Messier 108

Object Name: Messier 108
Alternative Designations: M108, NGC 3556
Object Type: Sc Spiral Galaxy
Constellation: Ursa Major
Right Ascension: 11 : 11.5 (h:m)
Declination: +55 : 40 (deg:m)
Distance: 45000 (kly)
Visual Brightness: 10.0 (mag)
Apparent Dimension: 8×1 (arc min)

Locating Messier 108: M108 is easily located about one quarter the distance between Beta Ursa Majoris and Gamma Ursa Majoris… but locating doesn’t mean it’s always easily seen! At nearly edge-on in presentation, this mottled streak of light is a rather difficult catch for smaller telescopes and requires good, dark sky to see any details. Larger instruments will make out both faint and bright patches in structure.

What You Are Looking At: Located about 45 million light years away from Earth and running away from us at 772 kilometers per second, this disturbed looking galaxy is rich in dark dust, star forming regions and a supershell. “We present the first high resolution HI maps of the nearby edge-on galaxy, M 108 (NGC 3556). This galaxy is known to have a radio continuum thick disk and we have now found HI arcs and extensions protruding from the plane on kpc scales. Two HI arcs, positioned at either end of the optical major axis have the signature of expanding shells and, in the context of energy input from supernovae and stellar winds, the required input energy for the eastern shell is > 2.6 times 10^56 erg, making this the most energetic HI supershell yet detected.” says D. L. Giguere and J. Irwin.

“Since this galaxy is isolated, the supershells are unlikely to have been created through impacting external clouds, yet the required input energy is also greater than that available from the observed internal star formation rate. Thus it would appear that some form of energy enhancement (such as magnetic fields) must also be important in creating these features. The supershells are so dominant that they distort the outer major axis. Without a knowledge of the resolved structure of these features, the galaxy would mistakenly be considered warped. We have also modeled the underlying smooth density and velocity distributions of this galaxy by reproducing the line profiles in the HI cube.”

What else is unusual about Messier 108? Try a water maser that disappeared. “NGC 3556: is a nearby spiral galaxy located at a distance of 12Mpc. Its FIR luminosity, LFIR 1010 L?, is similar to that of the Milky Way. The detected H2O maser line initially had a central velocity of 738 kms?1. With a peak flux of 20–40mJy, the maser had an isotropic luminosity of 1 L. More recently, the maser feature disappeared and another weaker component, at 708 kms?1, was detected.” says A. Tarchi (et al). “The high rate of maser detections in our sample of galaxies strongly suggests that a relationship between FIR flux density and maser phenomena exists.”

What else is hiding? Perhaps an intermediate mass black hole, you say? “We present a 60 ks Chandra ACIS-S observation of the isolated edge-on spiral galaxy NGC 3556, together with a multiwavelength analysis of various discrete X-ray sources and diffuse X-ray features. Among 33 discrete X-ray sources detected within the IB = 25 mag arcsec-2 isophote ellipse of the galaxy, we identify a candidate for the galactic nucleus, an ultraluminous X-ray source that might be an accreting intermediate-mass black hole, a possible X-ray binary with a radio counterpart, and two radio-bright giant H II regions.” says Q. Daniel Wang (et al). “The diffuse X-ray emission exhibits significant substructures, possibly representing various blown-out superbubbles or chimneys of hot gas heated in massive star-forming regions. This X-ray-emitting gas has temperatures in the range of ~(2-7) × 106 K and has a total cooling rate of ~2 × 1040 ergs s-1. The energy can be easily supplied by supernova blast waves in the galaxy. These results show NGC 3556 to be a galaxy undergoing vigorous disk-halo interaction. The halo in NGC 3556 is considerably less extended, however, than that of NGC 4631, in spite of many similarities between the two galaxies. This may be due to the fact that NGC 3556 is isolated, whereas NGC 4631 is interacting. Thus, NGC 3556 presents a more pristine environment for studying the disk-halo interaction.”

History: According to SEDS, Charles Messier’s hand-written preliminary and unpublished version of his catalog, M108, similar to M109, was discovered by Pierre Méchain shortly after M97 (which he had found February 16, 1781): Méchain discovered M108 3 days after M97 on February 19, 1781, and M109 on March 12, 1781. Both objects were apparently also observed by Charles Messier when he measured the position of M97 (March 24, 1781), but apparently he didn’t find occasion to obtain positions for these objects at that time. Messier listed this object, M108, under number “98” in his preliminary manuscript version of his catalog, without giving a position.

M108 was catalog again by William Hershel in 1789, but best described by Admiral Smyth who said: “A large milky-white nebula, on the body of the Great Bear, with a small star at its sp [South Preceding, SW] apex, and an 8th-magnitude preceding [W] it at double the distance; there is also a brightish group in the np [North Preceding, NW] quadrant. It is easily found, since it lies only about 1 deg south-east of Beta, Merak. This object was discovered by H. [William Herschel] in April, 1789; and is No. 831 of his son’s Catalogue. It is faint but well defined, being much elongated with an axis-major trending sp [South Preceding, SW] and nf [north following, NE] across the parallel, and a small star, like a nucleus, in its center. As H. [WH] considers this star to be unconnected with the nebula, it follows that it is between us and it, and therefore strengthens to confirmation our belief in the inconceivable remoteness of those mysterious bodies.”

Enjoy every inch of this mysterious body!

Top M108 image credit, Palomar Observatory courtesy of Caltech, M108 Hubble Image, M108 courtesy of Ole Nielsen (Wikipedia), M108 GALEX image and M108 image courtesy of NOAO/AURA/NSF.