P05: Nebular Extinction

Observing Goals:

In this project you will observe a nebula or edge-on galaxy and try to determine the extinction due to dust in the nebula or galaxy. Useful Background and References:

Observation Planning:

  • Observation Time

    • You will be observing HALF a night.

  • Target Selection

    • Depending on your group, you should select either an edge-on spiral galaxy or a nebula.

    • Your target should be as high as possible in the sky (i.e. lowest airmass) for the duration of your observations.

    • Your target should fit nicely within the frame of the CCD.

  • Observations

    • Take images of your nebula in the \({\rm H\beta}\), \({\rm V}\), \({\rm H\alpha}\), and \({\rm R}\) filters.

    • Note that the :math:`{rm Hbeta} filter images will need to be much longer (total exposure time) than the other images.

    • We strongly suggest that you take all of your images, including the calibration images (flat fields and bias frames), with the imaging CCD in \({\rm 3 \times 3}\) binning mode. (This reduces the resolution of your images but greatly improves the signal-to-noise.)

Data Analysis:

  • Data Reduction Basic Steps

  • Further for this project

    • Create an \({\rm H_{\alpha}/H_{\beta}}\) image.

    • To make it even more precise, you might want to first subtract the R-band image from the \({\rm H\alpha}\) image and the V-band image from the \({\rm H\beta}\) image.

  • Further Analysis for this project

    • Refer to this page for the flux knowledge R03: Calculating Narrow Lines

    • Compare the values of the \({\rm H_{\alpha}/H_{\beta}}\) ratio across the image. In a fully calibrated image the ratio will be 2.74 in fully calibrated flux units in the case of emission from 10000K nebula with pure \({\rm H}\) and \({\rm He}\) (“case B recombination”; see Osterbrock & Ferland 2005).

    • Any variation from this number means that there is extinction by dust in the nebula.

Results:

Can you answer these questions with your work?

  • Q1: Is there any variation in the ratio \({\rm H_{\alpha}/H_{\beta}}\)?

  • Q2: If so, why? If not, why not?

Presenting your results: