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Orientation dependency of broad-line widths in quasars and consequences for black-hole mass

a r X i v :a s t r o -p h /0603231v 1 9 M a r 2006

Mon.Not.R.Astron.Soc.000,000–000(0000)Printed 5February 2008

(MN L A T E X style ?le v2.2)

Orientation dependency of broad-line widths in quasars

and consequences for black-hole mass estimation

Matt J.Jarvis 1?and Ross J.McLure 2

Astrophysics,Department of Physics,Keble Road,Oxford,OX 3RH,U.K.

2Institute

for Astronomy,University of Edinburgh,Royal Observatory,Edinburgh EH93HJ,U.K.

5February 2008

ABSTRACT

In this paper we report new evidence that measurements of the broad-line widths in quasars are dependent on the source orientation,consistent with the idea that the broad-line region is ?attened or disc-like.This reinforces the view derived from radio-selected samples,where the radio-core dominance has been used as a measure of orientation.The results presented here show a highly signi?cant (>99.95per cent)correlation between radio spectral index (which we use as a proxy for source orien-tation)and broad-line width derived from the H βand MgII emission lines.This is the ?rst time that this type of study has used quasars derived from a large optically selected quasar sample,where the radio-loud quasars (RLQs)and radio-quiet quasars (RQQs)have indistinguishable distributions in redshift,bolometric luminosity and colour,and therefore overcomes any biases which may be present in only selecting via radio emission.

We ?nd that the mean FWHM for the ?at-spectrum (αrad <～0.5)radio-loud quasars (FSQs)to be

FWHM SSQ =6464±506km s ?1.We also ?nd that the distribution in FWHM for the FSQs is indistinguishable from that of the radio-quiet quasars (RQQs),where

2M.J.Jarvis&R.J.McLure

superposition of many relativistically beamed synchrotron self-absorbed spectra.These?at-spectrum sources usually exhibit the optical characteristics of quasars,i.e.they are unresolved in optical imaging observations and have broad permitted emission lines(FWHM>2000km s?1).High-frequency(ν>1GHz)radio surveys(e.g.Drinkwater et al. 1997)preferentially pick-out these FSQs due to the fact that their radio emission does not become fainter at high frequen-cies,as is observed in optically thin synchrotron sources.

Conversely,the other extreme is a source with its ra-dio jets aligned along the plane of the sky.In this case the bulk of the emission arises from the optically thin lobes which can extend up to Mpc scales,and these sources are classi?ed as radio galaxies.The radio spectra of the lobes which dominate the radio spectrum of these sources are pre-dominantly steep withαrad>0.5.These sources dominate the low-frequency radio surveys,such as the complete sam-ples derived from the3CRR(Laing,Riley&Longair1983), 6CE(Eales et al.1997)7CRS(Willott et al.2002)and the Westerbork Northern Sky Survey(WENSS;Rengelink et al.1997).Optically these sources appear as normal galax-ies,usually with some extended line emission in the direc-tion of the radio jets;the so-called alignment e?ect(e.g.van Breugel,Heckman&Miley1984;Inskip et al.2002).The spectra of these objects only contain narrow emission lines (FWHM<2000km s?1)that are believed to reside beyond the obscuring torus.

Between these two extreme cases,where the jets lie at an angle<～45degrees(e.g.Barthel1989)to the line-of-sight we still see broad emission lines and also some emission from the?at-spectrum core,however there is also a large con-tribution from the large scale,steep-spectrum radio lobes. In this case the total radio spectra are somewhere in be-tween the?at-spectrum core dominated sources and the core-less,lobe-dominated radio galaxies and can thus exhibit both components,these are the steep-spectrum radio-loud quasars(SSQs).

Using this argument,the total radio spectral index is therefore an indicator of the orientation of the source on the sky.This is also reinforced by another orientation estima-tor based around the same argument.The strength of the core radio emission in quasars,when compared to the ex-tended emission,has been used extensively as an indication of source orientation(e.g.Wills&Browne1986;Brotherton 1996).

