---
title: "Galaxy colors from photons to redshift distributions"
description: "Understand galaxy colors, derive filter fluxes and redshift posteriors, then reproduce a fixed real SDSS sample and read EAZY and Euclid calibration."
canonical_url: "https://sandboxphysics.com/stories/galaxy-colors/"
language: "en"
page_type: "story"
alternate_languages:
  zh-CN: "https://sandboxphysics.com/zh/stories/galaxy-colors/"
last_modified: "2026-10-28"
---

AC04 · Galaxy photometry

# Galaxy colors from photons to redshift distributions

Sandbox Physics · 2026-10-28

A red photograph leaves several explanations open. Follow the light from a stellar population through dust, expansion and measured filters. Then ask how five uncertain fluxes become several possible redshifts, and how a real survey checks them.

[01 · L1 Why a red galaxy can have several possible distances Redshift, old stars and dust all redden galaxies. Follow a spectral break through five filters to see how astronomers separate them.](https://sandboxphysics.com/stories/galaxy-colors/why-galaxies-look-red/) [02 · L1 What a filter measures when a detector counts photons Derive photon-weighted AB fluxes from spectral densities, keep wavelength units straight and calculate a real SDSS filter count rate.](https://sandboxphysics.com/stories/galaxy-colors/counting-light-through-filters/) [03 · L2 Redshift stretches a spectrum and dust reshapes it Derive frequency and wavelength redshift factors, a flat-spectrum K correction, and a rest-frame dust model with explicit stellar-age assumptions.](https://sandboxphysics.com/stories/galaxy-colors/dust-and-moving-spectra/) [04 · L3 Keep both answers when a redshift distribution has two peaks Build a normalized multimodal redshift posterior, integrate a positive amplitude exactly and distinguish peak height from probability mass.](https://sandboxphysics.com/stories/galaxy-colors/when-redshift-has-two-answers/) [05 · L3 Reproduce redshift distributions for twenty-four real SDSS galaxies Use fixed SDSS DR17 fluxes, negative measurements and withheld spectroscopic redshifts to reproduce a small photo-z validation with failure cases.](https://sandboxphysics.com/stories/galaxy-colors/sdss-five-band-tutorial/) [06 · L4 Read EAZY and a Euclid calibration paper with the selection visible Follow pinned EAZY sections and Euclid 2026 figures to separate template fitting, posterior summaries and population redshift calibration.](https://sandboxphysics.com/stories/galaxy-colors/reading-photoz-reliability/)

## Sources and calculation scope

1. [Hogg et al., The K correction, astro-ph/0210394v1 (2002)](https://arxiv.org/abs/astro-ph/0210394v1)

   Equations (2)–(12): spectral densities, photon response, redshift factors and the AB K correction. We use luminosity distance and define wavelength densities per Angstrom.

2. [Doi et al., Photometric Response Functions of the SDSS Imager, arXiv:1002.3701v1 (2010)](https://arxiv.org/abs/1002.3701v1)

   Table 4 response curves as distributed by speclite at commit 8159ea61f1c8539136660b4c2e87fbbf94a70097. The sdss2010 quantum throughputs already include an atmosphere at airmass 1.3; source files and BSD notice are retained.

3. [Bruzual & Charlot, Stellar population synthesis at the resolution of 2003, astro-ph/0309134v1](https://arxiv.org/abs/astro-ph/0309134v1)

   Sections 2–3 describe the stellar-population model and its spectral evolution. The frozen solar-metallicity Salpeter SSP file is distributed by EzGal; this course fixes its model choices rather than inferring a general galaxy age.

4. [EzGal model distribution, commit 133c62b383fae964701b673e62517293161b41d5](https://github.com/cmancone/easyGalaxy/tree/133c62b383fae964701b673e62517293161b41d5)

   tests/data/bc03\_ssp\_z\_0.02\_salp.model, primary image and the vs/ages FITS tables. README acknowledgements explicitly describe the model authors’ redistribution permission; the repository MIT notice is retained. Spectra are converted from fnu at ten parsecs into relative shapes.

5. [Calzetti et al., The Dust Content and Opacity of Actively Star-forming Galaxies, astro-ph/9911459v1 (2000)](https://arxiv.org/abs/astro-ph/9911459v1)

   Section 3.3, equations (2)–(5): empirical starburst stellar-continuum attenuation from 0.12 to 2.2 microns, with RV=4.05. The stellar continuum color excess differs from that of nebular gas.

6. [SDSS DR17 imaging fluxes and flux calibration](https://www.sdss4.org/dr17/algorithms/fluxcal/)

   The AB-offset and physical-flux sections state the approximate u shift, tentative z shift and their uncertainties. The tutorial freezes TOP 24 clean primary GALAXY spectra with zWarning=0 ordered by specObjID, keeping linear modelFlux values and inverse variances.

7. [SDSS public-release use and acknowledgement policy](https://www.sdss4.org/collaboration/)

   The released catalogue is public data. Credit the Sloan Digital Sky Survey and its collaboration; the tutorial uses catalogue numbers, not survey images.

8. [Brammer, van Dokkum & Coppi, EAZY: A Fast, Public Photometric Redshift Code, arXiv:0807.1533v1 (2008)](https://arxiv.org/abs/0807.1533v1)

   Sections 2.1–2.5: template combinations, template error, priors and output redshifts. Section 3.3 and section 4, especially Figures 8–11, connect the error model and priors to real validation. Our finite SSP posterior is a separate teaching calculation.

9. [EAZY manual and source, commit f651e94d91042a84906090a66d2a3fec75504e21](https://github.com/gbrammer/eazy-photoz/tree/f651e94d91042a84906090a66d2a3fec75504e21)

   Read doc/eazy\_manual.tex alongside the pinned paper. Record the filter file indices and the selected definition of z\_peak or marginalized output before comparing catalogues. The course does not redistribute its HDFN catalogue or default templates.

10. [Euclid Collaboration, Improving redshift distribution reconstruction using a deep-to-wide transfer function, arXiv:2601.02005v1 (2026)](https://arxiv.org/abs/2601.02005v1)

    Sections 2.1–2.3 define transfer and selection; sections 3.1–3.2 define the Flagship 2.1 mock validation and color-cell calibration. Figures 5–6 compare distribution calibration. These are simulation tests, not a reproduced Euclid sky measurement.