The discovery of a previously unknown 21cm HI line source identified as an ultra–compact high velocity cloud in the ALFALFA survey is reported .
The HI detection is barely resolved by the Arecibo 305m telescope \sim 4′ beam and has a narrow HI linewidth ( HPFW of 24 km s ^ { -1 } ) .
Further HI observations at Arecibo and with the VLA corroborate the ALFALFA HI detection , provide an estimate of the HI radius , \sim 1 ′ at the 5 \times 10 ^ { 19 } cm ^ { -2 } isophote , and show the cloud to exhibit rotation with an amplitude of \simeq 9.0 \pm 1.5 km s ^ { -1 } .
In other papers , Rhode et al .
( 22 ) show the HI source to have a resolved stellar counterpart and ongoing star forming activity , while Skillman et al .
( 27 ) reveal it as having extremely low metallicity : 12 + \log ( O / H ) = 7.16 \pm 0.04 .
The HI mass to stellar mass ratio of the object is found to be 2.6 .
We use the Tully–Fisher template relation in its baryonic form ( 17 ) to obtain a distance estimate D _ { Mpc } = 1.3 ^ { +0.9 } _ { -0.5 } .
Additional constraints on the distance are also provided by the optical data of Rhode et al .
( 22 ) and McQuinn et al .
( private communication ) , both indicating a distance in the range of 1.5 to 2.0 Mpc .
The three estimates are compatible within their errors .
The object appears to be located beyond the dynamical boundaries of , but still in close proximity to the Local Group .
Its pristine properties are consistent with the sedate environment of its location .
At a nominal distance of 1.75 Mpc , it would have an HI mass of \simeq 1.0 \times 10 ^ { 6 } M _ { \odot } , a stellar mass of \simeq 3.6 \times 10 ^ { 5 } M _ { \odot } , and a dynamical mass within the HI radius of \simeq 1.5 \times 10 ^ { 7 } M _ { \odot } .
This discovery supports the idea that optically faint — or altogether dark — low mass halos may be detectable through their non-stellar baryons .