HI Galactic Halo Cloud G21.2+2.2 (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 (G21.2+2.2) 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 (emission and absorption). It can be estimated that this cloud is about 8 kpc away from us and 300 pc above the Galactic plane. Its width chnages from 15 to 30 pc. This cloud contains 1350 solar masses of hydrogen and its density reaches more than 10 atoms per cubic cm. This cloud is special because there is a bright continuum source located behind it (marked by a white cross). Measuring how the cloud absorbs its radiation allows to understend more about the cloud's physics. 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 integrated over all the cloud's linewidth. Cloud's peak column density is 4.2x10^+20 cm^-2 and the line's FWHM is 4.6 km/s, as measured from the VLA data. Distance is estimated assuming the cloud resides at the tangent point. The absorption source is (J2000) 1822-096 with flux of 5.2 Jy. The absorption experiment allowed to measure cloud's opacity of 0.07 and spin temperature of 150 K. A comparison to the thermal linewidth has shown that the cloud contains little turbulence.


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 G21.2+2.2

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 18:22:35.000, Dec: -9:36:0.000 (J2000)

Field of View

2.000000 x 5.000000 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 G21.2+2.2 (collage),” NRAO Archives, accessed December 3, 2021, https://www.nrao.edu/archives/items/show/33446.