The Universal Rotation Curve of Disk Galaxies

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Patel, Raj A.
Astronomy & Astrophysics , Extragalactic Astrophysics , Dark Matter in Galaxies , Galaxy Evolution
In the standard model of cosmology, the formation and evolution of dark matter halos plays an essential role in galaxy development. However, the response of the halo to internal baryonic processes is difficult to observe. We tackle this problem through an investigation of the Universal Rotation Curve (URC) hypothesis. A URC is an analytic function that can predict the rotation curve (RC) of any disk galaxy and, through the relations used to define it, directly correlate stellar properties to dark halo parameters. By examining these dark-luminous relations for different galaxy morphologies, we can study how dark halo shapes change with variations in stellar distributions. As a practical tool, a well-characterized URC can predict RCs for any galaxy with photometric data, greatly expanding the data available for dynamical analyses. We utilize our extensive data set of 3,846 disk galaxy RCs with matching surface brightness profiles to test two popular URCs proposed by Persic et al. (1996), Karukes and Salucci (2017), and Di Paolo et al. (2019). We find that neither URC candidate achieves an adequate level of accuracy to qualify as truly "universal". Specifically, we note systematic under-predictions over outer regions of galaxies, implying the modelled dark halo is not dense enough at galaxy outskirts to reproduce observed RCs. Furthermore, we develop neural network equivalents for each URC which predict RCs with higher accuracy - showing URC inaccuracies are not due to insufficient data, but rather non-optimal formulations. We extend the neural network application to determine the feasibility of a URC in general, and conclude that a URC with an acceptable level of accuracy (less than 15 per cent) is not possible with the data available to us. Key improvements for future URC iterations are an extension to two-dimensional bulge-disk decompositions, allowing for a reliable bulge component in a URC, and the inclusion of additional galaxy properties (e.g., star formation rate, environment data). While noting the shortcomings of our tested URCs and the need for additional data for conclusive results, we speculate that the halos for galaxies in our sample show signs of halo contraction driven by both adiabatic evolution and mergers.
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