Question

Observations of a galaxy located 20 Mpc away give us the total flux and the radial velocity of stars within circles on the sky that correspond to radii of 100 and 1000 pc.

Within these two regions, the results are as follows:

100 pc radius -- Total flux (W m^-2):  2.1×10^-16,  v_r (km s^-1): 290

1000 pc radius -- Total flux (W m^-2): 1.5×10^-14,  v_r (km s^-1): 250

Use these data to answer the following questions about the galaxy.

A) Calculate M, L, and M/L, all in solar units, for the region of the galaxy contained within a 1 kpc radius.

B) Repeat for a 100 pc radius.

C) Based on your answers for the first two parts, do you think there is a supermassive black hole at the center of this galaxy? Estimate the mass of the possible black hole.

Answer 1

(1) conversion factors;

(A) for R = 1 kpc (i.e 1000 pc) radii:

Calculation for Mass;

......................................................................................................................................

Calculate luminosity (L);

where r = distance of elliptical galaxy = 20 Mpc = 20 x 10^6 pc;

and f = total flux = 1.5 x 10^{-14} W/m^2;

.....................................................................

calculate M/L;

...........................................................................................

(B) for R = 100 pc radii:

Calculation for Mass;

......................................................................................................................................

Calculate luminosity (L);

where r = distance of elliptical galaxy = 20 Mpc = 20 x 10^6 pc;

and f = total flux = 1.5 x 10^{-14} W/m^2;

.....................................................................

calculate M/L;