An Upper Limit to Differential Magnification Effects in Strongly Gravitationally Lensed Galaxies

Serjeant, Stephen (2024). An Upper Limit to Differential Magnification Effects in Strongly Gravitationally Lensed Galaxies. Research Notes of the AAS, 8(2), article no. 52.

DOI: https://doi.org/10.3847/2515-5172/ad2bfe

Abstract

Differential magnification is now well-known to distort the spectral energy distributions of strongly gravitationally lensed galaxies. However, that does not mean that any distortions are possible. Here I prove an analytic upper bound to differential magnification effects. For example, a thermal or sub-thermal CO ladder cannot be made to appear super-thermal just from gravitational lensing, and the Balmer decrement emission line ratio Hα:Hβ cannot reduce below the case B prediction just from differential magnification. In general, if a physical model of a galaxy predicts upper and/or lower bounds to an emission line ratio, then those bounds also apply to the differentially magnified strongly gravitationally lensed case. This applies not just for velocity-integrated emission lines, but also for the line emission in any rest-frame velocity interval.

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