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Register a new userThe prevalence of FRI radio quasars
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We present deep, multi-VLA-configuration radio images for a set of 18 quasars, having redshifts between 0.36 and 2.5, from the 7C quasar survey. Approximately one quarter of these quasars have FRI-type twin-jet structures and the remainder are a broad range of wide angle tail, fat double, classical double, core-jet and hybrid sources. These images demonstrate that FRI quasars are prevalent in the universe, rather than non-existent as had been suggested in the literature prior to the serendipitous discovery of the first FRI quasar a few years ago, the optically powerful radio quiet quasar E1821+643. Some of the FRI quasars have radio luminosities exceeding the traditional FRI / FRII break luminosity, however we find no evidence for FRII quasars with luminosities significantly below the break. We consider whether the existence of such high luminosity FRI structures is due to the increasingly inhomogeneous environments in the higher redshift universe.
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We present 1-7 GHz high-resolution radio imaging (VLA and e-MERLIN) and spatially-resolved ionized gas kinematics for ten z<0.2 type~2 `obscured quasars (log [L(AGN)/(erg/s)]>~45) with moderate radio luminosities (log [L(1.4GHz)/(W/Hz)]=23.3-24.4). These targets were selected to have known ionized outflows based on broad [OIII] emission-line components (FWHM~800-1800 km/s). Although `radio-quiet and not `radio AGN by many traditional criteria, we show that for nine of the targets, star formation likely accounts for <~10 per cent of the radio emission. We find that ~80-90 per cent of these nine targets exhibit extended radio structures on 1-25 kpc scales. The quasars radio morphologies, spectral indices and position on the radio size-luminosity relationship reveals that these sources are consistent with being low power compact radio galaxies. Therefore, we favour radio jets as dominating the radio emission in the majority of these quasars. The radio jets we observe are associated with morphologically and kinematically distinct features in the ionized gas, such as increased turbulence and outflowing bubbles, revealing jet-gas interaction on galactic scales. Importantly, such conclusions could not have been drawn from current low-resolution radio surveys such as FIRST. Our observations support a scenario where compact radio jets, with modest radio luminosities, are a crucial feedback mechanism for massive galaxies during a quasar phase.
We built a catalog of 219 FRI radio galaxies (FRIs), called FRICAT, selected from a published sample and obtained by combining observations from the NVSS, FIRST, and SDSS surveys. We included in the catalog the sources with an edge-darkened radio morphology, redshift $leq 0.15$, and extending (at the sensitivity of the FIRST images) to a radius $r$ larger than 30 kpc from the center of the host. We also selected an additional sample (sFRICAT) of 14 smaller (10 $<r<$ 30 kpc) FRIs, limiting to $z<0.05$. The hosts of the FRICAT sources are all luminous ($-21 gtrsim M_r gtrsim -24$), red early-type galaxies with black hole masses in the range $10^8 lesssim M_{rm BH} lesssim 3times10^9 M_odot$; the spectroscopic classification based on the optical emission line ratios indicates that they are all low excitation galaxies. Sources in the FRICAT are then indistinguishable from the FRIs belonging to the Third Cambridge Catalogue of Radio Sources (3C) on the basis of their optical properties. Conversely, while the 3C-FRIs show a strong positive trend between radio and [OIII] emission line luminosity, these two quantities are unrelated in the FRICAT sources; at a given line luminosity, they show radio luminosities spanning about two orders of magnitude and extending to much lower ratios between radio and line power than 3C-FRIs. Our main conclusion is that the 3C-FRIs just represent the tip of the iceberg of a much larger and diverse population of FRIs.
There is now unequivocal evidence that the jets in FR I radio galaxies are initially relativistic, decelerating flows. On the assumption that they are axisymmetric and intrinsically symmetrical (a good approximation close to the nucleus), we can make models of their geometry, velocity, emissivity and field structure whose parameters can be determined by fitting to deep VLA observations. Mass entrainment - either from stellar mass loss within the jet volume or via a boundary layer at the jet surface - is the most likely cause for deceleration. This idea is quantitatively consistent with the velocity field and geometry inferred from kinematic modelling and the external gas density and pressure profiles derived from X-ray observations. The jets must initially be very light, perhaps with an electron-positron composition.
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.
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