A Two-month, Galaxy-targeted NIR Follow-up of the Sub-solar-mass Gravitational Wave Candidate S251112cm: Probing Electromagnetic Counterpart Scenarios

Abstract

S251112cm is a nearby ($93 \pm 27$~Mpc) compact-binary merger candidate with a high inferred probability that at least one component is below $1\,M_\odot$. This event provides a rare test case for electromagnetic follow-up of a theoretically predicted but unconfirmed sub-solar-mass merger candidate. Motivated by scenarios predicting NIR-bright emission over weeks to months, we carried out a deep, galaxy-targeted UKIRT $J$-band campaign over approximately two months, prioritizing host candidates by a ranking that combines the three-dimensional localization probability with WISE W1 luminosity as a stellar-mass proxy. Of $\sim9000$ candidate hosts, we monitored 59, corresponding to 7.7\% of the cumulative weighted host prior. Difference imaging reveals no convincing transient down to typical depths of $J\sim22$--23~mag (AB). At this distance, both a Type~Ic-BL supernova template and an AT\,2017gfo-like KN template---including time-shifted variants up to $\Delta t \sim 60$~days---would have remained above our median depth for multiple epochs within the monitored hosts. Comparison with a canonical KN model grid, summarized through conditional weighted consistent-fraction maps, shows that bright configurations with high dynamical and wind-ejecta masses are strongly disfavored, while less luminous configurations remain largely permissible. While these constraints are host-limited and do not constitute an event-wide exclusion, this campaign provides the first empirical case study of long-baseline, deep NIR follow-up for a sub-solar-mass merger GW class and demonstrates a host-weighted framework applicable to future galaxy-targeted campaigns.