9361( 8) - 09/08/03 08:32 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 9361 Version: 8 Check-in Date: 08-Sep-2003 08:31:36 1.Proposal Title: Searching for Primeval Galaxies: the promising case of SBS 1415+437 ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 11 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Alessandra Aloisi The Johns Hopkins University CoI: Dr. Mark Clampin Space Telescope Science Institute N CoI: Dr. Guido de Marchi Space Telescope Science Institute N CoI: Dr. Claus Leitherer Space Telescope Science Institute N CoI: Dr. Antonella Nota Space Telescope Science Institute N CoI: Dr. Marco Sirianni The Johns Hopkins University N CoI: Dr. Monica Tosi Osservatorio Astronomico di Bologna N CoI: Dr. Roeland van der Marel Space Telescope Science Institute N ------------------------------------------------------------------------------------ 5. Abstract Do primeval galaxies exist in the local Universe? The best candidates are extremely metal-poor (Z < 1/20 Zo) blue compact dwarf (BCD) galaxies whose photometric and chemical properties are consistent with a stellar population younger than 100 Myr. SBS 1415+437 (Z = 1/21 Zo) is the closest candidate: its proximity (d = 11.4 Mpc), detailed spectroscopic knowledge of its HII regions and low metal content, used to infer the primordial He^4 abundance, make it the best target for this investigation. We propose to take deep exposures of SBS 1415+437 with the ACS in the F814W and F606W filters. This instrument has resolution and magnitude limits allowing us to reach and resolve with the required accuracy individual stars 1 mag below the tip of the red giant branch (RGBT). If present, these stars will provide a clear sign of an old stellar population (with ages > 1 Gyr) and an independent distance indicator. If absent, this will unambiguously show that the system has started to form stars only recently and can be considered the first robust case of local primeval galaxy. We propose to take exposures in the F658N (HAlpha Lambda6563) and FR505N (HBeta Lambda4861) filters to study the morphology of the ionized gas through HAlpha emission and map the dust content with the HAlpha/HBeta ratio in order to solve the age-dust degeneracy of the photometry by constraining reddening effects. ------------------------------------------------------------------------------------ 9361( 8) - 09/08/03 08:32 - [ 2] Observations Description ------------------------ Due to its distance (d ~ 11.4 Mpc), corresponding to a distance modulus (m-M)_0 ~ 30.3, the search for a resolved stellar population older than 100 Myr in SBS 1415+437 can be performed only with the use of HST. This project implies a quite reasonable number of orbits if performed with the ACS/WFC, thanks to its higher sensitivity with respect to the WFPC2. The target (~ 50"x30") will be centered on one of the two WFC detectors (either WFC1 or WFC2), with a resolution of 0.05" per pixel and a field of view (FOV) 202"x102". The two broad band filters F606W (broad V) and F814W (broad I) will be considered to construct an I vs V-I CMD where to search for the RGBT. The choice of the two optical filters V and I is almost straightforward: the literature suggests that the RGBT is best calibrated in the Johnson-Cousins I band. Moreover our data on NGC 1705 (Tosi et al. 2001, AJ 122, 1271) have demonstrated the better efficiency of the optical WFPC2 V and I bands to reach the RGBT than the near- infrared J and H filters of NICMOS. Finally, the ACS/HRC is not preferred to the ACS/WFC, despite the higher resolution, for i. its smaller FOV (29"x26"), that does not allow us to sample the whole body of the target, as well as the regions surrounding it, ii. its PSF in the F814W filter that, due to its extended halo, implies higher integration times, despite a comparable sensitivity, and more difficulties during the photometric reduction process with the PSF-fitting technique. At the estimated distance, the RGBT (M_I = - 4 at Z = 1/21 Zo, see Bellazzini et al. 2001, ApJ 556, 635) is expected to be at I_RGBT, 0 ~ 26.3 in the Johnson-Cousins photometric system. We want to detect stars in F606W and F814W at least one magnitude fainter than the RGBT with a photometric error Sigma < 0.1 (S/N > 10): this