The 2175 Å bump features in FeLoBAL quasars: One indicator of MW-like dust in the nuclear region of quasars

¹ Shanghai Key Lab for Astrophysics, Shanghai Normal University, Shanghai 200234, PR China
e-mail: zhangshaohua@shnu.edu.cn
² Key Laboratory for Research in Galaxies and Cosmology, Shanghai Astronomical Observatory, Chinese Academy of Sciences, Shanghai 200030, PR China
³ Key Laboratory of Polar Science, Polar Research Institute of China, Ministry of Natural Resources, Shanghai 200136, PR China
⁴ Key Laboratory for the Structure and Evolution of Celestial Objects, Yunnan Observatories, Chinese Academy of Sciences, Kunming, Yunnan 650011, PR China
⁵ Key Laboratory for Research in Galaxies and Cosmology, Department of Astronomy, University of Science and Technology of China, Chinese Academy of Sciences, Hefei, Anhui 230026, PR China
⁶ School of Astronomy and Space Science, University of Science and Technology of China, Hefei, Anhui 230026, PR China

Received: 19 October 2021
Accepted: 28 March 2022

Abstract

To investigate the properties of dust in the nuclear region of quasars, we explored the extinction curves of the iron low-ionisation broad absorption line (FeLoBAL) quasar SDSS J163004.29+311957.6 and its two analogues. The parametrised extinction curves indicated the Milky Way-like 2175 Å bump features in underlying extinction, which are similar to those seen in the Local Group and in a subset of high-redshift star-forming galaxies. Compared to the bump features in the Large Magellanic Cloud (LMC), the detections in this work are much closer to those in the Milky Way (MW). These bump features, as well as those in the high- and low-ionisation broad absorption line (BAL) quasars with 2175 Å bumps, are probably the counterpart of the 2175 Å bump features in the quasar environment. This type of dust grain is generally small, easily disrupted by high-energy photons, and has difficulty surviving in the radiation field of the active galactic nucleus. However, due to the presence of absorption-line outflows, the 2175 Å bump feature in quasars, which should be rare, is seen many times in BAL quasars. The shielding effect of outflow clouds allows the MW-like dust grains to be assembled or extends the survival period in the quasar nuclear region. The process and physical and chemical conditions deserve further observational study and investigation.