Multiple H I 21-cm absorbers towards blazar PKS 1158+007
Abstract
We report an interesting line of sight towards the blazar PKS 1158+007 (J1201+0028) (zem = 1.369), revealing the rare detection of two intervening HI 21-cm absorption systems within a narrow redshift interval (Δz ∼ 0.04) using the MeerKAT radio telescope. The stronger absorber at zabs =0.5653 (∫τdv = 1.31±0.04 \kms) is associated with a strong MgII λ2796 system seen in the Sloan Digital Sky Survey (SDSS) and Dark Energy Spectroscopic Instrument (DESI) spectra, with rest-frame equivalent widths of W2796 = 1.40±0.07~Å and 1.21±0.11~Å, respectively. In stark contrast, the second detection at zabs = 0.5305 reveals a distinct HI 21-cm absorption feature (∫τdv = 0.31 ± 0.04 \kms) 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 ∼0.53±0.06) 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.
Related
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BibTeX
@misc{mohapatra2026multiple,
title = {{Multiple H I 21-cm absorbers towards blazar PKS 1158+007}},
author = {Abhisek Mohapatra and R. Srianand and R. Dutta},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.07273},
eprint = {2610.07273},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.07273},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Multiple H I 21-cm absorbers towards blazar PKS 1158+007
AU - Abhisek Mohapatra
AU - R. Srianand
AU - R. Dutta
PY - 2026
DA - 2026/10/05
PB - arXiv
DO - 10.48550/arxiv.2610.07273
UR - https://arxiv.org/abs/2610.07273
AB - 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 ∼ 0.04) using the MeerKAT radio telescope. The stronger absorber at z_abs =0.5653 (∫τdv = 1.31±0.04 \kms) is associated with a strong MgII λ2796 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±0.07~Å and 1.21±0.11~Å, respectively. In stark contrast, the second detection at z_abs = 0.5305 reveals a distinct HI 21-cm absorption feature (∫τdv = 0.31 ± 0.04 \kms) 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 ∼0.53±0.06) 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.
ER -