Arecibo imaging of compact high-velocity clouds

Ten isolated compact high-velocity cloud (CHVCs) of the type cataloged by B raun & Burton (1999) were imaged with the Arecibo telescope and were found to have a nested core/halo morphology. We argue that a combination of high- resolution filled-aperture and synthesis data is crucial to determining the intrinsic properties of the CHVCs. We identify the halos as Warm Neutral M edium surrounding one or more cores iii the Cool Neutral Medium phase. Thes e halos are clearly detected and resolved bs the Arecibo filled-aperture im aging, which reaches a limiting sensitivity (1 sigma) of N-HI similar to 2 10(17) cm(-2) over the typical 70 km s(-1) linewidth at zero intensity. The FWHM linewidth of the halo gas is found to be 25 km s(-1), consistent with a WNM thermal broadening within 10(4) h: gas. Substantial asymmetries are found at high N-HI (> 10(18.5) cm(-2)) levels in 60% of our sample. A high degree of reflection-symmetry is found at low N-HI (< 10(18.)5 cm(-1)) in a ll sources studied at these levels. The column-density profiles of the enve lopes are described well Ly the sky-plane projection of a spherical exponen tial in atomic. volume density, which allows estimating the characteristic central halo column density, N-HI(0) = 4.1 +/- 3.2 10(19) cm(-2), and chara cteristic exponential scale-length, h(B) = 420 +/- 90 arcsec. For plausible values of the thermal pressure at the CNM/WNM interface, these edge profil es allow distance estimates to be made for the individual CHVCs studied her e which range between 150 and 850 kpc. Bn alternate method of distance esti mation utilizing the mean exponential scale-length found in nearby low mass dwarf galaxies, h(B) 10.6 +/- 4.0 kpc, yields distances in the range 320 t o 730 kpc. A consequence of having exponential edge profiles is that the ap parent size and total flux density of these CHVCs will be strongly dependen t on the resolution as well as on the sensitivity of the data used; even a relatively deep observation with a limiting sensitivity of similar to 10(19 ) cm(-2) over 70 km s(-1) will detect only the central 30% of the source ar ea and less than 50% of the total flux density. The exponential profiles al so suggest that the outer envelopes of the CHVCs are not tidally truncated. Several CHVC cores exhibit a kinematic gradient, consistent with rotation. The halos appear kinematically decoupled fi om the cores, in the sense tha t the halos do not display the velocity gradients shown by the dense cores: the gradients are therefore not likely to be due to an external cause such as tidal shear. The much higher degree of symmetry observed in the halos r elative to the cores also argues against an external cause of asymmetries i n the cores.