broad line region

A broad line region (BLR or broad emission-line region) is whatever region produces broader emission lines at an active galactic nucleus (AGN), in contrast to a narrow line region (NLR), which is whatever region produces the narrower lines. "Narrow" is relative: the narrow lines have considerable broadening, but significantly less than the broader lines. The broadening in both cases is Doppler broadening, a sign of the temperature of the gas, and also for the BLR, its velocity dispersion and orbital speed.

The attractive current theory (termed the unified model) is that a substantial torus-shaped cloud with considerable dust surrounds the supermassive black hole (SMBH) and its accretion disk. Material in that inner, partially-hidden region has a high velocity and temperature, leading to the broad lines. Material further from the SMBH is somewhat heated by the accretion emission, forming the narrower lines that are not blocked from our view by the torus. This explains the fact that observed AGNs generally show the narrow lines but only some show the broader lines, the latter which we see when our viewing angle is such that we see what is encircled by the torus.