Galaxies where V(obs) is less than V(bar)

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In summary, the observed velocities in the SPARC dataset may be lower or higher than expected from baryonic matter due to various factors such as the distribution of dark matter, non-spherical distribution of baryonic matter, or incomplete/inaccurate data or models.
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bakerjay
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I'm confused by some galaxies from the SPARC dataset which seem to have velocities which are lower than expected from observed matter, instead of higher
I've been looking through the SPARC dataset (in particular, this part: http://astroweb.case.edu/SPARC/MassModels_Lelli2016c.mrt), and looking at the observed velocities vs velocities expected from baryonic matter.

While in some cases I see the sort of rotation curves I'd expect, with observed velocities higher than expected due to DM:
1679093989916.png
1679093998216.png


In lots of other cases I see the opposite - observed velocities far lower than expected from the baryonic matter:
1679094108188.png
1679094116873.png
1679094125116.png


What is going on here? I assume I'm missing something obvious/doing something wrong, but I can't figure out what...
(graphed here is the Vobs column, and (Vgas + Vdisk + Vbulge) as V(baryonic))
 
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There could be several explanations for this. First, it is possible that the baryonic matter is not distributed in the same way as the dark matter, and therefore the velocity due to baryonic matter is not a good estimate. Additionally, some of the galaxies in the dataset may have more extended dark matter halos than expected, leading to lower observed velocities than expected from the baryonic matter. Finally, it is also possible that the baryonic matter is not distributed in a spherically symmetric way. This could lead to a decrease in the rotational velocity even if the total mass is the same.
 
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  • #3
It's possible that the observed velocities are actually lower than expected due to baryonic matter because you haven't taken into account the effects of non-baryonic dark matter. Dark matter is believed to make up the majority of the mass in galaxies, and its gravitational influence can cause the observed velocities to be lower than expected. Additionally, if there is a large amount of gas or dust present in the galaxy, this could also reduce the observed velocities. Finally, it's also possible that the data or model you are using is incomplete or inaccurate, resulting in incorrect predictions for the velocities.
 
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