Surface Brightness of Disc Galaxies –
In this project you image some galaxies, comment scientifically on your images, and then obtain
surface brightness profiles which you analyse using a spreadsheet tool. This will enable you to see
how the surface brightness decreases with distance from the centre of the galaxy and make
measurements of some relevant parameters. This information can then be correlated with the
Hubble galaxy classification.
The basic project is to examine the surface brightness of a disc galaxy (lenticular or spiral) and see
if it decreases exponentially with distance outside of the central bulge. A specially written
spreadsheet enables makes this easy to do.
There are also more advanced activities examining the surface brightness of elliptical galaxies and
the bulge as well as the disc in a disc galaxy, and these are described on separate worksheets.
However, you are advised to try this basic project first however.
Summary of Activities
Planning In planning you will have to decide upon the galaxies that you want to image.
Observing You use the telescope (in real time or off-line) to obtain JPEG images of spiral
galaxies in different wavebands (colours). A full colour (colour composite) image is also obtained.
Commenting and classifying You print out the images of each galaxy and comment in scientific
terms on their features, including the differences between images in different colours. You also try
to classify your galaxies using a modified form of Hubble's classification scheme.
Producing surface brightness plots You use the free DS9 software to open the full data files
produced by the telescope ('FITS' files) and produce surface brightness plots which you then save
in a form that can be read into a spreadsheet.
Analysing You use a specially written spreadsheet to analyse the surface brightness and see if it
decreases exponentially with distance outside of the central bulge region of your galaxies.
Evaluating You evaluate your results, comment on anything that went well or went badly, find
explanations for anomalous results and suggest reasons and solutions for these
What images are required
Images for this project can be obtained in either real time or off-line.
Each galaxy to be measured and classified should, at the very least, be imaged with the V and R
waveband (green and red) filters, so that a three-colour composite image can be created. In
addition, taking images using different filters, including possibly U (ultraviolet) and I (infrared) as
well, will allow you to compare the proportions of hot, blue, young stars and older, cooler, more
yellow stars in different parts of the galaxy.
JPEG images for each waveband are sent back to you straight away by the telescope software, as
well as a full colour JPEG image.
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These JPEG images are all that is needed to observe and classify a galaxy. However, if you want
to carry out the full astronomical image processing required to produce a high quality image for the
Faulkes Atlas of Nearby Galaxies, you will need to make use of the FITS files returned to you
several hours after the actual observing session. Details are given as a separate project.
1 Choose suitable galaxies
Ideally these should be one that are neither too small nor too large for the field of view and
sufficiently bright to give reasonably good images given appropriate choices of exposure time. A
list of suitable galaxies can be downloaded from the resources page for this project, along with an
Observation Planner - a spreadsheet showing which galaxies are visible when.
2 Choose when to observe
Good images of faint deep sky objects such as galaxies can only be obtained when there is a low
level of background sky brightness. Such conditions are called photometric because they are
suitable for making brightness measurements of objects. To observe under photometric conditions
with the Faulkes Telescopes, you need to choose times when the Moon is not up and it is not
3 Choose suitable filters
To obtain a true colour image you will need to produce images using the blue, green and red (B, V
and R) filters. These are broadband filters because they allow through a range of wavelengths
centred on the blue, green or red parts of the spectrum respectively.
However, you may also want to use the ultra-violet (U) filter to show up any bright regions
containing bright blue young stars more clearly. You might also want to use the near infra-red fliter
(I) to show up old cooler stars more clearly.
4 Choose suitable exposure times
Having chosen when to observe, and through which filters, you next need to decide how long to
observe each galaxy through each filter. The telescope software will help you do this.
5 Plan you observing slot
Draw up an observing schedule taking account of the time required for each exposure and the time
required for the telescope to slew from one galaxy to the next.
Submit this to the Telescope Control Centre and book your observing slot.
6 Save the images
Save the JPEG images you have obtained from your observing session in a suitable folder on your
7 Viewing images
You can use JPEG images taken in different colours (wavebands) and you can also use the full
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If you want to take your analysis further than is possible simply using JPEGs, an alternative is to
use astronomical image processing software to view the full astronomical data files (FITS files).
The advantage of doing this is that you can selectively enhance different aspects of a galaxy image
in order to study it. You could, for example, enhance less bright regions of an image so that fainter
outer parts of the galaxy can be seen clearly. However, this would saturate the bright central
regions so that no detail was visible there, so you might also want to change the settings in such a
way that detail in these regions can be studied.
Make sure that you save the originals as well as the processed images. In some cases you may
find that it is a good idea to process an image in different ways to bring out different features (e.g.
detail in fainter outer parts, detail in bright parts of the galaxy).
