------ 6738( 4) - 02/15/96 11:03 - [ 2] Observations Description ------------------------ A. Choice of Filters. For each of the galaxies in our sample, we propose to obtain WFPC2 images through the filters F547M, F658N, and F814W, placing the nuclear ring on the PC detector. The choice of the PC is essential because the clusters and ionH2 regions will be crowded together and would not be well resolved by the WFC. All four rings are small enough to fit entirely on the PC detector. The HAlpha emission line falls at or near the peak of the F658N bandpass for these four galaxies, making it an obvious choice for observing the ionH2 regions. Ideally, the continuum would be subtracted using a narrow off- band filter slightly displaced from the wavelength of HAlpha, but even if such a filter were available, the exposure times would be prohibitively long. Our tests using SYNPHOT indicate that a single broad off-band filter would not be adequate for continuum subtraction, because the underlying continuum in the nuclear rings arises from a spatially varying combination of bright, compact, blue clusters and a much redder background of galactic bulge starlight. A combination of two broad-band filters will determine the continuum level underlying HAlpha much more accurately. We will obtain two broad-band images, one on either side of the emission line of interest, and create an appropriate linear combination of the two off-band images to subtract from the narrow-band image. The optimal filters for application of this method to the HAlpha line are F547M and F814W. We prefer F547M over F555W or F569W, despite its lower throughput, because its bandpass is entirely free of emission lines. F814W is the recommended I-band WFPC2 filter and it is perfectly suitable for our purposes. We have used SYNPHOT to test whether a simple weighted average of the two broad-band images will provide an adequate continuum subtraction given the range of background colors that may occur in the data. We find that the sum (0.6 * F547M flux) + (0.4 * F814W flux) can be used to predict the 6500 Angstrom continuum flux to an accuracy of better than 8% for stellar colors ranging from O to M-type; thus, we will achieve an excellent continuum subtraction with this technique, and also determine the clusters' (reddening- free) ratio of HAlpha emission to 6500 Angstrom continuum flux. Our tests indicate that this method is at least as accurate as using an R-band filter for the continuum subtraction (as described by Waller 1990), and the two-band method has the advantage that none of the HAlpha flux is subtracted off along with the continuum. Furthermore, we will be able to measure V and I magnitudes for the clusters, yielding additional constraints for comparison with the cluster evolution models. B. Estimation of Exposure Time. Broad-band images. Our goals for the F547M and F814W images are to obtain an accurate continuum subtraction that does not add noise to the narrow- band image, and to determine cluster magnitudes accurately (S/N ~ 20) down to at least V = 23 mag; at this level, we will begin to detect individual luminous stars in the target galaxies. The exposure time calculator on the World-Wide Web indicates that to achieve S/N=20 at V=23 requires only 350 seconds in F547M or F814W for a point source on a dark background. However, the nuclear ring clusters are slightly extended, and are superimposed on a bright, varying background, making detection of faint objects more difficult. By comparison with WFPC2 images of nuclear rings in NGC 2997 and NGC 6951, we estimate that exposure times of 800 s each in F547M and F675W will yield the desired result. To ensure adequate cosmic-ray cleaning we will split the 800 second total exposure time for each filter into two 400 s exposures, and additionally obtain one 100 s exposure to guard against saturation of bright clusters or potential PSF stars. In 100 s, point sources fainter than V=16 mag will be in no danger of saturation. These broad-band images will occupy a total of one orbit per galaxy, and the resulting signal will be more than adequate for purposes of continuum subtraction of the F658N image. Narrow-band images. In order to make a meaningful comparison with ionH2 region LFs measured from the ground, it is necessary to have good photometry at least down to Halpha+ionN2 luminosities of 10^38 erg s^-1, and preferably to 10^37 erg s^-1, to determine whether the characteristic LF break at 10^39 erg s^-1 occurs. The exposure time calculation is uncertain because the surface brightnesses of nuclear ring ionH2 regions have never been measured at high resolution; it is not known whether they will appear nearly pointlike at HST resolution (radii <=sssim3 pc) or if they are more extended, but we have examined HST narrow-band images of other types of galaxies to estimate the ionH2 region sizes. Archival WFPC2 F658N images of the ionH2 nucleus galaxy NGC 4536 show that the circumnuclear ionH2 regions are slightly extended, with typical sizes of ~0.3 * 0.3". Taking this size as a reasonable guess, we find that L(Halpha+\\\\\\\\ionN2) = 10^37 erg s^-1 (our target faint limit) corresponds to a surface brightness of 5.6 * 10^-15 erg s^- 1 cm^-2 arcsec^-2, or a count rate of 0.02 electrons pixel^-1 s^-1 in F658N. Examination of WFPC2 images of nuclear rings in NGC 2997 and NGC 6951 shows that the expected underlying galaxy background light at the nuclear ring is V ~ 18 mag arcsec^-1, yielding a count rate of 0.02 electrons pixel^-1 s^-1 for the galaxy background in F658N. Noise sources include the dark count rate of 0.005 electrons pixel^-1 s^-1 and the read noise of 5.2 electrons pixel^-1 in both the narrow-band and continuum images. We find that we will reach S/N = 5 per pixel in the continuum-subtracted image (total S/N ~ 35 for the entire ionH2 region) in 5000 seconds of total exposure time, where the total exposure is split into four individual exposures to ensure adequate cosmic-ray removal. If the background region includes a bright cluster, or if the ionH2 region were more extended, then the S/N would be correspondingly lower. Perhaps the best indication of the required exposure time is comparison with the archival F658N exposure of NGC 4536 (2 exposures * 1500 s). We have