We study the dependence of the M BH -M host relation on the redshift up to z = 3 for a sample of 96 quasars, the host galaxy luminosities of which are known. Black hole masses were estimated assuming virial equilibrium in the broad-line regions, while the host galaxy masses were inferred from their luminosities. With these data, we are able to pin down the redshift dependence of the M BH -M host relation along 85 per cent of the Universe age. We show that, in the sampled redshift range, the M BH -L host relation remains nearly unchanged. Once we take into account the ageing of the stellar population, we find that the M BH /M host ratio ( ) increases by a factor of ∼7 from z = 0 to z = 3. We show that evolves with z regardless of the radio loudness and of the quasar luminosity. We propose that the most massive black holes, living their quasar phase at high redshift, become extremely rare objects in host galaxies of similar mass in the Local Universe.

Context. We present B-band imaging of 18 low redshift (z ≤ 0.3) BL Lac objects for which their host galaxies were previously resolved in the R-band and the near-infrared H-band. For a subset of the objects, U-and V-band imaging also is presented. Aims. These multiwavelength data are used to investigate the blue-red-near-infrared colours and the colour gradients of the host galaxies of BL Lacs in comparison with other elliptical galaxies with and without nuclear activity. Methods. For all the BL Lacs observed in the B-and V-bands, and all objects at z < 0.15 in the U-band, the host galaxy is clearly resolved. In all cases galaxies are well represented by an elliptical model, with average absolute magnitude M B = −21.6 ± 0.7 and average scale length R e = 7.6 ± 3.2 kpc. BL Lac host galaxies are therefore luminous (massive) elliptical galaxies, in agreement with previous studies in other bands. Results. The best-fit B-band Kormendy relation of (µ e = 3.3 log R e (kpc) + 18.4 mag arcsec −2 ) is in reasonable agreement with that obtained for normal ellipticals and radio galaxies. This structural and dynamical similarity indicates that all massive elliptical galaxies can experience nuclear activity without significant perturbation of their global structure. The distributions of the integrated blue/nearinfrared colour (with average B − H = 3.5 ± 0.5) and colour gradient (with average ∆(B − R)/∆(log r) = −0.14 ± 0.75) of the BL Lac hosts are much wider than those for normal ellipticals, and most BL Lac objects have bluer hosts and/or steeper colour gradients than those in normal ellipticals. Conclusions. The blue colours are likely caused by a young stellar population component, and indicates a link between star formation caused by an interaction/merging event and the onset of the nuclear activity. This result is corroborated by stellar population modelling, indicating a presence of young/intermediate age populations in the majority of the sample, in agreement with low redshift quasar hosts. The lack of strong signs of interaction may require a significant time delay between the event with associated star formation episodes and the start of the nuclear activity.

We present the results of a high resolution (0.27 ′′ px −1 ) near-infrared H band (1.65 µm) imaging survey of a complete sample of 20 flat radio spectrum quasars (FSRQ) extracted from the 2Jy catalogue of radio sources (Wall & Peacock 1985). The observed objects are intrinsically luminous with median M(B) = -25.5. The median redshift of the objects in the sample is z = 0.65. At this redshift, the H band observations probe the old stellar population of the hosts at rest frame wavelength of ∼1 µm.We are able to detect the host galaxy clearly for six (30 %) FSRQs and marginally for six (30 %) other FS-RQs, while the object remains unresolved for eight (40 %) cases. We find the galaxies hosting FSRQs to be very luminous (M(H)∼-27). Compared with the typical galaxy luminosity L* (M*(H)∼-25) they appear ∼2 mag brighter, although the undetected hosts may reduce this difference. They are also at least as bright as, and probably by ∼1 mag brighter than, the brightest cluster galaxies (M(H)∼-26). The luminosities of the FSRQ hosts are intermediate between host galaxies of low redshift radio-loud quasars and BL Lac objects (M(H)∼-26), and the hosts of high redshift radio-loud quasars (M(H)∼-29), in good agreement with current unified models for radio-loud AGN, taking into account stellar evolution in the elliptical host galaxies. Finally, we find an indicative trend between the host and nuclear luminosity for the most luminous FS-RQs, supporting the suggestion based on studies of lower redshift AGN, that there is a minimum host galaxy luminosity which increases linearly with the quasar luminosity.

Aims. We present a study of the very high-energy (VHE; E > 100 GeV) γ-ray emission of the blazar PKS 1424+240 observed with the MAGIC telescopes. The primary aim of this paper is the multiwavelength spectral characterization and modeling of this blazar, which is made particularly interesting by the recent discovery of a lower limit of its redshift of z ≥ 0.6 and makes it a promising candidate to be the most distant VHE source. Methods. The source has been observed with the MAGIC telescopes in VHE γ rays for a total observation time of ∼33.6 h from 2009 to 2011. A detailed analysis of its γ-ray spectrum and time evolution has been carried out. Moreover, we have collected and analyzed simultaneous and quasi-simultaneous multiwavelength data. Results. The source was marginally detected in VHE γ rays during 2009 and 2010, and later, the detection was confirmed during an optical outburst in 2011. The combined significance of the stacked sample is ∼7.2σ. The differential spectra measured during the different campaigns can be described by steep power laws with the indices ranging from 3.5 ± 1.2 to 5.0 ± 1.7. The MAGIC spectra corrected for the absorption due to the extragalactic background light connect smoothly, within systematic errors, with the mean spectrum in 2009-2011 observed at lower energies by the Fermi-LAT. The absorption-corrected MAGIC spectrum is flat with no apparent turn down up to 400 GeV. The multiwavelength light curve shows increasing flux in radio and optical bands that could point to a common origin from the same region of the jet. The large separation between the two peaks of the constructed non-simultaneous spectral energy distribution also requires an extremely high Doppler factor if an one zone synchrotron self-Compton model is applied. We find that a two-component synchrotron self-Compton model describes the spectral energy distribution of the source well, if the source is located at z ∼ 0.6.