arXiv:1503.02235v2 [astro-ph.CO] 17 Apr 2015 NAPRN ESITDPNEC FQAA OTNU:IMPLI CONTINUUM: QUASAR OF DEPENDENCE REDSHIFT APPARENT AN loCege l 91.Cmae oohrattempts other to Compared 1991). al. et Cheng also where a value cal α usr a ewl ecie yapower-law a by described well be can quasars ocue htteaeaeln fsgthscsi ex- cosmic has and sight dust, of cosmic line of the average tinction of the effect that red- the to higher concluded toward Wright detected spectra they Indeed, spectra redder shift quasar the redshifts. attributed the higher (cos- (1981) makes at intergalactic redder which the statistically extinction, is one dust mechanism them, Among mic) interesting of component. the evolution dust of the the and/or of 2008) e.g., Bursick evolution & the continuum, (Kennefick process quasar accretion the the or of 2003) dence al. et Pentericci 1981). 2001; (Wright any reverse al. hardly even et find (Kuhn others 1999; while evolution al. 2008), et Bursick (Carballo & the Kennefick redshift of higher existence hardening slight toward the con- the hand, continuum quasar suggest on the studies other debate Some of (evolution) the running tinuum. dependence long On redshift see the a however, of been 2007, 2008). has al. al. Bursick et et there & (Carballo Davis Kennefick 2002; harder also al. is et Telfer quasars 1999; luminosity higher of continuum. inlns(eesn19) twvlntslne than emis- longer broad wavelengths At of Ly series the 1997). a (Peterson and lines characterized continuum sion be featureless can which a 1992), by al. et (Francis spectra ν rpittpstuigL using 2018 typeset 8, Preprint November version Draft † eea ehnsscudcueterdhf depen- redshift the cause could mechanisms Several continuum UV the that agreement general a is There bevtoal,qaasso eysmlrUV-optical similar very show quasars Observationally, oenegative more , [email protected] 1 e aoaoyfrRsac nGlxe n omlg,Shan Cosmology, and Galaxies in Research for Laboratory Key α α ν htcudpouesc ednn ffc aeas endiscusse been also have effect reddening headings: a Subject such produce could that ednyadfida ffcieds density dust effective an find and tendency ihrrdhf) esgetta h omcds xicini ver is extinction dust con cosmic quasar the the that suggest (i.e., α We redshift redshift). with higher anti-correlations significant show te ad hnqaasaefrhrgopdit uioiybins luminosity into grouped further are quasars when hand, other k uvy usrcniumsoe nteU-p adaemeasu are band UV-Opt the samp in quasar i.e., slopes radio-quiet ranges, continuum non-BAL Quasar large Survey. a Sky using spectrum composite ly h Vsetasoebcmshre (higher harder becomes slope spectra UV the ally, ie from line, − ∼ ν hrceie h otnu lp ihatypi- a with slope continuum the characterizes eivsiaetelmnst n esitdpnec ftequasa the of dependence redshift and luminosity the investigate We − A 2 v 0 z rdaeUiest fteCieeAaeyo cecs No. Sciences, of Academy Chinese the of University Graduate . 0 = Vne eke l 01.I em of terms In 2001). al. et Berk (Vanden 5 eain ebidasml omcds xicinmdlt uniythe quantify to model extinction dust cosmic simple a build We relation. α 1. . ∼ 85 ν α A INTRODUCTION en tee o edr softer) redder, (or steeper a means T ν 30t 5000 to 1300 E ± 12 tl mltajv 03/07/07 v. emulateapj style X 0 (1000 . ut xicin—qaas general quasars: — extinction dust, 1ott = hwvr see (however, z=3 to out 51 ioiXie Xiaoyi ∼ 2000 adnRa,Saga 000 hn;[email protected] China; 200030, Shanghai Road, Nandan ,tecniumof continuum the A, ˚ 3 e a o srpyis hnhi203,China 200234, Shanghai Astrophysics, for Lab Key )and A) ˚ 1,2 hynShen Shiyin , rf eso oebr8 2018 8, November version Draft f α ν ν 24 ∝ EXTINCTION? (2000 ν ABSTRACT nσ α ν v , 1,3 ∼ ∼ † hny Shao Zhengyi , hiAtooia bevtr,CieeAaeyo Science of Academy Chinese Observatory, Astronomical ghai α 10 4000 ν aea es n msinln ihFH agrthan larger FWHM with line s km emission 1000 one least at have ffcso h esitt h ee f3 ms km 30 of level the systematic to the recalcu- redshift reduced the (2010) and of quasars Wild effects of & redshift Hewett the lated catalogs. our value-added for database basic the provides study. catalog This tures. rcini h e aeeghbn.Atrmeasuring After sub- band. sky wavelength the red improved the (2010) in Hewett traction & Wild redshift. ape hticue htmtisin photometries includes that samples Ω rsn u ocuin nScin6 hogotthis Ω Throughout parameters we cosmological 6. the Finally, Section adopt we possibilities. in Letter, other conclusions