Seven broad absorption line quasars with excess broad band absorption near 2250 \AA

TL;DR

This paper reports the discovery of excess broad band absorption near 2250 Å in seven BAL quasars, suggesting a possible link to early quasar evolution and the 2175 Å bump phenomenon.

Contribution

The study identifies and verifies EBBA features in BAL quasars, highlighting their potential connection to quasar evolution and dust properties in the early Universe.

Findings

EBBA detected with > 4σ significance in seven quasars.

High fraction (~70%) of LoBALs in the sample.

EBBA features resemble the LMC 2175 Å bump.

Abstract

We report the discovery of excess broad band absorption near 2250 A (EBBA) in the spectra of seven broad absorption line (BAL) quasars. By comparing with the statistical results from the control quasar sample, the significance for the detections are all above the > 4{\sigma} level, with five above > 5{\sigma}. The detections have also been verified by several other independent methods. The EBBAs present broader and weaker bumps at smaller wavenumbers than the Milky Way, and similar to the Large Magellanic Cloud. The EBBA bump may be related to the 2175 A bump seen in the Local Group and may be a counterpart of the 2175 A bump under different conditions in the early Universe. Furthermore, five objects in this sample show low-ionization broad absorption lines (LoBALs), such as Mg II and Al III, in addition to the high-ionization broad absorption lines (HiBALs) of C IV and Si IV. The fraction of LoBALs in our sample, ~70%, is surprisingly high compared to that of general BAL quasars, ~10%. Although the origin of the bump is still not clear, the coexistence of both BALs and bumps and the significantly high fraction of LoBALs may indicate the bump carriers is closely related to the early evolution phase of quasars.