BASS-HI

The BASS-HI survey is the largest resolved HI imaging survey of local hard X-ray-selected AGN host galaxies to date, built on the BASS survey. Comprising ~100 targets selected from the parent BASS sample based on distance and observability, the survey combines new VLA and ATCA observations with archival data from WSRT, GMRT, and other facilities. Data collection is now complete (2026.09), and a catalog paper presenting the full sample is currently in preparation. The survey is designed to address two key questions: how the presence of an AGN affects the surrounding HI gas reservoir, and how Seyfert galaxies experience macro-scale (kpc–Mpc) feeding processes.

1) BASS: LI. Cool gas supply of HI-massive local Seyfert galaxies (ADS)

DECaLS optical images, HI intensity contours and HI velocity fields for the AGN hosts Z102-48 and UGC 12741
HI extent relative to the optical disc plotted against bolometric luminosity, star formation rate, Eddington ratio and stellar mass

Core Question: How is cool gas supplied to AGN hosts, and does the environment or gas accretion history play a role in fueling nuclear activity?

Key Findings:

  • As a pilot study drawn from the HI-rich subsample of BASS-HI (22 hard X-ray-selected Seyferts with MHI ≳ 109.7 M☉), most hosts reside in group-like, gas-rich environments, with 80% showing HI-detected neighbors.
  • Over 60% of these systems exhibit lopsidedness, one-sided gas tails, or HI structures connecting to companion galaxies, pointing to a history of gas exchange.
  • The sample follows known HI size–mass and star formation scaling relations, showing no systematic deviation, and are typically the most massive systems within their groups — more likely to accrete gas than lose it.
  • Galaxies with more extended HI disks show stronger AGN activity, and this becomes even tighter when neighboring galaxies are HI-poor.

Implications: Extended HI disks are commonly linked to external processes such as galaxy interactions. The correlation between HI extent and AGN activity — strengthened when nearby galaxies are likely the gas source — suggests that environmentally accreted gas may play a direct role in fueling AGN activity in these systems.

2) BASS-HI catalog

Current ongoing project (TBU).

HI-bearing Dwarfs

LEDA 136976: A Caterpillar in a Cabbage. An HI-rich dwarf galaxy caught in an active phase of gas accretion and star formation

HI moment-0 and moment-1 maps of LEDA 136976 overlaid with optical and H-alpha imaging, highlighting two star-forming clumps and a disturbed northern HI component

Core Question: Can gas-rich, optically faint dwarf galaxies with unusually extended HI reservoirs represent systems in an early, active stage of mass assembly?

Key Findings:

  • LEDA 136976 hosts an unusually large HI reservoir, comparable in extent and mass to the Milky Way's, despite having less than 10% of its stellar mass (M⋆ = 3.38 × 109 M☉).
  • Both HI and Hα emission show consistent velocity gradients, confirming rotationally supported dynamics.
  • The HI disk shows a disturbed morphology and a kinematically distinct gas component in the north, suggesting ongoing gas accretion.

Implications: LEDA 136976 appears to be a gas-rich system in an early stage of stellar mass growth, recently accreted gas which might trigger/enhance star formation. Such systems may be systematically underrepresented in optical surveys, motivating HI-based searches for similar low surface brightness dwarfs.

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