is a conservative request in order to obtain an estimate of the distance modulus with an uncertainty less than 0.25 mag. Based on the ACS ETC, this implies 16x1200 s in F814W and 16x1200 s in F606W. With the conservative assumption of an orbit lasting 40 minutes (overheads have been already excluded), these numbers imply 8 orbits in F814W and 8 orbits in F606W. We prefer the F606W over the F555W filter (Johnson V), due to integrations ~ 3 times longer in the latter. The exposure times have been calculated to detect a red giant star with an effective temperature of ~ 4400 K (V-I ~ 1.4) and magnitude I ~ 27.3 mag with signal-to-noise ~ 10 in the central pixel: this is the minimum requirement for a reliable, robust measurement of the stellar magnitude with the PSF-fitting technique. The 16 exposures in each filter will be executed with a CR-SPLIT technique and a dithering pattern of an integer+fraction of pixels in order to get rid of cosmic rays, remove hot/bad pixels and improve the PSF sampling. Since our goal can be achieved only if we can really resolve individual stars at the faintest reachable magnitudes, we have estimated the amount of stellar crowding predictable in our target at such magnitudes. We have used the LMC field around SN1987A kindly made available by N. Panagia. LMC is an irregular galaxy with a quite moderate SF activity, like SBS 1415+437, and we can take its stellar density as reasonably similar to that of our object. Its proximity, however, allows one to safely resolve single stars down to much fainter absolute magnitudes than in any other external system. Projecting the stellar density per unit area in the LMC to the distance of SBS 1415+437, we calculate an expected stellar density in the WFC of 0.2 stars per px^2 at one magnitude fainter than the RGBT. This implies that no severe crowding problems will affect our data and that the resolution of single stars at the requested magnitudes is guaranteed. Another concern might be contamination by Galactic 9361( 8) - 09/08/03 08:32 - [ 3] foreground stars. We have considered the field star densities predicted in the direction of Galactic globular clusters by a two-component (disk+spheroid) Galaxy model (Ratnatunga & Bahcall 1985, ApJS 59, 63). The prediction is ~ 2 stars brighter than V = 28 per arcmin^2 at the position of SBS 1415+437. This translates into about 10 Galactic stars within the field of view of one of the two WFC detectors, and only 1 on an area corresponding to the body of our target, suggesting that foreground stars are not an issue. In addition to the broad band imaging in F606W and F814W, we also require narrow band imaging in F658N and FR505N. F658N is a regular narrow band filter centered around HAlpha Lambda6563, while FR505N is a ramp filter that we will center around HBeta Lambda4861. This will allow us to map the distribution of the ionized gas via the HAlpha emission, and the spatial variation of the reddening through the HAlpha/HBeta ratio. Subtraction of the stellar continuum from the narrow band filters will be possible with the use of broad band images. The narrow band images, on the other hand, will be very helpful to subtract the contribution of the ionized gas from the broad band images in F606W, contaminated by many emission lines. However, gas contamination in this band should not be a serious problem for the goal of our project, since the older stellar populations are typically less concentrated than the star-forming regions, and their photometry should not be heavily affected by dust or gaseous emission. The small systemic velocity of SBS 1415+437 (607 km per s) places the HAlpha emission line well inside the highest sensitivity region of the response curve (DeltaLambda ~ 13 Angstrom). The NIILambda6584 emission line will be inside the F658N band as well, but its flux is practically negligible with respect to the HAlpha one, being SBS 1415+437 a very metal-poor BCD. For the ramp filter FR505N, we will prefer to center it at the 10 Angstrom redshifted HBeta position (Lambda = 4871 Angstrom). From the HST/FOS optical spectra published by Thuan, Izotov & Foltz (1999, ApJ 525, 105) we derive the following typical nebular fluxes over a 0.86" circular aperture: F(HAlpha) ~ 1.5 x 10^-15 ergs