Astr212 Galaxy Projects, Spring 2009
Spindle Galaxy (NGC 5866),
NGC 5866 is an SO_3 type galaxy. This means that it has some characteristics of an elliptical galaxy and some of a spiral galaxy. NGC 5866 has the bulge of an elliptical galaxy and the disk of a spiral galaxy. NGC 5866 is 45 million lightyears away from the earth and has a linear size (distance across) of 40 kly. If lightyears are converted into kpc, we see that NGC 5866 is 12 kpc across. The Milky Way Galaxy (our galaxy) has a linear size of about 50 kpc: four times the size of NGC 5866! NGC 5866 has had an identity crisis throughout its life since it’s been discovered here on earth. It has been claimed that NGC 5866 is the same galaxy as M102. This is unsure because of controversy over Charles Messier’s entry of the galaxy without a position in the publication of the catalog after the discovery of the galaxy by Pierre Méchain in 1781. Pierre Méchain denied the discovery two years later, and so it is not known whether NGC 5866 was the galaxy observed as M102 or if it was in fact another galaxy within a couple of degrees of NCG 5866 (SEDS).
The light being emitted from this galaxy is produced by the stars in the galaxy. As can be seen from image of the galaxy, most of the light coming from the galaxy is a faint yellow color. This is probably due to the fact that a lot of the stars are low mass, older stars that have not yet reached the stage of their life span in which they become red giants. This means that they are not as hot as some other stars and so the light tends to be more yellowish. On the right side of the galaxy there is a patch of blue light (marked in images below). Blue light signifies extremely hot, high-mass stars. High mass stars do not live as long as low mass stars and so blue light signifies stars that were recently formed. A region that is more blue than surrounding areas, means that star formation is faster in that region. Faster star formation would began occurring if there was just some trigger to ignite the molecular clouds to start forming stars. This trigger could be the shockwaves of a supernova explosion. Once the process is starting, a chain reaction occurs and more and more stars are formed in the same region creating an area of blue light. NGC 5866 is almost exact edge on and has a prominent dust lane right across the middle of the galaxy. This dust lane has a reddish tint to it. Dust tends to scatter and absorb light. It affects blue light more than red and so the red light is the light that gets through the dust lane to be viewed by us on Earth.
The picture above was taken by the 2MASS telescope that is 1.3 m in diameter. It was taken in the near infrared. The image shows barely the dust lane that is visible in the image taken with Calvin’s Rehoboth telescope. This is because infrared light has a large enough wavelength that the light can pass through the dust mostly unaffected. The light that is being emitted from the galaxy at this wavelength is dominated by the older stars and the galactic bulge.
The picture below was taken by the GALEX telescope in the near ultraviolet part of the spectrum. Images in the ultraviolet can give a lot of info about star formation. In this image, the yellow light is due to all the old stars in the galaxy. The areas of blue light are due to the young hottest stars that are being formed. This blue light is obvious in the galactic plane and a patch on the right side of the galaxy. Star formation is common in the galactic plane, but it is interesting to note that there is a separate region of star formation in NGC 5866. The reason for this was discussed above. This region is even visible in the image taken with Calvin’s telescope (see second image below).
S. Pellegrini (1994) conducted research of NGC 5866 using the ROSAT to observe the galaxy in the x-ray part of the spectrum. Previous studies had shown that some SO galaxies had a lower x-ray luminosity than elliptical galaxies. Pellegrini observed NGC 5866 to see if this was true and to determine a reason for this to be the case. Possible sources for x-ray emission are the hot diffuse gas in between the stars that is known as interstellar medium (ISM) or the hot gas left over from supernova explosions. These supernova remnants don't radiate as much heat as the ISM and so the energy received from that type of radiation is lower. Radiation on the low end of the x-ray/high end of UV part of the spectrum is known as the Very Soft thermal Component (VSC). This means that the wavelengths of the light perceived from VSC emission has slightly longer wavelength than hard x-ray emissions (high energy x-rays). There is some debate over whether the VSC emission observed from SO galaxies are due to the ISM or stellar sources. Pellegrini came to the conclusion that it is most likely not the ISM that dominates the VSC of the x-rays emission. A reason for this to be true is that SO galaxies rotate at a faster rate than elliptical galaxies. This allows for there to be a presence of wind in the SO galaxies that strip the ISM from the galaxy. If there is not as much ISM in the galaxy, then it is possible for the most dominant source of VSC to be from stellar emissions such as the supernova remnants mentioned earlier.
The Light Profiles.
Light profile are a useful tool used to categorize galaxies by allowing astronomers to observe the shapes and features of a galaxy. A light profile is a plot of the surface brightness against the radius from the center of the galaxy. It shows how the light decreases as the distance from the center of the galaxy increases. Each type of galaxy has a unique light profile because each type of galaxy has distinctive features that can show the light dropping off at different rates throughout the galaxy.
