A Redshift [CLC][ITAL]z[/ITAL][/CLC] = 6.56 Galaxy behind the Cluster Abell 370

Abstract

We report the discovery of a redshift z = 6.56 galaxy lying behind the cluster Abell 370. The object HCM 6A was found in a narrowband imaging survey using a 118Å bandpass filter centered at 9152Å in the LRIS camera on the 10 m Keck II Telescope. Candidate Lyα emitters were identified by the equivalent width of the emission and the absence of lower wavelength flux in ultradeep broadband images. HCM 6A is the first galaxy to be confirmed at redshift z > 6, and has W λ (observed)=190Å, flux = 2.7 × 10 −17 erg cm −2 s −1 . Spectra obtained with LRIS confirm the emission line and the continuum break across the line, and show an asymmetric line profile with steep fall-off on the blue side. Deep Subaru nearinfrared CISCO images in J, H and K ′ which extend the sampled continuum to longer wavelengths give a consistent estimate of the continuum flux density in these line-free regions of 2.6 ± 0.7 × 10 −30 erg cm −2 s −1 Hz −1 . The line width and strength, asymmetric profile, and very deep spectral break are only consistent with the interpretation of the line as a redshifted Lyα feature. From the detailed lensing model of this cluster, we estimate a lensing amplification of 4.5 for this galaxy, which is located slightly over an arcminute from the center of the cluster, for an unlensed flux of 6.5 × 10 −18 erg cm −2 s −1 . The presence of such a galaxy suggests that the reionizing epoch is beyond z = 6.6.

Luminous Lyman Break Galaxies atz > 5 and the Source of Reionization

“…Those selected by lensing (Ellis et al 2001) or narrowband imaging (Hu et al 2002;Rhoads et al 2003) have continuum magnitudes that are generally below our I-band limits. Others were serendipitously discovered or based on optical/IR colors (Weymann et al 1998).…”

“…With this in mind there has been an explosion in the number of galaxies and quasi-stellar objects (QSOs) discovered at the highest redshifts (z > 5: e.g., Hu et al 2002;Ellis et al 2001;Stern & Spinrad 1999;Dey et al 1998;Fan et al 2001;Rhoads et al 2003). These galaxies have been found using a variety of techniques ranging from narrowband imaging, to accidental alignments of spectroscopic slits on high-redshift objects, to using the dropout technique (Lyman break galaxies; LBGs) pioneered by Steidel et al (1999).…”

SPITZER ULTRA FAINT SURVEY PROGRAM (SURFS UP). II. IRAC-DETECTED LYMAN-BREAK GALAXIES AT 6 ≲ z ≲ 10 BEHIND STRONG-LENSING CLUSTERS

“…4. From the photometric redshifts and IRAC colors, we identify two galaxy candidates that likely have strong 24 HCM6A was first reported by Hu et al (2002), and later Chary et al (2005) published its IRAC fluxes. 25 Abell 370 is one of the Hubble Frontier Fields cluster.…”

Hubble Space TelescopeImaging and Keck Spectroscopy ofz≈ 6i‐Band Dropout Galaxies in the Advanced Camera for Surveys GOODS Fields

“…While large ground-based telescopes have made possible spectroscopic confirmation of redshifts for galaxies beyond z $ 6 (e.g., Hu et al 2002), this process is expensive in telescope time and is only possible for those objects with strong emission lines or that are lensed by intervening objects (e.g., Ellis et al 2001). As a result, the use of ''photometric redshift'' selection from broadband colors (such as the Lyman break technique of Steidel et al 1995) has become a widespread method of identifying large samples of high-redshift galaxies.…”