The first identification of Lyman $\alpha$ Changing-look Quasars at high-redshift in DESI

TL;DR

This paper reports the first detection of Lyman alpha changing-look quasars at high redshift, revealing new insights into quasar variability and accretion processes in the early universe.

Contribution

It presents the discovery of high-redshift Lyα changing-look quasars using DESI and SDSS, providing new observational evidence of CL phenomena at z > 2.

Findings

Confirmed variability through significant r-band magnitude change.

Estimated low accretion rates indicating transition between accretion states.

Observed persistence of broad C IV emission after Lyα CL event.

Abstract

We present two cases of Ly$\alpha$ changing-look (CL) quasars (J1306 and J1512) along with two additional candidates (J1511 and J1602), all discovered serendipitously at $z >2$ through the Dark Energy Spectroscopic Instrument (DESI) and the Sloan Digital Sky Survey (SDSS). It is the first time to capture CL events in Ly$\alpha$ at high redshift, which is crucial for understanding underlying mechanisms driving the CL phenomenon and the evolution of high-redshift quasars and galaxies. The variability of all four sources is confirmed by the significant change of amplitude in the $r$ band ($|r_{\rm DESI}-r_{\rm SDSS}| >0.5 \ \rm mag$). We find that the accretion rate in the dim state for these CL objects corresponds to a relatively low value ($\mathscr{\dot M} \approx 2\times10^{-3}$), which suggests that the inner region of the accretion disk might be in transition between the Advection Dominated Accretion Flow ($\mathscr{\dot M}<10^{-3}\sim 10^{-2}$) and the canonical accretion disk (optically thick, geometrically thin). However, unlike in C {\sc iv} CL quasars in which broad Ly$\alpha$ remained, the broad C {\sc iv} may still persist after a CL event occurs in Ly$\alpha$, making the physical origin of the CL and ionization mechanism event more puzzling and interesting.