We have searched the absorption spectra of five quasars for the presence of redshift system dominated by the highly ionized doublets C iv, N v, and O vi, which could be the strongest lines produced by absorbing clouds with collisional ionization temperatures between 10^5 ° and 10^6 °K. There is at most marginal evidence for one such system apiece in the spectra of PHL 957 and 4C 05.34, which are the two quasars with the largest known emission redshifts. Highly ionized redshift systems of this type are not widespread among the five quasars we investigated; the number of redshifts found in the observed spectra is not significantly larger than the number found in similar random-number spectra. Less than 5 percent of the observed absorption lines are identified in a statistically significant way by redshift systems of this type

Bahcall, J. N.

Cohen, J. G.

Joss, P. C.

Caltech Authors - Main

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The Astrophysical Journal, 184:57-63, 1973 August 15 
© 1973. The American Astronomical Society. All rights reserved. Printed in U.S.A. 
A SEARCH FOR HIGH-IONIZATION REDSHIFT SYSTEMS 
IN THE ABSORPTION SPECTRA OF FIVE QUASARS 
J. N. Bahcall and P. C. Joss 
Institute for Advanced Study, Princeton, New Jersey 
AND 
J. G. Cohen 
Department of Astronomy, University of California at Berkeley 
Received 1973 January 22; revised 1973 March 26 
ABSTRACT 
We have searched the absorption spectra of five quasars for the presence of redshift systems 
dominated by the highly ionized doublets C iv, N v, and O vi, which could be the strongest lines 
produced by absorbing clouds with collisional ionization temperatures between 105 ° and 106 ° K. 
There is at most marginal evidence for one such system apiece in the spectra of PHL 957 and 
4C 05.34, which are the two quasars with the largest known emission redshifts. Highly ionized 
redshift systems of this type are not widespread among the five quasars we investigated; the 
number of redshifts found in the observed spectra is not significantly larger than the number found 
in similar random-number spectra. Less than 5 percent of the observed absorption lines are 
identified in a statistically significant way by redshift systems of this type. 
Subject headings: quasi-stellar sources or objects — redshifts 
I. INTRODUCTION 
Systematic searches for redshift systems in the rich absorption spectra of quasars 
(see Bahcall 1968; also Bahcall and Joss 1973, and references therein) have been 
carried out under the assumption that low-ionization lines (especially La) will be 
present if they are in the accessible part of the spectrum; this is equivalent to assuming 
that the absorbing cloud is not extremely highly ionized. Cohen (1973) has recently 
pointed out, on the basis of a collisional ionization model for an absorbing cloud, 
that the low-ionization lines may not be among the strongest absorption lines if 
T ^ 105 ° K, where T is the ionization temperature. In fact, for a plausible abundance 
of heavy elements within the cloud, the strongest lines for 105 ° ^ T ^ 106 ° K will 
be due to C iv, N v, and O vi (similar results have been obtained by McKee, Tarter, 
and Weisheit 1973). If redshift systems due to such clouds are actually present in the 
spectra of quasars, they may well have been overlooked in previous analyses by 
Bahcall and his collaborators. 
In the present paper, we present the results of an investigation of the rich absorption 
spectra of five quasars (PKS 0237-23, TON 1530, PHL 938, 4C 05.34, and PHL 957) 
to determine whether high-ionization redshift systems may be present. In § II we out- 
line our method of analysis, in § III we present our results for the above five quasars, 
and in § IV we estimate, with the aid of random-number spectra, the probabilities for 
apparent high-ionization systems to arise by chance in the spectra of the quasars we 
investigated. In the Appendix, we discuss in more detail the method of construction of 
the random-number spectra. 
