8234( 4) - 03/16/00 15:04 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 8234 Version: 4 Check-in Date: 12-May-1999 14:05:22 1.Proposal Title: Calibrating Star Formation with the Metal-Rich Starburst Galaxy M83 (NGC5236) ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 8 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Daniela Calzetti Space Telescope Science Institute CoI: Prof. John S. Gallagher III University of Wisconsin N CoI: Dr. Denise A. Smith Space Telescope Science Institute N CoI: Dr. Anne L. Kinney Space Telescope Science Institute N CoI: Christopher J. Conselice University of Wisconsin N ------------------------------------------------------------------------------------ 5. Abstract A large fraction of the active star-formation in the local and intermediate-redshift Universe happens in relatively metal- rich regions of galaxies. We propose to study the structure of the prime nearby example of a metal-rich, actively star- forming environment: the starburst located in the nuclear region of the spiral galaxy M83. In M83 the PC gives 1 pc resolution so we can chart the detailed structure of this dusty but UV-bright system. For this purpose we will obtain broad-band images in the mid-UV, V, and I bands to locate and characterize stars, star clusters, and obscuring dust clouds, as well as HAlpha, HBeta, OIII, and SII narrow band images to diagnose conditions in the ionized ISM. We will combine these datasets to investigate the presence of sequential/propagating star formation and establish whether the starburst is a large- scale coherent or a random process. For this purpose we will measure: (1) the recent star formation history of both stellar clusters and young diffuse population; (2) the dust distribution; (3) the structures in the shocked and photo- ionized gas. This program will allow us to quantitatively place M83 in the broader context of starbursts in terms of the energetics, evolution, and internal structures of a key class of burst events, while also exploring its impact on the surrounding galaxy. ------------------------------------------------------------------------------------ 8234( 4) - 03/16/00 15:04 - [ 2] Observations Description ------------------------ Strategy, Intrument and Choice of Filters. The strategy we propose for this observation has been already successfully employed for the HST observations of the dwarf starburst NGC5253 (GO-6524, P.I. Calzetti, see Calzetti et al. 1997). The center of M83 will be observed with the PC camera, to achieve maximum spatial resolution, as 1 PC pixel corresponds to 1 pc at the distance of the galaxy (~4.5 Mpc). Young stellar clusters, which have half-light radii ~1.5--5 pc will be resolved (Meurer et al. 1995, Calzetti et al. 1997). Our ground- based HAlpha image shows that the starburst is <40^ in size, perfectly matched to the PC Field-of-View (cf. Fig. 1). The WF chips will provide photometry and colors of the stellar population underlying the starburst, which we will use to subtract this component from the starburst population. The starburst will be observed in the mid-UV (3, 000 Angstrom), V, and I filters, and in HBeta, OIII(5007), HAlpha, and SII(6717, 6731). At the distance of M83, the PC resolves the stellar clusters, but not the individual stars (except the brightest); we will thus derive ages by comparing the observations with models of stellar populations (Fig. 2). The massive, young stars produced by the starburst emit the bulk of their energy in the UV (Lambda<0.35Mum) and the intrinsic spectral energy distribution longward of 2000 Angstrom of young populations is weakly dependent on the metallicity and the IMF details (Leitherer & Heckman 1995). The flux at 3000 Angstrom is dominated by stars with M>5 M_\odot, probing star formation averaged over ~100 Myr. The V band maps the older stars, while the I band probes the red supergiants. The HAlpha emission maps the ionized gas and the star formation over <10 Myr. The HBeta/HAlpha ratio maps the dust reddening distribution of the ionized gas and its variations, which we will convert to reddening corrections for the stellar population via the recipe of Calzetti et al. (1997), in order to remove the age -reddening degeneracy. Reddening-corrected color-magnitude and color-color diagrams (see Fig. 2), and the number of ionizing photons map the age and age variations of the diffuse and clustered young stellar population, in the range 0--1 Gyr and 0--100 Myr, respectively. The precision will be Deltaage/age=0.2-- 0.3 for the clusters and Deltaage/age=0.4--0.5 for the diffuse population (Calzetti et al. 1997), which is adequate for drawing a coherent picture of the star formation history in the starburst. The brightest isolated stars will be resolved, but investigations of the high-mass stellar function will be hampered by its close entanglement with the star formation history (Massey et al. 1995). OIII/HBeta ratios will map the sub-arcsec distribution and intensity of the most active star forming regions, while SII/HAlpha is sensitive to shocks and will map the shock structure. The WFPC2 broad- and medium- band filters F300W, F547M, and F814W will be employed as mid- UV, ~V, and ~I. The narrow-band filters F487N, F502N, F656N, and F673N will map the line emission of HBeta4861Angstrom, OIII5007Angstrom, HAlpha6563Angstrom+NII6548Angstrom, and SII6717, 6731Angstrom. The three continuum filters are selected to avoid strong emission lines