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

## TL;DR

This study reports the first confident optical detection of a low-mass galaxy associated with an ultra-compact high velocity cloud, confirming its nature and properties through deep imaging and HI data analysis.

## Contribution

First optical counterpart detection of an UCHVC, linking it to a nearby low-mass galaxy with detailed stellar and HI property analysis.

## Key findings

- Optical counterpart found at >98% confidence.
- Galaxy at 1.64 Mpc with low luminosity and high HI-to-stellar mass ratio.
- Properties similar to gas-rich Ultra-Faint Dwarfs.

## Abstract

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 ALFALFA blind neutral hydrogen survey. UCHVCs are compact, isolated HI 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$\pm$0.45 Mpc. Matching our optical detection with the HI synthesis map of AGC 249525 from Adams et al. (2016) shows that the stellar overdensity is exactly coincident with the highest-density HI contour from that study. Combining our optical photometry and the HI properties of this object yields an absolute magnitude of $-7.1 \leq M_V \leq -4.5$, a stellar mass between $2.2\pm0.6\times10^4 M_{\odot}$ and $3.6\pm1.0\times10^5 M_{\odot}$, and an HI 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.