Galaxy rotation curves Dark matter

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rotation curve of typical spiral galaxy: predicted (a) , observed (b). dark matter can explain flat appearance of velocity curve out large radius.




comparison of rotating disc galaxies in distant universe , present day. imaginary galaxy on left in nearby universe , stars in outer parts orbiting rapidly due presence of large amounts of dark matter around central regions. on other hand, galaxy @ right, in distant universe, , seen ten billion years ago, rotating more in outer parts dark matter more diffuse. size of difference exaggerated in schematic view make effect clearer. distribution of dark matter shown in red.


the arms of spiral galaxies rotate around galactic centre. luminous mass density of spiral galaxy decreases 1 goes centre outskirts. if luminous mass matter, can model galaxy point mass in centre , test masses orbiting around (similar solar system). kepler s second law, expect rotation velocities decrease distance centre, similar our solar system. not observed. instead, galaxy rotation curve remains flat distant centre data available.


if assume validity of kepler s laws, obvious way resolve discrepancy conclude mass distribution in spiral galaxies not similar of solar system. in particular, there lot of non-luminous matter in outskirts of galaxy ( dark matter ).

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