7881( 4) - 04/29/98 14:40 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 7881 Version: 4 Check-in Date: 29-Apr-1998 14:38:44 1.Proposal Title: Starbursts: The First Generation ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 7 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: MONICA TOSI OSSERVATORIO DI BOLOGNA CoI: Alessandra Aloisi Universita di Bologna N CoI: Mark Clampin Space Telescope Science Institute N CoI: Laura Greggio Universita di Bologna N CoI: Claus Leitherer Space Telescope Science Institute N CoI: Antonella Nota Space Telescope Science Institute N CoI: Livia Origlia Osservatorio Astronomico di Bologna N CoI: Marco Sirianni Universita di Padova N ------------------------------------------------------------------------------------ 5. Abstract We propose to perform deep NICMOS imaging in the F110W and F160W bands of the Blue Compact / Dwarf Irregular galaxies NGC1569 and NGC1705 to trace their major star formation episodes over a Hubble time. These two galaxies are among the closest starbursting dwarfs and therefore among those very few in which old, i.e. faint, stars can be resolved and measured, if present. They therefore provide an optimal opportunity to verify directly from individual star detection whether or not some starbursting galaxies have started to form stars only in relatively recent epochs (as suggested for IZw18). If so, their contribution to galaxy count samples at early epochs (i.e. high redshifts) would be quite lower than estimated by extrapolating their current/local features. The only instrument capable of achieving this goal is NICMOS on HST, whose NIC2 camera provides the angular resolution and the magnitude limits necessary to reach and resolve with the required accuracy individual stars at the tip of the red giant branch (i.e. with age between 1 and 15 Gyr). These stars have proven to be unreachable in the optical bands. Infrared color- magnitude diagrams (J, J--H) will allow us to evaluate the mass and the age of these stars. Combined with the available optical data and with our method of a theoretical interpretation based on synthetic color-magnitude diagrams, these data will firmly establish at what epochs these galaxies have formed stars. ------------------------------------------------------------------------------------ 7881( 4) - 04/29/98 14:40 - [ 2] Observations Description ------------------------ sectionThe Targets NGC1569 has a distance modulus of (m-- M)_0=26.71, corresponding to 2.2 Mpc. Its extended loops of HAlpha emission, the high frequency cut-off for non-thermal emission and the general paucity of early O stars and Wolf- Rayet stars have led to the suggestion that NGC1569 is in a post-burst evolutionary phase (Hodge 1974; Israel 1988) although many bright HII regions indicate that the rate of SF is still high (Gallagher et al. 1984; Hunter et al. 1993). Using the aberrated HST to probe the stellar structure of NGC1569, O'Connell et al. (1994) have partially resolved two super star clusters similar in characteristics to R136 in the LMC, but with a much higher central brightness. More recently, our group has obtained deep U, B and V WFPC2 images of this galaxy, showing that one of the super star clusters actually results from the projected superposition of two distinct clusters (DeMarchi et al. 1997). From the analysis of the CMD we have inferred that the SF in the field has stopped around 8 Myr ago, but in the last 0.2 Gyr has been rather continuous and 10^3 times higher than in the solar neighbourhood (Greggio et al. 1997). NGC1705 has a distance modulus of (m--M)_0~eq28. It shares two important characteristics with NGC1569: nebular emission around the galaxy suggests a galactic superwind, and the nucleus of NGC1705 hosts a luminous super star cluster (Meurer et al. 1992; O'Connell et al. 1994). Earlier ground- based work suggested the presence of a ~1 Gyr old field population which defines the NGC1705 morphology. We will study the recent SF history of this galaxy, as done for NGC1569, as soon as we get the U, B, V and I WFPC2 images planned for HST cycle 7. sectionThe Observing Strategy We propose to perform deep J and H photometry of NGC1569 and NGC1705 using NIC2 and the F110W, F160W filters. NIC2, with its magnitude limit 24.7, is the only instrument capable to reach the RGBT in our target galaxies and its pixel size (0.075 arcsec) allows us to resolve individual stars at those distances. The NIC2 FOV (19" * 19") is large enough to observe all IZw18, the two SSCs in N1569 plus a sufficient field region around them, and most of N1705, including its SSc. Orienting NIC2 along the galaxy major axis can also allow parallel observations with NIC1 (~ 33" from the NIC2 center) in a more external field (but still on the main body) of NGC1569 and NGC1705. Even if the RGBT is out of reach of NIC1, these