II. METHOD 
We used the method of analysis of rich absorption-line spectra developed by Bahcall 
(1968). We included 16 lines in our list of standard lines: O vi ÀÀ1031.95, 1037.63; 
57 
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58 J. BAHCALL, P. JOSS, AND J. COHEN Vol. 184 
N v ÀÀ1238.81, 1242.80; C iv AÀ1548.20, 1550.77; C iv (av) À1549.50; Si iv ÀÀ1393.76, 
1402.77; C m À977.03; Fe m Al 122.53; Si m A1206.51; Al m AA1854.72, 1862.78; 
La; and L/3. We required that the absolute wavelength discrepancy between an 
observed line and a redshifted standard line be less than or equal to 2 Á for an 
acceptable identification. 
In order to search for high-ionization redshift systems, we established a set of formal 
rules to determine whether a candidate system would be accepted. The rules were : 
1. At least two complete doublets among the three doublets (C iv, N v, O vi) 
must be present. 
2. The ratio of line strengths for each accepted doublet must be consistent with 
atomic physics and spectroscopic limitations. 
3. The two accepted doublets must contain at least two lines of strength greater 
than or equal to 2. 
By trial and error, we found that less stringent rules resulted in an undesirably large 
number of acceptable systems within random-number spectra having the same essential 
characteristics as the observed spectra (see § IV and the Appendix). The same difficulty 
was encountered when we attempted to include in our analysis the absorption lines in 
PHL 957 observed by Lowrance et al. (1972) with a high-resolution integrating 
television system. In other respects, our method of search was identical to that of 
previous analyses (see Bahcall and Joss 1973, and references therein). 
The observational data were taken from Bahcall, Greenstein, and Sargent (1968) 
(for PKS 0237 — 23), Bahcall, Osmer, and Schmidt (1969) (for TON 1530), Burbidge, 
Lynds, and Stockton (1968) (for PHL 938), Lynds (1971) (for 4C 05.34), and Lowrance 
et al. (1972) (for PHL 957). 
III. RESULTS FOR FIVE QUASARS 
We have searched the observed spectra of five quasars for acceptable high-ionization 
redshift systems between z = 4.0 and a minimum redshift zmin, the value of which 
TABLE 1 
High-Ionization Absorption Redshift Systems in Two Quasars* 
Observed Wavelength Identifications in 
Wavelength Discrepancy Low-Ionization 
(Â) Strength Identification*}* (Â) Systems 
4C 05.34: emission redshift zerri = 2.877; absorption redshift zabs = 2.5711 
3683.54  3 O vi (2) 1031.95 +1.7 Lj3 1025.72 (z = 2.5925) 
fLy 972.54 (z = 2.8106) 
3706.10  3D O vi (1) 1037.63 -0.6 ^ O vi 1031.95 (z = 2.5925) 
lC ii 1334.53 (z = 1.7758) 
[4010-48  1 Fe m 1122.53 -1.8 {| HO^z = S)] 
4422.83  4B N v (2) 1238.81 +1.1 None 
4436.74  3 N v (1) 1242.80 +1.4 Si iv 1393.76 (z = 2.1819) 
PHL 957: zem = 2.69; zabs = 2.6181 
3735.0....... 2W O vi (2) 1031.95 -1.3 None 
3753.9  2W O vi (1) 1037.63 +0.4 LjS 1025.72 (z = 2.6613) 
4482.5  1 Nv (2) 1238.81 -0.4 None 
4495.0  1 N v (1) 1242.80 +1.6 Si iv 1393.76 (z = 2.2250) 
* Letters following the line strengths stand for diffuse (D), broad (B), and wide (W), as denoted 
by the observers. The identification in brackets is questionable and was not considered as evidence 
supporting the other identifications. 
t Relative intrinsic strengths between the members of each doublet are given in parentheses. 
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No. 1, 1973 SEARCH FOR HIGH-IONIZATION REDSHIFTS 59 
was set equal to the minimum redshift for which two of the three doublets C iv, N v, 
and O vi fall within the observationally accessible portion of the spectrum. The value 
of ¿min varies from ~ 1.5 for TON 1530 to ~ 1.8 for 4C 05.34. We found one acceptable 
system apiece in the spectra of PHL 957 and 4C 05.34 at redshifts of z = 2.6181 and 
z = 2.5711, respectively; the properties of these systems are summarized in table 1. 