With these measures of orientation which are presented by radio-loud sources we are able to probe the geometry of other properties inherent to powerful quasars.One such property is the geometry of the broad-line region.Thus far, studies of this type have been restricted to radio-selected samples which are di?cult to reconcile with the more abun-dant radio-quiet quasar population.However,with the re-cent large-scale quasar surveys coupled with all-sky radio surveys,we are now able to construct large samples of both radio-quiet and radio-loud quasars with consistent selection criteria.

In this paper we use a sub-sample of the Sloan Digi-tal Sky Survey quasar catalogue to investigate the depen-dency of broad-line width on source orientation in a sam-ple of radio-loud quasars.The important di?erence of this study to previous studies is that the radio-loud sources are selected with exactly the same criteria as the larger popula-tion of radio-quiet sources in the sample.Thus we are able to directly infer the orientation e?ects which may present a distorted view of the radio-quiet population.Such an ef-fect may be very important if we are to continue using the broad-line measurements as an important component in de-termining black-hole masses of quasars,via the virial esti-mate(e.g.Kaspi et al.2000;Vestergaard2002;McLure& Jarvis2002),over the history of the Universe(e.g.McLure &Dunlop2004).

The paper is set out as follows,in Section2we pro-vide a brief description of the sample of quasars de?ned by McLure&Jarvis(2004).In section3we present evidence for a correlation between broad line width and radio spec-tral index,and then go on to determine the parameters of a disc-like geometry for the broad-line region in RLQs.In section4we attempt to relate the broad-line width distri-bution in the radio-loud quasars(RLQs)with that of the radio-quiet quasars(RQQs),and outline the implications that this would have in using broad-line widths to estimate black-hole masses.We summarise our results in section5. All cosmological calculations presented in this paper assume ?M=0.3,?Λ=0.7,H?=70km s?1Mpc?1.

2THE SAMPLE

For this analysis we use quasars drawn from the full sample of McLure&Jarvis(2004).These were selected from the SDSS quasar catalogue II(SQCII;Schneider et al.2003). Only those quasars which fell within the SDSS/FIRST over-lap region were included2.This ensures complete coverage in the radio wave band and both radio-loud and radio-quiet quasars were completely matched in terms of optical lumi-nosity and redshift distributions.Therefore,any biases that could be present due to targeted selection of FIRST(Becker, White&Helfand1995)radio sources are negated.

For the purposes of this paper we use the de?nition of radio-loudness of Miller,Peacock&Mead(1990;i.e. L1.4>1024W Hz?1sr?1),which results in a total sam-ple size of409.However,to ensure that we have an indica-tor of orientation through measurement of the radio spectral index,we cross-correlate our sample with the WENSS(Ren-gelink et al.1997),and to ensure a matching of spatial res-olution also with the NVSS(Condon et al.1998).Both the NVSS and WENSS have spatial resolutions of～30?40arc-sec at325MHz and1.4GHz respectively.

The need for each source to be matched with a source in the WENSS means that the sample size is reduced signif-icantly due to the fact WENSS only covers the sky north of 30degrees in declination and at a signi?cantly higher?ux-density limit of S325>18mJy(5σrms).Of the409objects in the radio-loud sample,121lie in the area covered by the WENSS,53of which have?ux-densities greater the WENSS ?ux-density limit.

2We note that using the quasar catalogue from SDSS DR3does not enhance our data set in any way as there are the same number of radio-loud quasars with our selection criteria.

Orientation dependency of broad-line widths in quasars

Figure 1.The relation between the radio-core dominance and the radio spectral index from the sample de?ned by Brotherton (1996).The solid line is the best ?t to the data where the ?tting takes account of uncertainties in both αrad and R .

ORIENTATION DEPENDENCE OF BROAD-EMISSION LINES?

3.1

Radio spectral index as a proxy for source orientation

The radio sources in our sample are derived from the op-tically selected SDSS quasar catalogue.Therefore in-depth studies have not yet investigated the radio structure of these sources,unlike other widely used radio surveys,such as the 3CRR (Laing et al.1983).For this reason we are limited in our analysis,due to the fact that we have no morphological constraint on the source orientation.Even the FIRST radio survey does not have enough spatial resolution (the spatial resolution of FIRST is ～5arcsec)to measure the core-to-lobe ?ux ratio (the R parameter;Hine &Scheuer 1980;Orr &Browne 1982;Hough &Readhead 1989)for the majority of our sample.