additional images will be very useful since they will show whether or not the red stellar populations of intermediate age are uniformly distributed over the whole galaxy. In the Local Group dwarf irregulars, despite their small size, the SF episodes have been quite different from one galactic region to another; we would now be able to verify if this occurs also in galaxies where the SF activity is much more intense. As shown in Fig.1, in NGC1569 red supergiants are brighter than Vsimeqm_555=23 while AGB stars are visible at Vsimeqm_555>=23. The RGBT of stars with 0.6leqM/M_\\odot<=1.8 is visible at m_555=26 and 2leqB--V~eqm_439-m_555<=2.6. Since red giants have V--J~eq3 (e.g. Johnson 1966, Bessel & Brett 1988), the RGBT in NGC1569 is expected at J=23. In addition, the IR images of the super star cluster A in NGC1569 will firmly establish if indeed it is made of two distinct projected clusters, one, red, providing the red supergiant features observed in the spectra (Gonzalez-Delgado et al. 1997) and the other, blue and therefore much fainter with NICMOS, providing the Wolf-Rayet spectral features. NGC1705 has a higher distance modulus (m-- M)$_0~eq28 but a much lower reddening E(B--V)=0.045 (Meurer et al. 1992) than NGC 1569, and the RGBT turns out to be at J=22.5 (i.e. V=25.5). Also for this galaxy, NIC2 will thus allow us not only to detect the RGBT stars if present, but also to measure the red giant branch down to two magnitudes below its tip and test with extreme reliability the age of the galaxy. According to the NICMOS Exposure Time Calculator estimates, 2 orbits (about 2*3000 = 6000s on source total integration time) per filter and galaxy allows us to reach J, H=24--25 with a S/N>= 10, that is to perform photometry 1--2 mag below the RGBT in NGC1569 and NGC1705. Short exposures of ~ 10s will be also secured in order to avoid saturation of the brightest red supergiants. For both long and short exposures we choose the MULTIACCUM readout mode for its larger dynamical range. The 3000s exposures can be split in 10 single integrations of about 300s each, which will provide the J and H photometry of stars in the mag range ~16 to ~25. The 10s exposures will result from ten ~1s integrations allowing photometry in the range ~11 to ~16 mag. The former will guarantee reaching the RGB stars, while the latter will avoid saturating even the brightest SSCs in the field. Since our goal can be achieved only if the individual stars can be resolved at the faintest reachable magnitudes, we have estimated the amount of stellar crowding predictable in our targets at such magnitudes. To do this we have used the LMC field around SN1987A kindly made available by N.Panagia. Since the LMC is also an irregular galaxy, although with moderate SF activity, we can take its stellar space density as reasonably similar to that in our target galaxies. Its proximity, however, allows one to safely resolve single stars down to much fainter absolute mags than in any other external system. Projecting the stellar density per unit area in the LMC to the distance of our target galaxies, we calculate an expected number of stars per square pixel of NIC2 of 0.2 stars/px^2 in all of them. This shows that no sever crowding problems will affect our data and that the resolution of single stars at the requested magnitudes is guaranteed. Furthermore, we have to take into account that we are looking for red stars in fields dominated by blue objects. Despite their larger number, the expected global contribution of main sequence stars to the near IR luminosity is negligible, since at most their fluxes can reach ~1/1000 of the red giants flux. This maximum contribution is provided by stars around 2 M_\\odot with temperatures around 10000 K, all the others being either too hot or too faint. Another concern might be contamination by Galactic foreground stars, in particular for NGC1569, which is close to the Galactic plane. We ran simulations with the ``sky'' model described by Wainscoat et al. (ApJS, 83, 111 1992). It is based on a multi-component Galaxy model including bulge, disk, halo, a 4-arm spiral, and molecular ring components. Extinction is modeled as an exponential disk. The prediction is 8 * 10^4 stars brighter than J=23 per square degree at the position of NGC1569. This translates into about 2 Galactic stars within the NIC2 field of view, suggesting that foreground stars are not an issue. Real Time Justification ----------------------- No supporting observations are planned. Optical WFPC2 images of NGC1569 (proposal 6111), as well as high resolution spectra for NGC1569 (prop 6408) and NGC1705 (prop 3591) are available from the HST archive. WFPC2 imaging of NGC1705 will be performed during the HST cycle 7 (prop 7506). Calibration Justification ------------------------- Real_Time_Justification Additional Comments ------------------- ------------------------------------------------------------------------------------ 7881( 4) - 04/29/98 14:40 - [ 3] Data Distribution Media: DAT Blocking Factor: 10 Ship To: PI_Address Ship Via: Email: ------------------------------------------------------------------------------------ 7881( 4) - 04/29/98 14:40 - [ 4] TARGET LIST a) Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC1705 EXT-CLUSTER, OPEN RA=4H 54M 13.3261S +/- 0.1S, 2000 V = 15.0 CLUSTER DEC=-53D 21' 38.374" +/- 1.0", V-I = 1.0 PLATE-ID=061F Comments: coordinates from WF/PC1 image w0ou0504t ID2389 PI O'Connell 2 NGC1569 EXT-CLUSTER, OPEN RA=4H 30M 48.6743S +/- 0.1S, 2000 V = 14.9 CLUSTER DEC=64D 50' 55.237" +/- 0.6", V-I = 1.0 PLATE-ID=01MS Comments: coordinates from WFPC2 image u32f0111t ID6111 PI Leitherer 7881( 4) - 04/29/98 14:40 - [ 5] Visit: 01 Visit Priority: Visit Requirements: ORIENT 80D TO 100D 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 NGC1705 NIC2 MULTIAC NIC2 F110W SAMP-SEQ=STEP256,NSAMP=12, 1 DEF PAR 2 WITH 1 CUM NUM-POS=10,PATTERN=SPIRAL-DITH, DITH-SIZE=0.2 ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC1705 NIC1 MULTIAC NIC1 F110W SAMP-SEQ=STEP256,NSAMP=12 1 DEF CUM ------------------------------------------------------------------------------------------------------------------------------------ 3 NGC1705 NIC2 MULTIAC NIC2 F160W SAMP-SEQ=STEP256,NSAMP=12, 1 DEF PAR 4 WITH 3 CUM NUM-POS=10,PATTERN=SPIRAL-DITH, DITH-SIZE=0.2 ------------------------------------------------------------------------------------------------------------------------------------ 4 NGC1705 NIC1 MULTIAC NIC1 F160W SAMP-SEQ=STEP256,NSAMP=12 1 DEF CUM ------------------------------------------------------------------------------------------------------------------------------------ 7881( 4) - 04/29/98 14:40 - [ 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 NGC1569 NIC2 MULTIAC NIC2 F110W SAMP-SEQ=STEP256,NSAMP=12, 1 DEF PAR 2 WITH 1 CUM NUM-POS=10,PATTERN=SPIRAL-DITH, DITH-SIZE=0.2 ------------------------------------------------------------------------------------------------------------------------------------ 2 ANY NIC1 MULTIAC NIC1 F110W SAMP-SEQ=STEP256,NSAMP=12 1 DEF CUM ------------------------------------------------------------------------------------------------------------------------------------ 3 NGC1569 NIC2 MULTIAC NIC2 F160W SAMP-SEQ=STEP256,NSAMP=12, 1 DEF PAR 4 WITH 3 CUM NUM-POS=10,PATTERN=SPIRAL-DITH, DITH-SIZE=0.2 ------------------------------------------------------------------------------------------------------------------------------------ 4 ANY NIC1 MULTIAC NIC1 F160W SAMP-SEQ=STEP256,NSAMP=12 1 DEF CUM ------------------------------------------------------------------------------------------------------------------------------------ 7881( 4) - 04/29/98 14:40 - [ 7] Visit: 51 Visit Priority: Visit Requirements: ORIENT 260D TO 280D 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 NGC1705 NIC2 MULTIAC NIC2 F110W SAMP-SEQ=STEP256,NSAMP=12, 1 DEF PAR 2 WITH 1 CUM NUM-POS=10,PATTERN=SPIRAL-DITH, DITH-SIZE=0.2 ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC1705 NIC1 MULTIAC NIC1 F110W SAMP-SEQ=STEP256,NSAMP=12 1 DEF CUM ------------------------------------------------------------------------------------------------------------------------------------ 3 NGC1705 NIC2 MULTIAC NIC2 F160W SAMP-SEQ=STEP256,NSAMP=12, 1 DEF PAR 4 WITH 3 CUM NUM-POS=10,PATTERN=SPIRAL-DITH, DITH-SIZE=0.2 ------------------------------------------------------------------------------------------------------------------------------------ 4 NGC1705 NIC1 MULTIAC NIC1 F160W SAMP-SEQ=STEP256,NSAMP=12 1 DEF CUM ------------------------------------------------------------------------------------------------------------------------------------ 7881( 4) - 04/29/98 14:40 - [ 8] Summary Form for Proposal 7881 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures NIC2 NIC1 ------------------------------------------------------------------------------------------------------------------------------------ Configurations NIC2 NIC1 ------------------------------------------------------------------------------------------------------------------------------------ Opmodes MULTIACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters SAMP-SEQ=STEP256 NSAMP=12 NUM-POS=10 PATTERN=SPIRAL-DITH DITH-SIZE=0.2 ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ORIENT 80D TO 100D PAR 2 WITH 1 PAR 4 WITH 3 ORIENT 260D TO 280D ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F110W F160W ------------------------------------------------------------------------------------------------------------------------------------ Target Names NGC1705 NGC1569 ANY ------------------------------------------------------------------------------------------------------------------------------------