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Register a new userA comparison of the optical properties of radio-loud and radio-quiet quasars
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P. Goldschmidt
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We have made radio observations of 87 optically selected quasars at 5 GHz with the VLA in order to measure the radio power for these objects and hence determine how the fraction of radio-loud quasars varies with redshift and optical luminosity. The sample has been selected from the recently completed Edinburgh Quasar Survey and covers a redshift range of 0.3 < z < 1.5 and an optical absolute magnitude range of -26.5 < M_{B} < -23.5 (h, q_{0} = 1/2). We have also matched up other existing surveys with the FIRST and NVSS radio catalogues and combined these data so that the optical luminosity-redshift plane is now far better sampled than previously. We have fitted a model to the probability of a quasar being radio-loud as a function of absolute magnitude and redshift and from this model infer the radio-loud and radio-quiet optical luminosity functions. The radio-loud optical luminosity function is featureless and flatter than the radio-quiet one. It evolves at a marginally slower rate if quasars evolve by density evolution, but the difference in the rate of evolutions of the two different classes is much less than was previously thought. We show, using Monte-Carlo simulations, that the observed difference in the shape of the optical luminosity functions can be partly accounted for by Doppler boosting of the optical continuum of the radio-loud quasars and explain how this can be tested in the future.
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Studies of radio-loud (RL) broad absorption line (BAL) quasars indicate that popular orientation-based BAL models fail to account for all observations. Are these results extendable to radio-quiet (RQ) BAL quasars? Comparisons of RL and RQ BAL quasars show that many of their properties are quite similar. Here we extend these analyses to the rest-frame ultraviolet (UV) spectral properties, using a sample of 73 RL and 473 RQ BAL quasars selected from the Sloan Digital Sky Survey (SDSS). Each RQ quasar is individually matched to a RL quasar in both redshift (over the range $1.5 < z < 3.5$) and continuum luminosity. We compare several continuum, emission line, and absorption line properties, as well as physical properties derived from these measurements. Most properties in the samples are statistically identical, though we find slight differences in the velocity structure of the BALs that cause apparent differences in CIV emission line properties. Differences in the velocities may indicate an interaction between the radio jets and the absorbing material. We also find that UV FeII emission is marginally stronger in RL BAL quasars. All of these differences are subtle, so in general we conclude that RL and RQ BAL QSOs are not fundamentally different objects, except in their radio properties. They are therefore likely to be driven by similar physical phenomena, suggesting that results from samples of RL BAL quasars can be extended to their RQ counterparts.
We discuss 6 GHz JVLA observations covering a volume-limited sample of 178 low redshift ($0.2 < z < 0.3$) optically selected QSOs. Our 176 radio detections fall into two clear categories: (1) About $20$% are radio-loud QSOs (RLQs) having spectral luminosities $L_6 gtrsim 10^{,23.2} mathrm{~W~Hz}^{-1}$ primarily generated in the active galactic nucleus (AGN) responsible for the excess optical luminosity that defines a emph{bona fide} QSO. (2) The radio-quiet QSOs (RQQs) have $10^{,21} lesssim L_6 lesssim 10^{,23.2} mathrm{~W~Hz}^{-1}$ and radio sizes $lesssim 10 mathrm{~kpc}$, and we suggest that the bulk of their radio emission is powered by star formation in their host galaxies. Radio silent QSOs ($L_6 lesssim 10^{,21} mathrm{~W~Hz}^{-1}$) are rare, so most RQQ host galaxies form stars faster than the Milky Way; they are not red and dead ellipticals. Earlier radio observations did not have the luminosity sensitivity $L_6 lesssim 10^{,21} mathrm{~W~Hz}^{-1}$ needed to distinguish between such RLQs and RQQs. Strong, generally double-sided, radio emission spanning $gg 10 mathrm{~kpc}$ was found associated with 13 of the 18 RLQ cores having peak flux densities $S_mathrm{p} > 5 mathrm{~mJy~beam}^{-1}$ ($log(L) gtrsim 24$). The radio luminosity function of optically selected QSOs and the extended radio emission associated with RLQs are both inconsistent with simple unified models that invoke relativistic beaming from randomly oriented QSOs to explain the difference between RLQs and RQQs. Some intrinsic property of the AGNs or their host galaxies must also determine whether or not a QSO appears radio loud.
Although radio-quiet quasars (RQQs) constitute >90% of optically-identified quasar samples their radio properties are only poorly understood. In this paper we present the results of a multi-frequency VLA study of 27 low-redshift RQQs. We detect radio emission from 20 objects, half of which are unresolved (< 0.24). In cases where significant structure can be resolved, double, triple and linear radio sources on scales of a few kpc are found. The radio emission (typically) has a steep spectrum (alpha ~ 0.7), and high brightness temperatures (T_B > 10^5 K) are measured in some of the radio components. The RQQs form a natural extension to the radio luminosity - absolute magnitude distribution of nearby Seyfert 1s. We conclude that a significant fraction of the radio emission in RQQs originates in a compact nuclear source directly associated with the quasar. There are no significant differences between the radio properties of RQQs with elliptical hosts and those in disc galaxies within the current sample.
Although radio-quiet quasars (RQQs), which constitute the majority of optically-identified quasar samples, are by no means radio silent the properties of their radio emission are only poorly understood. We present the results of a multi-frequency VLA study of 27 low-redshift RQQs. In general, we find that the properties of the radio sources in RQQs are consistent with them being weak, small-scale (~1 kpc) jets similar to those observed in nearby Seyfert galaxies. We conclude that a significant fraction of the radio emission in RQQs is directly associated with the central engine and is not a result of stellar processes in the surrounding galaxy. There appears to be no difference between the radio properties of RQQs in elliptical and disc galaxies, implying that the relationship between the host galaxy and the `radio loudness of the active nucleus is not straightforward.
We present the study on the X-ray emission for a sample of radio-detected quasars constructed from the cross-matches between SDSS, FIRST catalogs and XMM-Newton archives. A sample of radio-quiet SDSS quasars without FIRST radio detection is also assembled for comparison. We construct the optical and X-ray composite spectra normalized at rest frame $4215,rm AA$ (or $2200,rm AA$) for both radio-loud quasars (RLQs) and radio-quiet quasars (RQQs) at $zle3.2$, with matched X-ray completeness of 19%, redshift and optical luminosity. While the optical composite spectrum of RLQs is similar to that of RQQs, we find that RLQs have higher X-ray composite spectrum than RQQs, consistent with previous studies in the literature. By dividing the radio-detected quasars into radio loudness bins, we find the X-ray composite spectra are generally higher with increasing radio loudness. Moreover, a significant correlation is found between the optical-to-X-ray spectral index and radio loudness, and there is a unified multi-correlation between the radio, X-ray luminosities and radio loudness in radio-detected quasars. These results could be possibly explained with the corona-jet model, in which the corona and jet are directly related.
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