However,as previously discussed,many studies in the literature have shown that the radio-spectral index αrad may be a good indicator of the source orientation.This stems from the uni?cation picture of radio-loud AGN and has been con?rmed observationally by both Wills &Browne (1986)and Brotherton (1996).Brotherton used a sample 60radio-loud quasars to investigate the relation between radio-core dominance and broad-line FWHM.In Fig.1we plot the ra-dio spectral index and the radio core-dominance parameter R ,from Table 1of Brotherton (1996).This shows a highly signi?cant correlation (>>99.99per cent)between these two parameters with a dispersion in αrad of σα=0.37for a given R parameter (and σR =0.87for a given αrad ).Given that the R parameter has been used extensively to estimate the orientation of radio sources we use αrad as a proxy for R and thus source orientation for the purposes of this study.

Table 1.Radio-loud sources in the sample used in this analysis.All radio luminosities are given in W Hz ?2sr ?1and the survey from which they are derived are also listed.z αrad FWHM log 10(L 325)log 10(L 1.4)log 10(L 1.4)km s ?1WENSS NVSS FIRST 0.320.53533324.0323.5122.720.340.60330425.3124.8724.630.340.901053125.5924.7523.720.410.42255424.4924.2024.120.430.64190224.7124.2924.240.450.77940425.0124.5024.460.560.51569524.8524.4424.220.580.51565525.6825.3225.220.590.23483626.9326.7626.710.640.59254924.6224.2924.370.69-0.04247824.8024.8524.910.700.60620125.2024.5924.200.73-0.92408224.5725.0224.810.770.43561325.1724.8924.870.79 1.26483124.6924.3124.930.810.63649125.7725.1024.710.820.80467226.3625.4424.840.840.34269925.8825.6025.510.850.47354526.0525.6925.600.880.48735825.8825.5625.550.890.85751925.5924.9624.830.93-0.13500125.4325.4725.421.000.71594626.2125.3824.971.02-0.35233624.9025.1325.131.030.65585426.1525.2224.681.07 1.23872227.6626.8526.831.140.49512225.6825.3625.361.170.77498625.8425.1124.901.170.23361825.2125.0325.001.320.45706527.1526.7626.681.330.73649626.6425.9725.821.350.55467825.6125.2225.201.390.43880125.6925.3525.311.410.87702027.0525.8625.431.440.971569427.7826.2025.581.440.221340926.0225.8625.841.460.41716525.9725.6325.591.480.77612227.2026.6926.681.55-0.18309725.0825.2025.201.570.71789826.7325.9225.731.59 1.06915626.5625.4425.201.630.21310925.6925.4625.411.710.18336626.7126.5226.491.710.53550425.8425.1625.001.720.30537125.5725.3325.311.740.47284825.8925.6425.661.790.39289726.1325.8625.851.800.87471628.4526.7626.281.850.47646225.7125.3625.341.850.67820826.2525.6025.511.910.94589127.3526.4826.381.990.36547326.7226.4126.382.03

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4M.J.Jarvis&R.J.

McLure

Figure2.The radio spectral index(measured between1400and

325MHz)versus the measured broad

emission-line FWHM for

the sample of radio-loud quasars discussed in section2.The open

stars represent those sources where the integrated?ux measured

from the FIRST catalogue is within a factor of two of the inte-

grated?ux measured from the NVSS catalogue.The solid sym-

bols are sources where the ratio of FIRST/NVSS?ux-density is

less than two,implying that there is a signi?cant extended com-

ponent.The solid line represents the best?t to the correlation

accounting for the uncertainties in bothαrad and FWHM.One

can immediately see that the detection of extended emission is

heavily biased to those sources with steep(αrad>0.5)spectral

index as expected in the orientation-based uni?cation scheme.

