CHANDRA OBSERVATIONS OF SIX QSOs AT z % 31 - IOPscience

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The Astronomical Journal, 131:659–665, 2006 February # 2006. The American Astronomical Society. All rights reserved. Printed in U.S.A.

CHANDRA OBSERVATIONS OF SIX QSOs AT z
31 Takamitsu Miyaji Physics Department, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, PA 15213; [email protected]

Gu¨nther Hasinger and Ingo Lehmann 2 Max-Planck-Institut f u¨r extraterrestrische Physik, Postfach 1312, 85741 Garching, Germany

and Donald P. Schneider Department of Astronomy and Astrophysics, Pennsylvania State University, 525 Davey Laboratory, University Park, PA 16802 Receivved 2005 May 16; accepted 2005 October 9

ABSTRACT We report the results of our Chandra observations of six QSOs at z  3 from the Palomer Transit Grism Survey. Our primary goal is to investigate the possible systematic change of
ox between z > 4 and z  3, between which a rapid rise of luminous QSO number density with cosmic time is observed. The summed spectrum showed a powerlaw spectrum with a photon index of

1:9, which is similar to other unabsorbed active galactic nuclei. Combining our z  3 QSOs with X-ray observations of QSOs at z > 4 from literature or archives, we find a correlation of
ox with optical luminosity. This is consistent with the fact that the luminosity function slope of the luminous end of the X-ray-selected QSOs is steeper than that of optically selected QSOs. We discuss an upper limit to the redshift dependence of
ox using a Monte Carlo simulation. Within the current statistical errors, including the derived limits on the redshift dependence of
ox , we found that the behaviors of the X-ray and optically selected QSO number densities are consistent with each other. Key words: galaxies: active — galaxies: high-redshift — galaxies: luminosity function, mass function — quasars: general — X-rays: galaxies Online material: color figure

1. INTRODUCTION

Ueda et al. 2003; Barger et al. 2005) showed that the number density curve is luminosity-dependent and a trend that as luminosity goes lower the density peak shifts to lower redshifts (which some authors call an antihierarchical active galactic nucleus [AGN] evolution and others call a downsizing of the AGN activity). However, these studies did not sample well the highredshift, high-luminosity regime, in which optically selected QSOs show a decline at z > 3. In a recent study based on an updated soft X-ray sample including Chandra and XMM-Newton surveys, Hasinger et al. (2005, hereafter SXLF3) also measured the number densities in various luminosity bins with a better accuracy over a large range in the redshift-luminosity space. The study revealed this early ‘‘growth’’ of AGN number density (or a decline with increasing z) at z k 3 for AGN/QSO luminosities at log LX P 44 45. The results from the Chandra Multiwavelength Project survey, which was designed to optimally trace the high-redshift, highluminosity (log LX > 44:5) regime with improved statistics, found that the density curves of X-ray-selected QSOs ( log LX > 44:5) decline with increasing redshift at z > 3. This decline is, however, shallower than that seen in optically selected QSOs. The dependence of the optical–to–X-ray flux ratio (customarily expressed by the quantity
ox , the effective spectral index between the rest-frame 2500 8 and 2 keV )3 on redshift and luminosity has been a key issue in X-ray observations of highredshift QSOs and has important implications for possible differences in the AGN evolution traced by X-ray and optical samples. Some authors (e.g., Vignali et al. 2003a; Strateva et al. 2005) found that
ox strongly depends on luminosity, with LX / L0:75 opt

Large-scale optical surveys show that the luminous QSO number density peaks at 1:7  z  2:7, before which (in cosmic time) the QSO number density grows rapidly and after which the density steadily decays until the present epoch (e.g., Boyle et al. 1988; Warren et al. 1994; Schmidt et al. 1995, hereafter SSG95). The first indication of a density decline (with redshift) at z k 3 was reported in the pioneering work of Osmer (1982). The Palomar Transit Grism Survey (PTGS; Schneider et al. 1994) was designed to investigate this possible ‘‘redshift cutoff’’ and produced a sample of 90 z > 2:7 QSOs; an analysis of the PTGS (SSG95) revealed a very rapid growth, by a factor of 3–5, of the number density of luminous QSOs from z  4:5 to 2.7. This result was verified by Fan et al. (2001) using a sample of 3:6 < z < 5:0 quasars found in the Sloan Digital Sky Survey (SDSS; York et al. 2000). This redshift cutoff has also been found in the number densities of radio flat-spectrum QSOs (Shaver et al. 1996; Wall et al. 2005). In the X-ray regime, however, the size of a sample based on the combination of various ROSAT surveys (Miyaji et al. 2000, hereafter SXLF1) was not sufficient to accurately probe this ‘‘growth’’ phase of the number density of X-ray-selected luminous ( log LX > 44:5) QSOs. While this study appears to show a flat number density in z > 2:7, the uncertainties produced by the small number of quasars limited the robustness of the conclusions. More recent studies including the results from Chandra and XMM-Newton surveys (Cowie et al. 2003; Fiore et al. 2003; 1