The acceptability of both systems relies upon the identification of the doublets of N v 
and O vi. Another system (z = 2.2055) in the spectrum of PHL 957 obeys all of our 
formal rules, but we rejected it because of an unacceptably large wavelength dis- 
crepancy (3.6 Â) between the two lines of the closely spaced doublet of O vi. (If this 
doublet is discounted, the system is unacceptable.) This system is virtually identical 
to the system z = 2.2056 in table 5 of Lowrance et al (1972). We found no acceptable 
systems in the spectra of three other quasars (PKS 0237 — 23, TON 1530, and PHL 
938). 
We also note that, among the eight identified lines in the two acceptable systems, 
five have already been identified in previous searches for low-ionization systems. 
Moreover, among these five previously identified lines, four were essential to the 
acceptability of low-ionization systems in which they were identified. 
IV. RANDOM-NUMBER SPECTRA 
In order to estimate the probability that an acceptable high-ionization redshift 
system will occur by chance, we have generated and analyzed 20 random-number 
spectra corresponding to each quasar. These spectra were required to have the same 
essential characteristics as the observed spectra, e.g., the minimum separations 
between lines were the same as those imposed on the observed spectra by the spectro- 
scopic techniques and observational conditions. Further details concerning the char- 
acteristics of the random-number spectra are given by Bahcall (1968) (for PKS 
0237-23), Bahcall et al. (1969) (for TON 1530), Bahcall and Feldman (1970) (for 
PHL 938), Bahcall and Goldsmith (1971) (for 4C 05.34), and Bahcall and Joss (1973) 
(for PHL 957). (See also the Appendix to the present paper.) In all cases, we checked 
to ensure that acceptable redshift systems in these spectra would not have been 
accepted in previous searches for low-ionization systems. 
The results of our analysis of the random-number spectra are summarized in table 2. 
On the basis of the number of acceptable systems in the random-number spectra, we 
estimate that the probability of the two acceptable systems having occurred by chance 
in the observed spectra of PHL 957 and 4C 05.34 is ~ 10 percent, and that the prob- 
ability of two acceptable systems having occurred by chance among all five quasars 
is ~25 percent. We note that five out of the 14 acceptable systems in the random- 
number spectra have redshifts appreciably larger than the emission redshifts of the 
respective quasars, while no acceptable absorption redshift much in excess of the 
emission redshift has yet been found in any observed quasar spectrum. However, this 
consideration is of limited value in the context of the present probability estimates, 
because high-ionization redshift systems have also not yet been reported in the 
absorption spectra of quasars. 
The total number of lines in the quasar absorption spectra we have studied is 267. 
Since the two candidates for high-ionization absorption redshift systems explain at 
most nine lines, we conclude that the percentage of absorption lines which can be 
explained with statistical validity as having been formed under the conditions of high 
collisional ionization considered by Cohen (1973) and McKee et al. (1973) is less than 
5 percent. Another way of summarizing this conclusion is to note that a total of 23 
redshifts have been previously identified in these quasars using the same techniques 
employed in the present paper. Thus, at most 2/25 of the redshifts identified in this 
statistically significant way are likely to have been produced under conditions of high 
collisional ionization. 
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60 J. BAHCALL, P. JOSS, AND J. COHEN 
TABLE 2 
Acceptable Redshifts among the Random-Number Spectra 
Number of Acceptable 
Acceptable Redshifts in Redshifts per Random- 
Object 20 Random-Number Spectra* Number Spectrumt 
PHL 957 2.55 0.20 ± 0.51 
2.49 
2.31Î 
1.96Î 
4C 05.34  3.48 0.15 ± 0.36 
2.91 
2.28 
PHL 938  2.69 0.20 ± 0.40 
2.55 
1.79 
1.72 
TON 1530  None 0 
PKS 0237-23 2.76 0.15 ± 0.36 
1.79 
1.70 
* Italicized redshifts are larger than the observed emission redshift of the quasar, 
t The quoted errors are one standard deviation per random-number spectrum. 
t These two redshifts were in the same random-number spectrum. This is the only case where 
more than one acceptable redshift was found in a single spectrum. 
