Spectroscopically-Confirmed z > 6 Galaxies with Extremely Blue UV Slopes: Possible Pop III dominated targets for JWST spectroscopy

Fist submission of Jiang et al. HST Cycle 25 proposal on: ``Spectroscopically-Confirmed z > 6 Galaxies with Extremely Blue UV Slopes: Possible Pop III dominated targets for JWST'' Please confirm its receipt. Thanks! -40333531 Assigned ID: 60 ----- Attempting Submission 1 (Sun Apr 02 08:05:24 GMT 2017) ----- Here is our first attempt for phase II. It's all single orbit per filter with no orient constraints or parallels, so it is straightforward. I (seth.cohen@asu.edu) am the responsible party for phaseII. -34871694 ----- Attempting Submission 1 (Mon Jul 17 22:41:06 GMT 2017) ----- Spectroscopically-Confirmed z > 6 Galaxies with Extremely Blue UV Slopes: Possible Pop III dominated targets for JWST spectroscopy We propose to use HST/WFC3 to study 7 galaxies at z>6 with much bluer rest-frame UV continua than models of stellar populations and galaxy formation can accommodate. They were selected from a dedicated, large Subaru survey of bright spectroscopically-confirmed galaxies at z>6, and have extremely steep UV-slopes around beta=-3. Such slopes have never been found in low-z galaxies, nor have they been predicted by cosmological simulations. Although recent HST observations found some extreme UV-slopes in photometrically-selected galaxies at z>6, this is controversial, since it could be caused by contamination and bias in photometrically-selected samples. Our galaxies are bright (J=25-26.5 AB-mag) with secure spectroscopic redshifts, so their measurements of beta are NOT subject to these effects. They are the most promising candidates of truly extremely blue galaxies. Their uncertainties in beta are 0.3-0.5, mainly due to the short UV continuum baseline from previous HST data. We propose to add critical WFC3 near-IR images over a larger wavelength baseline to significantly improve the beta measurements by reducing uncertainties to <0.2. The new images will conclusively confirm the existence of beta = -3 galaxies at z>6. Such ultra-blue beta-values would suggest the existence of very young stellar populations with extremely low metallicity and dust content, which are possibly Pop III star dominated. Their confirmation will have significant impact on early galaxy formation, their star formation history, associated metallicity, dust, and even initial mass function. They would make very compelling targets for JWST spectroscopy. The HST/WFC3 data therefore must be completed now. Our observing strategy is straightforward (simple broad-band photometry). We will observe 7 spectroscopically confirmed galaxies at z >= 6 with 12 HST orbits covering 7 pointings. Five galaxies are in the the Subaru Deep Field (13h24m, 27d30m) and two are in the UKIDSS Ultra-Deep Survey field (2h18m, -5d10m). We will observe using specific WFC3-IR filters needed to complement previous observations, and provide the optimally wide wavelength baselines to measure the SED rest-frame UV slope of the target galaxies. Each pointing will be observed in a single orbit per filter and repeated for one, two, or three WFC3 bands, chosen from F105W, F125W, F140W, and F160W. The choice of filter depends on galaxy redshift and existing data. According to our existing WFC3 data, these galaxies span a magnitude range of 25.16 < J_AB < 26.66, and WFC3 is able to detect them with one-orbit integrations. The integration of each object per band will be broken into four individual exposures with sub-pixel dithering, to improve PSF and to remove bad pixels and CRs. These galaxies were chosen so that all of them can be observed within 12 HST orbits regardless of rolling angles (with fixed coordinates). We do not have any scheduling constraints or special requirements. Galaxies and the IGM Galaxies and the IGM 0 false false false false true false false SUPPORTED GALAXY HIGH REDSHIFT GALAXY J2000 J2000 J = 26.66 AB mag GALAXY HIGH REDSHIFT GALAXY J2000 J2000 J = 25.84 AB mag GALAXY HIGH REDSHIFT GALAXY J2000 J2000 J = 26.11 AB mag GALAXY HIGH REDSHIFT GALAXY J2000 J2000 J = 26.23 AB mag GALAXY HIGH REDSHIFT GALAXY J2000 J2000 J = 25.93 AB mag GALAXY HIGH REDSHIFT GALAXY J2000 J2000 J = 25.16 AB mag GALAXY HIGH REDSHIFT GALAXY J2000 J2000 J = 25.22 AB mag WFC3/IR Imaging No No F105W WFC3/IR Imaging No No F160W WFC3/IR Imaging No No F140W WFC3/IR Imaging No No F160W WFC3/IR Imaging No No F140W WFC3/IR Imaging No No F160W WFC3/IR Imaging No No F105W WFC3/IR Imaging No No F105W WFC3/IR Imaging No No F160W WFC3/IR Imaging No No F105W WFC3/IR Imaging No No F105W POS-TARG DITHER 0.572 LINE 0.365 false false 74.653 4 18.528 WFC3-IR-DITHER-BOX-MIN 1

false

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