Study of BL Lac objects in far infrared and submillimetre region

Abstract

Here 35 BL Lac objects are studied from far infrared to submillimetre region (60 μm–850 μm) using the observational data from IRAS, ISO and SCUBA. Twelve Fanaroff–Riley radio galaxies are also studied for comparison. Using the flux densities at the far infrared and submillimetre bands, the luminosities, dust temperatures and dust masses are estimated. Total infrared luminosities LTIR (8–1000 μm) (L⊙) are derived from total flux densities (F8–1000). The flux ratios at 100/60 vs. 170/100 plane show almost no correlation whereas the flux ratios at 60/850 vs. 100/850 plane show good correlation. About 50% of these sources show high F170/F100 flux ratios indicating the presence of cold dust. Dust temperatures estimated from far infrared flux densities (F60/F100), vary from 25 K to 50 K, considering emissivity β=1.5. The variation of dust temperature with total infrared luminosities LTIR (L⊙) shows good correlation whereas the variation of flux densities at 850 μm with total infrared luminosities LTIR (L⊙) shows weak correlation. The submillimetre luminosities at 850 μm (L850 (L⊙)) and cold dust masses at 850 μm (Md) are estimated using the flux densities at 850 μm. The variation of the cold dust masses at 850 μm (Md (M0)) with submillimetre luminosities (L850 (L⊙)) shows very good correlation. The submillimetre flux densities of BL Lac objects are much higher than the flux densities of radio galaxies or quasars but comparable to those of FR I radio galaxies. So the total IR luminosities (LTIR (L⊙)), Luminosity at 850 μm (L850 (L⊙)) and cold dust mass (Md (M0)) of FR I radio galaxies are compared with the BL Lac objects. It is possible to compare the molecular gas mass MH2 with cold dust mass (Md) for only one BL Lac object W Com, due to unavailability of proper observational data.