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 lled{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 lled{aperture imaging, which reaches a limiting sensitivity (1 )o fNHI 21 0 17 cm 2 over the typical 70 km s 1 linewidth at zero intensity. TheFWHM linewidth of the halo gas is found to be 25 km s 1 , consistent with a WNM thermal broadening within 10 4 K gas. Substantial asymmetries are found at high NHI (>10 18:5 cm 2 ) levels in 60% of our sample. A high degree of reflection-symmetry is found at low NHI (<10 18:5 cm 2 ) in all sources studied at these levels. The column{density proles 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, NHI(0) = 4:1 3: 21 0 19 cm 2 , and characteristic exponential scale{length, hB = 420 90 arcsec. For plausible values of the thermal pressure at the CNM/WNM interface, these edge proles 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, hB =1 0:6 4:0 kpc, yields distances in the range 320 to 730 kpc. A consequence of having exponential edge proles 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 of10 19 cm 2 over 70 km s 1 will detect only the central 30% of the source area and less than 50% of the total flux density. The exponential proles 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.