Based on observations using the Chandra X-Ray Observatory. Current address: LPS-Berlin, Ko¨pernickerstrasse 325, 12555 Berlin, Germany. 2

3

659

We use f / 
XX , where XX is any subscript to
.

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TABLE 1 Log of Chandra Observations and Sample Properties

Name PC PC PC PC PC PC

0041+0215 ................. 0947+5628 ................. 1000+4751 ................. 1015+4752 ................. 1035+4747 ................. 1447+4750 ................. a

ObsID 4150 4151 4152 4153 4154 4155

Date 2003 2003 2002 2003 2003 2003

Sep 1 Jan 25 Dec 18 Jan 1 Mar 16 Jul 28

Exposure ( ks)

z

AB1450a

RL

9.2 9.0 13.9 8.1 10.0 7.0

2.93 2.91 2.95 2.92 2.96 2.93

19.5 19.5 20.0 19.4 19.6 19.3

2:7 at this luminosity range. In order to make a parent distribution of
ox from which the simulations draw objects, we used all the QSOs in the current sample except for upper limits. In addition, the
ox values of QSOs in each group have been shifted so that all groups have the same mean value. The standard deviation of these 58
ox values is consistent with those of groups A and D. In each run we randomly took six QSOs to represent group A and 11 to represent group D. The mean of each of the six and 11 random
ox values has been calculated, and the distribution of the difference of these means (DAh
oxi) for 2000 simulations has been investigated. As a result, the range in which 90% of the simulations fall is jDA h
ox iDA j < 0:13. This limits the systematic difference in log (LX /Lopt ) between z  3 (group A) and z > 4 (group D) to 0.34. For the slope of the luminosity function (LF ) of X ¼ 2, this corresponds to a number density difference of jlog ( opt / X )j < 0:68, where opt and X are the number densities of QSOs above optical and X-ray luminosities, respectively. Figure 3 shows the evolution of the comoving number density of soft-X-ray-selected QSOs (SXLF3) as a function of redshift plotted with those of optical QSOs from SSG95 and Fan et al. (2001) at z > 2:7 (their original values are converted to our default cosmology and
o). The luminosity limit for the softX-ray QSOs is set at log LX > 44:95, where LX is the observed frame 0.5–2 keV luminosity in ergs s1 corresponding to approximately the 28 keV luminosity at the rest frame. Note that this is a rest-frame 0.5–2 keV luminosity under the assumption of a
¼ 2:0 power-law spectrum, which is representative of X-ray-selected type 1 AGNs, and SXLF3 treats the luminosities as such. We also show the results from Silverman et al.’s (2005) sample for the same observed 0.5–2 keV luminosity cut. This luminosity (Fig. 3, thick lines and filled symbols) was selected such that the space density became equal to that of optical QSOs at z  2:9. For reference, we show the number density curves from SXLF3 and Silverman et al. (2005) (open symbols with thin lines) in lower luminosity ranges (see labels) representative of the respective samples. These show declines for z k 3. The large error bars in the log LX > 44:95 data at z > 2:7 show that the QSOs in this regime are still underrepresented by current X-ray surveys. The limits of the number density curve of the X-ray-selected sample are shown in the shaded area in Figure 3, which corresponds to jh
ox iDA j < 0:13. A limitation of the above investigation is the uncertainties in the effects of the sample selection and variability. The X-ray and optical luminosities have been measured in different epochs. Thus, the variability of AGNs has a net effect of increasing the variance of the
ox distribution. Our underlying assumption is

