Variation of QSO color indices with redshift

Chin. Astron. Astrophys. (1994)18/4, 393-402 A translation of Acta AstrophysSin. (1994)14/3,197-206 Copyright @ 1994 Elsevier Science Ltd Printed in Great Britain. A)1 rights reserved 0275-1062/94$24.00~.00

Variation of QSO color indices with redshiftt FAN Xiao-hui Beijing

Astronomical

Observatory,

CHEN Jian-sheng Chinese

Academy

of Sciences,

Beijing

100080

Abstract In this paper we discuss the effect of features in the spectrum of QSOs on their color indices and the consequent variation of the latter with the redshift. As the redshift increases, the various absorption features shortwards of the Lye emission line enter into the visible region and alter the way in which the color indices vary with the redshift, while the effects of the intrinsic power-law spectrum and emission lines on the color indices become secondary. The largest effect is due to the Lyman limit absorption system. These results are checked using the color indices calculated from IUE spectra. Key words:

QSO-color

indices-redshifts

1. INTRODUCTION When QSO redshift exceeds 2.2, the Lycv emission lines enters the visible region, the U - B color greatly increases and the method of UVX sky patrolling is no longer adequate. Using multi-color photometric survey, researchers have had some success in finding high-redshift QSO ca.ndidates, but the situ&ion is far more complicated than in the case of low-redshift (z < 2.2) using the UVX method. Many important issues await solutionl’-31. The most important issue is, when the redshift is greater than 2.2, the rich array of spectral features in the QSO spectrum enter the visible region. As the redshift changes, these features enter different color bands. Consider &SOS with the same redshift. On one hand, their intrinsic spectral features (emission lines and the continuum) may differ from one another; on the other hand, their absorption spectra differ because of different locations of the absorbing matter. These complicating factors are responsible for the marked changes in the color indices with the redshift, as well as the very large dispersion in the color indices for objects of the same redshift 141. The ilnplication is that, when we use multi-color photometry for searching for high-redshift &SOS, there are serious selection effects. A quantitative analysis of these complicated selection effects is necessary for improving the sky patrol of high-redshift &SOS. In principle, we have two approaches: (1) We collect as many as possible unbiassed QSO spectra to investigate their location in the color index space. (2) We make use of the various known statistical regularities in the QSO spectrum t Supported by National Natural Science Foundation Received 1993 March 13; revised Lersion 1994 February 18

FAN Xiao-hui & CHEN Jian-sheng

394

and carry out Monte Carlo simulation on the computer, and this will equally lead to the position of the QSOs in the multi-dimensional color index space. The aim of the present paper is to investigate how the statistical properties of the spectral features of &SOS affect their color indices,- the basis of quantitative numerical simulation. We shall also use the IUE data bank to study the observed IUE spectra of QSOs, while pointing out the limitations of this approach. Detail of the Monte Carlo method and its results will be published in another paper. First, we discuss the effect of the various spectral features on the color indices and their dispersion, so as to provide a reliable basis for simulating the QSO spectrum and for analyzing the selection problem. Then, we shall translate the low-redshift WE spectra to high redshifts and discuss qualitatively the variations of the color indices with the redshift, separately caused by the various spectral features.

2, FACTOR

AFFECTING

THE COLOR INDICES

Let US now discuss in detail the various intrinsic and extrinsic spectral features that affect the color indices. These are all related to the redshift but in quite different ways. These include: (1) The Continuum a) Effect of the same power-law exponent a different redshifts; b) Effect of different power-law exponents; c) Effect of the non-power-law component; (2) Emission Lines a) Effect of the width of the emission lines; b) Effect of the intensity of the emission lines and of its variation with the redshift; (3) Absorption Systems a) The Gunu-Peter~n effect; b) The Lyman-o forests; c) The Lyman Limit Systems (LLS); d) The Lyman-o damped line systems; e) Narrow metal absorption lines; f) Broad ab~rptio~ lines (BAL). We must first define the photometric system. At present, international effort in searching for QSOs with multi-color photometry mostly uses broad band photometry, This is because the survey data mainly come from large field photographs taken by Schmidt telescopes. The color system selected is close to the Johnson-cousins system. Therefore, our calculation below of color indices will be on the Jol~nson-Cousins system. In another paper we shall point out that this system is not, particularly effective for searching for high-redshift &SOS. Our simulated Johnson-Cousins system is based on the following steps: (1) For U - B and B -V, the response function and equation of transformation are taken from Mathews and Sandage’s paper151. (2) For V - R and R - I, the -,_-3posse function is taken from Bessel’s paperlsl, and the zero-point is determined from the continuum of Vega given by Tug et al.171.

