Astronomy & Celestial Mechanics
the sky, computed
Concept

Stellar halo — where it appears

The faint, extended population of old stars around a galaxy's disc and bulge, much of it the debris of smaller galaxies torn apart and absorbed. Its structure records the galaxy's history of mergers.

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

A sky wrong by a thousandth, and the disc that ends or does not. The surface-brightness profile of an exponential disc with a central surface brightness of 21.65 magnitudes per square arcsecond, against radius in units of its scale length, drawn straight — a pure exponential is a straight line in magnitudes — and then as it would be measured after subtracting a sky of 21.5 magnitudes per square arcsecond wrong by +0.1%, −0.1%, +0.3%, −0.3%. A sky left in bends the profile up into a shallower outer part, the shape called an anti-truncation; a sky taken out too hard bends it down into a break and then drops it out of existence where the residual goes negative. The +0.1% error departs by 0.2 magnitudes at 5.2 scale lengths, 27.3 magnitudes per square arcsecond; The −0.1% error departs by 0.2 magnitudes at 5.0 scale lengths, 27.1 magnitudes per square arcsecond; The +0.3% error departs by 0.2 magnitudes at 4.1 scale lengths, 26.1 magnitudes per square arcsecond; The −0.3% error departs by 0.2 magnitudes at 3.9 scale lengths, 25.9 magnitudes per square arcsecond. Real discs are observed to break both ways, at 24 to 27 magnitudes; every one of those breaks lies in the range where a sky error of a few tenths of a per cent would make one.

Where the sky is the measurement

The outskirts of a galaxy at 29 or 30 magnitudes per square arcsecond are a thousand times fainter than the night sky they are seen through. Photon noise averages down with exposure; an error in the sky subtracted from under them does not. A sky wrong by a tenth of a per cent draws a break into a disc that has none, or a halo around a galaxy that has none, and the telescope's own scattered light draws another.

galaxies · Surface brightness A compact galaxy with a faint envelope, seen at redshift two. The surface-brightness profile of a model galaxy at redshift 2, in magnitudes per square arcsecond against radius in kiloparsecs on a logarithmic axis (solid): a compact de Vaucouleurs core of 1 kpc half-light radius and an exponential envelope of 10 kpc holding 30 per cent of the light, dimmed by 4.77 magnitudes by the expansion. The dotted curve is the core alone. Horizontal lines mark survey limits of 27 and 30 magnitudes per square arcsecond. A single Sérsic profile fitted to what is brighter than each limit (dashed) gives a half-light radius of 1.3 kpc and index 4.6 above 27, and a half-light radius of 1.5 kpc and index 5.1 above 30. The galaxy's true half-light radius, core and envelope together, is 2.1 kpc. The envelope is above the deeper limit out to 16 kpc, and the fit still does not follow it: a single profile weighted towards the bright centre cannot bend to two components, and more depth only nudges the compromise it settles on.

One profile for a galaxy that has two

The worry about the compact galaxies of ten billion years ago was that their growth since might be partly a faint envelope the sky hides. Modelled, the envelope does hide — but not mainly under the sky. A single profile fitted to a core with an envelope reports a compromise at any depth, too large for a tight envelope and too small for a wide one, and the growth it measures is an understatement.

galaxies · Surface brightness

Named alongside it

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

Surface brightnessDisc truncationExponential discThe half-light radiusLow-surface brightness galaxyMinor mergerThe point-spread functionQuiescent galaxyScattered lightSersic profileSize evolutionSky background

All concepts