discuss our effects. cosmic we present reddening simple 5, the a Section quantify build to In we model 4, extinction Section dust In redshifts. higher pcrm nScin3 erpr h nigo signifi- a of finding the of composite report dependence we the cant 3, building Section of In method spectrum. and data the its describe explore dust. further to cosmic then spectrum the for dependence, composite implications redshift the quasar use its the and probe 2000) of al. slope et York continuum (SDSS, Survey Sky therefore, Digital redshifts. higher size; to sample explored of large be advantage can and al. they take luminosity et quasars More high (e.g., 2010a), their extinction al. M´enard dust et cosmic 2009; the probing on usr htaemr uiosthan luminous more 105,783 DR7 are SDSS compiled the (2010) that In al. quasars 9200 et 2000). al. Schneider to et catalog, (York quasar 3800 2000 about from of tions spectra and bands − oadhge ooerclmnst.O the On luminosity. bolometric higher toward ) M copnigteD7qaa aao,teeaeother are there catalog, quasar DR7 the Accompanying h DSlgc uvypoie aaaeo quasar of database a provides survey legacy SDSS The hsLte sognzda olw.I eto ,we 2, Section In follows. as organized is Letter This Sloan from sample quasar large a take we Letter, this In 5 h .,and 0.3, = )drvdfo oe-a tig Gener- fitting. power-law a from derived A) ˚ Mpc 9 uunRa,Biig104,Ciaand China 100049, Beijing Road, Yuquan 19A − − 1 1 rhv neetn/ope bopinfea- absorption interesting/complex have or at 1,3 d. ieytecueo hsobserved this of cause the likely y efidta both that find we , h u Yin Jun , edanfo h la Digital Sloan the from drawn le iumbcmsrde toward redder becomes tinuum < z 0 = e ttodffrn wavelength different two at red otnu ymaso the of means by continuum r α . 1 7. ν . .Teohrpossibilities other The 5. nterdhf:rde pcr at spectra redder redshift: the on 2. AINFRCSI DUST COSMIC FOR CATION 1 , DATA bevdreddening observed α ν 12 and M ihresolu- with A ˚ i u,g,r,i = α − ν − 24 1 22 and , Λ e unit per ,80 s, . 0.7, = and 0 z 2 Xie et al. the spectroscopic features, Shen et al. (2011) calculated 1350-1365 A˚ and 4200-4230 A,˚ have been adopted, which the implied physical parameters (e.g., the bolometric lu- avoid the Lyα line at shorter wavelengths and contamina- minosity, black hole mass, and Eddington ratio) for each tion from the host galaxy at longer wavelengths. How- quasar. ever, this range is too long to be covered by any sin- In this study, we take the spectra from Wild & Hewett gle SDSS spectrum at any given redshift. Similar to (2010) and use the improved redshifts from Davis et al. (2007), we add a new continuum window Hewett & Wild (2010). We exclude the quasars 2210-2230 A.˚ With this configuration of continuum win- with broad absorption lines from our analysis according dows and the 3800-9200 A˚ wavelength range of SDSS to the flags in Shen et al. (2011). The radio-loud quasars quasars, the far-UV slope αν12 and the near-UV slope show significant redder continua than radio-quiet ones αν24 can be fitted for quasars in the redshift intervals (Labita et al. 2008) and the fraction of them changes 1.80 blue line in Fig. 1) and cording to the redshifts of Hewett & Wild (2010). Next, normalized at 3000 A.˚ The results are shown in Fig. 2. we follow Vanden Berk et al. (2001) to make the quasar The quasar composite spectra change with Lbol sig- composite and calculate the statistical error for the com- nificantly, where the higher Lbol quasars have system- posite spectrum by dividing 68% semi-quantile to the atically harder continua. We calculate the continuum square of the contributing number at each wavelength. slopes (αν12 and αν24) for each composite spectrum and The composite spectrum of all 91,131 quasars is shown plot them as a function of Lbol in the sub-panel of the in Fig. 1 as the blue line. For comparison, the composite figure. The αν12 and αν24 values are close at high Lbol spectrum of Vanden Berk et al. (2001) is shown as the and show similar trends with luminosity. At the low red line. As can be seen, these two composites show good luminosity side (log Lbol < 47), the change of the con- consistency. Because of a much larger sample size, our tinuum slope with luminosity is very significant, while at composite spectrum has a much higher signal-to-noise the high luminosity side (log Lbol > 47), quasar continua ratio (shown in the sub-panel of Fig. 1). roughly keep a constant slope (−0.5).

