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	Figure 1: SDSS spectrum ( ) of SDSS J0011+0055. Top: the full spectrum. is in units of 10-17 ergs cm-2 s-1 Å-1. Bottom: a portion of the spectrum illustrating the Si IV  1393.75, 1402.77 and C IV   1548.20, 1550.78 doublets. Absorption lines at z = 2.29071 (part of system A) and at z = 2.05844 (part of system B) are indicated.
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In the text

Figure 2: Portions of the UVES spectrum of SDSS J0011+0055 illustrating the absorption line systems A and B defined in Table 1. System B is best seen in C IV and system A in Si IV (C IV at 2.29 is outside the observed UVES spectral range). The following transitions are indicated: N V  1238.82, 1242.80, C IV  1548.20, 1550.78, Si IV  1393.75, 1402.77, Mg II  2796.35, 2803.53, Si II  1260.42, 1264.74, 1265.00, C II  1334.53, 1335.66, 1335.71, and Ly 1215.67. For clarity, the two reddest and very close transitions of Si II and C II are represented by single marks at the mean wavelengths; all four of those transitions are excited-state transitions. Absorption from the high-velocity system B is not detected in Si II or C II, and therefore is not plotted although it lies well within the observed UVES spectral range (Si II 1260.42 could be present in system B1 but the detection is not convincing). Narrow intervening Mg II 2796.35, 2803.53 absorption contaminates the Si II absorption of system A2. Narrow intervening C IV 1548.20 lines are seen slightly longward of the Si IV absorption from system B, along with a broader instrumental artifact. All spectra are plotted on a velocity scale of v/c = (R2-1)/(R2+1) where , being the observed wavelength, the laboratory vacuum rest wavelength of the red transition of the multiplets, and the systemic redshift of the quasar.

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In the text

Figure 3: The covering factor and opacity of resonance doublets seen in the low-velocity absorption system A and computed from Eq. (2). From top to bottom: the normalized residual intensities (blue) and (red); the normalized residual intensity (blue) and the square of the normalized residual intensity (black); the covering factor ; the opacity expressed as e . Horizontal dashed lines indicates the range [0, 1] over which these quantities have a physical meaning. Unphysical values of and e have been cut to a maximum of 1.1 and a minimum of -0.1.

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In the text

	Figure 4: Same as Fig. 3 but for the high-velocity absorption system B.
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In the text

	Figure 5: Part of the spectrum illustrating the C II 1334.53, C II 1335.70, Si II 1260.42, and Si II 1264.76 transitions in system A1 (cf. Fig. 2). In order to emphasize these features, the spectra are divided by a local continuum, and smoothed using median filtering on 20 km s-1 boxes. The vertical marks indicate the velocities predicted for these absorption lines at z = 2.29071 (A1).
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In the text

	Figure 6: C II and Si II column density ratios as a function of the electron density in cm-3. The Si II curves are labeled with the electron temperature; the C II curves follow the same trend.
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In the text