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Astron. Astrophys. 346, L21-L24 (1999)

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3. Absorption lines in J2233-606

3.1. Q433 at [FORMULA]

We searched the HST spectrum for absorptions around [FORMULA]. The wavelength ranges of H I Ly[FORMULA] together with C IV [FORMULA]1548, 1550 and N V [FORMULA]1238, 1242 around this redshift are shown in Fig. 3 on a relative velocity scale, v. Strong H I Ly[FORMULA] and Ly[FORMULA] absorption lines are detected at [FORMULA]. The Ly[FORMULA] line however is redshifted in a region of poor S/N below the Lyman break of the moderately optically thick system at [FORMULA] and is most certainly blended with Ly[FORMULA] absorption at a different redshift. More than one component are probably present since the continuum level at the bottom of Ly[FORMULA] goes to zero over about 150 km s-1 but neither damping wings nor an associated Lyman break are present. The total equivalent width of the Ly[FORMULA] line, [FORMULA] Å, suggests a neutral hydrogen column density of at least [FORMULA]H I[FORMULA] cm-2. A one-component fit gives [FORMULA]H I[FORMULA] cm-2 and a Doppler parameter [FORMULA] km s-1. The latter large value of b provides additional evidence for multiple structure. We tentatively fit the line with three components at [FORMULA], [FORMULA] and [FORMULA] km s-1 with [FORMULA]H I[FORMULA], [FORMULA], [FORMULA] cm-2 and [FORMULA], 33 and 25 km s-1 respectively. There might be a C IV [FORMULA]1548 component at [FORMULA] km s-1 but with no obvious C IV [FORMULA]1550 counterpart; the latter could be below the detection limit. N V [FORMULA]1242 absorption could be present at [FORMULA] km s-1. The N V [FORMULA]1238 counterpart is unseen because it is blended with a strong Ly[FORMULA] line; and the associated C IV absorption is not detected. An absorption line is seen at the expected position of O VI [FORMULA]1031 but with no obvious O VI [FORMULA]1037 counterpart; the corresponding part of the spectrum has a poor S/N ratio however. The presence of metals in the cloud is thus questionable; better data in the optical range will help decide this issue.

[FIGURE] Fig. 3. Absorptions for the [FORMULA] absorption-line system in the normalized J2233-606 spectrum. The Ly[FORMULA] line profile suggests a multi-component structure. From bottom to top, H I Ly[FORMULA], N V 1238, N V 1242, C IV 1548, C IV 1550.

The good correspondence between the redshift of Q433 and the Lyman absorption redshift in the J2233-606 spectrum ([FORMULA], [FORMULA] km s-1) might only be coincidence. The absorption could in fact be due to gas associated with an object in the QSO's immediate environment. We note that the number density of Ly[FORMULA] lines with [FORMULA]H I[FORMULA] cm-2 is about 5 per unit redshift (Petitjean et al. 1993). Assuming no dependence on redshift, the probability that a randomly placed Ly[FORMULA] cloud with [FORMULA]H I[FORMULA] cm-2 is observed within 200 km s-1 from the redshift of Q433 along the line of sight to J2233-606 is smaller than 0.01. This probability is not highly significant since it is an a-posteriori statistical argument. Note that Savaglio et al. (1999) have shown that the region spanning [FORMULA]-1.460 has a low density of absorption lines with five lines detected when 16 are expected from the average density along the line of sight. This possible `transverse proximity effect' is at odds with the presence of the strong line at the same redshift as Q433. A similar situation has been observed along the lines of sight to Q1026-0045A,B, two QSOs at [FORMULA] and 1.520 respectively, with an angular separation of 36", corresponding to an impact parameter of [FORMULA] kpc (Petitjean et al. 1998). A metal-poor associated system is seen at [FORMULA] along the line of sight to A, with a complex velocity profile. A strong Ly[FORMULA] absorption is detected along the line of sight to B, redshifted by only 300 km s-1 relative to the associated system in A.

Follow-up spectroscopic studies of the field will investigate whether this QSO/absorption association is a consequence of the presence of a gaseous disk, halo or other gaseous structure of radius larger than 200[FORMULA] kpc around Q433 or is due to a galaxy at a similar redshift to Q433.

