H5 - practical considerations

acquiring galactic spectra can be done via two main procedures

• long slit spectroscopy involves placing a thin aperture to slice a galaxy along one of its axis

• it is shifted/rotated to cover the entire galaxy

• a spectrum is obtained for each position along the slit

• integral field spectroscopy is where the spatial distribution of the galaxy is mapped on the plane of the sky, and a spectrum per pixel is produced

• a better resolution element is called a spaxel

• the velocity components being referred to can be:

  • bulk velocity: the radial velocity of the entire galaxy
  • rotational velocity: the orbital velocity of a material around the centre, and can be obtained by subtracting the bulk velocity from the measured velocity
  • velocity dispersion: $({\sigma }_r)$ the intrinsic 'spread' in the local velocity, ie. the velocity measured at a certain position

• eg: for the solar neighbourhood: $v_{rot}\approx 200-250$ km s${}^{-1}$, ${\sigma }_r\approx 40$ km s${}^{-1}$

• for unresolved galaxies, $v(r)$ cannot be measured, so a velocity profile is measured using the global shift/broadening of spectral lines due to all velocities combined