Surface-Brightness Dimming as a Selection Wall: Redshift Limits for Three Galaxy Classes

This paper addresses a specific question in the accounting of the cosmic optical and infrared backgrounds: at what redshift does cosmological surface-brightness dimming push a normal galaxy below a stated detection threshold, and how strongly does that limit depend on the galaxy's intrinsic surface brightness? The approach is a transparent computation from stated inputs drawn from the published literature, carried through in a deposited script whose output supplies every number quoted here. The principal computed values are: z limit mu0 21 lim 26.5 = 1.96; z limit mu0 22 lim 26.5 = 1.4; z limit mu0 23 lim 26.5 = 0.956; z limit mu0 22 lim 28.5 = 2.65; dimming at z1 mag = 3.01; dimming at z3 mag = 6.02; mu at z2 mu0 22 = 27.6. These are compared with published measurements and budgets at the same wavelengths, and the agreement or disagreement is stated in each case. The study is computed from stated assumptions, not a new measurement; its contribution is arithmetic that can be checked line by line, with sensitivities reported wherever an input is uncertain.