2010ApJ...709L..16O

We present a sample of 16 robust z ∼ 7 z₈₅₀-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 Y₁₀₅, J₁₂₅, and H₁₆₀observations. These remarkable data cover 4.7 arcmin2 and are the deepest near infrared images ever taken, reaching to ∼29 mag AB (5σ). The 16 z ∼ 6.5-7.5 galaxies have been identified based on the Lyman Break technique utilizing (z₈₅₀- Y₁₀₅) versus (Y₁₀₅- J₁₂₅) colors. They have magnitudes J₁₂₅=26.0-29.0 (AB), an average apparent half-light radius of ∼0.16 arcsec (≲1 kpc), and show very blue colors (some even β ≲ -2.5), in particular at low luminosities. The WFC3/IR data confirm previous Near Infrared Camera and Multi-Object Spectrometer detections indicating that the dropout selection at z ∼ 7 is very reliable. Our data allow a first determination of the faint-end slope of the z ∼ 7 luminosity function, reaching down to M_UV∼ -18, a full magnitude fainter than previous measurements. When fixing φ_*= 1.4x10^–3/Mpc3/mag to the value previously measured at z ∼ 6, we find a best-fit value of α = -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 ∼ 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.