Fig. 1.— Composite quasar spectrum generated from 91,131 Fig. 2.— Ratios of the composite spectra in different bolometric SDSS DR7 quasars (this work, blue line) and the result of luminosity bins to that of the whole sample. All the composites (Vanden Berk et al. 2001, red line). The signal-to-noise ratios of are normalized to the flux at 3000 A.˚ The range of the bolometric these two composite quasar spectra are plotted in the sub-panel. luminosity bin (in log Lbol) and the number of contributing spectra The dotted lines mark the regions of continuum windows, i.e., 1350- for each composite are listed in the legend. In the sub-panel, the 1365 A,˚ 2210-2230 A˚ and 4200-4230 A,˚ which are used to calculate continuum slopes αν12 (diamonds) and αν24 (triangles) are plotted αν12 and αν24 (see the text for details). against the median bolometric luminosity of each bin.

In the UV-Opt band, the quasar continuum can be The redder spectra of lower luminosity quasars can very nicely characterized by a single power law (fν ∝ hardly be explained by the accretion disk model, where ναν ) and is usually fitted from continuum windows. the peak of the big blue bump moves toward lower fre- In Vanden Berk et al. (2001), two continuum windows, quency (longer wavelength) with increasing luminosity An apparent redshift dependence of quasar continuum 3

(Hubeny et al. 2000; Davis et al. 2007). As discussed in redshift with diamonds (αν12) and triangles (αν24). The Davis et al. (2007), this discrepancy could be alleviated continuum slopes within the same Lbol bin are connected by assuming a higher intrinsic dust extinction in low lu- with lines, while different Lbol bins are represented by minosity quasars. Indeed, there is observational evidence different colors. We find that in any given Lbol bin, that the low luminosity AGNs have higher intrinsic dust quasars at higher redshifts have systematically redder extinction (e.g., Gaskell et al. 2004). Such a luminosity UV continuum slopes (for both αν12 and αν24). dependent intrinsic dust extinction scenario is also con- This result clearly identifies the existence of the red- sistent with a receding dust torus model (Simpson 2005; shift dependence of quasar spectral slopes and implies Gu 2013). However, even if the dust extinction is in- that the previous debates are probably driven by red- dependent of the quasar luminosity, quasars with higher shift selection bias. extinction will also show lower luminosity and redder UV spectra. Such a selection effect may also partly explain the αν − Lbol relation we observed.

3.2. Trend of αν with redshift In this section, we test whether the quasar continuum slope shows systematical variance with redshift. As a first step, we divide the sample into five redshift bins with bin sizes of 1 from z = 0 to 5 and make composites. Similar to Fig. 2, we calculate the ratios of the com- posites to the global one for different redshift bins and show the results in Fig. 3. Consistent with the finding of Pentericci et al. (2003), we find little dependence of quasar spectra on redshift.

Fig. 4.— αν12 (diamonds) and αν24 (triangles) of quasar com- posites in different Lbol bins (different colors) as a function of red- 5 1 shift. Dust effective density nσv is fixed to 2 × 10− h Mpc− and two extinction models: GB07 and SMC are drawn in dotted and dash lines. αν is probed by αν24 below z = 1.5 and by αν12 above z = 1.5.

