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Astronomical Observatory: Cool Images

Astr212 Galaxy Projects, Spring 2007

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M66 (NGC 3627), Kathy Hoogeboom


M66 is a barred spiral galaxy 46000 light years (14 kpc) in diameter and 35,000,000 light years distant. This is about a quarter the size of the Milky Way and of course much farther away.

When Charles Messier was cataloguing all the "uninteresting" (non-comet) objects he observed, he nearly missed this one, as well as nearby M65. In 1773 a bright comet passed between these two galaxies, probably overwhelming the light from the galaxies. On March 1, 1780, he returned to the region and catalogued both.
Leo Triplet

M65 and M66 comprise two thirds of the Leo Triplet, the dominant forces in the M66 group of galaxies. The Triplet takes up about one square degree in the sky. M66 shows up in the lower left, M65 in the lower right (nearly 200,000 light years away) and edge-on NGC 3628 in the upper left. The proximity of these galaxies suggest important gravitational interaction between them, which can cause interesting shape evolution, like warped spiral disks.

[Image from REU program/NOAO/AURA/NSF]

Three supernovae have appeared in M66. Astronomers like to watch these because they can sometimes be used to find distances to faraway objects or to study star and remnant evolution and formation. Type II supernovae were observed here in December 1973 and April 1997. A Type Ia supernovae was seen in January-February 1989.

The bright center of the galaxy is called the bulge. It represents a concentration of stars and likely a black hole in the middle. Most of the light-producing mass (stars) of the galaxy is accounted for here. The elongated shape of M66's bulge suggests the presence of a bar in this spiral. The spiral arms in the disk around the center are thought to arise from density waves caused by the gravity of the material in the disk. According to this model, the galaxy formed with a disk of material mostly uniform in density. Through time or disruption from an outside gravitational force, some of the material began to clump together, creating denser regions. As material continued to orbit the galactic center, the dense regions slowed it down for a time like a slow-moving semi on a highway causing a backup in traffic. In this way the dense region could remain dense even as the material moved on and other material took its place.

The increase in density in the arms can compact the dust and clouds of molecules orbiting the central bulge enough to spark star formation. This is why bright blue regions are particularly apparent in the spiral arms, especially on the leading edge. Blue for a star means it is hot. But stars that burn hotter use up their material faster than cooler stars, so we know blue stars must be relatively young. Therefore we know the blue regions in the image represent regions of star formation. The dark lanes in the arms, especially noticeable on the sides opposite the blue regions, are caused by dust which absorbs the light coming from behind it.

M66 is classified as an SAB(s)b LINER Sy2 galaxy. The odd series of letters tells a lot about its structure, particularly relative to other galaxies. SAB means M66 is an "ovally distorted barred spiral." (s) tells us that the spiral arms come out from the central bulge rather than from a ring around the bulge. b suggests a moderately sized bulge and moderately tightly wound spiral arms. LINER (low ionization nuclear emission line regions) refers to the prominence of emission lines from ionized gas clouds near the central black hole. Sy2, or Seyfert 2, characterizes a mildly active galactic nucleus.

M66 Tail


A long tail of material trails off the top and to the right of M66, toward nearby M65. As part of the Leo Triplet, it is likely that this tail and other distortions in the disk of M66 are the results of interaction with the other nearby galaxies. This tail is not easy to find--it is only .016 magnitudes brighter than the background. But it is quite large: about 6.4 kpc long (21,000 light years).



To further study the structure revealed by the images of a galaxy, we can use light profiles to closely examine how brightness per pixel varies across the galaxy.

M66 Light Profile - Bar

This plot examines the brightness as it changes along the bar of M66. It is brightest at the center of course, but the way in which it slopes off from there yields lots of interesting information. The flattening of the curve just outside the center confirms that this spiral has a flat bar. The peak soon thereafter represents the spiral arm where light-producing material is more concentrated than between the arms. The brightness then drops off along a mostly straight line on this plot. Since the y-axis displays the logarithm of the brightness values, this represents an exponential decay. The orange line represents a best-fit line for this decay with equation: ln (counts per pixel) = -.74 (+/- .02) x radius + 7.24 (+/- .06).

