Arecibo imaging of compact high-velocity clouds

Abstract

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 NHI ∼ 2 10 17 cm −2 over the typical 70 km s −1 linewidth at zero intensity. The F W HM 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 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, NHI(0) = 4.1 ± 3.2 10 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 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, hB = 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 ∼10 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 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.

Effelsberg H I observations of compact high-velocity clouds

“…They simulated both a Local Group population of CHVCs with a typical distance of 750 kpc and a circumgalactic distribution with a distance of 150 kpc. Sternberg et al (2002) compared their simulations with the observed properties of CHVCs according to high-resolution observations with the Westerbork Synthesis Radio Telescope (WSRT) by Braun & Burton (2000) and Arecibo observations by Burton et al (2001). They found the observed properties to be consistent with the results of their circumgalactic model so that CHVCs might be associated with an extended Galactic corona.…”

“…Line widths systematically increase towards the apparent leading edge which may indicate heating or growing turbulence of the gas in the course of ram-pressure interaction. Burton et al (2001) also reported significant asymmetries in the high column density regime (N H i > 3 × 10 18 cm −2 ) of their 10 CHVCs observed with the Arecibo telescope. In the low column density regime below about 3 × 10 18 cm −2 , however, they found a high degree of reflection symmetry which they took as evidence against an external mechanism producing the observed asymmetries in the high column density cores.…”

“…As the distance of CHVCs cannot be determined directly we have to apply indirect distance estimates. Burton et al (2001) used an estimate of the thermal pressure at the interface of cold and warm neutral medium to obtain distances between 150 and 850 kpc for a number of CHVCs observed with the Arecibo telescope. Such distances would place the CHVC population throughout the entire Local Group.…”

Erratum: Arecibo imaging of compact high–velocity clouds

“…We have discovered an arithmetic error in our recent publication (Burton et al 2001), corresponding to exactly a factor of ten in the value of the exponential scale lengths, h e , of outer disk H i in the sample of 15 nearby late-type dwarf galaxies which we took from Swaters (1999). The mean exponential scale-length value for this sample is actually h e = 1.17 ± 0.44 kpc.…”

Galactic cold cores

“…09-39.46 This field is outside the CO data by Dame et al (2001), and the extinction method failed producing distance estimates. The distance estimate reported by Burton et al (2001) is for the high velocity cloud CHVC092-39-367, located one degree apart from G91.09-39.46. There are no evidence for a potential connection between the two clouds.…”