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Astronomical Observatory: Cool Images

Astr212 Galaxy Projects, Spring 2009

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M61 (NGC 4303), Nick Schuck

M61 spiral galaxy

(Calvin Rehoboth Telescope)

M61 also called NGC 4303 is a spiral galaxy found in the constellation Virgo. It was discovered in 1779 and was initially thought to be a comet. The name M61 was coined by French astronomer Charles Messier who in the late 1700’s created a list of deep sky objects to help comet hunters distinguish them from moving comets. Galaxies consist of massive amounts of stars, gas, dark matter and dust and are often part of a cluster of many galaxies. M61 is part of the nearby Virgo cluster of galaxies and is 60 million light-years away from us. M61 has an angular diameter 4.6 arc minutes which gives a diameter of  72,000 light years which is about half of the diameter of our galaxy, the Milky Way. It is a SAB type galaxy according to the Hubble classification system. The S stands for spiral galaxy and the AB tells us that it has a slight bar of stars and matter coming out of either side of the center of the galaxy where there is most likely a black hole hiding.

Many interesting and exciting features are visible in M61. The grand spiral arms are clearly visible in the image sweeping out and around the core.  The arms are blue in color which is due to areas of hot new stars. The gas and dust are denser in the arms and if you notice some parts of the arms are more clumpy than others.  This is where the star forming regions are in the galaxy. These clumps appear slight pinkish red which indicates clouds of molecular hydrogen where hot new stars are being born. As the gas cloud’s gravity pulls inward it becomes denser and also much hotter. As the cloud becomes hotter and denser fusion begins to occur and stars start to turn on for the first time. Areas in between the spiral arms are not as blue in color indicating much older stars that have drifted away from the star forming regions. The core of the galaxy shows a yellow-white color which comes from the large number of old stars. There is much less gas and dust towards the core of the galaxy and therefore few new stars are being born. Coming out from the core of M61 is a bar of stars which can channel gas inward from the spiral arms and may indicate it has an active galactic nucleus. Also to the upper left is a smaller galaxy NGC 4303A, a companion spiral galaxy to M61. It also has blue colored spiral arms indicating star formation.

NGC4303A galaxy

(NGC 4303A, Rehoboth Telescope)

M61 in UV

This image of M61 was taken in the far to near ultraviolet, (1516-2267 angstroms) part of the electromagnetic spectrum. Blackbody emission from hot material radiates in the UV. The ultraviolet part of the spectrum shows areas where the young hot OB stars are located in the galaxy. The spiral arms are very well defined in the image and you can see the clumps of stars that have just formed. M61 shows up very well in the UV indicating it is very active in star formation which agrees with the observation in the visible part of the spectrum.

M61 in microwave

(Schinnerer, Maciejewski, Scoville 2002)

This particular image of M61 was taken in the infrared and another image taken in CO 2.6mm (microwave) (red contours) is superimposed on top of it. The CO image shows much more detail towards the core. Microwave radiation is produced when CO molecules rotate and electrons make transitions between energy levels. Radiation in CO shows giant molecular clouds in the young thin disk of M61. Molecular clouds near the core show the bars of M61 very clearly and you can see clumps of brighter emission. There are two particular areas of higher concentration which seem to indicate the ends of the bar of M61.

Schinnerer, Maciejewski, Scoville (2002) discussed the molecular gas properties in the central parsec of the galaxy. As the spiral arm travels around the core a density wave builds up on the front of the arm.They found that most of the molecular gas emission comes from the leading edge of the dust lanes on the central bar. Here is an image taken in the CO of the core of M61 showing the intensity of the molecular clouds(high=white, low=blue). The article mentions that M61 is a double barred spiral and in this image a second bar is visible going horizontal in addition to the bar going vertical.

M61 core in microwave

An interesting calculation I performed on M61 was finding the pitch angle on two of the spiral arms. The variations in pitch angle can provide information about the origins of the spiral structure of M61. In order to perform the calculation of pitch angle I printed out a negative of my image and drew several circles on my galaxy using a compass centered on the core. I then drew lines lies tangent to the spiral arm in question and also drew lines tangent to the circle passing through that particular part of the spiral arm. By measuring the angle between the two lines I could calculate the pitch angle of that part of the spiral arm. I calculated the pitch angle for two different spiral arms and found that in one arm the angle was 36 degrees at the end of the bar and then decreased to 7 degrees along a straight segment and then rose to 18 degrees. It then makes a sharp turn and the pitch angle drops to 6 degrees and then rises back up to 18 degrees. In the other arm I found the pitch angle to decrease from about 30 degrees to 20 degrees and then to 17 degrees towards the outer part of the arm.

M61 pitch angle

M61 light profile

A light profile of a galaxy can provide some important information about the structure of the galaxy. I made it by using a Maxim DL to measure the pixel counts along a line through the galaxy. In my light profile I plotted the surface brightness on a log scale versus radius in kiloparsecs. The sharp decrease on my graph from 0 to 1pc indicates the bulge of the galaxy and then after that the light of the disk decreases exponentially shown by the constant slope line of data points. The humps in the data points indicate where the spiral arms are located. My galaxy is face on and I found the major axis to be very similar to the minor axis.


Elmegreen, Debra M. Galaxies and Galactic Structure. New Jersey: Prentice Hall, 1998.

Z. Frei & J. Gunn Princeton. "M61: Virgo Spiral Galaxy".NASA Astronomy Picture of the Day. 22 September 1998. <>

Kutner, Marc L. Astronomy: A Physical Perspective. 2nd ed. Cambridge, UK: University Press, 2003.

Schinnerer, Eva , Witold Maciejewski, and Nick Scoville. "TOWARD THE SECONDARY BAR: GAS MORPHOLOGY AND DYNAMICS IN NGC 4303." Astrophysical Journal 575.2 (2002). 30 Apr. 2009 <>.

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 SpaceAdministration. <>

Right Ascension (J2000) 12:22:6.4
Declination (J2000) 04:29:14
Filters used blue(B), green(V), red(R), and clear(C)
Exposure time per filter 19x60 seconds in C, 19x300 seconds in B, 9x300 seconds in V, 5x300 seconds in R

Date observed


February 20, 2009 (CBVR) , March 2, 2009 (C)
March 14, 2009 (BVR)

Calvin College .4m OGS Ritchey Chretien (Rehoboth, NM)

My image was processed with the Maxim DL software using dark, bias, and flat files. The dark, flat and biased files were used to remove artificial artifacts in the images. I median combined the multiple calibrated images in each filter and then color combined the 4 images in C,B,V, and R to produce a final image using values of Red=1, V(green)=1.5, and Blue=7. Using the screen stretch I used a gamma value of .45 with a min value of 35 and a max value of 1543 pixel counts.




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