per s per cm^2 per Angstrom and F(HBeta) ~ 0.5 x 10^-15 ergs per s per cm^2 per Angstrom. Exposure times for the narrow band filters have been calculated with the ACS ETC, considering the detection of HAlpha and HBeta fluxes per arcsec^2 50 times fainter than the above values with a S/N ~ 6 within a square aperture of 2x2 pixels: this implies 2x250 s in F658N and 4x800 s in FR505N, making a total of 2 HST orbits. The exposures in each narrow filter will be acquired with CR- SPLIT, while dithering will not be considered: we are dealing with extended sources, thus we do not need to have a good sampled PSF, and the missing information from hot/bad pixels will be easily recovered by interpolation. The ramp filter has a much smaller corrected field of view (65"x100"). Its aperture covers thus a smaller area of the chosen WFC chip, but the location will be maintained fixed on the detector. For the narrow band observations, we will reposition the target at the center of the smaller aperture of the ramp filter FR505N and we will take the exposures in both FR505N and F658N. In order to facilitate the registration of narrow and broad band images during the data reduction process, we will acquire in the same position one very short exposure (50 s) in both F814W and F606W to detect the yellow-red supergiants brighter than I = 23.5 and with V-I = 1 (T_eff ~ 5000 K) with a S/N ~ 5. Each of the 2 short exposures in the broad band filters will be easily inserted in one of the 2 orbits already assigned to the narrow bands (taking into account all the overheads), and will be very useful to build an unsaturated image of the brightest objects in the BCD. Summarizing, we require 8 orbits in F606W and 8 orbits in F814W for the study of the resolved stellar population in SBS 1415+437; 2 orbits in total in the narrow band filters F658N and FR505N, plus 9361( 8) - 09/08/03 08:32 - [ 4] the short exposures in the broad band F606W and F814W, for the study of the ionized-gas morphology and reddening map. This makes a total of 18 HST orbits. Real Time Justification ----------------------- There are no special requirements. None are planned. Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 9361( 8) - 09/08/03 08:32 - [ 5] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 SBS1415+437 GALAXY, DWARF COMPACT, RA=14H 17M 1.88S +/- 0.02S, J2000 V = +609 V = 15.55+/-0.01 EMISSION LINE NEBULA, DEC=43D 30' 18.3" +/- 0.2", I = 15.47+/-0.01 STAR FORMING REGION PLATE-ID=00XZ SURF-BKG(V) = 23.0+/-0.5 SURF-BKG(I) = 22.2+/-0.5 SURF(V) = 21.00+/-0.04 SURF(I) = 20.69+/-0.03 SURF(6563)=2.8+/-0.1e-15 SURF(4861)=1.1+/-0.1e-15 E(V-B) = 0.009 Coordinate Source: HST_IMAGE Comments: 1. Coordinates from WFPC2 image u2kd0601t of proposal ID 5408 (PI Thuan). 2. V and I isophotal magnitudes have been obtained by integrating the surface brightness profiles from 0 to an equivalent radius of 16" (isophotal radius at the surface brightness level of 25 mag/arcsec^2). 9361( 8) - 09/08/03 08:32 - [ 6] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 99 Primary_Pattern Secondary_Pattern Pattern_Type ACS-WFC-DITHER-BOX Pattern_Purpose DITHER Number_Of_Points 4 Point_Spacing 0.265 Coordinate_Frame POS-TARG Pattern_Orient 20.67 Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ 9361( 8) - 09/08/03 08:32 - [ 7] 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 ------------------------------------------------------------------------------------------------------------------------------------ 1 SBS1415+437 ACS/WF ACCUM WFC2 F606W CR-SPLIT=2 1 2520 S PATTERN 99 1 C ------------------------------------------------------------------------------------------------------------------------------------ 9361( 8) - 09/08/03 08:32 - [ 8] Visit: 02 Visit Priority: Visit Requirements: SAME ORIENT AS 01 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 ------------------------------------------------------------------------------------------------------------------------------------ 9 SBS1415+437 ACS/WF ACCUM WFC2 F606W CR-SPLIT=2 1 2520 S PATTERN 99 9 POS TARG C 0.124, 0.084 ------------------------------------------------------------------------------------------------------------------------------------ 9361( 8) - 09/08/03 08:32 - [ 9] Visit: 03 