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Register a new userGiant radio quasars: composite optical spectra
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Agnieszka Ku\\'zmicz
Publication date
2021
fields
Physics
and research's language is
English
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We present the composite optical spectrum for the largest sample of giant radio quasars (GRQs). They represent a rare subclass of radio quasars due to their large projected linear sizes of radio structures, which exceed 0.7 Mpc. To construct the composite spectrum, we combined 216 GRQs optical spectra from Sloan Digital Sky Survey (SDSS). As a result, we obtained the composite spectrum covering the wavelength range from 1400 {AA} to 7000 {AA}. We calculated the power-law spectral slope for GRQs composite, obtaining $alpha_{lambda}=-1.25$ and compared it with that of the smaller-sized radio quasars, as well as with the quasar composite spectrum obtained for large sample of SDSS quasars. We obtained that the GRQs continuum is flatter (redder) than the continuum of comparison quasar samples. We also show that the continuum slope depends on core and total radio luminosity at 1.4 GHz, being steeper for higher radio luminosity bin. Moreover, we found the flattening of the continuum with an increase of the projected linear size of radio quasar. We show that $alpha_{lambda}$ is orientation-dependent, being steeper for a higher radio core-to-lobe flux density ratio which is consistent with AGN unified model predictions. For two GRQs, we fit the spectral energy distribution using X-CIGALE code to compare the consistency of results obtained in the optical part of the electromagnetic spectrum with broad-band emission. The parameters obtained from the SED fitting confirmed the larger dust luminosity for the redder optical continuum.
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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.
The composite spectra of quasars are widely used as templates for redshift determination, as well as for measurements of the mean transmission in Lyalpha-forest studies, and for investigation of general spectral properties of quasars. Possibility of composite spectra utilisation in these fields is related to remarkable similarity of quasar spectra in UV-optical range. But despite of general similarity in spectral shapes, they differ in several parameters, one of which is the spectral index. In the present paper we study the possible effects, related to neglect of this difference. We compiled 16 composite spectra from subsamples of individual SDSS DR7 quasar spectra with different spectral indices alpha_lambda within the wavelength range 1270-1480 AA, and show that (i) the redshifts measured for a test sample of high signal-to-noise ratio quasar spectra using these composites as templates appear to be systematically higher than those calculated with a traditional template, compiled from spectra with different alpha_lambda, with 1.5 times smaller errors in the former case; (ii) the difference in alpha_lambda in individual spectra used for compilation of composites can yield the mean transmission uncertainty up to 20%; (iii) a number of emission lines indistinguishable in ordinary composites, but seen in individual high-resolution spectra, can be detected in such composites. It is also shown, that there is no dependence of alpha_lambda on quasar luminosity in SDSS u, g, r and i bands, and monochromatic luminosity at 1450 AA.
This paper reports on the extreme ultraviolet (EUV) spectrum of three low redshift ($z sim 0.6$) radio loud quasars, 3C 95, 3C 57 and PKS 0405-123. The spectra were obtained with the Cosmic Origins Spectrograph (COS) of the Hubble Space Telescope. The bolometric thermal emission, $L_{bol}$, associated with the accretion flow is a large fraction of the Eddington limit for all of these sources. We estimate the long term time averaged jet power, $overline{Q}$, for the three sources. $overline{Q}/L_{bol}$, is shown to lie along the correlation of $overline{Q}/L_{bol}$ and $alpha_{EUV}$ found in previous studies of the EUV continuum of intermediate and high redshift quasars, where the EUV continuum flux density between 1100 AA, and 700 AA, is defined by $F_{ u} sim u^{-alpha_{EUV}}$. The high Eddington ratios of the three quasars extends the analysis into a wider parameter space. Selecting quasars with high Eddington ratios has accentuated the statistical significance of the partial correlation analysis of the data. Namely. the correlation of $overline{Q}/L_{mathrm{bol}}$ and $alpha_{EUV}$ is fundamental and the correlation of $overline{Q}$ and $alpha_{EUV}$ is spurious at a very high statistical significance level (99.8%). This supports the regulating role of ram pressure of the accretion flow in magnetically arrested accretion models of jet production. In the process of this study, we use multi-frequency and multi-resolution Very Large Array radio observations to determine that one of the bipolar jets in 3C 57 is likely frustrated by galactic gas that keeps the jet from propagating outside the host galaxy.
The radio-loud/radio-quiet (RL/RQ) dichotomy in quasars is still an open question. Although it is thought that accretion onto supermassive black holes in the centre the host galaxies of quasars is responsible for some radio continuum emission, there is still a debate as to whether star formation or active galactic nuclei (AGN) activity dominate the radio continuum luminosity. To date, radio emission in quasars has been investigated almost exclusively using high-frequency observations in which the Doppler boosting might have an important effect on the measured radio luminosity, whereas extended structures, best observed at low radio frequencies, are not affected by the Doppler enhancement. We used a sample of quasars selected by their optical spectra in conjunction with sensitive and high-resolution low-frequency radio data provided by the LOw Frequency ARray (LOFAR) as part of the LOFAR Two-Metre Sky Survey (LoTSS) to investigate their radio properties using the radio loudness parameter ($mathcal{R} = frac{L_{mathrm{144-MHz}}}{L_{mathrm{i,band}}}$). The examination of the radio continuum emission and RL/RQ dichotomy in quasars exhibits that quasars show a wide continuum of radio properties (i.e. no clear bimodality in the distribution of $mathcal{R}$). Radio continuum emission at low frequencies in low-luminosity quasars is consistent with being dominated by star formation. We see a significant albeit weak dependency of $mathcal{R}$ on the source nuclear parameters. For the first time, we are able to resolve radio morphologies of a considerable number of quasars. All these crucial results highlight the impact of the deep and high-resolution low-frequency radio surveys that foreshadow the compelling science cases for the Square Kilometre Array (SKA).
Based on the miscellaneous published radio and optical data, SDSS and APM catalogue we consider the various properties of the giant radio sources (gRS) with the aim of refining the conditions leading to the formation of these objects. We compare gRSs with the regular-sized radio sources in radio and optical bands, yielding the following results: 1. The fraction of broad line objects among gRSs with high excitation spectrum is the same as for the RSs from isotropic samples. According to Unified Scheme this leads to the isotropic angle distribution of gRSs jets, thus gRSs cannot be characterized as objects with jets lying in the plane of sky. 2. gRSs do not differ from normal sized RSs in apparent asymmetry distribution of their extended radio components (ERC). However the fact that asymmetry distributions for gRSs and giant radio quasars (gQSS) are essentially the same leads within the Unification Scheme to the conclusion that the origin of this asymmetry is in the non-uniform environment. 3. The richness of the environment for gRSs is the same as for normal sized RSs. This contradicts the opinion that the low density of the environment is the single reason for gRSs formation. 4. About 10% of FRII RSs have by order of magnitude longer lifetimes and eventually evolve to gRSs. 5. The observed relative quantity of radio quasars in gRSs population (~0.1) can be interpreted as the presence of long-living population of radio loud QSSs as ~0.1 of all radio quasars.
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