Gas around black holes and interstellar medium distribution are key factors in understanding the growth of supermassive black holes and the evolution of their host galaxies. However, as a crucial parameter, gas density is hard to be determined reliably, because the general method is not applicable to all quasars.
Researchers from the University of Science and Technology of China (USTC) of the Chinese Academy of Sciences (CAS) for the first time detected a "sharp rise" signature in the detection rate of broad absorption line (BAL) variations, which in turn deduced ionized gas density. The work was published in The Astrophysical Journal Letters on January 11, 2021.
The ionization state of a gaseous outflow requires a period of time (the recombination timescale, trec) to respond to changes in the ionizing continuum for the ionized outflows. trec is inversely proportional to the gas density.
According to that, a previous study reported by the group of Prof. WANG Tinggui and Prof. LIU Guilin from USTC of CAS proposed that the gas density can be determined by measuring trec.
They assumed the probability of detecting the variability of a BAL with trec at observational time interval (ΔT) is a step function. In other words, the BAL variability can be detected when the trec is shorter than the ΔT.
Following the same method, "sharp rise" phenomena are detected in detection rate of several different BALs in a quasar SDSS J141955.26+522741.1 from the Sloan Digital Sky Survey Data Release 16 (SDSS DR16), which indicates that this measuring method is reliable.
Researchers first found that the detection rate curve could be used to distinguish gaseous components with different density but the same velocity and location, optimizing the group's work on the new method measuring gas density by trec.