4. MODELING THE REDSHIFT DEPENDENCE OF THE QUASAR CONTINUUM SLOPE WITH COSMIC DUST EXTICTION As we have mentioned in the Introduction, the cosmic dust extinction is a very interesting mechanism that can make the quasar continua systematically redder at high redshift. Therefore, it is quite natural to introduce a cosmic dust extinction model to explain the results we Fig. 3.— Ratios of composite spectra from different redshift bins have obtained in Fig. 4. Other possibilities will be to that of the whole sample. discussed in Section 5. We start from a toy model, which assumes that dust is However, the SDSS quasar sample is roughly a magni- uniformly distributed along the line of sight. Considering tude limit sample and thus high redshift quasars are bi- the intervening dust has a comoving number density of ased to high luminosity ones. According to the results in n, the optical depth of the cosmic dust to the photons Section 3.1, if there are no evolutionary effects of quasar with wavelength λ0 emitted from a quasar at redshift zQ is spectra, high redshift quasars should also have harder zQ 2 1+ zQ (1 + z) UV spectra. Therefore, the apparent non-evolutionary τ(λ0)= nσ( · λ0)DH dz (1) quasar spectra implies another redshift dependent mech- Z0 1+ z E(z) anism that could compensate for the luminosity bias. where DH = c/H0, E(z) = H(z)/H0, and σ(λ) is To recover this hidden effect, we make composite quasar the cross section of dust absorbers at wavelength λ spectra at different redshift bins after Lbol controlled. (More et al. 2009). We assume that the properties of the The quasars are grouped into two-dimensional bins of absorbers do not evolve with redshift and σ(λ) is scaled L and z. The bin width of L is set to 0.5 dex in bol bol by the cross section of the absorber at 5500A,˚ σv: logarithm while the width of z is set to either 0.1 or · ˚ 0.2 according to the number of the quasars available for σ(λ)= σv el(λ)/el(5500A) (2) composite. The minimum number of quasars to make a where el(λ) stands for the extinction law. composite spectrum is set to 20. The results are shown We also assume that the intrinsic continuum of quasars in Fig. 4, where the spectral slopes are plotted against in the UV-Opt band can be characterized by a pure 4 Xie et al.

