Astronomy & Celestial Mechanics
the sky, computed
Concept

Galactic centre — where it appears

The dynamical centre of the Milky Way, about eight kiloparsecs from the Sun in Sagittarius, hidden behind thick dust at visible wavelengths. It holds a black hole of four million solar masses, weighed from the orbits of the stars closest to it.

Named by 3 essays across 2 fields — each of them below, with the objects they name alongside it.

Fitted to one curve, three halo histories agree at the Sun and not at the centre. The dark-matter density from 1 to 15 kpc from the Galactic centre, on logarithmic axes in GeV per cubic centimetre, for three halos fitted to the same flat rotation curve with the same disc of 5·10¹⁰ solar masses: one as dark-matter-only simulations produce it, and two contracted by the disc under different assumptions about the dark matter's orbits. At the Sun's 8.2 kpc they give 0.45, 0.41, 0.40 GeV/cm³, a spread of 12 per cent. At 2 kpc they give 3.2, 4.6, 4.6, a spread of 44 per cent. Inside the region the curve constrains, a fit has no freedom to move dark matter about, and the history drops out; inward of it, where the disc and bulge dominate the curve and the dark matter is a small part of the total, the history decides the answer.

A history the rotation curve absorbs

Whether the Milky Way's dark halo was pulled inward by its disc changes the dark matter at the Sun by half again — if the halo is taken from a simulation. Fit it to the Galaxy's own rotation instead, and the history drops out at the Sun's radius and survives only near the centre. The local density that detection experiments depend on is set by the disc's mass, not by the halo's past.

galaxies · Halo profiles A cusp steepened into a spike by the black hole growing in it. The dark-matter density near the Galaxy's central black hole of 4.3·10⁶ solar masses, in solar masses per cubic parsec on logarithmic axes. The dashed line is the halo's NFW cusp, falling inward as one over the radius. If the black hole grew slowly at the centre of that cusp, the dark matter would have followed it inward on adiabatically shrinking orbits, steepening the cusp inside about a fifth of the black hole's sphere of influence — here 0.4 parsecs, with the sphere of influence at 1.8 — into a spike falling as r to the minus 7/3, measured off the drawn curve. Close in, the dark matter's own annihilation would cap the density at the particle mass over the annihilation rate times the Galaxy's age — for a 100 GeV particle with the thermal cross-section, 2.8·10⁸ solar masses per cubic parsec — the flat plateau. Between the spike's edge and its plateau the density rises by 6 orders of magnitude.

A cusp a black hole would sharpen

A black hole that grows slowly at the centre of a dark-matter cusp drags the dark matter in after it, turning a density that rises as one over the radius into a spike rising as the radius to the minus seven-thirds. Whatever the cusp was, the spike comes out nearly the same — and the annihilation signal it would produce is set less by the particle than by whether the spike survived the stars around it.

galaxies · Halo profiles A year that asks for more than a day on some days. The observing time requested per day through the year, with each target's request spread evenly over the days it can be seen, for the same programme filling 90 per cent of the year. The telescope supplies one day per day (dashed). On 104 days the visible targets ask for more than that, peaking at 1.6 days per day when the Galactic centre is in its window; on the rest there is slack. The annual total fits; the seasons do not.

A schedule written by where the Sun is

A space telescope that must keep the Sun at a fixed range of angles sees each part of the sky in windows set by the date, and the sky its users want is not spread evenly. A schedule can fill around a crowded window as long as the crowd fits inside it, by sending everything else to other dates. Once the demand for one region exceeds the share of the year that region is visible, no choice of proposals elsewhere can recover the excess — the limit is each window, not the total.

sky · Sidereal time

Named alongside it

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

Dark matter haloNfw profileAdiabatic contractionAdiabatic invariantContinuous viewing zoneDark matter annihilationDirect detectionEcliptic coordinatesField of regardLocal dark matter densityObserving windowsOversubscription

All concepts