Volume 525, January 2011
|Number of page(s)||13|
|Published online||01 December 2010|
The spectra of three blazars show very weak emission lines yielding new emission redshifts. They are presented in Fig. A.1.
In PKS 0332−403 (Fig. A.1, top panel), the weak emission at λ = 6580 Å is identified as Mg ii at zem = 1.351. As reported in Table 2, there is a weak Mg ii absorption doublet at zabs = 1.2083 with associated Fe ii absorptions (the λ2600 doublet and the λ2382 triplet); the other Mg ii system at zabs = 1.0791 is too weak to be included in our statistical Mg ii sample.
In PKS 1008+013 (Fig. A.1, central panel), there is an asymmetric emission line centred at λ = 6368 Å identified as Mg ii at zem = 1.275, which is also clearly present in the SDSS spectrum of this source. We thus do not confirm the “uncertain” redshift, zem = 0.8615, given by Collinge et al. (2005). We do not detect any absorption line in the FORS1 spectrum of this blazar.
In PKS 1407+022 (Fig. A.1, bottom panel), there are two weak emission lines at λ = 5238 and 6308 Å that we identified as C ii] and Mg ii at zem = 1.253. There is a weak Mg ii absorption doublet at zabs = 1.1123 (see Table 2) with possible associated Fe ii absorptions (marginal detections of Fe ii2382 and Fe ii2600).
For one blazar (the weak FIRST radio source SDSS J094257.8−004705), there is an unusual broad absorption line with a FWHM ≃ 6000 km s-1 as shown in Fig. B.1. This feature cannot be the signature of a dust shallow absorption at 2175 Å associated with the blazar (zem = 1.362), since it should be at λr > 2285 Å. Thus, it is identified as a Mg ii BAL at zabs = 0.929 or Δv = 0.20c. Such highly detached BALs, although very rare, have already been detected at low redshift (e.g. the high ionization BAL in PG 2302+029 at zabs = 0.695 reported by Jannuzi et al. 1996). In sources at 1.5 ≤ z ≤ 3 from the FIRST Bright Quasar Survey (FBQS), there is a 5% fraction of low ionization (Lo) BALs (Becker et al. 2000); the frequency of LoBAL QSOs is dependent of radio-loudness and decreases for the most radio-luminous QSOs. We thus expect at most two LoBAL QSOs in our blazar sample since 33 of the 45 blazars are strong (~1 Jy) radio sources, which is consistent with our single LoBAL detection.
There are also two narrow absorption systems in the blazar spectrum. The strong Mg ii doublet at zabs = 0.8182 has Mg i, Mn ii and Fe ii associated strong absorptions. The weak system at zabs = 1.0231 shows Mg ii absorption only.
BL0942-0047 with C iv and Mg ii emission lines and a Mg ii BAL at λ ~ 5400 Å.
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Appendix C: Not confirmed strong Mg ii absorption in one source of the Stocke & Rector BL Lac sample
PKS 0426−380 is one of the ten sources presented by Stocke & Rector (1997), sample for which an excess of strong Mg ii in BL Lac objects had first been reported. The high S/N (270) of our FORS1 spectrum enables us to detect very weak absorption lines. The absorber at zabs = 1.0283 is not confirmed as a strong Mg ii system (wr(2796) = 0.56 Å) and has weak Fe ii(2382, 2600) and Al iii doublet associated absorptions.
There is a stronger Mg ii doublet (wr(2796) = 0.93 Å) at zabs = 0.5592 with associated Mg i absorption together with the five stronger Fe ii lines. We also detect Galactic Ca ii absorption, as well as the only intervening (zabs > 0) Ca ii absorber in our blazar sample. The latter is at zabs = 0.1940 with a marginal detection of the weaker line of the doublet but exactly at the same redshift.
In this BL Lac the Mg ii absorption at zabs = 1.0283 is weaker than reported by Stocke & Rector (1997).
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Classical Damped Lyα systems (DLAs) have H i column densities N(H i) > 2 × 1020 cm-2. The fraction of DLAs among intervening QSO absorbers, with constraints on Mg ii2796 and Fe ii2600 (or the ratio of their equivalent widths) and on Mg i2852 is about 40% (Rao et al. 2006). Is the DLA fraction similar for blazars? Among our sample of 45 blazars, N(H i) can be derived from Lyα absorption line profiles for eight sources only, i.e. those with HST/FOS or STIS spectra. Since most of our selected blazars are strong radio sources, the DLA subpopulation which traces a cold gas phase can also be identified by 21 cm absorption.
Three of the strong Mg ii blazar absorbers have UV spectra. One is a DLA, the zabs = 0.5245 absorber towards PKS 0235+164, and two are sub-DLAs with N(H i) < 1 × 1020 cm-2, the intervening absorber at zabs = 1.3439 towards PKS 0215+01 and the possibly associated absorber at zabs = 0.9107 towards PKS 0823−223. The other properties of DLAs and sub-DLAs mentioned above are their large wr(Fe ii2600)/wr(Mg ii2796) ratio (> 0.5) and Mg i2852 strength (e.g. Bergeron & Stasińka 1986; Rao et al. 2006). Four strong Mg ii absorbers do not satisfy these criteria and thus cannot be DLAs: (i) two intervening ones at zabs = 0.7753 in SDSS J024156.4+004351 and zabs = 1.4247 in SDSS J094827.0+083940; (ii) two possibly associated systems at zabs = 1.0851 in PKS 1008+013 and zabs = 1.6832 in SDSS J141927.4+044513.
This blazar (PKS 1406 − 076) is a variable FSRQ, with superluminal motions, and also a gamma-ray emitter (see Sect. 2.1 and Table 1). It has strong emission lines (Mg ii and C iii]) and two Mg ii absorption systems: a strong one at zabs = 1.2735, and a very weak one at zabs = 1.2913 thus not included in our statistical Mg ii sample. The former is a 21 cm absorber and a DLA since, in addition to Mg i and the five stronger Fe ii lines, the very weak transitions of Fe ii2249, 2260, Zn ii2026, 2062blend and Cr ii2056, 2062blend, 2066 are detected.
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Two strong Mg ii systems in the BL Lac sample of Stocke & Rector (1997), also in our sample, were known 21 cm intervening absorbers, at zabs = 0.5245 (DLA mentioned above) and zabs = 0.6850 towards the lensed source B2 0218+357. There are three other intervening, strong Mg ii absorbers towards bright radio sources, with DLA signatures (detection of weak transitions of Fe ii, Zn ii and Cr ii). We have recently obtained GMRT and GBT time to observe these sources. The GMRT data show 21 cm absorption at zabs = 1.2753 in PKS 1406−076 and zabs = 1.1158 in PKS 2029+121 (Gupta et al., in prep.). The FORS1 spectrum of PKS 1406−076 is presented in Fig. D.1. Analysis of the GBT data for PKS 0454−234 is in progress to search for 21 cm absorption at zabs = 0.8922.
In our blazar sample, 10 strong Mg ii systems satisfy the criteria of high N(H i): four are confirmed DLAs and one is a probable DLA, thus yielding a DLA fraction of 40% compatible with that determined for QSOs absorbers. Additional UV and/or 21 cm data are needed to ascertain whether the incidence of DLA in our strong Mg ii-Fe ii blazar sample is even higher or not than for QSOs.
© ESO, 2010
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