αν0 power-law form fν = ν . Quasars are put to their redshifts zQ, and then, their observational spectral con- tinuum can be rebuilt and slopes αν,z can be measured after considering the cosmic dust extinction from z = zQ to z = 0. To mimic the observation, we also fit the con- tinuum slopes in different wavelength ranges for quasars at different redshifts (αν12 for zQ > 1.5 and αν24 for zQ < 1.5). In this toy model, besides the extinction law, we have two free parameters, one is the combined pa- rameter nσv, which represents the effective density of the cosmic dust particles, and the other is αν0, the intrinsic quasar continuum slope. We have little prior knowledge of the template extinc- tion law of the cosmic dust. As the 2175 A˚ extinction bump is rarely seen in other galaxies, an SMC-type ex- tinction curve is preferred. In this study, we test the featureless SMC extinction law first. We choose sets of representative values of αν0 and show the predicted αν,z as a function of zQ, which are represented by the dashed Fig. 5.— Extinction magnitude AV as a function of redshift −5 −1 parallel lines in Fig. 4. Here, nσv =2 × 10 h Mpc is using the GB07 extinction model in Fig. 4. The open circle shows − the result of M´enard et al. (2010a), where the dashed line shows tuned to match the observed αν24 z relation. their extrapolation from a constant comoving dust density model. As can be seen, our simple cosmic dust extinction The other observational constraints are extracted from Fig. 9of model can reproduce the observed αν24 − z relation at M´enard et al. (2010a). In the bottom panel, the model-predicted the low redshift range (z < 1.5) very well. At high dust reddening E(B − V ) is plotted against z. redshift, our model (SMC extinction curve and nσv = 5. DISCUSSION: OTHER POSSIBILITIES −5 −1 2 × 10 h Mpc ) predicts a much steeper αν12 − z rela- tion than that is observed. This discrepancy can be easily In this section, we discuss whether there are any other explanations to the redshift dependence of the quasar solved by adopting either a lower nσv at z > 1.5 or a shal- lower extinction curve at a far-UV wavelength. To keep continuum slopes other than the cosmic dust. our model simple, we test the later possibility. Indeed, 5.1. there are studies suggesting that the extinction curves Evolution of intrinsic properties? in quasars are flatter in the UV (Czerny et al. 2004; In Section 3.1, we have argued that the internal dust Gaskell et al. 2004). We take the flat reddening curve extinction might be the cause of the αν -luminosity trend from (Gaskell & Benker 2007, hereafter, GB07), which that we observed. Could it also be responsible for the is quite similar to the Milky Way extinction law, except αν − z relation? In general, we know that metal and for the lack of a 2175 A˚ bump. We show the GB07 dust dust are accumulated through various processes in the −5 −1 extinction models with the same nσv =2×10 h Mpc galaxy evolution, e.g., dust formation in Type II su- as the dotted lines in Fig. 4. As expected, the GB07 ex- pernova and asymptotic giant branch stars (Asano et al. tinction curve matches the observation much better than 2014). Therefore, galaxies at lower redshifts will typi- the SMC extinction curve at z > 1.5. cally have more dust. This global trend is opposite to In order to have a more intuitive impression of the the αν − z relation we observed for quasars. cosmic dust extinction, we take the GB07 dust ex- However, we know that quasar host galaxies are atypi- tinction model and calculate the observer-frame extinc- cal. Many observations have shown the presence of large tion AV and reddening E(B − V ) as a function of dust mass in high redshift (z > 4) quasars; though, the source redshift. The AV and E(B − V ) are plotted origin of the large amount of dust in such early epochs in the upper and lower panels of Fig. 5 respectively. is unclear (Valiante et al. 2011, and references therein). In this plot, we also show the cosmic extinction con- Moreover, there are also clues that the extinction curves strained from a few other measurements for compar- of redshift (z > 4) quasars might be different from their ison. The dashed line shows the constant comoving lower redshift counterparts (e.g., Nozawa et al. 2015). dust extinction model of M´enard et al. (2010a), which Despite the variations, we have not yet found any sta- is constrained from the statistical detection of dust red- tistical evidence that quasars at higher redshifts show dening around galaxies up to large scales. The other systematically higher intrinsic extinctions, at least for observational constraints are also extracted from Fig. the redshift range we probed (0.7 starlight might still be significant, especially at the near-UV band, i.e., α . ν24 6. CONCLUSION For a log Lbol ∼ 45.75 quasar, a bright star-forming host galaxy with Mr = −23 contributes ∼ 20% of the flux in In this Letter, we use a large non-BAL radio-quiet the 2000 ∼ 4000 A˚ range. More than that, the redshift quasar sample from SDSS DR7 to study the redshift range of the near-UV band we probed is from z =0.7 to and bolometric luminosity dependence of quasar contin- z = 1.1. This is the time when the cosmic star forma- uum slope by means of the composite spectrum. Con- tion rate (SFR) starts to decrease. Therefore, it is pos- sistent with previous studies, we find that the UV-Opt sible that there are larger contributions of the starlight spectrum becomes harder toward higher bolometric lu- at higher redshift, so that makes the quasar spectra look minosity, and we suggest that the internal dust reddening redder. might be the main reason for this luminosity dependence. To test this possibility, we generate a series of model When the bolometric luminosity is controlled, we find galaxy spectra at different redshifts according to the cos- that the quasar UV continuum slope is anti-correlated mic star formation history given in Madau & Dickinson with redshift (redder at higher redshift). We propose (2014) and their luminosities are assumed to be Mr = that the redshift dependence of the quasar UV slope −23. We then combine the galaxy spectra at differ- might be caused by the reddening of the cosmic dust. We ent redshifts with the composite spectrum of log Lbol ∼ build a simple cosmic dust extinction model to quantify 45.25 quasars. Again, we measure the αν24 for the syn- the reddening of the quasars. Based on our model, we ∼ −5 −1 thetic quasar+galaxy spectra at different redshifts. We find that an effective dust density nσv 10 h Mpc − find that, globally, α becomes smaller (redder) when at z < 1.5 can reasonably reproduce the αν24 z rela- ν24 − the galaxy spectra is combined into the quasar contin- tion. The αν12 z relation at high redshift (z > 1.5) can uum. However, on the other hand, we do not find that also be easily reproduced by using a shallow extinction the combined spectra get redder toward high redshift. curve or adopting a smaller effective density of the cos- The reason is that, while going to the higher redshifts mic dust. The cosmic dust extinction constrained from from z ∼ 0.7 to 1.1, because of the higher SFR, not our model is globally consistent with the constrains from only does the contribution of the starlight in the near- other independent measurements, and has reached to an UV band become larger, but the near-UV band spectrum unprecedented high redshift (z ∼ 3.2). of the galaxy itself also becomes bluer.

5.3. Quasar color selection bias? We thank the referee for a careful reading and highly SDSS quasars are selected using a complicated color- appreciate the comments and suggestions. X.X.Y. based algorithm (Richards et al. 2002). Quasar candi- thanks Peng Jiang in USTC for help with our code writ- dates at high and low redshift are selected based on differ- ing, Hengxiao Guo and Fangting Yuan in SHAO for help- ent color-color diagrams. Therefore, the color-selection ful discussions, and Brice M´enard in JHU for provid- criteria might introduce biases to the intrinsic colors of ing their model data. This work was supported by the quasars at different redshifts. For example, is it possible Strategic Priority Research Program “The Emergence of that the redder quasar continua at higher redshifts are Cosmological Structures” of the Chinese Academy of Sci- caused by the missing of red quasars at low redshift or ences (CAS; grant XDB09030200), the National Natural vice versa? Science Foundation of China (NSFC) with the Project To test this possibility, we take the reddest (the last Numbers 11433003, 11390373, and 11103058, and the magenta diamond in Fig. 4) composite spectrum and put “973 Program” 2014 CB845705.

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