8 Print the images out
You will find it helpful to print out your images so that they can be studied more easily. This will
also enable you to label them with relevant comments for inclusion in a written report, if
HINT If you can use a photographic quality printer and high resolution or photographic quality
paper, you will get much better results than with a cheap printer and ordinary paper.
Commenting and classifying
9 Analyse the images
Examine the images carefully, looking in particular at the following features:
Examine the images carefully, looking in particular at the following features:
whether you think that it is an elliptical galaxy or a disc galaxy (such as a spiral galaxy)
which is flattened like a disc, or whether it is hard to tell (as can sometimes be the case)
if the galaxy is a disc galaxy, whether it is face-on to our line of sight or tilted
if the galaxy is a spiral:
whether it is barred or not
the number of spiral arms, how long they are, and how tightly wound
whether the spiral arms are clearly defined, or vague and ragged in appearance
regions where there are a lot of bright, hot, young blue stars
regions where there are dark dust clouds
"H2" regions which are glowing with a pinky colour because ionised hydrogen gas is
glowing due to nearby hot young stars giving off copious amounts of ultra-violet radiation
(these regions are also known as emission nebluae)
globular clusters also containing older yellow stars
any other features you think are worth pointing out
10 Hubble classification
Using the preceding information to decide how well each of your galaxies fits the published
properties for galaxies in the Hubble Classification. As there are several modern variations on
Hubble's original scheme, a guide to one appropriate for this project has been written for you. This
can be downloaded from the resources page for this project.
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11 Download FITS files
Save the red, green and blue FITS files on which your colour image is based to a suitable location
on your computer. Use file names which will enable you to easily identify the three files.
12 Generate and save surface brightness profiles
Open each of your three FITS files in turn from within SAOImage DS9. Produce surface brightness
plots and save these as data files. Instructions for doing this can be downloaded from the
resources page for this project.
13 Analyse the profiles
Import the surface brightness data files for each of your slices into the profiles_disc.xls analysis
spreadsheet. Details of how to use this software can be printed out directly from it.
Using the spreadsheet, for each slice:
o estimate the background sky brightness
o make estimates of the apparent disc and bulge radii in arcmin
o produce a best fit of an exponential model to the brightness in the disc region and estimate
the scale length in units of 1/arcmin
14 Comment on your brightness profiles
Examine the best fits of your exponential models to the brightness profiles along your slices and
comment on the following aspects:
o what the sharp peak on the centre of the galaxy indicates about the density of stars there
(and it would be even sharper were it not for the blurring of the atmosphere or 'seeing')
o how the location of spiral arms and dark dust lanes in your image relates to features visible
in the surface brightness profile graphs (the log graphs show this more clearly because
they indicate brightness ratios)
HINT Make sure that if there were any bright stars along the slices that these are not
confused with bright spiral arms
o how the background brightness of the sky varies between the red, green and blue images
o how the amount of noise visible in the red, green and blue profiles varies
15 Final measurements
Print out the table of results in the analysis spreadsheet, and then:
o Calculate mean values for the disc and bulge radii in arcmin
o Calculate the disc to bulge ratio
o Convert the disc and bulge radii to thousands of light years (kly) using the relation
absolute size in kly = angular size in arcmin x distance away in Mly x 0.3
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o Calculate a mean value for the scale length in 1/arcmin
o Convert this to 1/kly using the relation:
scale length per kly = scale length per arcmin / (distance away in Mly x 0.3)
Comment on any sources of error that you are aware of and any ideas you have for improving on
the way that you carried out any aspect of the project, including both the observing and the
measurements you made. Include comments on how the different estimates of scale length vary.
Measuring spiral arms in spiral galaxies
There is a further investigation you can carry out which involves measuring the spiral arms in any
spiral galaxies you have observed. (You willl not need to repeat the initial sections of this additional
investigation, which involve Observing, commenting and classifying, as you will already have done
Faulkes Atlas of Nearby Galaxies (FANG)
If you wish to produce high quality colour images of your galaxies, you can do this by using a long
exposure time of (say) 5 minutes for each filter, or (better) you can take several colour images of
one minute per filter each and then 'stack' these (combine them together using software). The high
quality images so obtained could then contribute to the Faulkes Atlas of Nearby Galaxies which will
be a full-colour on-line atlas of galaxies that will be useful to both professional astronomers and
amateurs. For most galaxies, the images in this catalogue will be the first high quality colour
images ever obtained.
Eventually, the images will be put together to make a full colour atlas of nearby galaxies that will
make a very attractive "coffee table" book as well as being of great scientific value.
In both the on-line atlas and the book, credits will be printed alongside each image giving the name
of the school and the individuals involved in obtaining and processing the image.
Details can be obtained via the galaxy projects webpage.
Richard Beare 21st March, 2005
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