The first identification of Lyman $\hat{I}\pm$ Changing-look Quasars at high-redshift in DESI

Abstract

We present two cases of Ly$\hat{I}\pm$ 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$\hat{I}\pm$ 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 ($|r_{\mathrm{DESI}}-r_{\mathrm{SDSS}}|>0.5\mathrm{mag}$). We find that the accretion rate in the dim state for these CL objects corresponds to a relatively low value ($\dot{\mathcal{M}}\approx 2\times 10^{-3}$), which suggests that the inner region of the accretion disk might be in transition between the Advection Dominated Accretion Flow ($\dot{\mathcal{M}}<10^{-3}\sim 10^{-2}$) and the canonical accretion disk (optically thick, geometrically thin). However, unlike in C {\sc iv} CL quasars in which broad Ly$\hat{I}\pm$ remained, the broad C {\sc iv} may still persist after a CL event occurs in Ly$\hat{I}\pm$, making the physical origin of the CL and ionization mechanism event more puzzling and interesting.