3.2FWHM versus radio spectral index

In Fig.2we show the radio spectral index versus the FWHM

of the Hβor MgII emission lines,depending on the redshift

of the source(0.1

MgII)3.The sample used here is comprised of all of those

sources with a detection in the WENSS catalogue(see Ta-

ble1).One can immediately see a correlation between the

two parameters.The Spearman rank coe?cient for these two

parameters isρ=0.459with a signi?cance of the correla-

tion being present>99.95per https://www.doczj.com/doc/8617374726.html,ing Kendall’s rank

we?nd a similar statistical signi?cance withτ=0.324and a

signi?cance again of>99.95per cent.As these radio sources

are selected by exactly the same method i.e.SDSS optical

selection and present in the WENSS,with no biases that

would in?uence the measurement of both the FWHM and

spectral index,this correlation is undoubtedly real.

Moreover,if we now split the radio-loud sources in this

sample into?at-spectrum(αrad<0.5;FSQs)and steep-

spectrum quasars(αrad>0.5;SSQs),then the di?erence

in the measured FWHM for these two populations is highly

signi?cantly.The25FSQs have a mean and standard error

of FWHM=4990±536km s?1,whereas the28SSQs have

3The method of?tting the broad-line widths to these emission

lines is described fully in McLure&Dunlop(2004),where they

also show that the FWHM of Hβand MgII are consistent for

spectra where both are present.

Figure3.The best-?t models to the cumulative distribution

in FWHM for the RQQs(left)and the RLQs(right).The dis-

tributions from the data are represented by the solid lines and

the models are the dotted(RQQs)and dashed line(RLQs).The

best-?t model for the RLQs was obtained by freezingσorb and

θmax at the same values found for the RQQs(see text for details).

a mean FWHM=6464±506km s?1.The KS-test also

shows that the distributions in FWHM for FSQs and SSQs

are signi?cantly di?erent,with a probability of4per cent

that they are drawn from the same underlying distribution.

This is in line with previous studies in the literature

which have used both the spectral index and the ratio of

the core emission to the extended lobe emission to inves-

tigate the dependence of broad-line width on source orien-

https://www.doczj.com/doc/8617374726.html,ing a sample of radio-loud quasars derived from

various radio surveys Wills&Browne(1986)found evidence

for a signi?cant(>99.9per cent)correlation between radio-

spectral index and the FWHM of broad emission lines.These

authors also compiled the necessary data to investigate the

relation between the R parameter and the FWHM.Again

they found a signi?cant relation between FWHM and R,

and thus source orientation.This can be seen qualitatively

in Fig.2,where only the steep-spectrum sources exhibit an

excess of extended emission at the NVSS resolution when

compared to the higher resolution FIRST?ux-density.The

fact that our sample is derived from an optically selected

quasar sample provides additional constraints on the link

between orientation and radio spectral index,and also a

way of probing the distribution in orientation for RQQs.

3.3Do radio-quiet quasars have orientation

dependent broad lines?

The unique way in which our sample is de?ned means that

we are able to directly relate our?ndings for the radio-

loud quasars to the larger radio-quiet quasar(RQQ)pop-

ulation which have been selected from the same optical sur-

vey,namely the SDSS.In McLure&Jarvis(2004)we en-

sured that the radio-quiet and radio-loud quasar samples

Orientation dependency of broad-line widths in quasars5

possessed the same distribution in both redshift,bolometric luminosity and photometric colour.

However,the mean FWHMs and standard errors of the RQQs and RLQs from our sample are4831±25km s?1and 5858±129km s?1respectively.This was predominantly the reason for the di?erence in the estimates of the black-hole masses for RQQs and RLQs discussed in McLure&Jarvis (2004),where black-hole mass M bh∝FWHM2.

With the result that broad-line widths are dependent on spectral index and therefore source orientation,then it is interesting to investigate whether the distribution in broad-line FWHM is due to a disc-like geometry for the broad-line region.Wills&Browne(1986)quanti?ed the broad-line re-gion as comprised of a random isotropic velocity component v r and a second which is only directed in the place of a disc v p.In this scenario,the observed FWHM is given by FWHM?2(v2r+v2p sin2θ)1/2,whereθis the angle between the disc normal and the line of sight to the observer.McLure &Dunlop(2002)used a three-parameter model where the intrinsic distribution of orbital velocities in the disc plane is assumed to be Gaussian,with mean V orb and varianceσ2orb. The third parameter in this model is the maximum open-ing angle to the line-of-sight,θmax.The distribution in the opening angle is completely random within the solid angle de?ned betweenθ=0?→θmax,whereθ=0?represent a source that is pole-on along the line-of-sight.

