Parkes Half-Jansky Flat-spectrum Sample Research Articles

The host galaxies of flat-spectrum quasars

We present the results of deep VLT-ISAAC K s -band imaging of four z∼1.5 flat-spectrum quasars selected from the Parkes half-Jansky flat spectrum sample. We find that the hosts of these flat-spectrum quasars are consistent with lying on the K− z Hubble relation for radio galaxies. This implies that the flat-spectrum quasar hosts fall in line with the expectations from orientation based unified schemes and also that they contain black holes of similar mass. Moreover, the width of the H β broad emission line in these objects tends to be narrower than in their misaligned (low-frequency selected quasar) counterparts, implying that the width of the H β broad emission line depends on source inclination, at least for radio-loud quasars, in line with previous studies.

On the redshift cut-off for flat-spectrum radio sources

We use data from the Parkes Half-Jansky Flat-Spectrum (PHJFS) sample to constrain the cosmic evolution in the comoving space density ρ of radio sources in the top decade of the flat-spectrum radio luminosity function (RLF). A consistent picture for the high-redshift evolution is achieved using both simple parametric models, which are the first to allow for distributions in both radio luminosity and spectral index, and variants of the test, some of which incorporate the effects of radio spectral curvature. For the most luminous flat-spectrum objects, the PHJFS sample is extremely similar to that used by Shaver et al. to argue for an abrupt ‘redshift cut-off’: a decrease by a factor ∼30 in ρ between a peak redshift and Our analysis finds that the observable comoving volume is too small to make definitive statements about any redshift cut-off for the most luminous flat-spectrum sources, although both constant-ρ (no cut-off) models and models with cut-offs as abrupt as those envisaged by Shaver et al. are outside the 90 per cent confidence region. The inference that the decline in ρ is most likely to be gradual, by a factor ∼4 between and is in accordance with previous work on the RLF by Dunlop & Peacock, but different from the abrupt decline favoured by studies of optically selected quasars. Dust obscuration provides one explanation for this difference. We show that a significant fraction of the most radio-luminous flat-spectrum objects are Gigahertz Peaked Spectrum (GPS) rather than Doppler-Boosted (DB) sources, complicating any interpretation of the redshift cut-off. Studies based on objects extending into the next lower decade of the flat-spectrum RLF are likely to be more fruitful, but will require a separation of the GPS and DB populations, careful radio selection and analysis of K-corrections, and larger sky-area redshift surveys than those currently available.

Near Infrared Micro-variability of Radio-loud Quasars

AbstractWe observed three AGN from the Parkes Half-Jansky Flat-spectrum Sample at near infrared (NIR) wavelengths to search for micro-variability. In one source, the blue quasar PKS 2243–123, good evidence for NIR micro-variability was found. In the other two sources, PKS 2240–260 and PKS 2233–148, both BL Lacertae objects, no such evidence of variability was detected. We discuss the implications of these observations for the various mechanisms that have been proposed for micro-variability.

Red synchrotron jets in Parkes quasars

We present model fits to spectral energy distributions in the optical and NIR of >100 flat-spectrum radio quasars from the Parkes Half-Jansky Flat-spectrum Sample. We find that approximately 40% of the sources have power-law SEDs, while a similar number show evidence for two primary components: a blue power law and optical synchrotron emission. The blue power law is similar to the dominant component observed in the spectra of optically-selected QSOs. There is strong evidence that the synchrotron component has a turnover in the UV-optical rest frame of the spectrum. In the remaining sources it is likely that the synchrotron peaks at longer wavelengths. This mixture of two components is supported by optical polarisation measurements in a subgroup of the sources. The sources with power law SEDs show evidence for an excess number of red power law slopes compared to optically-selected quasars. There are additional spectral components in some of the sources, such as dust and the underlying galaxy, which have not been considered here.

Red Parkes Quasars: Evidence for Soft X‐Ray Absorption

The Parkes Half-Jansky Flat Spectrum Sample contains a large number of sources with unusually red optical-to-near-infrared continua. If this is to be interpreted as extinction by dust in the line-of-sight, then associated material might also give rise to absorption in the soft X-ray regime. This hypothesis is tested using broadband (0.1-2.4 keV) data from the {\it ROSAT} All-Sky Survey provided by Siebert et al. (1998). Significant ($>3\sigma$ confidence level) correlations between optical (and near-infrared)--to--soft X-ray continuum slope and optical extinction are found in the data, consistent with absorption by material with metallicity and a range in gas-to-dust ratio as observed in the local ISM. Under this simple model, the soft X-rays are absorbed at a level consistent with the range of extinctions ($0< A_{V}< 6$ magnitudes) implied by the observed optical reddening. Excess X-ray absorption by warm (ionised) gas, (ie. a `warm absorber') is not required.

Host galaxy contribution to the colours of 'red' quasars

We describe an algorithm that measures self-consistently the relative galaxy contribution in a sample of radio-quasars from their optical spectra alone. This is based on a spectral fitting method which uses the size of the characteristic 4000\AA~ feature of elliptical galaxy SEDs. We apply this method to the Parkes Half-Jansky Flat Spectrum sample of Drinkwater et al. (1997) to determine whether emission from the host galaxy can significantly contribute to the very red optical-to-near-infrared colours observed. We find that at around $2\sigma$ confidence, most of the reddening in unresolved (mostly quasar-like) sources is unlikely to be due to contamination by a red stellar component.

The Parkes half-jansky flat-spectrum sample

We present a new sample of Parkes half-Jansky flat-spectrum radio sources having made a particular effort to find any previously unidentified sources. The sample contains 323 sources selected according to a flux limit of 0.5 Jy at 2.7 GHz, a spectral index measured between 2.7 and 5.0 GHz of alpha > -0.5 (where S(v) is proportional to v to the power of alpha), Galactic latitude abs(b) > 20 deg and -45 deg < Declination (B1950) < +10 deg. The sample was selected from a region 3.90 steradians in area. We have obtained accurate radio positions for all the unresolved sources in this sample and combined these with accurate optical positions from digitised photographic sky survey data to check all the optical identifications. We report new identifications based on \R- and \Kn-band imaging and new spectroscopic measurements of many of the sources. We present a catalogue of the 323 sources of which 321 now have identified optical counterparts and 277 have measured spectral redshifts.