P01: Galaxy Radii

Observing Goals:

In this project you’ll determine the characteristic scales of one or two galaxies.

Useful Background and References:

Observation Planning:

  • Observation Time

    • You will be observing HALF a night.

  • Target Selection

    • We suggest first looking at the Messier objects to see if there’s one available at the appropriate right ascension and declination. If you can’t find an appropriate one, ask for help. Group 1 should pick a spiral galaxy and Group 2 should pick an elliptical galaxy (note that sometimes more than one galaxy can be observed simultaneously!).

  • Observations

    • We suggest that you take images in at least two bands: \(B\) and \(R\). If you have time, also take images in the \(V\) band.

    • To calibrate your data you should also observe standard star fields.

Data Analysis

  • Data Reduction Basic Steps

  • Further for this project

    • You need to register and combine your images together, so that you have one final image per band

    • You need to estimate (using the mode, rather than the mean or median) the sky background level in each band and take it off the image

    • Locate the center of the galaxy in the images, and estimate the radius of the galaxy

    • Next use and ellipse fitting program (in astropy) to fit ellipses from the center outwards, and create a surface brightness profile for the galaxy

  • Further Analysis for this project

    • Determine whether a de Vaucouleurs \(a^{\frac{1}{4}}\) profile or an exponential \(\exp(a)\) profile fits the surface brightness profile best

    • Determine the scale length of the galaxy (the effective radius in the de Vaucouleurs profile, or the scale length of an exponetial fit)

Results: Can you answer these questions with your work?

  • Q1 what is the surface brightness profile of the galaxy, which model fits best, and what is the scale length?

  • Q2 do the position angle and/or ellipticity of the isophotes change as a function of semi-major axis?  If so, why?

  • Q3 for the elliptical galaxies (only), are the galaxies boxy or disky?  Does this change with semi-major axis?

Presenting your results: