One‐Line Redshifts and Searches for High‐Redshift Lyα Emission

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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 10 diameter apertures), while the other line is associated with a faint (I814 24.4) red galaxy (B) offset by 27 and 7 Å spectrally. Both lines are broad (FWHM ≈ 700 km s-1), are extended spatially, and have high equivalent widths [W(A) > 1225 Å, 95% confidence limit; W(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 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.
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