----- Submission 1 (Tue May 16 18:26:19 GMT 2006) ----- ----- Attempting Submission 2 (Mon Apr 30 16:01:44 GMT 2007) ----- Assigned ID: 419 ----- Submission 1 (Fri Jan 27 21:15:28 GMT 2006) ----- The Largest Galaxies in the Local Universe: New Light on Disk Galaxy Formation? In the standard scenario of disk galaxy formation in a hierarchical Universe, large disks form late via the accretion of either hot or cold gas. Direct observational evidence for such late accretion-driven disk formation has not been forthcoming. In this proposal, we describe the discovery of a rare new type of galaxy that may be examples of massive disks in the process of assembly. We have identified a sample of three such galaxies selected from the SDSS DR4. They are extremely large (diameters over 100 kpc) and highly luminous systems with amorphous structures (no obvious spiral arms or bulges). They are larger than the largest normal spirals in the survey, and have significantly bluer colors, lower metallicities, lower dust extinctions, higher UV luminosities and higher total star formation rates than the most massive ordinary spirals. We request HST images in the rest-frame near-UV and red to provide detailed maps of the underlying structure of these galaxies as well as the distribution of the young stars. The interstellar medium of these galaxies is evidently quite different from that of normal large spirals and starburst galaxies and they may be experiencing a different mode of star formation. We believe they are worthy of further investigation with the high-resolution imaging capabilities of HST.
General Considerations We propose to observe 3 giant blue amorphous galaxies with the ACS WFC in two bands. We will use the F435W filter to trace the young star forming population. These images correspond to a rest-frame band centered at ~3400 Angstrom (below the 4000 Angstrom break, and dominated by light from stars younger than ~100 Myr). We will use the F814W filter (rest-frame ~6500 Angstroms) to map out the older (> Gyr) population of stars and elucidate the underlying structure of these galaxies. We have chosen these two specific filters to maximimize sensitivity and provide a large wavelength range to increase our sensitivity to variations in the stellar population. It is important to emphasize that the extremely blue colors of these galaxies imply that the amount of dust extinction in these galaxies is modest, ensuring that the images will provide reliable information about the distribution of young stars. These images will resolve structures as small as ~0.4 kpc at the median redshift of our sample (z ~ 0.29). These galaxies have half-light diameters of ~40 to 50 kpc, so the images should provide exquisite detail. Feasibility Our goal is achieve adequate signal-to-noise for compact (sub-arcsec) relatively high surface brightness features. We are proposing elsewhere to obtain the deep ground-based images to be used to study the faint outer regions of these galaxies. In what follows, we will couch our estimates in terms of the S/N per ACS resolution element (~0.1 arcsec) for a region having a surface brightness equal to the mean value interior to the galaxy half-light radius (e.g. the effective surface brightness). F435W: The galaxies have effective surface brightnesses in this band ranging from 1 to 2x10^-18 erg s^-1 cm^-2 Angstrom^-1 arcsec^-2. In a single orbit with two CR-Split observations the ACS ETC yields a S/N of 2.5 to 4 per 2-by-2 WFC pixel region. F814W: The galaxies have effective surface brightnesses in this band ranging from 1.0 to 1.5x10^-18 erg s^-1 cm^-2 Angstrom^-1 arcsec^-2. In a single orbit with two CR-Split observations the ACS ETC yields a S/N of 8.9 to 12.2 per 2-by-2 WFC pixel region. Cycle 16 modifications: =============== Visits 03 and 04 observe SDSSJ020707 for 4 orbits each in F450W and F814W with WFPC2. The aimpoint is PC, and we use a 4 point WFPC2 DITHER BOX pattern for every two orbits of observing time. Each 800 s exposure is split into two 400 s exposures. 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