Andrew Bunker

Abstract:

We report the serendipitous discovery of two objects close in projection with fairly strong emission lines at long wavelength (λ ∼ 9190 Å). One (A) seems not to be hosted by any galaxy brighter than V555 = 27.5, or I814 = 26.7 (Vega-based 3 σ limits in 1″.0 diameter apertures), while the other line is associated with a faint (I814 ≃ 24.4) red galaxy (B) offset by 2″.7 and 7 Å spectrally. Both lines are broad (FWHM ≈ 700 km s-1), are extended spatially, and have high equivalent widths [Wobsλ(A) > 1225 Å, 95% confidence limit; Wobsλ(B) ≈ 150 Å]. No secondary spectral features are detected for galaxy A. Blue continuum and the marginal detection of a second weak line in the spectrum of galaxy B is consistent with [O II] (the strong line) and Mg II (the weak line) at z = 1.466. By association, galaxy A is likely at z = 1.464, implying a rest-frame equivalent width of the [O II] emission line in excess of 600 Å and a projected separation of 30 h-150 kpc for the galaxy pair. Conventional wisdom states that isolated emission lines with rest-frame equivalent widths larger than ∼ 200 Å are almost exclusively Lyα. This moderate-redshift discovery therefore compromises recent claims of high-redshift Lyα emitters for which other criteria (i.e., line profile, associated continuum decrements) are not reported. We discuss observational tests to distinguish Lyα emitters at high redshift from foreground systems.

Abstract:

We present deep near-infrared Keck Near-Infrared Camera (NIRC) imaging of a recently discovered z = 4.04 galaxy. This is lensed by the rich foreground cluster Abell 2390 (z ≈ 0.23) into highly magnified arcs ″-5″ in length. Our H and K′ band NIRC imaging allows us to map the Balmer + 4000 Å break amplitude. In combination with high-quality archival Hubble Space Telescope (HST) WFPC2 data, we can spatially resolve stellar populations along the arcs. The WFPC2 images clearly reveal several bright knots, which correspond to sites of active star formation. However, there are considerable portions of the arcs that are significantly redder, consistent with being observed ≳ 100 Myr after star formation has ceased. Keck Low-Resolution Imaging Spectrograph (LRIS) long-slit spectroscopy along the arcs reveals that the Lyα emission is spatially offset by ≈1″ from the rest-UV continuum regions. We show that this line emission is most probably powered by star formation in neighboring H II regions and that the z = 4 system is unlikely to be an active galactic nucleus.