7879( 3) - 08/26/98 10:55 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 7879 Version: 3 Check-in Date: 26-Aug-1998 10:53:39 1.Proposal Title: Nuclear Rings in the IR: Hidden Super Star Clusters ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 7 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Dan Maoz Tel Aviv University CoI: Amiel Sternberg Tel-Aviv University N CoI: Luis Ho Center for Astrophysics N ------------------------------------------------------------------------------------ 5. Abstract We propose NICMOS broad-band (F160W, F187W) and Paschen Alpha (F187N) imaging of nuclear starburst rings in two nearby galaxies. We already have UV (F220W) FOC data, and are scheduled to obtain WFPC2 images in U, V, I, and Halpha+[NII] of these rings. The rings contain large populations of super star clusters similar to those recently discovered in other types of starburst systems. Nuclear rings contain large numbers of these clusters in relatively unobscured starburst environments. Measurement of the age, size, and stellar contents of the clusters can test the hypothesis that super star clusters are young globular clusters. Together with our UV and optical data, NICMOS images will provide the SED of numerous super star clusters over a decade in wavelength. Our already-approved observations will allow us to estimate, by comparison with evolutionary synthesis models, the masses and ages of the clusters. The proposed IR data will be sensitive to the number of supergiants (1.6 micron) and O-stars (Paschen Alpha) in each of the clusters. The observations will provide an independent determination of the reddening, mass, and age of each cluster. We expect to see in the IR numerous clusters that are obscured in the UV and optical. These clusters may be the younger ones, which are still embedded in their molecular clouds. By measuring the mass, age, and size of a large number of clusters, we can actually obtain an evolutionary picture of these objects at different stages in their lives. ------------------------------------------------------------------------------------ 7879( 3) - 08/26/98 10:55 - [ 2] Observations Description ------------------------ We propose to carry out F160W, F187W, and F187N (Paschen- Alpha) NICMOS Camera- 2 imaging of two nuclear ring galaxies, NGC 1512 and NGC 5248. The rings have diameters of 15'' and 11'', respectively, and so fit well on the Camera-2 19''x 19'' field of view. Accurate distances to these nearby galaxies are not presently known, but are of order 10 Mpc. In our FOC images (Maoz et al. 1996; see Fig. 2) we detect 43 super star clusters in NGC 1512, and 46 in NGC 5248, with similar apparent brightnesses. Based on our experience with NGC 2997, an order of magnitude more clusters may turn up in the IR. At the proposed imaging wavelengths, the Camera-2 resolution just matches the diffraction limit of 0.14'', corresponding to 7 pc at a distance of 10 Mpc. Based on our existing FOC UV images of these galaxies, most of the clusters will be unresolved, unless IR-selected clusters will turn out to be bigger than UV-selected ones. In all HST optical images of super star clusters, they stand up high above the diffuse background from the galaxy. We expect this to be true in the near-IR as well, so the main source of background will be thermal, and will determine the detection limit. The F160W filter is optimal for detecting faint, supergiant- dominated clusters; it is broad (FWHM 0.4 micron), its effective wavelength (1.6Mum) is red enough to be dominated by supergiant emission but is at the minimum of the background due to zodiacal light and thermal emission from the telescope. Based on the NICMOS Exposure Time Calculator, as updated on July 18 1997, a 45-min exposure reaches point sources of 5 * 10^-7 Jy with SNR=10. As seen in Fig. 3a, this limit allows us to detect clusters in most of the mass and age ranges indicated by previous observations. By comparison, F205W (comparable to K-band) reaches only 1/10 as deep in a similar exposure, because of the sharp rise in thermal background. Since we will be in the sky-background -dominated regime, and we are interested in measuring the IR flux from compact sources sitting on top of sky and diffuse-galaxy background, we will not need separate exposures for measuring background. The Paschen-Alpha line from these two galaxies (V_h=911 and 1156 km s^-1, respectively), falls well into the 1% width of the the F187N filter. Depending on the clusters' ages and masses, we calculate using our cluster synthesis models that the Paschen-Alpha luminosities will be 10^34- 10^39 erg s^-1, or fluxes of 10^-18-10^-13erg s^- 1cm^-2 at 10 Mpc. From the Exposure Time Calculator we see that in one orbit (47 min exposure time) we can reach a point source emission line flux of 2* 10^ -16 erg s^-1cm^-2 with SNR=10. Sources brighter than this limit will have a Paschen- Alpha/continuum flux ratio in the F187N bandpass of ~ 1-100. It is impractical to attempt detecting fainter Paschen-Alpha sources, since the noise becomes dominated by background from the sky and the continuum emission from the underlying clusters, and the exposure times become prohibitively long. Measuring the clusters in the Paschen-Alpha luminosity range 10^36-10^39 erg s^ -1 will nonetheless be very useful; if the IR-selected clusters are in fact young, many will have high Paschen-Alpha luminosity. Alternatively, an