In Fig.3we plot the cumulative distribution in FWHM for the complete sample of radio-quiet quasars from McLure &Jarvis(2004).We enforce a minimum FWHM cut-o?of 2000km s?1to ensure that any incompleteness at these line widths are minimised within the SDSS quasar catalogue4. Using the disc model,outlined above,to reproduce the cu-mulative distribution of the radio-quiet quasars drawn from our sample we?nd a best?t model with V orb=4700km s?1,σorb=1800km s?1andθmax=38.5?.To test whether such a disc model is fair representation of the data we use the 1-D KS-test which shows that the probability of the data being drawn from the best-?t model is40per cent.The largest di?erence between the data and the model occurs at the large FWHMs,where additional processes may alter the distribution.The most relevant of these would be a lumi-nosity dependent torus opening angle,i.e.the receding torus model(Lawrence1991).However,as the data and model are consistent we do not include more free parameters to include a description of a receding torus model.

Under the assumption that the disc-like model is a fair representation of the true nature of the broad-line regions in quasars,then we are able to constrain the actual di?erence in orbital velocity for RQQs and RLQs.As we have no reason to believe that the distribution of angles of the disk and the opening angle should be di?erent in RQQs and RLQs,we use the best-?t parameters ofσorb andθmax for the RQQs, and leave V orb as a free parameter to?t the whole RLQ population(both FSQs and SSQs).We?nd a best-?t of V orb=5950km s?1for the RLQs,shown in Fig.3.The KS-test shows that the probability of the data being drawn from this model is68per cent,and is therefore consistent. However,one can immediately see an excess of objects with 4We note that the formal FWHM limit for the SDSS to classify an object as a quasar is1000km s?1.FWHM<5000km s?1in the RLQ data when compared to the best-?t model distribution.

It is also worth noting that the distribution in FWHM for the RQQs is indistinguishable from that of the FSQs, with the KS-test giving a probability of75per cent that they are drawn from the same underlying distribution.On the other hand the KS-test shows that the probability of the SSQs being drawn from the same distribution as the RQQs is?0.01per cent.

We defer discussion of the excess number of RLQ sources at FWHM<5000km s?1to the next section,where we also discuss the reason for the agreement between the FWHM distributions of the RQQs and FSQs and their ob-served di?erence with the FWHMs of the SSQ broad-lines. 4DISCUSSION

4.1Flat-spectrum quasars as beamed radio-quiet

quasars

One possible explanation for the similarity in the FWHM distributions for RQQs and FSQs is that the FSQ popu-lation is predominantly comprised of RQQs.There is evi-dence to suggest that radio-quiet quasars have large-scale jets,similar to those found in FRI(Fanaro?&Riley1974) radio galaxies(Blundell&Rawlings2001).Due to the orien-tation of the source,the intrinsic low-luminosity of the radio jet may be Doppler boosted towards the observer.This was discussed fully in Jarvis&McLure(2002)where consider-ation of both the orientation and Doppler boosting of the radio emission was used to explain the apparent discrepancy of FSQs(Oshlack,Webster&Whiting2002)on the black-hole mass–radio luminosity correlation put forward by a number of authors(Franceschini,Vercellone&Fabian1998; Dunlop et al.2003;McLure et al.2004).

To estimate the amount of boosting that would be required to place the RQQs in the radio-loud domain at L1.4>1024W Hz?1sr?1(Miller,Peacock&Mead1990), we use the average luminosity of the FSQs,L1.4=8.7×1025W Hz?1sr?1.In order to boost these sources from the RQQ domain then Doppler factors ofΓ2～10?100are required.This is a relatively small amount of boosting.For example,as discussed in Jarvis&McLure(2002)factors of～100?1000are easily obtainable under reasonable as-sumptions of bulk Lorentz factor and,in the case of powerful FSQs,a maximum opening angle ofθmax～7?(Jackson& Wall1999).For the larger opening angle ofθmax～40?from our modelling in section3.3,then this would obviously re-duce the average amount of Doppler boosting5.However,a signi?cant fraction of sources would still exhibit substantial amounts of Doppler boosting as they would lie within～20?of our line-of-sight,which may still result inΓ2>～10as the distribution in angles follows1?cos(θ),i.e.proportional to the solid angle.Therefore,given a maximum opening angle ofθ～40?then,for a disc-like broad-line region,we would expect10?25per cent of the RLQ population to exhibit Doppler factors ofΓ2>10,i.e.those withinθ<20?.

