HI Galactic Halo Cloud G27.0+6.3 (collage)



Since 2002 hundreds of HI (neutral hydrogen) clouds have been discovered with the help of the new unique NRAO instrument -- 100 m Green Bank Telescope (GBT). These clouds are a completely new class of objects presenting a previously unknown phenomenon of the interstellar matter physics. Several brightest of them have been observed with another NRAO instrument, Very Large Array (VLA) which provides better angular resolution. This collage shows one of such clouds (G27.0+6.3) as seen at its peak by the GBT in the larger Galactic halo context and its VLA HI column density map with two VLA sample spectra taken at the cloud's core and envelope. The GBT image shows that the cloud is a part of a larger cloud-filament structure, while the VLA image reveals that the cloud has a "dense core - diffuse envelope" structure. The assymetry of the envelope might also suggest a downward motion. It can be estimated that this cloud is about 7.5 kpc away from us and 850 pc above the Galactic plane. Its size is approximately 20 pc with core size of about 7 pc. This cloud contains 500 solar masses of hydrogen and the density of its core exceeds 20 atoms per cubic cm. The images are false-color with blue corresponding to the least and red -- to the most hydrogen density.
The resolutions: GBT image -- 9 arcmin, VLA (taken in D-array) -- 1 arcmin. GBT exposure 5 sec per 3 by 3 arcmin pixel, VLA -- a few hours. The GBT image is taken from a single channel while the VLA's is imtegrated over all the cloud's linewidth. Cloud's peak column density is 3.5x10^+20 cm^-2 and the line's FWHM is 4.8 km/s, as measured from the VLA data. Distance estimated assuming the cloud resides at the tangent point.


Legacy Astronomical Images


NRAO/AUI/NSF does not hold full copyright for this image. Contact the archivist for details.


Legacy Astronomical Image

Object Name

HI cloud G27.0+6.3

Photo Credit

Yurii Pidopryhora, NRAO & Ohio University


H. Liszt, J. Lockman, M. Rupen, Y. Pidopryhora


Very Large Array (VLA)
Green Bank Telescope (GBT)

Observation Date


Type of Observation

spectral line

Center of Image

RA 18:18:0.000, Dec: -2:15:0.000 (J2000)

Field of View

3.000000 x 4.000000 degrees


Contact the archivist for a high resolution tif of this image.


Galactic Sources Series


Galactic Halo Unit


Legacy Astronomical Images, “HI Galactic Halo Cloud G27.0+6.3 (collage),” NRAO Archives, accessed December 3, 2021, https://www.nrao.edu/archives/items/show/33447.