in the passbands (e.g., HAlpha). The F300W is the best compromise between throughoput and dynamical range for the massive stars (Fig. 2; see below for why WFPC2 is chosen over STIS UV imaging). It has a small red leak, which is not a concern for this project, since we aim at mapping young stellar populations. The redshift of the galaxy (v= 516 km/s) is small enough that the emission lines fall almost at the center of the filters' bandpasses. In addition, the F656N filter is narrow enough that the strong NII6584Angstrom emission line is outside the band. The broad-band images will be used for the continuum subtraction, a very successful method with WFPC2 thanks to its photometric stability (e.g., Calzetti et al. 1997). Why Not STIS for UV? STIS is more sensitive in the UV than WFPC2, and could have provided maps of the blue stellar component at wavelengths shorter than 3, 000 Angstrom, for easier discrimination of the youngest stars. STIS UV imaging, however, is not an option for this project because three of the stellar clusters in the center of M83 approach or exceed the Bright Object Limit for point sources. Our estimates are power law extrapolations of the photometry on the WFPC1 images in F336W, F547M, and F814W (after correction for spherical aberration). Two of the clusters give F(1800 Angstrom)~eq1.4 E-14 erg/s/cm^2/Angstrom, well above the Bright Object Limit at this wavelength. As mentioned above, the WFPC2 F300W filter is still perfectly adequate to achieve the goals of the proposal. Exposure times. The maximum tolerable uncertainty in the ages requires to determine colors of stellar clusters and diffuse population with an accuracy of ~0.2 mag. Drawing experience from our previous HST observations of NGC5253 and rescaling to M83, we can achieve such accuracy with a 5 Sigma surface brightness detection limit of ABsimeq19.5 mag arcsec^-2 at 3000 Angstrom, 5500 Angstrom, and 8000 Angstrom, where the constant surface brightness with wavelength takes into account the blue spectral energy distribution of the starburst. This limit correponds to a 5 Sigma detection for a cluster of 3 main sequence B1-B2 stars at the distance of M83 (V=25.3), which is what is needed for the success of the program. S/N=5 per pixel for such magnitudes is obtained with CR-SPLIT exposures of 2000 sec in F300W, 1000 sec in F547M, and 700 sec in F814W (cf. the WFPC2 WWW exposure times calculator). We request relatively deep images for the emission lines HAlpha and HBeta, because they will be used to correct for dust reddening the stellar continuum, and thus need to reach relatively faint surface brightnesses. For the metal line emission of OIII and SII we request a more modest depth, as we are mostly interested in the small- scale emission structure in the high surface brightness regions in the starburst; for the large-scale structure, we will use our ground-based images. The average surface brightnesses of the nebular emission lines are F(hbeta)=10^ -14 erg s^-1 cm^-2 arcsec^-2, F(OIII)=5times10^-15 erg s^-1 cm^-2 arcsec^-2, F(halpha)=4*10^-14 erg s^-1 cm^-2 arcsec^-2, and F(SII)=5times10^-15 erg s^-1 cm^-2 arcsec^-2, as derived from our ground-based images and large-aperture spectroscopy of the galaxy. These average values are conservative estimates, since the nebular gas will be clumped (and thus brighter) on small scales. Taking into account the contamination of the underlying stellar continuum, S/N=5 per pixel is achieved in 6, 000 sec for HBeta and 1200 sec for HAlpha. For OIII and SII we request 2, 500 sec and 1, 200 sec of exposure (CR-SPLIT), respectively, which will achieve S/N=2--2.5 per pixel for the average surface brightness. With a typical target orbital visibility of 52^m, a total of 6 orbits are required for guide star acquisition, instrument set-ups, and sequence exposures. The 6 orbits are split in: 1.5 orbits for the three stellar continuum filters, 3 orbits for HBeta and HAlpha, and 1.5 orbits for OIII and SII. Real Time Justification ----------------------- We have recently obtained ground-based U, V, R and narrow- band (OIII, HAlpha, and SII) images of M83, which we are using to investigate the large-scale distribution of the photoionized and shock components of the gas, and its relation to the starburst. The ground-based maps are very deep, with typical detection limits of 10^-20 erg/s/cm^2/arcsec^2 in the continuum filters and 10^ -18 erg/s/cm^2/arcsec^2 in the narrow-band filters. Spatial resolutions are typical of ground-based observations, sim1^ . We have thus pushed the ground- based capabilities to the limit, and any step further in the quest for understanding evolution of the starburst in M83 requires the sub-arcsecond resolution of HST. We are also applying for observing time at ground-based facilities to obtain high resolution spectroscopy to perform a kinematical study of the ionized gas in M83. NICMOS observations in broad- and narrow-band filters have been recently obtained by the GTO (ID 7218); once public, we will use the infrared images to further constrain the starburst population and the dust geometry. The investigators in this proposal have been involved in various HST GO and GTO projects for the study of stellar populations and gas emission in starbursts with WFPC2. We are currently working on a HST archival program to investigate structures and evolution of starburst galaxies, with emphasis on UV imaging. Our search through the HST archive has yielded several suitable images, all of them of sub-solar metallicity starbursts. We plan to use the archival study as a term of comparison for the M83 project. Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 8234( 4) - 03/16/00 15:04 - [ 3] Data Distribution Paper Products: NO Media: ELECTRONIC Blocking Factor: 10 Ship To: PI_Address Ship Via: Email: ------------------------------------------------------------------------------------ 8234( 4) - 03/16/00 15:04 - [ 4] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 M83 GALAXY, STARBURST, RA=13H 37M 00.75S +/- 2.0S, DEC= 2000 V=+516 SURF(V) = 6.5e-18 SPIRAL -29D 51' 57.0" +/- 2" 8234( 4) - 03/16/00 15:04 - [ 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 ------------------------------------------------------------------------------------------------------------------------------------ 1 M83 WFPC2 IMAGE PC1-FIX F300W 2943 CR-SPLIT=NO 1 700 S ------------------------------------------------------------------------------------------------------------------------------------ 2 M83 WFPC2 IMAGE PC1-FIX F300W 2943 CR-SPLIT=NO 1 700 S ------------------------------------------------------------------------------------------------------------------------------------ 3 M83 WFPC2 IMAGE PC1-FIX F300W 2943 CR-SPLIT=NO 1 700 S ------------------------------------------------------------------------------------------------------------------------------------ 4 M83 WFPC2 IMAGE PC1-FIX F547M 5470 CR-SPLIT=NO 1 180 S ------------------------------------------------------------------------------------------------------------------------------------ 5 M83 WFPC2 IMAGE PC1-FIX F547M 5470 CR-SPLIT=NO 1 350 S ------------------------------------------------------------------------------------------------------------------------------------ 6 M83 WFPC2 IMAGE PC1-FIX F547M 5470 CR-SPLIT=NO 1 400 S ------------------------------------------------------------------------------------------------------------------------------------ 7 M83 WFPC2 IMAGE PC1-FIX F814W 7920 CR-SPLIT=NO 1 160 S ------------------------------------------------------------------------------------------------------------------------------------ 8 M83 WFPC2 IMAGE PC1-FIX F814W 7920 CR-SPLIT=NO 1 200 S ------------------------------------------------------------------------------------------------------------------------------------ 9 M83 WFPC2 IMAGE PC1-FIX F814W 7920 CR-SPLIT=NO 1 350 S ------------------------------------------------------------------------------------------------------------------------------------ 8234( 4) - 03/16/00 15:04 - [ 6] Visit: 02 Visit Priority: Visit Requirements: SAME ORIENT AS 1 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 M83 WFPC2 IMAGE PC1-FIX F487N 4865 CR-SPLIT=NO 1 1100 S ------------------------------------------------------------------------------------------------------------------------------------ 2 M83 WFPC2 IMAGE PC1-FIX F487N 4865 CR-SPLIT=NO 1 1200 S ------------------------------------------------------------------------------------------------------------------------------------ 3 M83 WFPC2 IMAGE PC1-FIX F502N 5012 CR-SPLIT=NO 1 1200 S ------------------------------------------------------------------------------------------------------------------------------------ 4 M83 WFPC2 IMAGE PC1-FIX F502N 5012 CR-SPLIT=NO 1 1200 S ------------------------------------------------------------------------------------------------------------------------------------ 5 M83 WFPC2 IMAGE PC1-FIX F673N 6732 CR-SPLIT=NO 1 1200 S ------------------------------------------------------------------------------------------------------------------------------------ 6 M83 WFPC2 IMAGE PC1-FIX F673N 6732 CR-SPLIT=NO 1 1200 S ------------------------------------------------------------------------------------------------------------------------------------ 7 M83 WFPC2 IMAGE PC1-FIX F487N 4865 CR-SPLIT=NO 1 1000 S ------------------------------------------------------------------------------------------------------------------------------------ 8 M83 WFPC2 IMAGE PC1-FIX F656N 6564 CR-SPLIT=NO 1 600 S ------------------------------------------------------------------------------------------------------------------------------------ 9 M83 WFPC2 IMAGE PC1-FIX F656N 6564 CR-SPLIT=NO 1 600 S ------------------------------------------------------------------------------------------------------------------------------------ 8234( 4) - 03/16/00 15:04 - [ 7] Summary Form for Proposal 8234 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures PC1-FIX ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFPC2 ------------------------------------------------------------------------------------------------------------------------------------ Opmodes IMAGE ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CR-SPLIT=NO ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements SAME ORIENT AS 1 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F300W F547M F814W F487N F502N F673N F656N ------------------------------------------------------------------------------------------------------------------------------------ Target Names M83 ------------------------------------------------------------------------------------------------------------------------------------ Wavelengths 2943 5470 7920 4865 5012 6732 6564 ------------------------------------------------------------------------------------------------------------------------------------