Multiple H I 21-cm absorbers towards blazar PKS 1158+007

We report an interesting line of sight towards the blazar PKS 1158+007 (J1201+0028) (z em = 1.369), revealing the rare detection of two intervening HI 21-cm absorption systems within a narrow redshift interval (Δz about 0.04) using the MeerKAT radio telescope. The stronger absorber at z abs =0.5653 (integralτdv = 1.31 plus or minus0.04) is associated with a strong MgII lambda2796 system seen in the Sloan Digital Sky Survey (SDSS) and Dark Energy Spectroscopic Instrument (DESI) spectra, with rest-frame equivalent widths of W 2796 = 1.40 plus or minus0.07 Å and 1.21 plus or minus0.11 Å, respectively. In stark contrast, the second detection at z abs = 0.5305 reveals a distinct HI 21-cm absorption feature (integralτdv = 0.31 plus or minus 0.04) with no associated metal absorption detectable in the optical spectra. This system represents a prime example of a "metal-poor'' HI 21-cm absorber. Using photometric redshifts from Hyper Suprime-Cam (HSC) PDR3 and DESI Legacy Surveys DR10, we identify a broad galaxy overdensity (at z about0.53 plus or minus0.06) coincident with these absorbers, suggesting the sightline pierces a complex large-scale structure or filament, which would be interesting to confirm with follow-up observations. These results underscore the power of blind HI 21-cm surveys to uncover the "invisible'' cold gas phase of the Universe that optical/UV metal-line selection may systematically miss.