USAAAO — 2026 First Round

Consider the following graph (below), which shows the mean orbital velocity of stars around a galaxy’s center as a function of distance from the center. The graph also shows the expected orbital velocities (dashed line) from observations of visible matter. The discrepancy between the observed and expected values is assumed to be due to invisible dark matter. Consider the following three assertions related to the techniques used to measure the overall rotation curve, in the context of this figure: (I) Interstellar dust is colder at lower values of r, making the dust obscure less starlight. (II) O/B type stars in the spiral arms of galaxies emit significant quantities of UV light, ionizing nearby H-I clouds and thus inhibiting the emission of 21-cm radiation in their vicinities. (III) The lower star number density for higher values of r means that the stars that do exist typically move at relativistic speeds, complicating Doppler-based stellar velocity measurements. Which of the above are TRUE?