395

Quasar Color Index

Table

1 Color Indices for Different Exponents

in the Power-Law

Spectrum

B-V

V-R

R-I

-0m90 -0.82

O'flO 0.20

OF18 0.25

OF23

-0.73

0.30

0.32

0.41

0.40

0.39

0.49

1.6

-0.64 -0.56

0.50

-0.47

0.59

0.48 0.54

0.58

2.0

a

U-B

0.0

0.4 0.8 1.2

2.1 The

Effect

of the

0.32

0.66

Continuum

The continuum of QSO is a superposition of a nonthermal power-law spectrum and a blackbody component (the 3000A bump). For high-redshift &SOS, the visible region corresponds to the far ultraviolet in the rest wavelength and the effect of the blackbody component is a decrease in the spectral index, it being generally assumed that the continuum in the visible region in the observed wavelength is always described by a single power-law[sl. Contribution to the color indices by a power-law pendent of redshift, the reason is as follows:

spectrum

of a given exponent

is inde-

Let the power-law spectrum be I, CKumQ, or I, = CX2-“, C a constant. Then, in a coordinate system at rest with respect to the &SO, corresponding to rest wavelength Xe we have I!& = CXima, while in the observer’s coordinate system, with &,bs = Xe( 1 + z), .z being the redshift. we have

again a power-law

with the sa.me exponent.

For a given &SO, the constant

C’ is independent

of the wavelength, hence it has no effect on any color index. The color indices to different values of the power-law exponent are shown in Table 1.

corresponding

For 59 QSOs with .z > 2.75, Sargent et al.lsl found (o) = 0.78 f 0.04, cr(cr) = 0.27. From Table 1 we see that the dispersion in the color index caused by the spectral exponent will of the accuracy of be less than O.lm, which is no greater than the order of magnitude photographic photometry for the fainter objects. 2.2 The

Effect

of Emission

Lines

The effect of width and intensity of emission lines on the color separately. Table 2 gives, for different redshifts, for a Lya emission 100 A in the rest frame, the changes in the B magnitude caused by line width from 2 000 km/s to 20 000 km/s. It shows that the effect to the emission line width over its range of variation, is negligible.

indices can be discussed line of equivalent width a change in the FWHM on the color indices due

As regards intensity, only a few strong emission lines can produce any observable effect. Table 3 gives, for the four main emission lines, Lya, CIV, C III] and Mg II with different equivalent widths, the maximum effect on the various color indices for the redshifts shown in brackets.

396

Quasar Color

397

Index

where fobs is the observed intensity and fini is the intensity extrapolated side of Lya, and suffix A refers to rest wavelength range 1025.&-1216a,

from the long wave and B, to 912A-

1025a. Of course, r>A and DB include also the Gunn-Peterson effect, and the effects of Lyac damped absorption line and the Lyman absorption system corr~ponding to the continuous LLS. Because of the large number of the forest lines, the contribution by the LLS will be the main one, unless very strong da.mped absorption lines are present. As an example, we considered the case where there is power-law continupm of cy = 0.78, and on the short wave side of Lycu, we have a DA = 0.4 and a &J = 0.6 (typical for a QSO of z = 4) and we calculated its effect on the color indices as the spectrum is redshifted. The results are shown in Table 4. These show that the effect of the Lyman forest is compxable to that, of the emission

lines in order of ma,gnitude. Table

4 Effect of Lyman Forest System on the color Indices

A l?xx

A (U-B) 0” .78

2

2.2

A (B-V)

A (V-R}

A (R-I)

