The Mass Distribution of Black Holes in Bl Lacertae Objects and Evolution of Active Galactic Nuclei

In this Letter, we make an attempt to estimate the masses of black holes and their distributions in BL Lac objects. We show the mass of black hole MBH ∝ ν −13/12 pk L 3/8 pk , where νpk and Lpk are the peak frequency and luminosity in the first bump of continuum, respectively. We find there are two populations of black hole masses in BL Lacs: the first has a lower-mass peak centred at ∼ 7.0× 10M⊙ composed of high frequency-peaked BL Lacs (HBLs) and the second is a higher-mass (> 10M⊙) population composed of low frequency-peaked BL Lacs (LBLs). We find that the black hole mass distribution in the high frequency-peaked BL Lacs is in good agreement with that of narrow line Seyfert 1 galaxies. Considering the fact that the orientation of BL Lacs close to the sight of observer’s line is the same with NLS1s as well as this agreement, we propose two evolutionary routes among the narrow line Seyfert 1 galaxies (NLS1s), low and high frequency-peaked BL Lacertae objects: 1) HBLs ↔ NLS1s (ṁ ≤ 0.1) ↔ IBLs, 2) HBLs → NLS1s (ṁ ≥ 5) → LBLs, strongly depending on the accretion rates in NLS1s. Subject headings: galaxy: active BL Lac object Narrow line Seyfert 1 galaxy