z ∼ 7 GALAXIES IN THE HUDF: FIRST EPOCH WFC3/IR RESULTS

We present a sample of 16 robust z similar to 7 z(850)-drop galaxies detected by the newly installed Wide Field Camera 3 (WFC3)/IR on the Hubble Space Telescope. Our analysis is based on the first epoch data of the HUDF09 program covering the Hubble Ultra Deep Field with 60 orbits of Y(105), J(125), and H(160) observations. These remarkable data cover 4.7 arcmin(2) and are the deepest near infrared images ever taken, reaching to similar to 29 mag AB (5 sigma). The 16 z similar to 6.5-7.5 galaxies have been identified based on the Lyman Break technique utilizing (z(850) - Y(105)) versus (Y(105) - J(125)) colors. They have magnitudes J(125) = 26.0-29.0 (AB), an average apparent half-light radius of similar to 0.16 arcsec (less than or similar to 1 kpc), and show very blue colors (some even beta less than or similar to -2.5), in particular at low luminosities. TheWFC3/IR data confirm previous Near Infrared Camera and Multi-Object Spectrometer detections indicating that the dropout selection at z similar to 7 is very reliable. Our data allow a first determination of the faint-end slope of the z similar to 7 luminosity function, reaching down to M(UV) similar to -18, a full magnitude fainter than previous measurements. When fixing phi(*) = 1.4 x 10(-3) Mpc(-3) mag(-1) to the value previously measured at z similar to 6, we find a best-fit value of alpha = -1.77 +/- 0.20, with a characteristic luminosity of M(*) = - 19.91 +/- 0.09. This steep slope is similar to what is seen at z similar to 2-6 and indicates that low-luminosity galaxies could potentially provide adequate flux to reionize the universe. The remarkable depth and resolution of these new images provide insights into the coming power of the James Webb Space Telescope.