upper limit on Paschen-Alpha/1.6micron translates to a lower limit on the age (see Fig. 3b). To obtain a continuum image to subtract from the Paschen-Alpha image, we will integrate for one orbit on each galaxy with the F187W filter. This is a broad-band filter that brackets the F187N Paschen-Alpha filter, and goes almost as deep as the F160W filter. A suitable linear combination of the images in the F187W and F187N filters with the F160W image will provide an accurate continuum image for subtracting from the F187N image. The F160W band is adjacent to the emission line, but does not include it (the F160W transmission at 1.88Mum is only 10^-3.) This measurement of the local slope of the continuum underlying Paschen-Alpha is essential for a proper continuum subtraction, since the hidden clusters for which we are searching will likely be affected by varying degrees of reddening. Therefore, a large range of IR slopes is likely, despite the small range in spectral slopes of the dominant stellar population. A continuum image with an adjacent narrow-band filter (e.g. F190N) would take a long exposure time and is not necessary; our experience with optical ground-based narrow-band imaging as well as others' with HST is that broad-band filters can provide adequate continuum measurement. Longer-wavelength broad-band filters are also inferior to F187W, since the rising thermal background limits the depth that can be reached within a reasonable time. 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. Orbit 1: Guide star acquisition (9min), F160W (44 min), overheads (<1min). Orbit 2: Guide star reacquisition (6min), F187N (47min), overheads (<1min). Orbit 3: Guide star reacquisition (6min), F187W (47min), overheads (<1min). Real Time Justification ----------------------- Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 7879( 3) - 08/26/98 10:55 - [ 3] Data Distribution Media: DAT Blocking Factor: 10 Ship To: PI_Address Ship Via: Email: ------------------------------------------------------------------------------------ 7879( 3) - 08/26/98 10:55 - [ 4] TARGET LIST a) Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC1512 GALAXY, EXT-CLUSTER, RA=04H 3M 54.29S +/- 0.10S, DEC= J2000 V=+911 F-CONT(16000)=50+/-40e-20 RING, STAR FORMING -43D 20' 56.7" +/- 1.0", PLATE- REGION ID=04FN 2 NGC5248 GALAXY, EXT-CLUSTER, RA=13H 37M 32.06S +/- 0.10S, J2000 V=+1156 F-CONT(16000)=50+/-40e-20 RING, STAR FORMING DEC=08D 53' 6.0" +/- 1.0", PLATE REGION -ID=01TK 7879( 3) - 08/26/98 10:55 - [ 5] Visit: 01 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 ------------------------------------------------------------------------------------------------------------------------------------ 10 NGC1512 NIC2 MULTIAC NIC2-FI F160W SAMP-SEQ=STEP64,NSAMP=18, 1 DEF CUM X PATTERN=SQUARE-WAVE-DITH, NUM-POS=4,DITH-SIZE=0.5 ------------------------------------------------------------------------------------------------------------------------------------ 20 NGC1512 NIC2 MULTIAC NIC2-FI F187W SAMP-SEQ=STEP64,NSAMP=18, 1 DEF CUM X PATTERN=SQUARE-WAVE-DITH, NUM-POS=4,DITH-SIZE=0.5 ------------------------------------------------------------------------------------------------------------------------------------ 30 NGC1512 NIC2 MULTIAC NIC2-FI F187N SAMP-SEQ=STEP64,NSAMP=18, 1 DEF CUM X PATTERN=SQUARE-WAVE-DITH, NUM-POS=4,DITH-SIZE=0.5 ------------------------------------------------------------------------------------------------------------------------------------ 7879( 3) - 08/26/98 10:55 - [ 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 ------------------------------------------------------------------------------------------------------------------------------------ 40 NGC5248 NIC2 MULTIAC NIC2-FI F160W SAMP-SEQ=STEP64,NSAMP=17, 1 DEF CUM X PATTERN=SQUARE-WAVE-DITH, NUM-POS=4,DITH-SIZE=0.5 ------------------------------------------------------------------------------------------------------------------------------------ 50 NGC5248 NIC2 MULTIAC NIC2-FI F187W SAMP-SEQ=STEP64,NSAMP=17, 1 DEF CUM X PATTERN=SQUARE-WAVE-DITH, NUM-POS=4,DITH-SIZE=0.5 ------------------------------------------------------------------------------------------------------------------------------------ 60 NGC5248 NIC2 MULTIAC NIC2-FI F187N SAMP-SEQ=STEP64,NSAMP=17, 1 DEF CUM X PATTERN=SQUARE-WAVE-DITH, NUM-POS=4,DITH-SIZE=0.5 ------------------------------------------------------------------------------------------------------------------------------------ 7879( 3) - 08/26/98 10:55 - [ 7] Summary Form for Proposal 7879 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures NIC2-FIX ------------------------------------------------------------------------------------------------------------------------------------ Configurations NIC2 ------------------------------------------------------------------------------------------------------------------------------------ Opmodes MULTIACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters SAMP-SEQ=STEP64 NSAMP=18 PATTERN=SQUARE-WAVE-DITH NUM-POS=4 DITH-SIZE=0.5 NSAMP=17 ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F160W F187W F187N ------------------------------------------------------------------------------------------------------------------------------------ Target Names NGC1512 NGC5248 ------------------------------------------------------------------------------------------------------------------------------------