8232( 6) - 05/12/99 14:05 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 8232 Version: 6 Check-in Date: 12-May-1999 14:04:41 1.Proposal Title: The Youngest Super-Star-Cluster ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 8 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Daniela Calzetti Space Telescope Science Institute CoI: Prof. Timothy Heckman The Johns Hopkins University N CoI: Dr. Claus Leitherer Space Telescope Science Institute N CoI: hristina Tremonti The Johns Hopkins University N ------------------------------------------------------------------------------------ 5. Abstract Local starbursts are the most immediate counterparts to the high-redshift, actively- star-forming galaxies, and the study of starbursts and their constituents can shed light on the initial phases of galaxy evolution. We propose to use STIS UV spetroscopy to derive an accurate age for the youngest super- star-cluster candidate known to date in a nearby starburst galaxy. The stellar cluster is located in the dwarf galaxy NGC5253 at a distance of 4 Mpc. Its HAlpha equivalent width (from WFPC2 imaging) gives an estimated age of ~2 Myr, and more accurate determinations are hampered by the presence of heavy dust reddening in the region. The cluster is bright enough in the UV to make the project feasible with STIS and produce an age determination with an accuracy +/-0.5 Myr. We will exploit the STIS long slit to derive an accurate age also for the oldest among the bright stellar clusters (~50 Myr, estimated from WFPC2 colors). We will use the new data to: (1) break the age/dust degeneracy, derive the intrinsic luminosity of the clusters, and verify whether they are proto-globular cluster candidates; (2) use the UV spectroscopy- derived ages to calibrate the color-derived ages of the other stellar clusters in the starburst; (3) use the spatial distribution of the calibrated ages to investigate the spatial and temporal evolution (e.g., propagation) of the starburst in NGC5253. ------------------------------------------------------------------------------------ 8232( 6) - 05/12/99 14:05 - [ 2] Observations Description ------------------------ We will obtain UV spectra for two bright stellar clusters in the starburst galaxy NGC5253 (recession velocity = 404 km/s), using the 52^prime *0.1^ aperture and the G140L grating, for the purpose of deriving accurate ages for the clusters. Our primary target is the young, dust-enshrouded cluster 5 (cf. Fig. 1), but we will exploit the STIS long-slit capability to obtain also the UV spectrum of an older stellar cluster (cluster 2), since the two clusters are less than 7^ apart. The targets are neither point nor completely extended sources, and the 0.1^ slit aperture is the best compromise between throughput and spectral resolution. We estimate that the resolution will be degraded to ~1.7--1.8 Angstrom per resolution element, or ~370 km/s. With this resolution, we will be able to determine the age of the youngest cluster with an accuracy of +/-0.5 Myr, since the spectra of ages <5--6 Myr are dominated by the strong stellar winds of O stars (v~2, 000 km/s). Age ranges between 3 and 7 Myr are also characterized by the presence of the HeII(1640) emission feature of Wolf- Rayet stars. For the older cluster (~50 Myr), the uncertainty will be ~20\ photospheric lines of B stars, which dominate at these ages, have typical velocity widths of ~300--400 km/s (Fig. 3 and Leitherer et al. 1998, in prep.). From aperture photometry on the WFPC2 images at 1700 Angstrom, we estimate that about ~20\ 0.1^ slit aperture, implying an entrance flux of 2 E-16 erg/s/cm^2/Angstrom in the slit at 1500 Angstrom. This value is similar for both clusters (see Fig. 2). Inputing the flux and a flat spectral continuum in the STIS Exposure Time calculator on the WWW, we find that we can get S/N~eq7-- 10 in the wavelength range 1, 200-1, 550 Angstrom in 4 exposures of 2, 300 seconds each. Previous experience has demonstrated that this level of S/N in the UV is perfectly adequate for a reliable measurement of lines strenghts and shapes (Fig. 4, see Heckman & Leitherer 1997). Overheads include guide star acquisition and re- acquisition (7 and 6 minutes, respectively), target acquisition and acquisition/peakup (3.5 minutes each), and instrument set-up (8 minutes). We thus estimate that we need 4 orbits to