Arecibo imaging of compact high-velocity clouds

Abstract

Ten isolated compact high-velocity clouds (CHVCs) of the type cataloged by Braun & 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 Medium surrounding one or more cores in the Cool Neutral Medium phase. These halos are clearly detected and resolved by the Arecibo filled-aperture imaging, which reaches a limiting sensitivity (1σ ) of N_HI ~ 210¹⁷ cm⁻² over the typical 70 kms⁻¹ linewidth at zero intensity. The FWHM linewidth of the halo gas is found to be 25 kms⁻¹, consistent with a WNM thermal broadening within 10⁴ K 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 (<1018.5 cm⁻²) in all sources studied at these levels. The column-density profiles of the envelopes are described well by the sky-plane projection of a spherical exponential in atomic volume density, which allows estimating the characteristic central halo column density, N_HI(0)=4.1±3.2 1019 cm⁻², and characteristic exponential scale-length, h_B=420±90 arcsec. For plausible values of the thermal pressure at the CNM/WNM interface, these edge profiles allow distance estimates to be made for the individual CHVCs studied here which range between 150 and 850 kpc. An alternate method of distance estimation 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 to 730 kpc. A consequence of having exponential edge profiles is that the apparent size and total flux density of these CHVCs will be strongly dependent on the resolution as well as on the sensitivity of the data used; even a relatively deep observation with a limiting sensitivity of ~10¹⁹ cm⁻² over 70 kms⁻¹ will detect only the central 30% of the source area and less than 50% of the total flux density. The exponential profiles also 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 from the cores, in the sense that 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 relative to the cores also argues against an external cause of asymmetries in the cores.