# Disk Scale Length Help: Troubleshooting for Science Fair

• SplinterIon
In summary, the speaker is working on a project for a science fair where they are analyzing galaxy pictures and plotting luminosity vs kpc. They are using a de vacouleurs luminosity profile with the disk scale length as a free parameter. However, they are finding that their results for h (the radius when the luminosity decays to 1/e of its original value) are between 7-14 kpc, while most galaxies have a textual value of 3-4 kpc for h. They are wondering if they are missing something fundamental or if this could be due to smeared pictures and dust effects. The speaker mentions that they are also considering photometric integration as an alternative. The conversation then shifts to discussing the surface
SplinterIon
Just a quickie

I'm taking a slice of a galaxy picture and plotting luminosity vs kpc. I'm fitting a luminosity profile (de vacouleurs) and keeping disk scale length as a free parameter. However, textual values for this (referred to as h from now on) range between 3-4 kpc for most galaxies. h is defined as the radius when the luminosity decays to 1/e of its original value. My plots for most galaxies end up with h between 7-14 kpc.

Is there something I'm doing wrong? Am I missing something fundamental? Can I just chalk this down to smeared pictures and dust effects?

If this doesn't work - I'm going to photometrically integrate and hope for the best

BTW: This is for a science fair (for anyone wondering)

By luminosity, I take it you mean "surface brightness". If not, make sure that's what you're measuring. What did you use to take these images? What's the width of your PSF? If it's comparable or greater than the scale length of your galaxy, you'll measure scale lengths that are too large.

Yes, I do mean surface brightness (the reason I mentioned luminosity is that my values are in solar luminosity assuming a thin infinite disk). I didn't take the pictures myself, got it from an online database - the name evades me (I'm at school and my home comp. has the link). I scaled the pictures and preserved ratio, so I don't think it's a stretching error. They aren't the best quality in the world, but they're passable. I selected galaxies which were more or less face on - and sort of corrected for angle deviations. I don't however understand what you mean by PSF - care to elucidate?

Thanks a million.

SplinterIon said:
Yes, I do mean surface brightness (the reason I mentioned luminosity is that my values are in solar luminosity assuming a thin infinite disk).

Not to split hairs or anything, but are you sure you're measuring the surface brightness? There's no need to convert things into physical units (like luminosity) if you're just measuring a surface brightness profile. I would expect your numbers to be in units of either intensity or magnitudes per square arcsecond. If you're measuring light integrated over the disk (as denoted by a luminosity), then you would expect a different shape of profile.

I didn't take the pictures myself, got it from an online database - the name evades me (I'm at school and my home comp. has the link).

When you get the info, let me know which database it is. It's not SDSS by any chance, is it?

I don't however understand what you mean by PSF - care to elucidate?

The PSF is the point spread function and it sort of describes the "blurring" of the light by the instrument. If there's a source of light that's emanating exactly from a point, then on your image it would be blurred out into something that doesn't look like a point (more like a circular halo), and this is described by the point spread function. A galaxy is not a point source, but its light will still be blurred somewhat by the PSF. The width of the PSF should be given approximately by the angular resolution of the instrument, so if you can find that (along with the distances to the galaxies you're looking at), I should be able to tell you right away whether or not it's a problem.

## 1. What is a disk scale length?

A disk scale length is a measurement used to determine the size of a spiral galaxy's disk. It is typically measured by the distance at which the brightness of the galaxy's disk drops to half of its maximum brightness.

## 2. How is a disk scale length calculated?

A disk scale length is calculated by fitting a curve to the brightness profile of a spiral galaxy's disk. This curve is called an exponential disk profile and the scale length is determined by the point where the curve drops to half of its maximum value.

## 3. What is the importance of studying disk scale lengths?

Studying disk scale lengths can provide insight into the structure and evolution of spiral galaxies. It can also help us understand the distribution of stars and gas within a galaxy and how they may be influenced by external factors.

## 4. What are common issues when measuring disk scale lengths?

Common issues when measuring disk scale lengths include inaccurate data due to factors such as light pollution, instrumental effects, and human error. It is also important to ensure that the galaxy being measured is not heavily obscured by dust or has a complex disk structure.

## 5. How can I troubleshoot for inaccurate disk scale length measurements?

If you are experiencing issues with inaccurate disk scale length measurements, it is important to carefully examine your data and make sure it is free from any external influences. You may also need to adjust your measurement techniques or use different tools to obtain more accurate results.

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