The constants in this equation correspond to two characteristic values used to describe galaxies with terms that are standard for all astronomers. The slope of the line is the negative inverse of the scale length. It quantifies at what radius the brightness has fallen to 1/e times its original value. Essentially, it describes how quickly brightness falls off from the center of the galaxy. For M66, the scale length was found to be 1.58 +/- .042 kpc. While lower than the typical values for spirals given by Elmegreen, this fits well into the range found by the Astr 212 Spring 07 class, and in fact on the high side.

With the assumption that the galaxy is roughly circular, comparing the scale length found for the major and minor axes can indicate how much the galaxy is tilted with respect to our line of sight--its inclination. A galaxy with 0 or 180 degree inclination would be observed face on; 90 degrees describes an edge-on galaxy. M66 has an inclination of angle of about 68 degrees.

M66 Light Profile Tail

Zooming in on the light profile near the background level, the tail which extends off the top of M66 can be observed: it is the knee which appears on the right but not on the left.

These images explore only optical wavelengths. Much more can be learned by examining images taken at longer or shorter wavelengths. Various emissions indicate the presence of certain materials or certain processes within the galaxy, and it is important to observe where these can be found.

M66 Infrared

[Image from NASA/JPL-Caltech/R, Kennicutt (University of Arizona) and SINGS team]

This image taken with the Spitzer infrared camera provides a beautiful display of glowing red disk and bright blue center These colors are opposite those in the optical images because they represent different wavelengths. Blue indicates emission at 3.6 microns, green corresponds to 4.5 microns, red to 5.8 to 8.0 microns. Put together, the blue now highlights a concentration of older stars while red indicates regions where the dust has been heated by hot, young stars to the point of infrared blackbody emission. Since this should correspond to optical blue hot stars, the structure visible in this image lines up very well with the blue optical image.


M66 Radio

This radio image from the VLA shows a very significant departure from the optical and infrared images. There are no spiral arms apparent here. Instead, the central bar is visible as the peak radio emission because of a concentration of neutral hydrogen emitting at 21 cm. Neutral hydrogen has two possible spin states. The proton and electron can either have their spins aligned (somewhat analogous to a planet and its moon spinning the same directions on their axes) or anti-aligned. The second represents a lower energy state, so the switch between the two requires a release of energy. The energy is released by emission of a 21 cm wavelength photon, which radio telescopes detect. The very closely spaced contour lines show how very quickly the radio emission falls off outside the bar.


M66 X-ray

An x-ray image overlayed with an optical image also has a few interesting features. The point sources (indicated by the small, highly contoured spots) may indicate x-ray binary stars or supernova remnants. The central bright emission highlights the active galactic nucleus.

[Image from "BeppoSAX observations of LINER-2 galaxies", Georgantopoulos, et al.]



"Messier 66" - Students for the Exploration and Development of Space

"NGC 3627" - Spitzer

Georgantopoulos, I., F. Panessa, A. Akylas, A. Zezas, M. Cappi, A. Comastri. "BeppoSAX observations of LINER-2 galaxies." Astronomy and Astrophysics. Vol. 386, p. 60-68. 2002.

Elmegreen. Galaxies and Galactic Structure. Prentice Hall. Upper Saddle River, NJ: 1998.

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.


Right Ascension (J2000) 11:20:14.4
Declination (J2000) +12:59:42
Filters used blue(B), green(V), red(R), and clear(C)
Exposure time per filter

18x60 seconds in C,
11x300 seconds in B,
6x300 seconds in V
3x300 seconds in R,

Date observed

February 21, 2007 (CBV)
March 13, 2007 (CBVR)

Data Reduction:

The raw images were calibrated using median-combined bias and dark files from the same night of appropriate exposure time and binning, and flat files for the appropriate filter and binning. A bloom from the star to the upper right of the galaxy was removed using Maxim's automatic tool. All the images from a single filter were aligned using the Manual 2 Star align mode and median-combined to create a "best image" from each filter. The four best images were then aligned in a similar manner and then color-combined with clear image for L, red filter image for R, visual filter image for G and blue filter image for B. Color balance values were best at R:1, G:1.7, B:10 with a saturation of 190 percent. Finally, the image was converted to jpg using a Gamma value of .7.




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