Astronomy & Celestial Mechanics
the sky, computed
Concept

Giant molecular cloud — where it appears

A cold cloud of molecular hydrogen of a hundred thousand to a few million solar masses and tens of parsecs across, where most stars in the Galaxy form. Such clouds live a few free-fall times and are dispersed by their own massive stars within a few million years of making them.

Named by 2 essays across one field — each of them below, with the objects they name alongside it.

A gap's depth is one number: 2GmT ÷ v b². The central density of the gap against impact parameter, 4 billion years after the encounter, for subhaloes of 10⁶, 10⁷, 10⁸ solar masses. Points are read off the drawn density profiles; the curves are 1/(1 + 2GmT/v b²), the stretch the map applies at the encounter point, and the two agree to 3.0 per cent wherever the histogram has enough stars left in the gap to measure a depth at all. Everything about the encounter enters the depth through that one combination. The consequence is the horizontal reading: a gap of a given depth is produced by every point along a locus on which the mass rises as the square of the impact parameter — half depth at 0.42, 1.33, 4.19 kiloparsecs for the three masses drawn, an exponent of 0.500 against the half the algebra requires. What breaks that particular degeneracy is the gap's width, which scales as the impact parameter itself while the depth does not: rescale s by b and the map is identical, so the profile is one shape stretched. Depth and width together give the impact parameter and the product mT. They do not give the mass.

A hole that says mass times time

A gap in a stellar stream is the strongest evidence available that dark subhaloes exist, and its depth depends on the perturber's mass, the impact parameter and the elapsed time only through one combination. Two of those three are unobservable, so a gap is a measurement of a product.

galaxies · Stellar streams Two ways of pointing an aperture, and the fork between them. The gas-to-young-star flux ratio inside apertures of size l, relative to the whole galaxy's, for apertures centred on peaks of molecular gas (upper branch) and on peaks of Hα from young stars (lower branch), if clouds live 20 Myr, the young stars shine in Hα for 4.3 Myr, the two overlap for 2 Myr and independent regions are 150 pc apart. On large scales every aperture averages many regions at all stages and both branches return to one. On small scales an aperture holds one region at one stage, and the branches split: ×2.1 on the gas peaks and ×0.10 on the stars at 10 pc. With the young stars' phase known from stellar evolution, the upper branch gives the overlap, the lower branch the clouds' lifetime, and the scale where the fork closes the regions' separation.

A lifetime read from how gas and stars sit apart

No molecular cloud has been watched for long enough to see it age, and every cloud looks the same whether it lives for one free-fall time or for ten. But a galaxy holds thousands of clouds at every stage at once, and how often molecular gas and young stars appear in the same place — measured in apertures of shrinking size, centred first on one and then on the other — turns a single snapshot into a clock, calibrated by how long the young stars shine.

galaxies · Molecular clouds

Named alongside it

The objects these essays reach for when they reach for this one.

Cold dark matterDark subhaloEpicyclic overdensityFree-fall timeGalactic barH alphaImpulse approximationInterstellar mediumJeans massMolecular cloudsStar formationStar formation efficiency

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