A&A 386, 865-868 (2002)
Tuorla Observatory, 21500 Piikkiö, Finland
Received 10 January 2002 / Accepted 20 February 2002
The presence and amount of dust in the halo matter producing narrow metal line absorption systems is still uncertain. We consider the parameter , summed over the detected aborption line system redshifts , and ask whether radio-loud quasars at z < 2 and with a large value of the parameter are redder in the B-V colour than those with a smaller . This is anticipated if the absorption lines originate in sites which contain "normal'' dust causing selective extinction. The answer is positive, and a mean intrinsic reddening e0 per halo is consistent with 0.05, or about 0.2 B-mag extinction, roughly as expected.
Key words: quasars: absorption lines - galaxies: haloes - ISM: dust
Massive galaxy haloes are generally thought to consist of some non-baryonic dark matter, with at least a minor component of baryonic gas. Dark matter is observed via its gravitational effect. The gas has been detected via narrow metal absorption lines in the spectra of background quasars, and absorptions appear to originate in chemically enriched gas haloes of typical galaxies having a wide range of luminosities and a gas halo radius about 100 h-1kpc (Chen et al. 2001).
The presence and amount of dust, and the dust/gas ratio, in the intermediate redshift halo matter producing narrow metal line absorption systems is still an uncertain issue, the best direct evidence being the marginal detection of the 2175 Å dust feature in the composite spectrum of MgII absorbers (Malhotra 1997; but cf. Pitman et al. 2000). There is evidence (Fall et al. 1989) for some dust in another kind of absorption system: damped Lyman- lines, thought to be associated with metal-poor progenitor galaxies, in the form of slightly redder absorption line quasars as compared to quasars with no such systems. This kind of compelling evidence is here presented for the narrow metal line systems, especially those containing CIV absorption, using radio-loud quasars at redshifts <2 as test objects, and readily available data.
We consider the parameter , summed over the detected aborption line system redshifts , and ask whether the quasars with a large value of the parameter are redder in the B-V colour than those with a smaller . This is expected if the absorption lines originate in sites which contain "normal'' cosmic dust causing selective extinction.
Ostriker et al. (1990) concluded that a typical absorption line system (with a rest frame equivalent width of 0.6 Å for CIV) may have dust corresponding to intrinsic , if the dust-to-gas ratio in these high-z objects is similar to that locally. Because of redshift and the extinction curve, the observed reddening and extinction are larger.
The extinction-to-reddening ratio
varies little with redshift for typical
quasar spectra, if the dust is in our Galaxy (Teerikorpi 1981).
If the dust lies at some redshift
between a quasar and us, one again does not expect a large
influence on RV, though similar
absorbers with an intrinsic extinction
at different z will cause significantly different effects on
the radiation received at z = 0 (Ostriker et al. 1990).
The idealized extinction curve
dependence leads to an especially simple
result (Teerikorpi 2000):
This means that a single low-
system is expected to show
little reddening, while quasars with several
systems including high
should be significantly reddened, if there is dust. In fact, if each
absorption redshift system is accompanied by some average
intrinsic (rest-frame) reddening e0, then Eq. (1) leads one to expect
Naturally, several problems can hinder a detection of the dust effect predicted by Eq. (2). Here we adopt a simple approach to the reddening question, which allows us to use readily-available data.
1) In order to see the dependence of reddening on one needs accurate photometry and, in particular, quasars with intrinsically similar colours. B-V naturally varies with redshift, due to the K effect, but this may be taken into account.
2) Fortunately, there is now evidence for intrinsically rather stable (at the level of mag) B-V colours for radio loud quasars (Teerikorpi 1981, 2000, 2001; Teerikorpi & Kotilainen 1989). This evidence has emerged from our work on the bright end of the optical luminosity function of radio quasars. The z-limit 1.65 was adopted in these studies in which Galactic reddening corrections were derived from . At higher redshifts the scatter in B-Vincreases. In fact, a large part of this scatter, even starting around , appears to be due to intervening halos, as will be seen.
3) Furthermore, being radio-detected there is little colour bias in a sample of radio quasars and also less bias towards small extinctions, a main motive when we initially came up with B-V as a Galactic reddening indicator.
4) It is difficult to gather a genuine comparison sample of "clean'' quasars with no absorption line systems. This problem is possible to bypass using quasars with 1 reported system: to have means that the quasar spectrum was at some level inspected for absorption lines.
5) It is known that at higher resolution individual absorption systems divide into subcomponents, apparently revealing clouds in a halo. For the present purpose, in the calculation of one should use data refering to entire haloes. The method of Eq. (2) breaks down if absorption systems emerging at high resolutions are counted, in addition to those of low resolution. On the other hand, the colour effect searched for is so strong that the scatter due to undetected independent weak absorption systems (which may correspond to smaller haloes and reddening) will not conceal the effect, if e0 is large enough.
In order to see whether there is a signal indicative of reddening as predicted, we use a sample containing radio-loud quasars with reliable narrow line systems (Tables 1-2). It was collected from the basic catalog by Junkkarinen et al. (1991), with a few additional objects from Steidel & Sargent (1992), having UBV photometry and z < 2. We take V and B-V from Veron-Cetty & Veron (1993). For each quasar we calculated the parameter . In this collection the absorption systems generally seem to refer to individual haloes.
