Another day, another Streamlit app. This one isn't of much use for public outreach, but is potentially quite the time-saver for me. It takes raw photometric measurements of galaxies (e.g. apparent magnitude, but also photon counts) and turns them into physical estimates of their stellar masses. Given an inclination angle estimate it also applies a correction for internal extinction (that is, all the gas and dust obscuring the stars as we look through the galaxy itself). It can even look up the extinction within our own Galaxy as well, given the sky coordinates.
All this is very simple to apply in practice, but there are so many little steps that it's very easy to make a mistake in one of them, which rapidly gets tedious. And if you just want a single measurement this can be no fun at all, so not having to do any of this - replacing it all with entering a couple of numbers in a table - is quite handy. I should probably write a version that can process multiple inputs as well, but this is at least a useful reference source for that.
Just how accurate these prescriptions are I know not. It's quite interesting to see how the mass estimates vary, even using the same photometric measurements gives values differing by a factor two or so according to different methods. As I understand it, these simple estimates are from fitting to different models of stellar populations; don't ask me how that works. Another major factor is that different wavebands (filters) often have different sensitivity levels... so using different colours can easily give estimates which differ by a factor of a few. Still, it's probably better than not doing it at all.
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