3.2. G486 at [FORMULA]

The line of sight to J2233-606 passes through the disk (seen approximatively face-on) of a late-type spiral galaxy at an impact parameter of only [FORMULA] kpc. This is a situation where conspicuous metal absorptions, and perhaps damped H I Ly[FORMULA], are expected. H I absorption associated with this galaxy is seen in the Lyman series (see Fig. 4) at [FORMULA]. Uncertainties are too large to reliably estimate the column density from fitting the lines. However, the fit of the Lyman limit (912Å) gives [FORMULA]H I[FORMULA] cm-2 (Outram, private communication). Because of the poor spectral resolution of the G140L spectrum, the presence of C III [FORMULA]977 and C II [FORMULA]1036 cannot be ruled out, and the C IV and Al III doublets are most certainly blended. There is no Fe II [FORMULA]2600 absorption at [FORMULA]4083.3 in the AAT spectrum (Outram et al. 1998) down to a 3[FORMULA] limit [FORMULA] Å. The lack of Fe II absorption is consistent with a low H I column density. Note that Fe II [FORMULA]2382 is lost in a strong Ly[FORMULA] complex.

[FIGURE] Fig. 4. Absorptions for the [FORMULA] absorption-line system in the normalized J2233-606 spectrum. From bottom to top, H I Ly[FORMULA], Ly[FORMULA], Ly[FORMULA] and C II 1036, C III 977.

It is well established that bright ([FORMULA]) galaxies within 40[FORMULA] kpc from the line of sight to a QSO produce strong ([FORMULA] Å) Mg II absorption (e.g. Bergeron & Boissé 1991, Steidel et al. 1994) whereas fainter galaxies with a similar range of impact parameters do not produce detectable metal-line absorptions (Steidel et al. 1997). In the present case, a weak absorption line at [FORMULA]4390.66 is detected both in the AAT spectrum and in a spectrum recently obtained at ESO (V. D'Odorico et al., private communication). In the ESO spectrum, [FORMULA] Å is observed. This line is probably Mg II [FORMULA]2796 at [FORMULA]. The limit on the corresponding weaker Mg II [FORMULA]2803 line is consistent with the optically thin case. The Mg II absorption is quite weak for a galaxy with [FORMULA] and such a small impact parameter: this is inconsistent with the correlation between the impact parameter and the strength of the absorption claimed by Lanzetta & Bowen (1990).

3.3. Other galaxies around J2233-606

A single component Mg II system is seen at [FORMULA] [FORMULA] km s-1 and [FORMULA]Mg II[FORMULA] (Outram et al. 1998). We observe two galaxies at a distance smaller than 40" (or 130[FORMULA] kpc) from J2233-606 at redshifts [FORMULA] and 0.4148, (G1096 and G496 in Table 1), while a third one with [FORMULA] (G1109) is slightly outside the 1´ radius ([FORMULA]"). The well-defined I-band selected CFRS redshift distribution gives 0.38[FORMULA]0.02 and 0.52[FORMULA]0.04 galaxies at [FORMULA] by square arcmin in the respective redshift ranges [0.30-0.40] and [0.40-0.50] (see Lilly et al. 1995). Thus the three galaxies observed in a 0.0001 redshift range represent a density far in excess of the expected mean. This overdensity of galaxies at [FORMULA] suggests that other objects closer to the QSO are responsible for the absorption. A possible candidate is object G484 (see Fig. 1), at a distance of 18.2", resolved in the HST image into an interacting pair of spirals.

For the other galaxies (G1143, G502, G483), no conspicuous Mg II is found, i.e. the [FORMULA] limit at 3[FORMULA] is 0.10, 0.13 and 0.05 Å respectively at [FORMULA], 0.227, 0.330. This is consistent with the halo radius-luminosity scaling-law found for Mg II absorption-selected galaxies (Bergeron & Boissé 1991, Steidel et al. 1994).

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© European Southern Observatory (ESO) 1999

Online publication: May 21, 1999
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