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

3. Absorption lines in J2233-606

3.1. Q433 at

We searched the HST spectrum for absorptions around . The wavelength ranges of H I Ly together with C IV 1548, 1550 and N V 1238, 1242 around this redshift are shown in Fig. 3 on a relative velocity scale, v. Strong H I Ly and Ly absorption lines are detected at . The Ly line however is redshifted in a region of poor S/N below the Lyman break of the moderately optically thick system at and is most certainly blended with Ly absorption at a different redshift. More than one component are probably present since the continuum level at the bottom of Ly 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 line, Å, suggests a neutral hydrogen column density of at least H I cm^-2. A one-component fit gives H I cm^-2 and a Doppler parameter km s^-1. The latter large value of b provides additional evidence for multiple structure. We tentatively fit the line with three components at , and km s^-1 with H I, , cm^-2 and , 33 and 25 km s^-1 respectively. There might be a C IV 1548 component at km s^-1 but with no obvious C IV 1550 counterpart; the latter could be below the detection limit. N V 1242 absorption could be present at km s^-1. The N V 1238 counterpart is unseen because it is blended with a strong Ly line; and the associated C IV absorption is not detected. An absorption line is seen at the expected position of O VI 1031 but with no obvious O VI 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.

Fig. 3. Absorptions for the absorption-line system in the normalized J2233-606 spectrum. The Ly line profile suggests a multi-component structure. From bottom to top, H I Ly, 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 (, 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 lines with H I cm^-2 is about 5 per unit redshift (Petitjean et al. 1993). Assuming no dependence on redshift, the probability that a randomly placed Ly cloud with H I 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 -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 and 1.520 respectively, with an angular separation of 36", corresponding to an impact parameter of kpc (Petitjean et al. 1998). A metal-poor associated system is seen at along the line of sight to A, with a complex velocity profile. A strong Ly 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 kpc around Q433 or is due to a galaxy at a similar redshift to Q433.

3.2. G486 at

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

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

It is well established that bright () galaxies within 40 kpc from the line of sight to a QSO produce strong ( 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 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, Å is observed. This line is probably Mg II 2796 at . The limit on the corresponding weaker Mg II 2803 line is consistent with the optically thin case. The Mg II absorption is quite weak for a galaxy with 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 km s^-1 and Mg II (Outram et al. 1998). We observe two galaxies at a distance smaller than 40" (or 130 kpc) from J2233-606 at redshifts and 0.4148, (G1096 and G496 in Table 1), while a third one with (G1109) is slightly outside the 1´ radius ("). The well-defined I-band selected CFRS redshift distribution gives 0.380.02 and 0.520.04 galaxies at 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 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 limit at 3 is 0.10, 0.13 and 0.05 Å respectively at , 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).

© European Southern Observatory (ESO) 1999

Online publication: May 21, 1999
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