This work was initiated while one of us (J. N. B.) was a visiting professor in the 
astronomy department of the University of California at Berkeley; J. N. B. wishes to 
express his gratitude for the hospitality and stimulation provided by the members of 
the Berkeley astronomy department. This work was supported in part by NSF grant 
16147 A#1 and the Miller Institute for Basic Research. 
APPENDIX 
In this Appendix, we discuss the method by which the random-number spectra are 
generated. 
The purpose of the random-number spectra is to determine the statistical validity of 
the candidate redshifts that are found in the observed spectra. Hélice, the random- 
number spectra are generated in a way that simulates the limitations imposed on the 
observed spectra by the spectroscopic techniques and observational conditions; the 
random-number spectra are thus superficially indistinguishable from the real spectra. 
In particular, the minimum and maximum wavelengths (Amln and Amax, respectively) 
allowed for the'lines in a random-number spectrum are set equal to the minimum and 
maximum wavelengths accessible to the observed spectrum; the minimum separation 
A Amin between each pair of lines in a random-number spectrum is set equal to the 
spectral resolution of the observed spectrum; the total number n of lines in each 
random-number spectrum is set equal to the total number of observed lines; and the 
distribution of line strengths and other line characteristics is set equal to the observed 
distribution (e.g., if there are two wide lines of strength 3 in the observed spectrum, 
then two randomly chosen lines in each random-number spectrum are considered to 
be wide and of strength 3). (See Bahcall 1968 and Bahcall and Joss 1973 for additional 
details concerning the generation of the random-number spectra.) 
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TABLE A-l 
A comparison of the observed ab sorption-line spectrum of 4C 05. 34 
with two typical random-number spectra (X. . = 3500&., X =6000$., AX . =6$, n = 93). 
mm max mm 
Observed Spectrum 
Wavelength(X) Strength^ 
Random-number 
Spectrum # 1 
Wavelength($) Strength1 
Random - numb e r 
Spectrum # 2 
Wavelength($) Strength'1 
3497.85 
3514.81 
3535.98 
3565.61 
3582. 46 
3592. 18 
3603.32 
3630.26 
3641. 86 
3668.39 
3683.54 
3706.10 
3727.17 
3744.74 
3766.27 
3779.43 
3816. 09 
3843.96 
3867.06 
3874.82 
3882. 94 
3894.82 
3908.29 
3965.40 
3974.34 
3985.83 
3997.07 
4010. 48 
4019.67 
4042. 23 
4053.04 
4061. 50 
4069.19 
4084.42 
4142. 34 
4154.96 
4163. 15 
4171. 36 
4185.46 
4197.38 
4215.43 
4223. 59 
4233. 72 
4246. 98 
4256. 62 
4296. 49 
4304.62 
2 
2 
2 
3D 
3 
0 
3 
0 