The light profile of NGC 5866 of the minor axis (the shortest distance for a straight line to bisect the galaxy) shows the extinction by the dust lane very nicely. The extinction due to the dust lane is very close to the center which shows that this galaxy is almost completely edge on. For a SO type galaxy, it is typical to have a slightly exponential fit to the profile since that is typical of most disk galaxies. However, SO galaxies also have the bulges of the elliptical galaxies and so towards the center of the galaxy, we should see a different function. The light profiles taken on the major axis (longest axis of the galaxy) and minor axis shown above have an almost perfectly exponential fit throughout the galaxy, uncharacteristic of a SO galaxy. This could be due to the fact that the dust lane to covering up the bulge. The light profile for the major axis was taken in from the center of the galaxy, which is where the dust lane is located in the image of this galaxy.
The Dust Lane:
Since the dust lane is so prominent in this galaxy, it would be interesting to know about how dense it is and how much it affects the light coming through. The amount of dust extinction can be calculated by using the equation , where A is the amount of extinction, is the intensity of the galaxy if there was no extinction (this was calculated assuming the exponential function continued to the center), and is the actual intensity. By this equation the extinction observed in the blue filter was 0.92 magnitudes , and 0.64 magnitudes in the visual filter. By convention,
, where R is 3.1 for typical dust grains. According to the values for extinction listed above, the calculated value of R is around 2.3. This is a relatively close to the commonly accepted value. From the extinction, the density of the dust lane can also be calculated using the equation , where is the optical depth, is the column density of the dust, and is the cross-section for the dust to be struck by light. The optical depth tells how easy it is for the light to pass through the dust, so a lower value means that it is easy for the light to pass though. The optical depth value for the blue filter is 0.843 and 0.589 for the visual filter. It can be seen the blue light has a higher value than green light. This explains the statement early mentioned that red light can pass through the dust easier and that was why the dust lane had a reddish tint to it. Now in order to solve for the density of the column of dust, it is necessary to make a few assumptions about the cross-section for striking of the dust. It will be assumed that the cross-section for striking is 1.5 times the area of a dust particle. The average radius for a dust particle is around 200 nm, so . This means that the column density is , which equals dust particles/ for the blue filter and dust particles/ for the visual filter . Getting the exact value for the column density is difficult due to the fact that the density is not constant throughout the entire dust lane. However, it is possible to calculate to within a factor of ten. So it can be said that the column density is around dust particles/.
The Reduction of the Data:
After taking the initial images, I calculated the exposure times and number of images I wanted in each filter. I then calibrated the images setting the calibration to take out the bias and darks taken the night of my data, and the flat taken in the specific filter. Once the calibrated data had been saved, I took out the bleeding from the overexposed stars in the field by using the Remove Bloom function in MaxIM. I, then, created a colored image in each filter, making sure they were aligned and that the output was set to Median. Then, I combined the color images after each filter was aligned with the others. I used the color combine in the LRGB filter option. I assigned the clear filter to the luminance slot, the red filter for the red slot, the visual filter for the green slot, and the blue filter for the blue slot. For the color balance, I had the red filter at a value of 1, the green filter at a value of 1.6, and the blue filter at a value of 10. I did a color saturation of about 100%. Finally, I saved the color image as a jpeg and stretched the image so that the maximum and minimum values were 27404 and 50, respectively, and a gamma value of 0.4. I decided to do another color saturation of around 350% after the screen stretch to bring out more of the color.
"NGC 5866/ Messier 102 ?" Society for the Exploration and Development of Space. <http://www.seds.org/messier/m/ngc5866.html>.
Kutner, Marc L. Astronomy: A Physical Perspective. 2nd ed. Cambridge, UK: University Press, 2003.
Pellegrini, S. "ROSAT PSPC observation of the X-ray faint early-type galaxy NGC 5866." Astronomy and Astrophysics 292.2 (1994): 395-403. 23 Apr. 2009 <http://adsabs.harvard.edu/abs/1994A%26A...292..395P>.
NASA/IPAC Extragalactic Database. NASA, IPAC, Jet Propulsion Lab, Caltech. 23 Apr. 2009 <http://ned.ipac.caltech.edu/cgi-bin/nph-imgdata?objname=NGC+5866>
"2560KB FITS from 2MASS 1.3m Image in 2.2 um." NASA/IPAC Extragalactic Database. NASA, IPAC, Jet Propulsion Lab, Caltech. 23 Apr. 2009 <http://ned.ipac.caltech.edu/img7/LGA/2MASS_NGC_5866_K.gif>.
" 121KB JPG from GALEX." NASA/IPAC Extragalactic Database. NASA, IPAC, Jet Propulsion Lab, Caltech. 23 Apr. 2009 <http://ned.ipac.caltech.edu/img15/GALEX_Atlas/NGC_5866:I:FUV:g2006.jpg>
This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.
red(R), and clear(C)
7 X 300 seconds in C
16 X 300 seconds in B
11 X 300 seconds in
6 X 300 seconds in
February 19, 2009 (CBVR)
March 2, 2009 (C)
March 14, 2009 (CB)
March 15, 2009 (VR)