5The Doppler factorΓis given byΓ=γ?1(1?βcosθ)?1,with β=v/c,where v the speed of the bulk motion,θis the angle of the jet to the line of sight,andγis the bulk Lorentz factor.

6M.J.Jarvis&R.J.McLure

Based on this reasoning,as many as half of the FSQs in our sample could be Doppler boosted RQQs.This is qual-itatively consistent with Fig.3where there is an obvious ridge in the cumulative distribution of RLQs where～20 per cent of the sample deviate from the best?t model at

<～5000km s?1.

It is worth noting that the radio-frequency at which the

RLQs are selected is the crucial factor which dictates the fraction of?at-spectrum,Doppler boosted sources,in any RLQ sample.For example,the sample used in Oshlack et al. (2002)was derived from the2.7GHz Parkes?at-spectrum sample of Drinkwater et al.(1997).Therefore,the Oshlack et al.sample would contain a much higher fraction of Doppler boosted sources than the sample discussed in this paper, due to the fact that by de?nition it is comprised solely of ?at-spectrum sources.Whereas the need for a low-frequency measurement from the WENSS means that the fraction of Doppler boosted sources in our sample is biased towards a lower value.This is due to the fact that many lower luminos-ity,?at-spectrum radio sources which are detected in FIRST will not be included here as they fall below the WENSS?ux-density limit.Therefore,we are enforcing an orientation in-dependent selection on the optical quasars in the SDSS by using the WENSS?ux-density limit at325MHz.

4.2Consequences for the virial black-hole mass

estimator

In this section we consider how the scenario outlined in sec-tion4.1would a?ect the estimation of the black-hole masses in quasars via the virial method(e.g.Wandel,Peterson& Malkan1999;Kaspi et al.2000;McLure&Jarvis2002).

The argument that the FSQs comprise objects that are Doppler boosted from the RQQ regime does?t in with ori-entation based uni?ed schemes.However,it is important to note that this argument is based on the average properties of the FSQ population.Some of them will no doubt be bona ?de radio-loud quasars with little or no Doppler boosting, and the fraction of such sources will be heavily dependent on the?ux-density limit and frequency of the survey from which the sample is derived,as discussed in section4.1.

With the disc model for the RQQ and RLQ populations determined in section3.3,then the di?erence in black-hole mass of RQQs and RLQs is0.18dex(as M bh∝V2orb),con-sistent with the0.16dex found by assuming FWHM≡V orb in the analysis of McLure&Jarvis(2004),where no consid-eration of source orientation was made.The sample of RLQs used in McLure&Jarvis comprised all of the radio sources with detections in FIRST,without the need for a WENSS detection,thus it is highly likely that the fraction of?at-spectrum sources is larger in the complete sample.Unfortu-nately this is currently impossible to test due to the lack of low-frequency radio data for all of the sources.Moreover,if higher-resolution,multi-frequency radio data were available for the sample used here then it is likely that the di?erence in black-hole mass for the RQQs and genuine RLQs would increase as the beamed RQQs could be removed from the RLQ sample.This would then push the di?erence closer to the value found from modelling of the host galaxies of radio galaxies and quasars.At low redshift(0.1

Therefore,our analysis reinforces the view that intrinsic radio power is dependent on the black-hole mass.However, there will be a coupling with extra parameters such as accre-tion rate and/or black-hole spin,along with the orientation e?ects discussed here.

