HI Emission Recovery in the Low-Redshift Radio Population: Host-Galaxy and Radio-Luminosity Dependence in FASHI and LoTSS

Abstract

Overlapping blind 21 cm and wide-area low-frequency continuum surveys make it possible to measure which radio sources an untargeted HI survey recovers. Cross-matching FASHI DR1 with the LoTSS DR2 value-added catalog yields 4392 one-to-one associations. Within an empirically defined comparison region of $2912$\,deg$^2$, we construct a low-redshift, spectroscopic, radio-selected parent of 25,573 LoTSS sources whose expected HI line falls in the FASHI band, and measure how often each is recovered. Sources classified as radio-loud AGN in the published LoTSS DR2 radio-AGN classification are recovered less often than star-forming ones: 143 of 1542 ($9.27\%$, $7.87$--$10.83$) against $14.88\%$ ($14.43$--$15.33$) of 24,031, an odds ratio of $0.585$ ($0.491$--$0.697$). The two classes, however, differ strongly in host properties, by $1.34$ dex in median specific star formation rate, and within their overlapping stellar-mass and star-formation ranges we detect no statistically significant residual class difference (adjusted odds ratio $1.60$, $0.87$--$2.94$). In contrast, 144 MHz luminosity remains positively associated with HI recovery after adjustment for the measured host properties, redshift and an approximate sensitivity proxy: within the same overlapping range, each dex raises the odds of recovery by a factor of $2.03$ ($1.54$--$2.68$), an association carried primarily by the star-forming population, not by the radio-AGN subsample. We interpret this as an empirical relationship with detectable HI emission, not as evidence that radio activity increases or decreases HI mass. The association catalog and the controlled recovery measurements characterize the radio population sampled by a blind HI survey.