Astronomy & Celestial Mechanics
the sky, computed
Concept

Spectral energy distribution — where it appears

The light an object emits as a function of wavelength or frequency, from which its temperature, composition, dust content and redshift can be inferred. For a galaxy it is the sum of the light of all its stars, gas and dust.

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

Four spectra, and 3.9 magnitudes between them at the same redshift. The K-correction — the magnitude that has to be added to compare a redshifted object with a nearby one through the same filter — against redshift, for four power-law spectra. At zero redshift every correction is zero by construction. Beyond that they diverge, because a filter at a fixed observed wavelength samples a different part of the source's own spectrum at every redshift, and how much flux is there depends on the spectrum. A flat spectrum needs no correction at all at any redshift, and the two extremes drawn differ by 3.9 magnitudes by z = 1.2. The circularity is the point: applying the correction requires the spectrum, and the spectrum is what a magnitude is being used to constrain. The dashed line is the way out — observe in a band chosen so that it lands on the rest-frame band of interest, and the spectral term cancels, leaving only the bandwidth stretch.

A magnitude in a band the source never had

A filter passes light at a fixed observed wavelength, and a redshifted source emitted that light at a shorter one. Comparing a distant galaxy with a nearby one through the same filter therefore compares two different parts of two spectra — and the correction between them needs the spectrum, which is what the magnitude was going to be used to find out.

starlight · Magnitudes Photometric against true redshift for a simulated sample. The redshift fitted from broad-band magnitudes against the true redshift, for 300 simulated galaxies — half old, half young — measured to a signal-to-noise of 10 in their brightest band in six optical bands, and fitted with the same two templates they were made from. 217 fall within 15 per cent in 1 + z of the diagonal, with a scatter of 0.053 in (the fitted z − z)/(1 + z) among them. 83 are catastrophic failures, more than 15 per cent off, and they are not scattered at random: they sit in clumps far from the diagonal, where a galaxy's break was mistaken for the other kind of break. The scatter can be improved by measuring better; the outliers are a different kind of error, and averaging does not remove them.

A redshift that is wrong by three

Most galaxies in the largest surveys will never have a spectrum. Their distances are read from five or six magnitudes, and the reading usually lands close. When it fails it does not fail by a little — a nearby galaxy is placed ten billion years away, because two different breaks in two different spectra fell into the same filter.

starlight · Magnitudes

Named alongside it

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

K-correctionBalmer breakBandpassBandwidth stretchBroad-band photometryCatastrophic outlierCircular inferenceCross filter photometryLikelihoodLuminosity distanceLyman breakPhotometric redshift

All concepts