Visit Priority: Visit Requirements: SAME ORIENT AS 01 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 ------------------------------------------------------------------------------------------------------------------------------------ 17 SBS1415+437 ACS/WF ACCUM WFC2 F814W CR-SPLIT=2 1 2520 S PATTERN 99 17 C ------------------------------------------------------------------------------------------------------------------------------------ 9361( 8) - 09/08/03 08:32 - [ 10] Visit: 04 Visit Priority: Visit Requirements: SAME ORIENT AS 01 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 ------------------------------------------------------------------------------------------------------------------------------------ 25 SBS1415+437 ACS/WF ACCUM WFC2 F814W CR-SPLIT=2 1 2520 S PATTERN 99 25 POS TARG C 0.124, 0.084 ------------------------------------------------------------------------------------------------------------------------------------ 9361( 8) - 09/08/03 08:32 - [ 11] Visit: 05 Visit Priority: Visit Requirements: SAME ORIENT AS 01 On Hold Comments: Additional Comments: All the exposures of this visit must be done with the target in the same position on the CCD. This means the target will be always centered on the same aperture used for the ramp filter FR505N, having the latter a fixed aperture. The aperture to use for all the exposures in this visit must be WFC1-MRAMP (the default for FR505N centered at 4871.2 Angstroms). Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 33 SBS1415+437 ACS/WF ACCUM WFC1-MR F606W CR-SPLIT=NO 1 60 S C AMP ------------------------------------------------------------------------------------------------------------------------------------ 34 SBS1415+437 ACS/WF ACCUM WFC1-MR F658N CR-SPLIT=NO 1 400 S C AMP ------------------------------------------------------------------------------------------------------------------------------------ 35 SBS1415+437 ACS/WF ACCUM WFC1-MR FR505N 4871.2 CR-SPLIT=2 1 1720 S NEW ALIGNMENT C AMP Comments: new alignment special requirement added as a test to see if it aids in scheduling this visit on the 03.202 SMS. -wj ------------------------------------------------------------------------------------------------------------------------------------ 37 SBS1415+437 ACS/WF ACCUM WFC1-MR F814W CR-SPLIT=NO 1 60 S SAME ALIGNMENT 37, 38 C AMP Comments: Same alignment special requirement added as a test to see if it aids in scheduling this visit on the 03.202 SMS. -wj ------------------------------------------------------------------------------------------------------------------------------------ 38 SBS1415+437 ACS/WF ACCUM WFC1-MR F658N CR-SPLIT=NO 1 400 S C AMP ------------------------------------------------------------------------------------------------------------------------------------ 39 SBS1415+437 ACS/WF ACCUM WFC1-MR FR505N 4871.2 CR-SPLIT=2 1 1720 S NEW ALIGNMENT C AMP Comments: new alignment special requirement added as a test to see if it aids in scheduling this visit on the 03.202 SMS. -wj ------------------------------------------------------------------------------------------------------------------------------------ 9361( 8) - 09/08/03 08:32 - [ 12] Summary Form for Proposal 9361 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures WFC2 WFC1-MRAMP ------------------------------------------------------------------------------------------------------------------------------------ Configurations ACS/WFC ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CR-SPLIT=2 CR-SPLIT=NO ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements PATTERN 99 1 SAME ORIENT AS 01 PATTERN 99 9 POS TARG 0.124, 0.084 PATTERN 99 17 PATTERN 99 25 POS TARG 0.124, 0.084 NEW ALIGNMENT SAME ALIGNMENT 37, 38 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F606W F814W F658N FR505N ------------------------------------------------------------------------------------------------------------------------------------ Target Names SBS1415+437 ------------------------------------------------------------------------------------------------------------------------------------ Wavelengths 4871.2 ------------------------------------------------------------------------------------------------------------------------------------