NRAO: National Radio Astronomy Observatory

About this Image

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.

Investigator(s):  Liszt, H., Lockman, J., Rupen, M. (NRAO), Pidopryhora Y. (NRAO & Ohio U)

Alternate Resolutions

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This image is available in the following downloadable versions:

• 650 x 323 color JPEG
• 1200 x 597 color JPEG
• 3000 x 1492 color TIFF

If you would like to obtain a higher resolution version of this image, please contact Pat Smiley.

For More Information

• Read the Journal article
• Up one level for more info on Galactic_Halo
• For unfamiliar terms, visit the NRAO Radio Astronomy Glossary

Technical Data

Telescope	VLA
Date of Observation	2004-07-20
Type of Observation	Spectral Line Observations
Center of Image	RA: 18:22:35.00, Dec: -9:36:0.00 (J2000)
Field of View	2.0000 x 5.0000 degrees
Technical Caption	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.



Astronomical database entries for HI cloud G21.2 2.2
• Query SIMBAD for more HI cloud G21.2 2.2 data

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