Ionization states of metallic absorption-line systems in continua of quasars (QSO's) are studied, assuming that the metallic lines arise in gaseous halos of high-redshift galaxies in photoionization equilibrium under the background UV radiation, and constraints on the intensity and spectral shape of the UV radiation are obtained. Then a structure of absorbers suitable for all of the metallic absorption line systems are discussed

Denda, Kiyomi

Ikeuchi, Satoru

English

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lonization States of Metallic Absorption-Line
Systems in Continua of Quasars
KIYOMI DENDA and SATORU IKEUCHI
National Astronomical. Observatory, Mitaka, Tokyo 181, Japan
ABSTRACT We study ionization states of metallic absorption - line systems in continua of quasars
(QSOs), assuming that the metallic lines arise in gaseous halos of high-redshift galaxies in photoionization
equilibrium under the background UV radiation, and obtain constraints on the intensity and spectral shape
of the UV radiation. Then we discuss a structure of absorbers suitable for all of the metallic absorption -
line systems.
1. INTRODUCTION
Metallic absorption - line systems in QSO spectra are generally supposed to be in photoionization equilibrium
under the diffuse UV background, whose existence is supported by the Gunn-Peterson test (e.g., Gunn &
Peterson 1965). These absorption systems can be classified into three groups, i.e., CIV system, Lyman
limit system (LLS) and Mgll system, according to the ionization state, in other words according to the
column density of neutral hydrogen. However, we can study the ionization states of metallic absorption
systems on the hypothesis that these difference in ionization states of three kinds of the absorption systems
is caused by the difference in lines of sight through the absorbing halos. For example, Lanzetta, Turnshek &
Wolfe(1987) suggested that Mgll - absorbing clouds and CIV - absorbing clouds are grouped together and
that the Mgll clouds are concentrated toward the center of the group, on the other hand, the CIV clouds
toward the edge. Furthermore, it is clear from observations at high spectral resolution that the metallic
absorption - line systems consist of several subcomponents (e.g., Blades 1988). Their Doppler parameters
are typically 6 = 5— 15km s"1 and the total velocity spread of subcomponents ranges from ~ 50km s"1 to
~ 800km s"1. There have been a few analyses on the clumpy structure of absorbers (e.g., Lanzetta & Bowen
1990). Lanzetta & Bowen (1990) found that a radial profile of the spatial density of the clump roughly
proportional to r~& , with J3 ~ 2 — 3.
In this paper, first, we assume a very simple model - absorber which has a density distribution HH oc r~2
and examine probable contributions to ionizing radiation 1) from QSOs ; /^ and/or 2) from early-type stars
in the central high-z galaxy ; If,. Then we find constraints on the UV radiation to repoduce the observed
ionization states of the metallic absorption-line systems. Second, we consider clumpy model - absorbers
irradiated by the UV radiation obtained here and discuss a structure of these absorption - line systems.
2. Calculation
We calculate the ionization states of model - absorber under the conditions of photoionization equilib-
rium and thermal equilibrium. We assume an intensity of ionizing radiation including a correction for
self-absorption due to halo gas: I(v;r) — 7ytL(i//i/£,/,)~orexp(— TO) -f 7*/(r/ro)2exp(— r/), where TO =
fr fij//rfr/1017'5cm~2, TJ = Jrr n#/dr/1017'5cm~2 (R : radius of absorber, TO : inner radius). I* is adopted
from Bregman & Harrington (1986) with upper cut-off at 500eV.
In order to compare with observations (summarized in Table 1 of Bergeron & Stasiriska 1986), we calculate
HI column density and column densities of several ions, e.g., CIV, MgH and SilV. When we consider a clumpy
model, we assume that an absorber consists of two components, with spatial density n1 and n2, respectively.
The filling factor of subcomponent 2; /(r) = EVc|ump/Vt0«a/ is used to modify the column density of each
ion on a line of sight at an impact parameter p, Nxt = 2/ (ni,-(r)(l - f(r)) + n2.,-(r)/(r))r/\/r2~^~p2cfr.
The optical depth is also modified according to the modification of NHI- But for simplicity, we assume that
/(r) is constant in this paper. The chemical elements considered in our calculations are H, He, C, N, O, Mg,
Si and Fe. We adopt the solar abundance from Allen (1972) and assume the metallicity of absorbing halos
to be Z/Z = 10~2.
3. Results and Discussions
In order to reproduce the observed ionization states, we find that the hydrogen Lyman - limit intensity of
137
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the diffuse UV background should be \ogIVLL(in ergs cm~2s-1Hz-1str~1) = -21.0 ± 0.5 at z ~ 2 and its
spectral shape should be /„ oc i/~° with spectral index l-5^a < 2. It appears that quasars would be the
dominant source of UV radiation at high redshifts and that the contributions from young galaxies should
be negligible (See Figure 1 and Denda & Ikeuchi 1992). This result is consistent with previous studies (e.g.,
Steidel and Sargent 1989).
However, it is difficult to reproduce the observed ionization states of all of the three absorption systems
at once, when we assume a spherical symmetric absorber with density distribution njj oc r~2. Then we
consider the cases of two - component models. The observed ionization states of all kinds of the metallic
absorption systems are well reproduced by the clumpy model that has a smoothly distributed and thicker
region, where nH oc r~2;r^:lOkpc, and a thin outer region with 'clumps', where a filling factor of 'clumps'
is about 0.1 (See Figure 2). Moreover, the absorber may have an inner dense core which shields its gaseous
halos from the stellar UV radiation of the central high-z (young) galaxy.
References
Bergeron, J., & Stasiiiska, G. 1986, AfcA, 169, 1. •
Blades, J.C. 1988, in QSO Absorption Lines: Probing the Universe, ed. Blades, J.C., Turnshek, D., & Norman, C.A.
(Cambridge University Press: Cambridge), 147.
Bregman, J.N., & Harrington, J.P. 1986, ApJ, 309, 833.
Denda, K., & Ikeuchi, S. 1992, submitted to ApJL.
Gunn, J.E., & Peterson, B.A. 1965, ApJ, 142, 1633.
Lanzetta, K.M., & Bowen, D. 1990, ApJ, 357, 321.
Lanzetta, K.M., Turnshek, D.A., fc Wolfe, A.M. 1987, ApJ, '322, 739.
Steidel, C.C., & Sargent, W.L.W. 1989, ApJ, 343, L33.
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Fig. 1
-3 -2.5 -2 -1.5
ft'*-1- log U H&.2
log !ULL versus log U for the suitable models to observations. The UV flux with spectral index a = 2
is adopted here. Open circles and closed circles represent values, for which both the observations of the CIV
systems and that of LLS can be well reproduced, respectively. Open triangles and closed triangles reprsent
values, for which the CIV systems or LLS isn't reproduced, respectively. From this figure, it is seen that
Io8 J"L
states of both the CIV systems and LLS.
erSs cm~2s~1Hz~ lstr~1) = -21 ± 0.5 and log U — -2.75 ± 0.75 are suitable for the ionization
Fig. 2 : The upper panel is the column density of CIV ion, N(CIV), versus the neutral hydrogen column density,
N(HI), and the lower one is the ratio of column densities N(SiIV)/ N(CIV) versus N(HI) for clumpy models for
(na< "w , /, ncH)= (0.1, 1.0, 0.1, 1.0), with different size of snoothly distributed region ; rc and UV radiation with
a spectral index or = 2 and intensity log LLL(in ergs cm~2s~1Hz~1str~1) = -21. The values of rc is labeled on
each curve and the hatched regions are excluded by the observational limits.
138


Ionization states of metallic absorption-line systems in continua of quasars

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