This can also be seen in the principal component analy-sis(PCA)of Boroson(2002).The median value for principal component1,which is believed to represent the accretion rate,for FSQs is shifted closer to the region in which the RQQs lie when compared to the SSQs.This implies that, even in these radio-selected samples,FSQs and RQQs may be drawn from the same population within～1dex of the boundary of radio-loud and radio-quiet classi?cation.Unfor-tunately,we do not have enough sources in our sample at the correct redshift to carry out the PCA discussed in Boroson (2002).However deeper low-frequency radio observations of the quasars over the FIRST area would allow us to increase the sample size at low redshift and thus carry out the PCA.

We have shown that the FWHM of broad lines mea-sured in both the RLQs and the RQQs will have an orien-tation dependency.Therefore,measuring black-hole masses via the virial estimate on small samples may lead to spurious results.However,the scatter due to the source orientation should be overcome if large samples of quasars are used, and this is evidently possible with quasar surveys such as the SDSS and2dfQSO(Croom et al.2004).

Decoupling orientation e?ects and black-hole mass di?erences will be a di?cult task.However,deep high-resolution multi-frequency radio observations of large sam-ples of RQQs will provide the best way of placing direct constraints on this problem.

5SUMMARY

We have used our well de?ned sample of radio-loud and radio-quiet quasars to investigate the dependence of the ob-served broad-line widths on orientation.To summarise ?Radio-quiet quasars and the?at-spectrum quasars ex-hibit the same distribution in broad-line widths,whereas the steep-spectrum population exhibit a distribution which is shifted signi?cantly to broader widths.

?The di?erence in FWHM distributions can be explained within uni?ed schemes where a substantial fraction of the ?at-spectrum sources are composed of Doppler boosted radio-quiet quasars.For the sample we use in this paper,this fraction could be as high as50per cent of the?at-spectrum quasars and25per cent of all radio-loud quasars.

?In light of this we agree with previous results which suggest that radio-quiet quasars contain less massive black holes than their radio-loud quasar counterparts.We?nd a

Orientation dependency of broad-line widths in quasars7

di?erence on the mean black-hole masses of0.18dex between radio-loud and radio quiet quasars.However,this is likely to be a lower limit due to the di?culty in accounting for beamed radio-quiet quasars which lie above the divide which separates radio-loud and radio-quiet quasars.

?Under this hypothesis,the measured broad-line widths are dependent on both orientation and black-hole mass.The level of orientation bias will be heavily dependent on the selection method,thus studies using the black-hole masses derived from the virial estimate need well de?ned quasar samples where any possible orientation bias is kept to a min-imum.

?We suggest that a method of decoupling the dependen-cies of orientation and black-hole mass and to keep orien-tation e?ects to a minimum would be to carry out deeper low-frequency radio observation across the FIRST area cov-ered by the SDSS.Such a survey could be easily achieved with future radio telescopes such as the low-frequency array (LOF AR;https://www.doczj.com/doc/8617374726.html,). ACKNOWLEDGEMENTS

MJJ acknowledges funding from a PPARC PDRA and RJM acknowledges funding from the Royal Society.We thank the referee Katherine Inskip for a detailed reading of the manuscript.This publication makes use of the material provided in the FIRST,NVSS and SDSS surveys.FIRST is funded by the National Radio Astronomy Observatory (NRAO),and is a research facility of the US National Science foundation and uses the NRAO Very Large Ar-ray.Funding for the creation and distribution of the SDSS Archive has been provided by the Alfred P.Sloan Founda-tion,the Participating Institutions,the National Aeronau-tics and Space Administration,the National Science Foun-dation,the U.S.Department of Energy,the Japanese Mon-bukagakusho,and the Max Planck Society.The SDSS Web site is https://www.doczj.com/doc/8617374726.html,/.The SDSS is managed by the Astrophysical Research Consortium(ARC)for the Partici-pating Institutions.The Participating Institutions are The University of Chicago,Fermilab,the Institute for Advanced Study,the Japan Participation Group,The Johns Hopkins University,the Korean Scientist Group,Los Alamos Na-tional Laboratory,the Max-Planck-Institute for Astronomy (MPIA),the Max-Planck-Institute for Astrophysics(MPA), New Mexico State University,University of Pittsburgh,Uni-versity of Portsmouth,Princeton University,the United States Naval Observatory,and the University of Washing-ton.

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