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A background galaxy at redshift $z_s=1$ is lensed by a foreground elliptical galaxy at $z_l=0.3$. The Einstein radius is measured to be $\theta_E = 2''$. Estimate the mass of the lens inside the Einstein radius. physics galaxy discussion questions solutions
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Two junglemen are standing on the two opposite banks of a river of width (l), facing each other. One of them starts beating a drum, and the sound reaches the other after time (t_1). Then, the second one starts beating the drum, and now the first one hears the sound after time (t_2). Calculate the velocity of sound relative to the air (v_s) and the velocity of wind (v_w), if it is blowing from the first bank to the other bank at a right angle to the river flow. The Einstein radius is measured to be $\theta_E = 2''$
For a Keplerian system (e.g., Solar System), orbital velocity (v(r) \propto 1/\sqrtr) outside the central mass. For spiral galaxies, (v(r)) rises steeply near the core, then becomes flat (constant) far beyond the visible stars’ edge.