achieve the desired science goals for an target orbital visibility of 52 minutes. The WWW ETC also shows that the proposed observations do not exceed the MAMA count rate limit. Real Time Justification ----------------------- We have already in hand WFPC2 images of the starburst in NGC5253. The images were obtained in broad-band filters (F170W, F255W, F336W, F547M, and F814W) and in narrow-band filters (F487N and F656N). They are mostly our own but we also have some archival (F170W and F336W) frames. We have used the images in a previous investigation (GO6524) to estimate the ages of the clusters using broad band colors, after correcting for the dust reddening (from the HAlpha/HBeta images provided by the narrow-band filters). The age/dust degeneracy suffered by the colors allowed us to constrain the ages within 30- 40\ use the STIS spectra to ``calibrate'' our color scale, and apply the calibration to all clusters in the starburst. Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 8232( 6) - 05/12/99 14:05 - [ 3] Data Distribution Paper Products: NO Media: ELECTRONIC Blocking Factor: 10 Ship To: PI_Address Ship Via: Email: ------------------------------------------------------------------------------------ 8232( 6) - 05/12/99 14:05 - [ 4] TARGET LIST a) Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 OFFSET-STAR STAR, G III-I RA=13H 40M 00.32S +/- 0.02S, 2000 V=+10 V = 16.1 DEC=-31D 39' 16.6" +/- 0.5", B-V = 0.80 PLATE-ID=02IZ Comments: The offset star position and colors have been measured from 1996 WFPC2 images. Proper motions have been checked against POSS1 and POSS2 plates, and found negligible. Uncertainty on V ~+/-25%, due to saturated core. Colors similar to a ~ K0 star. PLATE-ID comes from the 1996 WFPC2 images. 2 NGC5253- EXT-CLUSTER, OB RA=13H 39M 56.02S +/- 0.02S, 2000 V=+404 V = 17.85 CLUST5 Association DEC=-31D 38' 25.0" +/- 0.5", B-V = 0.40 PLATE-ID=02IZ Comments: The reddest and, possibly, youngest cluster in NGC5253. ST magnitudes for this cluster are F547M=17.87, F255W=16.61. The other cluster in the slit (CLUST2 or CLUST3) has F547M=17.46, F255W=16.68. 8232( 6) - 05/12/99 14:05 - [ 5] Visit: 01 Visit Priority: Visit Requirements: ORIENT 101.012D TO 101.236D On Hold Comments: Additional Comments: Used offset=45.02 degrees between STIS slit and U3 Clust 3 is selected for the slit position. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 OFFSET-STAR STIS/C ACQ F28X50L MIRROR ACQTYPE=POINT 1 3 S ONBOARD ACQ FOR 2 CD P ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC5253-CLU STIS/C ACQ/PEA 52X0.1 MIRROR 1 60 S ONBOARD ACQ FOR 3, 4, 5, ST5 CD K 6 ------------------------------------------------------------------------------------------------------------------------------------ 3 NGC5253-CLU STIS/F ACCUM 52X0.1 G140L 1425 1 1410 S POS TARG 0, 6 ST5 UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 4 NGC5253-CLU STIS/F ACCUM 52X0.1 G140L 1425 1 2945 S POS TARG 0, 6 ST5 UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 5 NGC5253-CLU STIS/F ACCUM 52X0.1 G140L 1425 1 2945 S POS TARG 0, 6 ST5 UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 6 NGC5253-CLU STIS/F ACCUM 52X0.1 G140L 1425 1 2945 S POS TARG 0, 6 ST5 UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 8232( 6) - 05/12/99 14:05 - [ 6] Summary Form for Proposal 8232 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures F28X50LP 52X0.1 ------------------------------------------------------------------------------------------------------------------------------------ Configurations STIS/CCD STIS/FUV-MAMA ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ ACQ/PEAK ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters ACQTYPE=POINT ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ORIENT 101.012D TO 101.236D ONBOARD ACQ FOR 2 ONBOARD ACQ FOR 3, 4, 5, 6 POS TARG 0, 6 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements MIRROR G140L ------------------------------------------------------------------------------------------------------------------------------------ Target Names OFFSET-STAR NGC5253-CLUST5 ------------------------------------------------------------------------------------------------------------------------------------ Wavelengths 1425 ------------------------------------------------------------------------------------------------------------------------------------