A&A 386, 865-868 (2002)
DOI: 10.1051/0004-6361:20020321
P. Teerikorpi
Tuorla Observatory, 21500 Piikkiö, Finland
Received 10 January 2002 / Accepted 20 February 2002
Abstract
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
with the
dependence leads to an especially simple
result (Teerikorpi 2000):
![]() |
(1) |
name | z | V | B-V | ![]() |
![]() |
0024+2225 | 1.118 | 16.6 | 0.33 | 1 | 2.109 |
0029-4124 | 0.896 | 17.8 | 0.57 | 1 | 1.781 |
0402-3613 | 1.417 | 17.2 | 0.15 | 1 | 1.797 |
0420-0127 | 0.915 | 17.8 | 0.58 | 1 | 1.633 |
0454-2204 | 0.534 | 16.1 | 0.06 | 1 | 1.475 |
0454+0356 | 1.345 | 16.5 | 0.23 | 2 | 4.013 |
0637-7513 | 0.651 | 15.8 | 0.33 | 1 | 1.152 |
0725+1443 | 1.382 | 18.9 | 0.43 | 1 | 2.388 |
0809+4822 | 0.871 | 17.8 | 0.57 | 2 | 3.308 |
0812+0204 | 0.402 | 17.1 | 0.18 | 2 | 2.728 |
0827+2421 | 0.939 | 17.3 | 0.36 | 1 | 1.524 |
0835+5805 | 1.534 | 17.6 | 0.48 | 3 | 7.512 |
0856+1703 | 1.449 | 17.9 | 0.40 | 2 | 4.848 |
0952+1757 | 1.472 | 17.2 | 0.08 | 1 | 1.238 |
0955+3238 | 0.530 | 15.8 | 0.10 | 2 | 2.518 |
0957+0019 | 0.907 | 17.6 | 0.47 | 1 | 1.672 |
1019+3056 | 1.316 | 17.5 | 0.27 | 1 | 1.346 |
1038+0625 | 1.270 | 16.8 | 0.16 | 1 | 1.441 |
1049+6141 | 0.422 | 16.5 | 0.10 | 2 | 2.618 |
1055-0434 | 1.428 | 17.8 | 0.07 | 1 | 2.256 |
1148+3842 | 1.303 | 17.0 | 0.18 | 1 | 1.213 |
1206+4356 | 1.400 | 18.4 | 0.58 | 3 | 6.185 |
1213-0630 | 1.410 | 17.7 | 0.20 | 1 | 1.789 |
1218+3359 | 1.519 | 18.6 | 0.19 | 1 | 2.500 |
1229-0207 | 1.038 | 16.8 | 0.48 | 1 | 1.395 |
1308+3236 | 0.997 | 15.2 | 0.37 | 1 | 1.879 |
1327-2040 | 1.169 | 17.0 | 0.63 | 3 | 4.724 |
1328+3045 | 0.849 | 17.3 | 0.26 | 1 | 1.692 |
1354+1933 | 0.720 | 16.0 | 0.18 | 1 | 1.456 |
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
![]() |
(2) |
name | z | V | B-V | ![]() |
![]() |
1416+0642 | 1.440 | 16.8 | 0.33 | 4 | 9.526 |
1421+1213 | 1.611 | 18.0 | 0.14 | 1 | 2.360 |
1510-0854 | 0.361 | 16.5 | 0.20 | 1 | 1.351 |
1511+1022 | 1.546 | 18.1 | 0.03 | 1 | 1.437 |
1602-0011 | 1.625 | 17.1 | 0.15 | 1 | 2.325 |
1622+2352 | 0.927 | 17.5 | 0.44 | 4 | 6.680 |
1704+6048 | 0.371 | 15.3 | 0.13 | 2 | 2.385 |
1821+1042 | 1.360 | 17.3 | 0.39 | 3 | 5.297 |
1912-5500 | 0.398 | 16.5 | 0.09 | 1 | 1.401 |
2020-3705 | 1.048 | 17.5 | 0.33 | 1 | 1.029 |
2128-1220 | 0.501 | 16.0 | 0.13 | 1 | 1.430 |
2135-1446 | 0.200 | 15.5 | 0.10 | 1 | 1.200 |
2142-7550 | 1.139 | 17.3 | 0.49 | 1 | 1.960 |
2145+0643 | 0.990 | 16.5 | 0.41 | 1 | 1.791 |
2201+3131 | 0.298 | 16.0 | 0.39 | 1 | 1.282 |
2223-0512 | 1.404 | 18.4 | 0.44 | 1 | 1.847 |
2326-4746 | 1.299 | 16.8 | 0.25 | 1 | 2.261 |
0002+0507 | 1.899 | 16.2 | 0.35 | 2 | 4.600 |
0058+0155 | 1.959 | 17.2 | 0.32 | 2 | 4.077 |
0119-0437 | 1.969 | 16.9 | 0.46 | 6 | 14.706 |
0802+1023 | 1.954 | 18.4 | 0.25 | 2 | 5.300 |
1023+0642 | 1.703 | 18.3 | 0.54 | 3 | 8.053 |
1148-0007 | 1.979 | 17.6 | 0.17 | 2 | 5.453 |
1157+0128 | 1.986 | 17.7 | 0.49 | 4 | 11.558 |
2044-1650 | 1.937 | 17.4 | 0.19 | 3 | 7.806 |
2120+1651 | 1.805 | 18.0 | 0.22 | 2 | 5.360 |
2249+1832 | 1.758 | 18.4 | 0.12 | 3 | 6.654 |
2354+1429 | 1.816 | 18.2 | 0.14 | 1 | 2.576 |
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:
![]() ![]() ![]() |
Open with DEXTER |
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
,
with
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
![]() ![]() |
Open with DEXTER |
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
values of
have z > 1.95.
![]() |
Figure 3:
Colour residuals
![]() ![]() ![]() |
Open with DEXTER |
The radio-loud quasars with narrow metal line absorption
display a clear signal of reddening, roughly as
expected:
.
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
![]() ![]() ![]() ![]() ![]() |
Open with DEXTER |
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.
Acknowledgements
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'').