3 
2 
3 
3D 
4B 
1 
3 
2 
2D 
1 
3D 
0 
1 
2 
ID 
1 
1 
2 
5 
1 
3 
1 
3 
1 
3 
ID 
0 
0 
1 
1 
2D 
2 
2 
4 
0 
3 
2 
3 
1 
3523. 30 
3591. 19 
3602. 92 
3705.78 
3763.98 
3792. 08 
3798.91 
3835.99 
3869.55 
3914.35 
3924.81 
3937.94 
3956.61 
3988.19 
4023.70 
4076.32 
4111. 23 
4152. 67 
4184.18 
4201.07 
4243.88 
4256. 00 
4266.18 
4324.60 
4338.26 
4352. 18 
4358.24 
4365.21 
4418. 41 
4432. 13 
4446.15 
4452. 67 
4519.05 
4532. 05 
4621. 73 
4656.89 
4662. 96 
4684.57 
4733.20 
4806.53 
4852. 62 
4861. 75 
4880.21 
4900.10 
4909.73 
4924.22 
4934.06 
0 
1 
0D 
1 
3 
1 
0D 
3 
0 
3 
1 
ID 
1 
3 
3D 
ID 
0 
3 
3 
3 
1 
1 
1 
2 
2 
0 
2 
ID 
2 
0 
5 
1 
3 
5 
1 
3 
1 
4 
1 
4 
2 
3 
3 
3 
2D 
0 
0 
3561. 24 
3572. 72 
3599. 68 
3611.45 
3627. 59 
3643.24 
3659.23 
3692. 97 
3709.26 
3747. 34 
3782. 47 
3793.04 
3823. 24 
3861. 85 
3901. 81 
3939.39 
3958. 61 
4018. 55 
4037.78 
4047.19 
4056.02 
4070.67 
4133.38 
4166.56 
4213. 48 
4249.75 
4259. 82 
4284.93 
4297. 61 
4313.47 
4328.88 
4338. 91 
4348.87 
4375.27 
4388. 52 
4418. 08 
4454.23 
4461. 34 
4503.57 
4519. 52 
4528. 46 
4573.64 
4595.82 
4613.70 
4620. 88. 
4635.74 
4642. 92 
2 
0 
4B 
4 
1 
1 
2 
2 
4B 
2 
1 
1 
3 
1 
2 
0 
3D 
2 
0 
1 
3 
1 
ID 
1 
3D 
4 
2 
3 
1 
2 
3 
1 
0D 
3 
1 
3 
1 
0 
2 
0 
3 
5 
5 
0 
2 
3 
4 
* Letters following line strength indicate "broad" (B) and "diffuse" (D), as denoted by 
Lynds (1971). 
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TABLE A-1—Continued 
Ob s e rved Sp e ct rum 
Wavelength(jl) Strength'1 
Random - numb e r 
Spectrum # 1 
Wavelength(ji.) Strength'1 
Random - numb e r 
Spectrum # 2 
Wavelength(5i) Strength'1 
4317.20 
4329. 81 
4366.50 
4380.43 
4391-09 
4422. 83 
4436.74 
4451. 95 
4465.08 
4479. 62 
4502. 44 
4507. 97 
4519. 12 
4529. 91 
4549. 09 
4557.29 
4582. 94 
4624. 25 
4631.03 
4639.10 
4653.94 
4665.62 
4676.18 
4688. 01 
4710. 81 
4728. 07 
4734.45 
4759. 55 
4800. 96 
4816.94 
4842. 29 
4972. 03 
5008. 10 
5032. 97 
5045.89 
5189.21 
5196.77 
5377. 37 
5386.67 
5402. 04 
5444.95 
5455.57 
5478. 41 
5839.00 
5903.80 
6005. 61 
0D 
0D 
5B 
2 
ID 
4B 
3 
0 
4D 
2 
1 
3 
1 
5 
2 
1 
5 
1 
4 
1 
2 
1 
1 
2 
4 
1 
1 
1 
2 
2 
1 
1 
1 
0 
1 
0 
0 
3 
3 
0 
1 
1 
0 
0D 
ID 
ID 
4940.95 
4985.14 
5053.17 
5075.71 
5096.67 
5113.49 
5164. 38 
5229. 52 
5259.22 
5265. 24 
5272. 59 
5278. 71 
5306.40 
5315.46 
5375.76 
5388.08 
5419. 25 
5431. 81 
5447. 91 
5475. 07 
5485.46 
5501. 72 
5510.89 
5537.20 
5550.57 
5585.54 
5591. 97 
5618. 99 
5628.92 
5635.19 
5643. 07 
5692. 85 
5708.55 
5729. 07 
5751.70 
5777.93 
5793.22 
5824.18 
5831. 38 
5854.38 
5860.73 
5869.98 
5885.25 
5925. 57 
5975.23 
5986.46 
3D 
1 
1 
5 
