tokens: , H β and H α , and narrow [O iii ] λλ 4959, 5007, and Balmer lines. The absorption features around 5450 Å are telluric. The arc spectrum shows some narrow emission lines, such as [O ii ] λ 3728, H β , [O iii ] λ 5007, H α , and [N ii ] λ 6583, and some weak absorption lines, such as Ca ii K λ 3933 and Ca ii H λ 3968. The arc spectrum is contaminated by the QSO, showing broad H α and Mg ii lines (see Section 2.2.2 for details). The redshift of the QSO is measured to be 0.3954 ± 0.0001 from the [O iii ] λλ 4959, 5007 emission lines. The redshift of the arc, using the QSO-contamination-subtracted spectrum, is measured to be 0.3970 ± 0.0001 via [O ii ] and H β emission lines, and consistent with the value of 0.3968 ± 0.0003 measured via the Ca ii K λ 3933 and Ca ii H λ 3968 absorption lines. The arc is likely located nearer than the QSO and moving toward it with a line-of-sight velocity of roughly 340 km s −1 . Figure 4. VLT FORS2 spectrum of the QSO PHL 6625 with model components. We fitted the spectrum from 2750 to 7100 Å, where the S/N is high. The windows used to fit the continuum are marked in black bars, and pixels with telluric absorptions around 5450 Å were excluded in the fitting. The dereddened spectrum is shifted to the rest frame of the QSO at z = 0.3954. Purple: power law; yellow: pseudo-continuum = power law + Fe ii templates; red: summed line model; blue: narrow-line components; green: broad-line components. Figure 5. VLT FORS2 dereddened spectrum (black) of the arc galaxy with the QSO contamination (blue), the galaxy population synthesis model (green), and the total model (red). All emission lines and telluric absorptions are masked during the population synthesis fit. The dereddened spectrum is shifted to the rest frame of the arc at z = 0.3970. 2.2.1. Decomposition of the QSO Spectrum The QSO spectrum is decomposed into multiple emission components following Shen et al. ( 2008 ). The fitting is performed in the rest-frame wavelength range of 2750–7100 Å, where the S/N is sufficiently high. First, a pseudo-continuum was fitted to the spectrum in some continuum windows 12 12 2925–3500 Å, 4200–4230 Å, 4435–4700 Å, 5100–5535 Å, 6000–6250 Å, and 6800–6900 Å at the rest frame. , consisting of a power-law component and Fe ii templates in both the Mg ii region (Salviander et al. 2007 ) and the H β region (Boroson Green 1992 ). The continuum-subtracted line spectrum was then fitted with multiple Gaussian components: three Gaussians for each of the H α , H β , and Mg ii broad components; five Gaussians for narrow lines in the H α region; 13 13 Two for [S ii ], two for [N ii ], and one for narrow H α . three near H β; 14 14 Two for [O iii ] and one for narrow H β . and one for narrow Mg ii . All of the narrow lines are imposed to have the same shift and width. The monochromatic continuum luminosity λL λ of the power-law component at the rest frame 3000 and 5100 Å, the power-law spectral index, and the broad-line width derived from the
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