CF.72

Om.67

IF.65

3.0

3.8

4.x

(3) Lyman Limit Absorption System (LLS). This is the absorption system of the continuous a.bsorption at the Lyma.n limit with optical depth 7 > 1 or &I > 1.6~ 10’7~m-2. It is the most important factor that a,ffects the U, B V ma.gnitudes of high-redshift QSOs. Because the locat,ion in the spectrum where LLS appears is random and because it eorresponds to r >> 1, the radiation at X < (1 + 2~~s) x 912A is almost completely absorbed, so the object in the U, B and other bands could be made fainter than the detection limit, thus greatly changing its position in the color index space and increasing the dispersions. Using the observed sta.tistical regularities, No = 0.78,~ = 0.68[sl and /3 = 1.25[1’l, we can calculate, for a QSO of z =I 5, the proba.bility that LLS has not yet entered the various color bands. The results are given in Ta.ble 5. Table

5 Probability

that the spectrum of a QSO of z = 5 at the given wavelengths is free of LLS (PI) or free of LLS with r > 10 (Pm)

x (A) 9 P In

5300 0.61 0.77

5000 0.27 0.50

4500 0.07 0.24

4000 0.02 0.12

3500 0.007 0.07

Thus, for the majority of high-redshift QSOs, radiation in the blue region of the visible range will be seriously, or even completely a.bsorbed, and large changes in the color indices will be ca.used. (4) Damped Line System. This corresponds to the Lyar absorption line where there is obvious da.mping in the wings. The corresponding Nm is generally greater than 10” cmm2, and there will be corresponding LLS with 7 >> 1. The effect on color of a damped Lycv line of a given equivalent width is the same as tha.t of a Lya emission line of the same intensity at the absorption redshift, with the sign reversed. Table 6 gives, for three values of the equivalent width, the maximmn effect of the Lycv damped line on the color indices,

FAN Xiao-hui & CHEN Jian-sheng

398

Table

6 Effect of Damped Lya Line on the Color Indices

10

0.05

0.05

0.05

0.05

20

0.01

0.10

0.10

0.10

30

0.15

0.14

0.15

0.15

(5) Narrow

Metal Absorption

Lines.

Since the narrow

metal absorption

lines rarely have

rest equivalent widths greater than 5A and moreover, there are not many of these, their effect can be neglected in the light of the foregoing discussion. (6) Broad Absorption Lines (BAL). BAL systems change the QSO spectrum greatly. As the morphology of BAL, their ejection velocity and intensity are all very complicated, the colors of BAL &SOS are very different from those of ordinary QSOs and there is also At present we have no consistent a large dispersion among the BAL objects themselves. understanding of the properties of BALs, especially their statistical regularities. Their effect on the colors and the distribution of BAL QSOs in the color index space can only be discussed after we have collected more spectra of the various BAL types.

3. QUALITATIVE

RESULTS

FROM

IUE

SPECTRA

The above analysis can be quantitatively checked using avialable IUE spectra. It should be noted, however, that, for objects fainter than magnitude 14, the spectra have poor signalto-noise ratios, and, also, all IUE spectra belong to QSOs with small redshifts. We saw above that it is the absorption systems that vary most with the redshift and are the most important factor in influencing the variation of color indices with the redshift. By translating the low-redshift IUE spectra to higher redshifts, we shall not get an adequate color variation, particularly, that part due to the absorption spectrum. On the other hand, the effects due to the continuum, the emission lines and the LLS can be gleaned from the translation. The IUE data were analysed using the relevant software in the IRAF package and color indices in the above system were obtained. We have selected three typical cases for discussion (Figs. l-3). Fig. 1 is the result obtained by combining the spectra of 18 &SOS with 0.045 < z < 2.219. This composite spectrum has the strongest emission lines and a LLS with an optical depth of about 1. Fig. 2 is the composite of E 1821+643 and PC 1115+080; its emission lines are the weakest and it has no LLS. Fig. 3 is a composite of 3C273 and The spectral index, emission lines and PG 1634+706; it has very strong LLS absorption. absorption features of these three composite spectra are listed in Table 7. From these three examples we derive the following conclusions: 1) Whether LLS is present or not, the U - B curve in all three cases lies below -0.4 for z < 2.3. This shows that we can use U - B 5 -0.4 as criterion to pick out low-redshift &SOS. This criterion is mainly determined by the continuum spectrum, for as the emission lines enter and leave the U and B bands, they change only the size of the colors but not whether U - B is or is not 5 -0.4. Considering that different spectral exponents may have some small effect on the threshold redshift value, present search for QSOs using the UVX technique has been restricted to redshifts below 2.2. In this case, incompleteness of sample

