Astronomical Observatory: Cool Images

Astr212 Galaxy Projects, Spring 2007

M58, Jessie Taylor

Galactic superclusters, gravitationally bound groups of hundreds of thousands of galaxies, dominate the macroscopic structure of the universe. Some of the largest structures known, they make patterns of filaments and voids throughout space. Our galaxy, the Milk Way, is just one member of the myriads of galaxies making up the Virgo Supercluster. At the center of this supercluster is the Virgo Cluster, a smaller cluster of approximately 2000 galaxies. M58, the galaxy pictured above, is one of the brightest members of the Virgo Cluster.

Located 60 million light years away, M58 has a diameter of 41 thousand light years, close to a third of the size of the Milky Way. To give you a sense of the scale of the image, the picture is 326 arcsec by 347 arcsec, which at the distance of M58 translates into 95 thousand light years by 101 light years.

Looking at the picture, you can see a bright bar of light running from the upper left of the galaxy to the lower right. The spiral arms start from the end of the bar and then curve around. The presence of the bar is what gained M58 its classification as a barred spiral galaxy, a classification it shares with roughly a third of the other spiral galaxies.

Also visible in the image above is a bluer region slightly up and to the right of the center of the galaxy. Since young stars are hotter and thus bluer than their older and cooler compatriots, the blueness of this area may indicate a star forming region. In such an area, interstellar gas would be gradually accreting into denser regions and those regions would continue to collapse in on themselves until dense enough to trigger fusion, igniting the new stars.

At the center of M58 resides a super massive black hole. Its mass has been estimated to be 6±1x10⁷ solar masses. Naturally, such a black hole, since it emits no light, cannot be observed directly. However, as matter rotates around the black hole, falling inward, it emits large amounts of radiation. This radiation is what is observed as a bright peak in the center of x-ray images (see the image below), not the optical image above. Galaxy like this are described as having Active Galactic Nuclei.

Light Profile

The image above also provides us with more quantitative information about M58. The brightness of the galaxy can be graphed as a function of the distance from the center of the galaxy. This is done, along several axes, in the graphs below. In these graphs, the brightness sharply increases as the radius goes to zero. This is because the core, or bulge, of the galaxy is so much brighter than the arms. The brightness decreases in a more linear fashion once we have moved away from the bulge. However, there is another bend in the light profile curve for the minor axis; this is due to the bar. Light profiles are compared by scale length, the radial distance needed for the light from the galaxy to decrease by approximately one third. The scale length for the major axis is 1.84±0.09 kpc and for the minor axis, it is 0.95±0.09 kpc. Other spiral galaxies examined by the class had scale lengths between 0.6 and 1.8 kpc, so M58 is on the very large end of the spectrum.

If we were looking at the galaxy face on, the major and minor axes would have the same scale length, since galaxies are roughly symmetric. As it is, we can use trigonometry to calculate the angle of inclination for M58. Unfortunately, such a calculation is complicated due to the influence of the bar. Using an approximation to isophotes, comparing the radii at the same brightness, the angle of inclination was found to be 44º.

All along, affects due to the bar have been mentioned. More information can be extracted from the image if we look at the light profile along the bar itself. Here is the characteristic radius is 0.954±0.0135 kpc. We can immediately see that this light profile is less smooth than for the other axes. The first bump is due to the bar ending and the second signifies some ring structure.

Other Wavelengths

	This image includes data taken both in the visible wavelengths and in the near infrared. The galaxy overall appears much redder, although the blue region is still visible. This increase in redness is due to light from older and cooler stars. The majority of the light is still concentrated in the bulge and along the arms, showing that the stars of all ages follow roughly the same distribution pattern.
Taken with Palomar1.5m by Zsolt Frei, Puragra Guhathakurta, James E. Gunn and J. Anthony Tyson
	In this image, taken in ultraviolet, the bar and central bulge almost completely disappear. Instead, a ring like structure becomes apparent. Ultraviolet light should highlight regions of star formation and younger stars. Note the greater brightness of the blue spot. This images implies that most of the younger stars are located away from the core and in the edges of the galaxy.
Taken with Galex, by A. Gil de Paz, et al.
	This image is taken in x-ray, with the curves showing lines of constant brightness. The contrast is reversed so brighter objects are darker. Here we can see none of the spiral or bar structure so prominent in other wavelengths. Instead, the galaxy is dominated by a bright point source at the center of the galaxy. This point source is the Active Galactic Nucleus and we are seeing the x-ray emission due to the matter falling into the central black hole.
Taken with the Einstein Observatory by G. Fabbiano, D.-W. Kim, and G. Trinchieri

Data Reduction

The images used to produce the main image above were taken with the 16 in telescope at the Calvin Rehoboth Remote Observatory. Data was taken in four filters: clear (C), blue (B), red (R) and green (V). Exposure times are listed below. Images were calibrated using flats, darks and bias to remove artifacts due to optical, digital and thermal effects from the camera and telescope. Next all the images in each filter were median combined in order to remove additional artifacts in individual images and to enhance faint structure. The images were then color combined, with the clear image used to define structure and each of the other three filters to determine the appropriate color. Color balance was adjusted to make the stars appear white. Saturation was increased to enhance the blue color of the galaxy. Finally, the gamma stretch was modified in order to both not overexposure the center of the galaxy and to bring out some of the faint structure in the arms.

References

Carroll, Bradley W., and Dale A. Ostlie. An Introduction to Modern Astrophysics. New York: Addison-Wesley Publishing Company, Inc, 1996.

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

Fabbiano, G., D.-W. Kim, and G. Trinchieri. "An X-ray Catalog and Atlas of Galaxies." Taken with the Einstein Observatory. <http://nedwww.ipac.caltech.edu/cgi-bin/nph-ex_refcode?refcode=1992ApJS...80..531F>.

Frei, Zsolt, Puragra Guhathakurta, James E. Gunn, and J. Anthony Tyson. " A Catalog of Digital Images of 113 Nearby Galaxies". Taken with Palamer 1.5. <http://nedwww.ipac.caltech.edu/cgi-bin/nph-ex_refcode?refcode=1996AJ....111..174F>.

Frommert, Hartmut, and Christine Kronberg. "Messier 58." 1999. Students for the Exploration and Development of Space. 26 Apr. 2007 <http://seds.org/messier/m/m058.html>.

Frommert, Hartmut, and Christine Kronberg. "The Virgo Cluster of Galaxies." 1999. Students for the Exploration and Development of Space. 26 Apr. 2007 <http://seds.org/messier/more/virgo.html>.

Gil de Paz, A., S. Boissier, B. F. Madore, M. Seibert, Y. H. Joe, A. Boselli, T. K. Wyder, D. Thilker, L. Bianchi, S. -C. Rey, R. M. Rich, T. A. Barlow, T. Conrow, K. Forster, P. G. Friedman, D. C. Martin, P. Morrissey, S. G. Neff, D. Schiminovich, T. Small, J. Donas, T. M. Heckman, Y. -W. Lee, B. Milliard, A. S. Szalay, S. Yi. "The GALEX Ultraviolet Atlas of Nearby Galaxies." Taken with GALEX. <http://adsabs.harvard.edu/cgi-bin/bib_query?2006astro.ph..6440G>.

Kutner, Marc L. Astronomy: A Physical Perspective. New York: Cambridge University Press, 2003.

Lewis, Karen T., and Michael Eracleous. "Black Hole Masses of Active Galaxies with Double Peaked Balmer Emission Lines." The Astrophysical Journal 642.2 (2006): 711-19.

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.