3D 
0D 
1 
2 
5B 
2 
4D 
1 
ID 
2 
1 
1 
1 
2 
0 
1 
0 
1 
4B 
1 
2 
2 
1 
1 
0 
0 
1 
2 
2 
2 
ID 
3 
0 
1 
2 
2 
2D 
4B 
3 
3 
4 
1 
4659. 06 
4696.95 
4713.98 
4739.39 
4790.58 
4805.96 
4874.45 
4902. 36 
4913.46 
4923.13 
4942. 07 
5000.06 
5042. 83 
5163.50 
5171. 03 
5195. 68 
5214. 36 
5250. 01 
5302. 86 
5312.43 
5325. 81 
5348. 65 
5374. 12 
5394.61 
5419. 63 
5438. 21 
5447. 22 
5464.87 
5490. 48 
5509. 81 
5523.15 
5544.10 
5563. 62 
5593.76 
5667.79 
5690.33 
5762. 73 
5791.96 
5815.52 
5825. 69 
5835.54 
5871.09 
5909. 62 
5942. 45 
5969.66 
5985.83 
1 
1 
1 
1 
0D 
2 
2 
2 
ID 
2 
3D 
1 
2D 
1 
3 
1 
1 
0 
5 
1 
0 
3 
1 
0 
0 
2D 
3 
0D 
1 
2 
1 
0 
1 
1 
2 
0 
3 
3 
2 
1 
4D 
ID 
ID 
ID 
3 
5B 
© American Astronomical Society • Provided by the NASA Astrophysics Data System 
SEARCH FOR HIGH-IONIZATION REDSHIFTS 63 
Fig. 1.—Flow chart for the computer program that generates the random-number spectra 
The random-number spectra are generated by a computer program whose operation 
is described in figure 1. Once generated, each spectrum is analyzed by exactly the 
same method that is used to determine acceptable redshift systems in the corresponding 
observed spectrum. Thus, the average number of acceptable systems in 10-20 random- 
number spectra is a statistically meaningful measure of the number of acceptable 
systems that arise by chance in each observed spectrum. 
In table Al, the observed spectrum of 4C 05.34 (Lynds 1971) is compared with two 
typical random-number spectra that were generated by the above method. 
REFERENCES 
Bahcall, J. N. 1968, Ap. 153, 679. 
Bahcall, J. N., and Feldman, U. 1970, Ap. 161, 389. 
Bahcall, J. N., and Goldsmith, S. 1971, Ap. 170, 17. 
Bahcall, J. N., Greenstein, J. L., and Sargent, W. L. W. 1968, Ap. /., 153, 689. 
Bahcall, J. N., and Joss, P. C. 1973, Ap. /., 179, 381. 
Bahcall, J. N., Osmer, P. S., and Schmidt, M. 1969, Ap. J. (Letters), 156, LI. 
Burbidge, E. M., Lynds, E. R., and Stockton, A. N. 1968, Ap. J., 152, 1077. 
Cohen, J. G. 1973, Ap. J. (in press). 
Lowrance, J. L., Morton, D. C., Zucchino, P., Oke, J. B., and Schmidt, M. 1972, Ap. /., 171, 233. 
Lynds, R. 1971, Ap. J. (Letters), 164, L73. 
McKee, C. F., Tarter, C. B., and Weisheit, J. C. 1973, Ap. J. (in press). 
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© American Astronomical Society • Provided by the NASA Astrophysics Data System 


A Search for High-Excitation Redshift Systems in the Absorption Spectra of Five Quasars

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A Search for High-Excitation Redshift Systems in the Absorption Spectra of Five Quasars

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