Quasar Color Index

399

comes mainly from photometric accuracy which is particularly For z > 2.3, this criterion is no long applicable, for the U -B affected by absorption lines on the short wave side of Lye.

poor for the fainter objects. curve will then be seriously

2) Effect of Emission Lines. With reference to Table 2, the points marked A and B can be seen to correspond to the values of the redshift where the effects of Lye and CIV emission lines on U - B are at a maximum. This shows that minor peaks and valleys in the color-redshift curve are caused by the strongest emission lines entering and leaving the various filter bands. As stated in the last paragraph, this does not alter the U - B 5 -0.4 criterion as long as z < 2.2. 3) Effect of Absorption Systems. There is no LLS in the spectrum of Fig.2. While the Lyman forest gives rise to a plateau in the U - B curve at the higher redshifts, 2.5 < z < 4.0, its value always remains below 0.0. At the same time, since the continuous absorption system 3

(a) -

V-R

2- -R-I

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

.?

6

(b)

1

Fig. 1 Composite spectrum of 18 QSOs (b) and its color-redshift diagram (a)

FAN Xiao-hui & CHEN Jian-sheng

400

is again dominated of the Lyman continuum is proportional to Y-~, the far ultraviolet by the power-law spectrum, so that for t > 4.0, the U-B curve falls again to below -0.4. The spectrum of Fig. 1 has a LLS with r M 1, making the hump more obvious with (V - B)Inax z 0.3. The spectrum of Fig.3 has a LLS with r >> 1, which means complete absorption of radiation short of 7908L and causes an indefinite rise in the U - B curve for t > 3.3. 4) The variation with certain

in the B - V, V - R, R - I curves is similar

to that in the U - B curve,

“lags”.

Fig. 2 Composite spectrum of E 1821-i-643 and PG 1115+080

(b) and its color-redshift, diagram (a)

Quasar Calor Index

TdAe 7 Spectral Features

4

3

x -z k,

2

I

in Three

Composite Spectra

402

FAN Xiao-hui & CHEN Jian-sheng

4. BRIEF The variation

of color with redshift

SUMMARY

for low-redshift

(z < 2.2) &SOS is determined

by intrinsic

features of the spectrum, the emission lines and the power-law spectrum. For high-redshift &SOS with z > 2.2, the situation is entirely different. This is because (1) the emission lines and the power-law are relative stable features when compared with the various absorption features. (2) At z = 2.2, Lya enters the B band; at z = 2.8, LLS enters the U band, and the large number of absorption features on the short wave side of Lycr greatly distort the shape of the spectrum in the visible region. (3) Most of the absorption systems vary considerably with the redshift and their effects become more obvious as the redshift is increased. In sum, for high-redshift &SOS, it is the extrinsic, absorption features of the spectrum that determines the way the colors vary with the redshift. Among these, the presence or otherwise of LLS is crucial. ACKNOWLEDGMENT FAN Xiao-hui wishes to express his sincere thanks Tan of Nanjing University and Dr JIANG Zhao-ji of Beijing astronomical their guidance and help.

References [II

Warren S. J., Hewett.

P. C., Irwin M. J. et al., ApJS,

1991, 76, 1

[21

Schneider D., Schmidt M., Gunn J. E., AJ, 1991,102,837

[31 [41 [51

Giallongo E., Trevese D., ApJ, 1990,353,

Koo D. C., Kron D. G., Cudworth

285

24

Mathews T. A., Sandage A. R., ApJ, 1963,138,30

[‘51Bessel [71 181 191 [lOI WI

K. M., PASP, 1986,98,

M. S., PASP, 1986, 98, 1303

Tiig H., White N. M., Lockwood G. M., A&A, 1977,61,679 Sargent W. L. W., Steidel C. C., Boksenberg Gunn J. E., Peterson

A., ApJS, 1989,63,

B. A., ApJ, 1965,142,1633

Giallongo E., Cristiani S., Trevese D., ApJ, 1992, 398, L9 Lanzettz

K. M., ApJ, 1991, 375, 1

703

to Professor Observatory

LU for