Detection of an Optical Counterpart to the ALFALFA Ultra-compact High-velocity Cloud AGC 249525

William Janesh, Katherine L. Rhode, John J. Salzer, Steven Janowiecki, Elizabeth A K Adams, Martha P. Haynes, Riccardo Giovanelli, John M. Cannon

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We report on the detection at >98% confidence of an optical counterpart to AGC 249525, an ultra-compact high-velocity cloud (UCHVC) discovered by the Arecibo Legacy Fast ALFA survey blind neutral hydrogen survey. UCHVCs are compact, isolated H i clouds with properties consistent with their being nearby low-mass galaxies, but without identified counterparts in extant optical surveys. Analysis of the resolved stellar sources in deep g- and i-band imaging from the WIYN pODI camera reveals a clustering of possible red giant branch stars associated with AGC 249525 at a distance of 1.64 ±0.45 Mpc. Matching our optical detection with the H i synthesis map of AGC 249525 from Adams et al. shows that the stellar overdensity is exactly coincident with the highest-density H i contour from that study. Combining our optical photometry and the H i properties of this object yields an absolute magnitude of -7.1 ≤ Mv ≤ -4.5, a stellar mass between and 2.2 ± 0.6 × 104 M⊙ and 3.6 ± 1.0 × 105 M⊙, and an H i to stellar mass ratio between 9 and 144. This object has stellar properties within the observed range of gas-poor ultra-faint dwarfs in the Local Group, but is gas-dominated.

Original languageEnglish
Article numberL16
JournalAstrophysical Journal Letters
Issue number2
Publication statusPublished - 10 Mar 2017


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