ASTRO: Finding large cold clouds of atomic hydrogen

From: Robert J. Bradbury (
Date: Sun Jul 22 2001 - 09:26:08 MDT

Nature is reporting on discoveries by the Canadian Galactic
Plane Survey and other observations by the Green Bank Radio
Telescope of finding several large-cold clouds of atomic hydrogen.

These "violate" the standard theories of how hydrogen is recycled
from hot atomic hydrogen generated by supernovas eventually
becoming cold *molecular* hydrogen that should eventually
collapse under gravity to form starburst clouds, like the Orion


>From the Dickey's summary:
> Knee and Brunt have discovered a new kind of cloud in data from
> the Canadian Galactic Plane Survey. The structure they found is
> made of atomic hydrogen so cold (about 10 K) that it is visible
> only because it absorbs background radiation essentially throwing
> a dark shadow across the sky.

> The newly discovered dark arc seems to be similar to these cold clouds,
> but much larger and more massive. Knee and Brunt1 estimate that it is
> 6,000 light years across, with a mass at least 20 million times that
> of the Sun.

> We do not yet know the origin of the dark arc. But in the meantime,
> others are finding similar clouds that are almost as cold. An immense
> atomic cloud discovered by astronomers using the Green Bank Telescope
> in the United States is somewhat smaller and warmer than the dark
> arc, but is similar in having very little star formation.

>From the paper:
> The conventional wisdom that cold, massive clouds in the
> interstellar medium are always predominantly molecular has
> achieved near-axiomatic status. The discovery that H I
> supershells can contain some of the coldest gas in the Galaxy
> will require a re-evaluation of this view.

Cold. Dark.. Large... No stars visible.... Hmmmm...

(Don't ask me any questions, I'm simply reporting the data...
Note that if it turns out that these are abundant, they may
account for some of the missing baryonic dark matter. But
they are going to have to be *really* abundant.)


  Astronomy: Dark arc casts a long shadow, John M. Dickey
  Nature 412:292-293 (2001)

  A massive cloud of cold atomic hydrogen in the outer Galaxy
  Knee, L. B. G. & Brunt, C. M.
  Nature 412, 308-310 (2001).

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