The Enigmatic ALFALFA (Almost) Dark Galaxy AGC 229101

Lukas Leisman, Hannah Pagel, Catherine Ball, Katherine Rhode, John Salzer, William Janesh, John M. Cannon, Martha P. Haynes, Gyula Józsa, Elizabeth A. K. Adams*, Steven Janowiecki

*Corresponding author for this work

Research output: Contribution to conferencePaperAcademic


We present deep HI and optical imaging of AGC 229101, an enigmatic and potentially unique source detected in the ALFALFA survey. Though it has an HI mass >109 solar masses, it is not detected in SDSS imaging, and has a very narrow HI line width. Deep follow up imaging with pODI on the WIYN 3.5m at KPNO detects a very blue, very low surface brightness optical counterpart with a stellar mass <107 solar masses, giving a gas fraction of MHI/M* in excess of 200. Low resolution WSRT HI imaging and higher resolution VLA B-array imaging reveal that AGC 229101 appears to consist of two connected HI components, with the optical counterpart associated with the peak column density in the northern component. The two components have approximately equal mass and radii, and together stretch over >80 kpc as projected on the sky. We compare the properties of AGC 229101 to other extreme HI-rich sources, and demonstrate that its properties appear to be unique relative to others sources in ALFALFA. We discuss potential explanations, including a tidal encounter between neighboring sources, a merger of two independent, almost dark sources, and gas in-fall along a filament.

Original languageEnglish
Publication statusPublished - 1-Jan-2019
EventAmerican Astronomical Society, AAS Meeting #233 - Seattle, Washington, United States
Duration: 6-Jan-201910-Jan-2019


ConferenceAmerican Astronomical Society, AAS Meeting #233
Country/TerritoryUnited States
CitySeattle, Washington

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