experimented with photometry of this image, and find that we can measure ionH2 regions down to a surface brightness of 5 * 10^-15 erg s^-1 cm^-2 arcsec^- 2, although these ionH2 regions are not superimposed on bright stellar backgrounds. The exposure time calculator indicates S/N ~ 5 per pixel would be expected for this Halpha+ionN2 flux in 3000 s on a dark background, in reasonable agreement with our measurements. To summarize, we propose to spend 3 orbits on each object. For a total visibility of 53 minutes per orbit, a sample visit would proceed as follows. noindent Orbit 1: Guide star acquisition (480 s), F658N (2400 s with CR-SPLIT), overhead (300 s). noindent Orbit 2: Guide star reacquisition (360 s), F658N (2500 s with CR-SPLIT), overhead (300 s). noindent Orbit 3: Guide star reacquisition (360 s), F547M (800 s with CR- SPLIT), overhead (300 s), F547M (100 s), overhead (180 s), F814W (800 s with CR-SPLIT), overhead (300 s), F814W (100 s), overhead (180 s). Real Time Justification ----------------------- None. We will propose to obtain ground-based spectra of the three southern galaxies in this study, since optical long-slit spectroscopy will provide important data in support of this work. (We have already obtained spectra of NGC 5248 as part of a large survey of northern galaxies; see Ho et al. 1995.) We will use the spectra to determine the contribution of ionN2 to the Halpha+\\\\\\\\ionN2 fluxes; to measure the Balmer decrement in the ionH2 regions so as to better constrain the extinction; and to measure rotation curves for the galaxies in order to locate inner Lindblad resonances, if they are present. Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 6738( 4) - 02/15/96 11:03 - [ 3] Data Distribution Media: 8MM Blocking Factor: 10 Ship To: PI_Address Ship Via: UPS Email: , ------------------------------------------------------------------------------------ 6738( 4) - 02/15/96 11:03 - [ 4] TARGET LIST a) Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC1512 GALAXY,SPIRAL,NUCLEUS,H RA=04H 03M 54.3S +/- 3",DEC=-43D J2000 V = 896 V = 11.1 OTSPOT,RING, 20' 57" +/- 3",PLATE-ID=04FN 2 NGC5248 GALAXY,SPIRAL,NUCLEUS,H RA=13H 37M 32S +/- 3",DEC=08D J2000 V = 1153 V = 11.0 OTSPOT,RING, 53' 08" +/- 3",PLATE-ID=01TK 6738( 4) - 02/15/96 11:03 - [ 5] Visit: 01 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 ------------------------------------------------------------------------------------------------------------------------------------ 10 NGC1512 WFPC2 IMAGE PC1-FIX F658N 1 2600S ------------------------------------------------------------------------------------------------------------------------------------ 20 NGC1512 WFPC2 IMAGE PC1-FIX F658N 1 2700S ------------------------------------------------------------------------------------------------------------------------------------ 30 NGC1512 WFPC2 IMAGE PC1-FIX F336W 1 300S ------------------------------------------------------------------------------------------------------------------------------------ 40 NGC1512 WFPC2 IMAGE PC1-FIX F336W 1 2200S ------------------------------------------------------------------------------------------------------------------------------------ 50 NGC1512 WFPC2 IMAGE PC1-FIX F547M 1 100S ------------------------------------------------------------------------------------------------------------------------------------ 60 NGC1512 WFPC2 IMAGE PC1-FIX F547M 1 900S ------------------------------------------------------------------------------------------------------------------------------------ 70 NGC1512 WFPC2 IMAGE PC1-FIX F814W 1 100S ------------------------------------------------------------------------------------------------------------------------------------ 80 NGC1512 WFPC2 IMAGE PC1-FIX F814W 1 1000S ------------------------------------------------------------------------------------------------------------------------------------ 6738( 4) - 02/15/96 11:03 - [ 6] Visit: 02 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 ------------------------------------------------------------------------------------------------------------------------------------ 10 NGC5248 WFPC2 IMAGE PC1-FIX F658N 1 2500S ------------------------------------------------------------------------------------------------------------------------------------ 20 NGC5248 WFPC2 IMAGE PC1-FIX F658N 1 2500S ------------------------------------------------------------------------------------------------------------------------------------ 30 NGC5248 WFPC2 IMAGE PC1-FIX F336W 1 300S ------------------------------------------------------------------------------------------------------------------------------------ 40 NGC5248 WFPC2 IMAGE PC1-FIX F336W 1 2000S ------------------------------------------------------------------------------------------------------------------------------------ 50 NGC5248 WFPC2 IMAGE PC1-FIX F547M 1 100S ------------------------------------------------------------------------------------------------------------------------------------ 60 NGC5248 WFPC2 IMAGE PC1-FIX F547M 1 800S ------------------------------------------------------------------------------------------------------------------------------------ 70 NGC5248 WFPC2 IMAGE PC1-FIX F814W 1 100S ------------------------------------------------------------------------------------------------------------------------------------ 80 NGC5248 WFPC2 IMAGE PC1-FIX F814W 1 1000S ------------------------------------------------------------------------------------------------------------------------------------ 6738( 4) - 02/15/96 11:03 - [ 7] Summary Form for Proposal 6738 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFPC2 ------------------------------------------------------------------------------------------------------------------------------------ Opmodes IMAGE ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Scientific Category GALAXIES & CLUSTERS ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F658N F336W F547M F814W ------------------------------------------------------------------------------------------------------------------------------------ Target Names NGC1512 NGC5248 ------------------------------------------------------------------------------------------------------------------------------------