A small young generation can change the light
A massive young star shines brightly and has a short lifetime. Long-lived, lower-mass stars keep contributing after its bright blue light has faded. A population therefore changes colour as it ages even if no star moves out of the aperture.
An SSP, or simple stellar population, is an ideal group born at one time with one specified chemical composition and a distribution of stellar birth masses. Its spectrum gives luminosity as a function of age. Multiplying that spectrum by an initially formed mass tells us how bright that generation is.
Our illustrative input is one fixed solar-labelled BC03/EzGal model with a Salpeter birth-mass distribution. It is normalized per initially formed solar mass. Its later remaining stellar mass is different because stars shed material as they evolve; multiplying by that remaining fraction again would apply mass loss twice.
Reading: Bruzual & Charlot, Stellar population synthesis at the resolution of 2003, astro-ph/0309134v1 · Original fixed-slice and static-restoring-force materials
Add generations at their present ages
Suppose the observation occurs million years after the common initial state. Stars born at an earlier time now have age minus that birth time. Multiply their present-age spectrum by the mass born during that interval, then add all birth intervals. This weighted sum is called a convolution.
The old population starts at an age of 10000 million years and ages along with the young population. Its initial formed mass is chosen using the fixed model’s remaining-mass convention at that initial age. The gravitational potential remains fixed in the gas exercise; this postprocessing does not feed stellar mass loss back into the gas.
There are two young-star input histories: continuing exponential consumption, and the same history stopped after 650 million years. The latter is a deliberately imposed experiment. The wind model did not predict this global shutdown.
Reading: Original fixed-slice and static-restoring-force materials · Bruzual & Charlot, Stellar population synthesis at the resolution of 2003, astro-ph/0309134v1
A filter counts a weighted part of the spectrum
A broadband filter admits a continuous range of wavelengths with different transmission weights. The g band samples a bluer region than r. For a photon-response curve, averaging a spectrum per unit frequency requires the corresponding wavelength weight; a plain unweighted arithmetic mean generally gives the wrong band flux.
Once the two band luminosities use the same AB convention, their ratio gives . Larger means relatively less blue-band light. Scaling every stellar formation mass by the same factor changes the total brightness but leaves the colour ratio unchanged.
The native EzGal file stores a rest-frame flux convention at 10 parsecs. Multiplying by the area of that sphere gives luminosity per Hz. An option labelled luminosity in the original source additionally multiplies by frequency; that quantity is frequency times luminosity per Hz, so its name alone is not enough to identify units.
Reading: Original fixed-slice and static-restoring-force materials
Dust changes colour without changing stellar ages
A foreground dust screen removes different fractions of light at different wavelengths. In the chosen empirical Calzetti prescription the bluer light is more strongly attenuated over these optical bands, so changing the screen can redden the same population.
The three specified screen strengths have colour-excess parameters 0, 0.2 and 0.4. They are inputs. Applying the screen does not infer its strength from a galaxy image, and the attenuation curve does not determine how much new dust the tail produces.
We apply this prescription only across its declared 1200–22000 Angstrom wavelength range. All used filter supports are covered, including the supplied response tails. There is no invented continuation outside that range or a new infrared emission calculation.
Reading: Calzetti et al., The Dust Content and Opacity of Actively Star-Forming Galaxies, astro-ph/9911459v1 / ApJ533 (2000) · Original fixed-slice and static-restoring-force materials
Use global light for the question it can answer
The public subset shows four exact ages:0, 325, 650 and 1300 million years. Points compare the continuing and separately stopped histories; they do not assign a colour to each visible gas sample. The independent package repeats the colour ratio, while the original native-spectrum convolution has its own source and numerical checks.
All five wind paths share the same global gas-consumption history and therefore the same global light in this experiment. A disk-only SFR curve cannot simply be substituted for a stellar population inside today’s disk: stars born earlier can later change position, and the gas birth inventories do not track those positions.
A colour change can motivate a history question, but age, recent formation and dust can produce overlapping effects. To infer a shutdown time, specify a population model, a dust model and data with enough independent information to compare them. First decide which stars and which region the colour actually includes.
Reading: Original fixed-slice and static-restoring-force materials · Bruzual & Charlot, Stellar population synthesis at the resolution of 2003, astro-ph/0309134v1