Object Name: Messier 99
Alternative Designations: M99, NGC 4254, Pinwheel Galaxy
Object Type: Type Sc Spiral Galaxy
Constellation: Coma Berenices
Right Ascension: 12 : 18.8 (h:m)
Declination: +14 : 25 (deg:m)
Distance: 60000 (kly)
Visual Brightness: 9.9 (mag)
Apparent Dimension: 5.4×4.8 (arc min)
Locating Messier 99: As part of the Virgo Cluster of Galaxies, M98 is best found by returning to our “galaxy hopping” ways we’ve learned. Begin with the bright M84/84 pairing located in the heavily populated inner core of the Virgo Cluster of galaxies about halfway between Epsilon Virginis and Beta Leonis. Once identified, stay at the eyepiece a move your telescope north until you locate M99. This face-on presentation will look like a round hazy patch to small optics and begin revealing its spiral arm pattern with mid-sized telescopes under dark skies.
What You Are Looking At: What’s in an Sc designation when it comes to a spiral galaxy? It means its rotating counterclock-wise. While that sounds very normal, you’ll also notice that M99’s mass seems to be just a little “off center”. What’s going on here? Let’s turn to the research of Victor P. Debattista and J. A. Sellwood: “We show that bars in galaxy models having halos of moderate density and a variety of velocity distributions all experience a strong drag from dynamical friction unless the halo has large angular momentum in the same sense as the disk. The frictional drag decreases the bar pattern speed, driving the co-rotation point out to distances well in excess of those estimated in barred galaxies. The halo angular momentum required to avoid strong braking is unrealistically large, even when rotation is confined to the inner halo only. We conclude, therefore, that bars are able to maintain their observed high pattern speeds only if the halo has a central density low enough for the disk to provide most of the central attraction in the inner galaxy. We present evidence that this conclusion holds for all bright galaxies.”
But what if it wasn’t just the galaxy itself, but a chance merger? “We present high-resolution H I and H? observations of the spiral galaxy NGC 4254. The observations were obtained with the VLA and the Maryland-Caltech Fabry-Perot camera, respectively. NGC 4254 is unusual in having a grand-design spiral structure with a strong m = 1 component for which there is no obvious cause in optical images. Our observations reveal that, in addition to the usual galactic disk component, there are H I clouds superposed on and beyond the H I disk, at velocities up to 150 km s^-1^ from those established for the disk. The mass in these clouds is ~2.3 x 10^8^ M_sun_, and they may be the remnants of an entity that was tidally disrupted by NGC 4254 and is now merging with it. The direct effects of the interaction between the cloud gas and the galaxy are limited to the region where the gas appears to be merging with the disk, where it may be causing a warp.” says Yuichi Terashima (et al).
“But the indirect effects of the infalling gas appear profound: it is the most likely cause for the unusual spiral structure of NGC 4254. If so, the m = 1 spiral structure of NGC 4254 is recent, and an internal amplification mechanism such as swing amplification has played a major role in its evolution. Since NGC 4254 does not appear to be exceptionally deficient in dark matter and is apparently a normal Sc galaxy, the nature of the interaction appears important in determining the susceptibility of the disk to various spiral modes (in particular the m = 1,3, and 5 modes of NGC 4254).”
Spiral modes, huh? T. Kranz (et al) knows a lot about that, and before there can be stars there has to be the material to make them – gas. “As a pilot project, we analyzed the data of NGC 4254 (M99). Assuming a constant stellar mass to light ratio, the gravitational potential due to the stellar mass fraction was calculated by direct integration over the whole mass distribution taken from the NIR-image. The mass to light ratio for the maximum disk contribution was scaled by the measured rotation curve. For the dark matter contribution we assumed an isothermal halo with a core. To combine the two components we chose a stellar mass fraction and added the halo with the variable parameters adjusted to give a best fit to the rotation curve.” says Kranz, “We used this potential as an input for the hydrodynamical gas simulations. Figure 2 presents the results for the resulting gas surface density, as it settles in the potenital. The morphology of the gas distribution is very sensitive to the velocity, with which the spiral pattern of the galaxy rotates (pattern speed).”
“Determining individual mass fractions of the luminous and dark matter is not a straightforward task. The rotation curve of a disk galaxy is only sensitive to the total amount of gravitating matter, but does not allow to distinguish the two mass density profiles,” continues Kranz. “Here we would like to exploit the fact, that the stellar mass in disk galaxies is often organized in spiral arms, thus in clearly non-axisymmetric structures.”
“On the other hand, in most proposed scenarios, the dark matter is non-collisional and dominated by random motions. It is not susceptible to spiral structures and distributed like the stars in elliptical galaxies. If the stellar mass dominates, the arms could induce considerable non-circular motions in the gas, which should become visible as velocity wiggles in observed gas kinematics. Using hydrodynamical gas simulations we are able to predict these velocity wiggles and compare them to the observations. Hence the contribution of the perturbative forces with respect to the total forces can be determined quantitatively and can be used to constrain the disk to halo mass ratio.”
History: M99 was discovered on March 15, 1781 by Messier’s colleague and friend, Pierre Mechain, together with the nearby situated M98 and M100. Charles Messier measured its position and included it in his catalog on April 13, 1781. In his notes he writes: “Nebula without star, of a very pale light, nevertheless a little clearer than the preceding [M98], situated on the northern wing of Virgo, and near the same star, no. 6, of Comae Berenices. The nebula is between two stars of seventh and of eighth magnitude. M. Mechain saw it on March 15, 1781.”
While M99 would be observed by both William and John Herschel, it would be Lord Rosse who finally brought it to light. Even though he didn’t truly understand the nature of what he was looking at, he was fascinated with knowing it had a spiral structure and M99 became his second “confirmed kill”. In his notes he writes: “In the following spring [of 1846] an arrangement, also spiral but of a different character [than in M51], was detected in 99 Messier, Plate XXXV. fig 2. This object is also easily seen, and probably a smaller instrument, under favourable circumstances, would show everything in the sketch.”
Top M99 image credit, Palomar Observatory courtesy of Caltech, M99 2MASS image, M99 by Hunter Wilson, M99 Spitzer images, M99 courtesy of Ole Nielsen, Rosse’s historical M99 sketch and M99 image courtesy of NOAO/AURA/NSF.