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Fig. 3.—Number densities of soft-X-ray–selected QSOs from samples used by SXLF3 ( filled hexagons) and Silverman et al. (2005; filled squares) plotted with those of optically selected QSOs (SSG95 and Fan et al. 2001; triangles), converted to our default cosmology. The X-ray luminosity cut ( log LX > 44:95) was chosen to give the same number density as the optical curve at z  2:9. These are represented by filled symbols and thicker error bars. The hatched area is the 90% confidence range of the expected evolution of soft-X-ray–selected QSOs from the constraint on the systematic
ox change with redshift as determined from the comparison between groups A and D. For reference, the density curves of QSOs from SXLF3 and Silverman et al. (2005) are overplotted for the original paper’s representative luminosity ranges as labeled and plotted by open symbols. The dotted data points from SXLF3 show the absolute maximum number densities at which all the optically faint unidentified X-ray sources were at the center of the redshift bin. [See the electronic edition of the Journal for a color version of this figure.]

that the variability does not cause a net systematic difference in its effect on the mean
ox between sample A and sample D, which are separated in redshift but not in luminosity. Both are optically selected samples, and h
oxi should be biased more toward larger (more optically luminous) values than toward the ‘‘true’’ h
oxi (i.e., the
ox value of time-averaged mean optical and X-ray luminosities) because optical selection is more likely to pick up the AGN when it is more optically bright, while the X-ray follow-up of the same object typically gives the average X-ray luminosity of the source. As long as both are selected in the optical and followed up by X-ray, the slopes of the LFs are the same at both redshifts, and there is no systematic difference in the variability amplitudes of AGNs with redshift, the effect of this ‘‘variability’’ bias should be equal in samples A and D. Thus, we do not expect that the variability bias plays a major role in our analysis of the redshift dependence of h
oxi. From a combination of our sample at z  3 and the z > 4 QSOs observed by Chandra, we have confirmed the apparent

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dependence of
ox on optical luminosity. This result, however, should be treated with caution, because our sample is optically selected and subject to the variability effect of preferentially picking up an optically brighter phase as described above. An X-ray-selected sample covering a much larger regime in z-L space instead showed LX / Lopt (Hasinger 2005). This is, however, subject to a similar effect working in the opposite sense. At this limited regime, on the other hand, there is a hint that our currently determined dependence might reflect the true behavior of the shift of
ox with QSO power. Expressing the LF as d/d log L / L at the luminosities of interest, the soft-X-ray LF has X ¼ 2:2  0:3 and (90% error) in 2:7 < z < 4:8 (from the same sample as SXLF3). This can be compared with the optical LF of opt ¼ 1:6  0:2 (1  error) by Fan et al. (2001) or opt ¼ 1:87 by SSG95. The trend that the optical LF has a flatter slope than the soft-X-ray counterpart is consistent with the relation opt ¼ X within errors. This is also consistent with the comparison between X-ray and optically selected AGN luminosity functions by Ueda et al. (2003; see their Fig. 20), in which a conversion of their hard-X-ray LF to the optical band assuming LX / L0:70 opt gave a good match to an observed optical QSO LF at high luminosities. However, our most recent comparison of the soft-X-ray LF (SXLF3; high-luminosity end) and optical QSO LF by Croom et al. (2004) at z < 2:1 is more consistent with LX / Lopt , and thus, more study is needed to investigate the relationship between direct comparison of LX and Lopt and the conversion between the X-ray and optically selected QSO LFs. 5. SUMMARY We have made Chandra ACIS-S observations of six QSOs at z  3, which marks the peak of the luminous QSO number density. These observations fill a redshift gap in the X-ray coverage of luminous QSOs. We found an average photon index of h
i ¼ 1:9  0:3 from the stacked spectrum, and we also found h
ox i ¼ 1:65  0:05. The h
oxi value is essentially the same as those at z > 4 in the similar UV luminosity range, and thus, we have found no systematic shift of X-ray-to-UV luminosity ratios with redshift above z ¼ 3. The density curves of MB < 26:5 optically selected QSOs and log LX > 44:92 softX-ray–selected QSOs, giving the same densities at z  2:7, are statistically consistent with each other within our limit of the systematic h
oxi shift at z > 3. We note that this regime is still underrepresented by X-ray surveys. Large-area, moderately deep X-ray surveys are needed to trace the rise in the number density of the most luminous QSOs at z > 3 in X-rays.

This work has been supported by Chandra General Observer Award GO3-4153X, NASA LTSA grant NAG5-10875 (T. M.), and NSF grant AST 03-07582 (D. P. S.). We thank John Silverman for calculating the space-density curves for our luminosity cuts and cosmological parameter choice.

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