Does Increasing Luminosity Affect the Percentage of Dark Matter in a Galaxy?

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Discussion Overview

The discussion centers around the relationship between the luminosity of a galaxy and the percentage of dark matter it contains. Participants explore whether an increase in luminosity implies a decrease in dark matter percentage, considering various factors such as star color and gravitational effects.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • One participant hypothesizes that an increase in luminosity could indicate a smaller percentage of dark matter, suggesting that as normal matter increases, dark matter's relative percentage decreases.
  • Another participant argues that dark matter interacts very rarely with normal matter, implying that dark matter should not significantly affect the luminosity of stars.
  • A different viewpoint suggests that while dark matter has minimal direct interaction with normal matter, its presence could influence the fraction of stars and gas, potentially affecting total luminosity.
  • One participant notes that there are galaxies with varying amounts of dark matter that do not show significant differences in luminosity or spectral features, questioning the impact of dark matter on luminosity.
  • It is mentioned that luminosity relates to the number of photons emitted, and while dark matter does not directly affect photons, its role in gravitational lensing is acknowledged, though not definitively concluded.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between dark matter and luminosity, with no consensus reached on whether an increase in luminosity affects the percentage of dark matter in a galaxy.

Contextual Notes

Participants highlight the complexity of interactions between dark matter and normal matter, noting that assumptions about gravitational effects and the nature of luminosity are not fully resolved.

Huej
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If luminosity increases, hypothetically, for a given galaxy, would the percentage of dark matter in that galaxy be smaller? What if the colors of the stars were to turn redder?

I was thinking if the luminosity were to increase, that would mean the percentage of the normal matters increases, thus the percentage of dark matter would decrease. But when I looked at the formula: Mass of stars: (M/L) ratio * Luminosity of stars, I was thinking the mass would be constant since the luminosities in the equation would cancel out.
 
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Other than through gravity, 'Dark matter' only interacts very rarely with 'Normal matter', (if it does at all).
Therefore It should not significantly affect the luminosity of stars.
 
rootone said:
Other than through gravity, 'Dark matter' only interacts very rarely with 'Normal matter', (if it does at all).
Therefore It should not significantly affect the luminosity of stars.
I don't think it's much of a interaction of dark matter and normal matter, but more of how one indirectly affects the other. So if there is higher fraction of dark matter, there would be smaller fraction of stars/gas (assuming total gravitational mass of the galaxy is constant) and impact the total luminosity. The question that I'm trying to answer does say smaller/larger, so I'm wondering which of the two, even if the impact is small. Wondering if my reasoning makes sense though.
 
I can't think of any reason the presence of dark matter would impact luminosity. We know of galaxies with great variance in the amount of dark matter present and it apparently has no apparent effect on their luminosity or spectral features.
 
Luminosity means the amount of photons which are getting emitted.
Apparently dark matter does not have any consequences at all for photons, (other than gravitational lensing)
... but that is by no means a dead certainty.
 
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