Astronomy & Celestial Mechanics
the sky, computed
Concept

Specific star formation rate — where it appears

A galaxy's star-formation rate divided by its stellar mass: the fraction of its stars it makes per unit time. Its inverse is the time the galaxy would take to double its stars at the present rate, and it sets how strongly fresh gas dilutes the metals the galaxy has made.

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

One star in 380 makes essentially all the ionising light. Three cumulative fractions against stellar mass, for a broken power-law initial mass function with slopes 1.3 and 2.3 breaking at 0.5 solar masses. Each curve says what share of one quantity is produced by stars heavier than the mass on the axis, and the three do not resemble one another. Only 0.41 per cent of the hydrogen-ionising photons come from stars below 15 solar masses, because the ionising output of a star climbs by five orders of magnitude between eight and twenty. One star in 380 is above that mass, and between them those stars hold 14 per cent of the mass. Those two numbers are the leverage in every star-formation rate quoted from an Hα line. What is measured is the light of a handful of very massive stars; what is reported is the mass of a whole population; and the number in between is an integral over a part of the mass function that no extragalactic observation reaches. The medians are marked but should be read with care, and the reason is visible in the curves: the mass-weighted median at 1.27 solar masses is a property of the population, while the light-weighted one at 58 is a property of where the plot stops — halving the upper mass limit moves it to 36. An integrand that rises with mass has its median wherever the axis ends.

A birth rate measured from light nothing young emitted

A star-formation rate is quoted in solar masses per year, and nothing in the measurement counts a star or weighs anything. What is measured is a luminosity produced by about one star in four hundred, and the conversion to a mass is an integral over a part of the mass function that no extragalactic observation reaches.

galaxies · Star formation A slope of 0.57 at the low-mass end, against the 0.59 and 0.24 two winds give there. Gas-phase oxygen abundance against stellar mass for star-forming galaxies. The solid measured curve is the relation found from electron-temperature abundances in stacked spectra, which rises as a power of mass below a turnover near 10^8.9 solar masses and saturates above it at 12 + log(O/H) = 8.798. The two model curves are a galaxy in equilibrium with its gas supply, whose metallicity is the yield divided by one plus the mass it ejects per unit mass of stars and one plus the dilution by the gas it must keep accreting. Only the ejection changes with mass. A wind driven by the energy of supernovae must lift gas out of a potential whose depth goes as v², so its loading goes as v⁻² and, with v ∝ M^(1/3), as M^(−2/3); a wind driven by momentum goes as v⁻¹ and M^(−1/3). Those set the limiting slopes far below the turnover, 0.67 and 0.33; at log M = 7.5, where dilution still matters, the drawn curves have slopes of 0.59 and 0.24. The measured slope there is 0.57. The dashed measured curve uses a strong-line calibration of the same galaxies' spectra; it sits higher, flattens sooner and has a slope of only 0.40 at log M = 9 — so which wind the data prefer is decided as much by the choice of abundance calibration as by the galaxies. Above the turnover every curve saturates, because a galaxy that ejects almost nothing keeps what it makes and its abundance is the yield, diluted.

The metals a galaxy keeps measure what it threw away

Small star-forming galaxies are metal-poor and large ones are not, in a relation tight enough to be a law. Read as an equilibrium between inflow, star formation and wind, its slope says how the wind is driven, its turnover says where galaxies stop losing what they make — and its redshift evolution says galaxies were poorer because they were still being filled.

galaxies · Chemical evolution

Named alongside it

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

BurstinessCalibration factorChemical evolutionCircumgalactic mediumDepletion timeDust correctionEffective yieldGalactic outflowGas infallGas regulatorIndicator timescaleInitial mass function

All concepts