Dust in BL Lac objects and Fanaroff-Riley radio galaxies: infrared region

Abstract

Here 28 BL Lac objects, 18 FR I type radio galaxies, 4 FR I/II type radio galaxies and 10 FR II type radio galaxies are studied from FIR (far infrared) to optical region (180 μm to 0.44 μm) to understand the nature of infrared emission from these objects and the physical properties of dust in the emitting region. Using the flux densities from 2MASS, IRAS, ISO, SCUBA (40 % samples), WISE All-sky Data, AKARI (10 % samples) data and optical (B) observations, the spectral energy distributions are constructed. FIR and NIR spectral indices (\(\alpha_{\mathrm{FIR}}\) and \(\alpha_{\mathrm{NIR}}\)) are estimated. The dust temperatures and dust masses of all the samples are estimated from FIR flux densities. The SEDs of most of the samples (90 %) show steep slopes from FIR to optical region and about 10 % of the samples show flat continuous spectra from FIR to NIR region. The SEDs of 80 % FR I type radio galaxies and 1 out of 4 FR I/II type radio galaxies and two RBLs show a bump in the NIR to optical region. The SEDs of these sources are compared with Radiative transfer models. From FIR to MIR region, the SEDs of 90 % of the objects studied here can be fitted to the models with luminosities \(\mathrm{L}\sim 10^{9.5} \mathrm{L}_{0}\), considering uncertainty from 10 % to 20 %. But the observable fluxes in the NIR region are higher and can be fitted to other models with higher \(\mathrm{L}\sim 10^{12.5} \mathrm{L}_{0}\). Since there is a difference in emission in NIR region, mainly for FR I radio galaxies, so the variation of apparent K magnitudes with logarithm of redshift z is also studied. The WISE colours, (\(\mathrm{W}_{1} - \mathrm{W}_{2}\)) and (\(\mathrm{W}_{2} - \mathrm{W}_{3}\)) are compared with isodensity contours. Comparing with radiative transfer models it can be suggested that, in the FIR and MIR region the infrared emission is from the dust containing large grains, small graphites and PAHs at temperature ∼50 K–100 K. In the NIR region hot dust is mainly due to small grains at temperature ∼1200 K and the emission is mainly from synchrotron radiation produced in the inner part of the relativistic jets. The dust masses of RBLs are higher than those of XBLs and IBLs. FR II radio galaxies also have higher dust masses and that is mainly due to the clumpy dust which is very seldom in FR I radio galaxies. Dust clearly plays an important role in the evolution of these objects.