First we show, in Fig. 1, B-V vs. redshift for our sample and indicate separately those narrow absorption line objects for which the normalized quantity is smaller than 1, between 1 and 2, or larger than 2. The curve (cf. Teerikorpi 2000) describes the lower envelope for radio quasars in general. For z > 1 the three categories are clearly segregated in the sense expected. Those with (usually having one ) are mostly found close to the lower envelope, while those with more absorption systems are systematically higher.
Though the sample of absorption line quasars is not homogeneous, as regards detection probabilities of absorption systems, it is difficult to think about some selection effect which would cause the pattern in Fig. 1.
|Figure 1: B-V vs. z for radio loud absorption line quasars in the redshift range 0.2-2.0. The curve describes the lower envelope for radio quasars in general. Dot: ; triangle: ; open circle: .|
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In order to see a typical value of the intrinsic reddening e0accompanying an absorption line system, we experimented with different e0,
and show in Fig. 2a the
vs. z diagram,
in which from each colour we subtracted
e0 = 0.045.
Now the quasars follow rather well a common colour vs. redshift relation.
In Fig. 2b we have excluded the faintest quasars with V > 18 mag.
|Figure 2: a) Corrected colours , with the intrinsic reddening/halo e0 = 0.045. b) As in a), but for bright quasars .|
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Figure 3 shows the colour residuals , as B-V minus the low envelope curve (Fig. 1), versus the parameter for z>1. Possibly, among the fainter quasars some absorption systems have gone undetected or are reddened in our Galaxy. In Fig. 4a, for the objects fainter than V = 18 mag, we have denoted quasars according to the presence of MgII and CIV in their spectra: those with MgII, but no CIV, those with CIV, but no MgII, and the rest, all containing CIV. The MgII and CIV objects have quite different distributions of (MgII usually observed at ), though their emission 's are rather similar. Nevertheless, the deviations from the common envelope follow the same expected relation.
We have also checked the diffuse Galactic reddening, from the dust IR emission maps of Schlegel et al. (1998). Almost all predicted B-V reddenings are small, between 0.03 and 0.10 mag. We show in Fig. 4b the diagram of Fig. 4a as corrected for Schlegel et al. reddenings.
Thus the observations are consistent with the conjecture that a typical absorption line system, encompassing one halo, has dust along the line of sight, corresponding to a local . The simplest interpretation is that the dust-to-gas ratio in galaxy halos at intermediate redshifts is of the same order as in low-z conditions, and the dust causes selective extinction roughly as in our Galaxy. This agrees with the conclusion from the 2175 Å feature (Malhotra 1997), but not with the damped Lyman- observations (e.g. Fall et al. 1989; Fall et al. 1996) in which the dust-to-gas ratio is smaller by a factor of ten, apparently because of an early evolutionary phase with less metals and worse conditions for dust.
In fact, reddening values obtained from (more or
less) magnitude limited samples, are expected to be lower limits to the
true average reddening (cf. Ostriker et al. 1990).
It is quite possible
that in Figs. 3-4 high reddenings are lacking for increasing
due to selection biases aided by an intrinsic spread
in the value of e0. For example, a data point at (
would involve a high-redshift quasar
obscured by about 2.5 mag. In Figs. 3-4 the quasars with the largest two
have z > 1.95.
|Figure 3: Colour residuals vs. , for quasars with z>1. Dots: bright objects, ; open circles: fainter objects, V > 18.0. The straight line has a slope of 0.045.|
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The radio-loud quasars with narrow metal line absorption
display a clear signal of reddening, roughly as
Typically, the rest frame
e0 for one system is about 0.05 mag, which must be
multiplied by about
to give the observed B-extinction.
Thus the quasars with several absorption systems are seen obscured by
dust of up to 1-2 B-mag.
|Figure 4: a) Residuals vs. , for quasars with z>1, ; diamonds: MgII objects, dots: CIV objects, circles: objects with at least one CIV system. b) , corrected for the diffuse reddening as in Schlegel et al. (1998), vs. .|
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After Webster et al. (1995) suggested that a large scatter (up to 5 mag) in the colours of Parkes radio quasars could be due to dust, there have been attempts to set an upper limit to the extinction towards radio quasars, including the host galaxy and intervening matter (Boyle & di Matteo 1995; Srianand & Kembhavi 1997; Benn et al. 2001). These studies have resulted in AB less than about 2 mag, consistent with our interpretation of the B-V vs. z diagram. Because here the reddenings are clearly correlated with intervening absorption, the scatter due to the extinction in the host galaxy is a minor factor.
The same may be said of the dust suggested to be in the medium giving rise to the Ly- forest, which according to Ferrara et al. (1999) may cause observed extinction in the optical of no more than 0.05 up to z =5.
I thank A. Ferrara for useful referee's comments. The study was supported by the Academy of Finland ("Cosmology from the local to the deep galaxy universe'').