A New Spin On NGC 2403

Written by Tammy Plotner

No. You're not looking at a Hubble image. This incredibly detailed photo was taken with a 14.5" telescope from right here on the surface of planet Earth. When Allan Sandage turned the Hale telescope its way, he discovered the first Cepheid variables beyond our local galaxy group. At the time he concluded its distance as about 8,000 light years away, but today it is believed to be as distant as 8,000,000. What's its name? NGC 2403...

Discovered in 1788 by Sir William Herschel, this intermediate spiral galaxy is part of the M81/M82 group... and like its contemporaries, is a product of a galaxy merger. Its northern spiral arm connects to NGC 2404 - riddling the halo with young stars. In this masterful astrophoto done by Warren Keller, the pink and red regions denote active star formation, while clusters of neophyte suns gather in the blue OB associations. Like a fine piece of Irish lace, holes appear where dark regions of dust block the light. But NGC 2403 doesn't follow the rules. Here the galaxy's arms rotate at a different speed.

"High sensitivity H I observations of the nearby spiral galaxy NGC 2403 obtained with the VLA are presented and discussed. The properties of the extended, differentially rotating H I layer with its H I holes, spiral structure and outer warp are described. In addition, these new data reveal the presence of a faint, extended and kinematically anomalous component. This shows up in the H I line profiles as extended wings of emission towards the systemic velocity. In the central regions these wings are very broad (up to 150 km/s) and indicate large deviations from circular motion." says F. Fraternali (et al). "We have separated the anomalous gas component from the cold disk and have obtained for it a separate velocity field and a separate rotation curve. The mass of the anomalous component is 1/10 of the total H I mass. The rotation velocity of the anomalous gas is 25-50 km/s lower than that of the disk. Its velocity field has non-orthogonal major and minor axes that we interpret as due to an overall inflow motion of 10-20 km/s towards the centre of the galaxy. The picture emerging from these observations is that of a cold H I disk surrounded by a thick and clumpy H I layer characterized by slower rotation and inflow motion towards the center. The origin of this anomalous gas layer is unclear. It is likely, however, that it is related to the high rate of star formation in the disk of NGC 2403 and that its kinematics is the result of a galactic fountain type of mechanism. We suggest that these anomalous H I complexes may be analogous to a part of the High Velocity Clouds of our Galaxy."

Does this different rotational curve have an cosmological implications? According to the work of E. Battaner and E. Florido: "We review the topic of rotation curves of spiral galaxies emphasizing the standard interpretation as evidence for the existence of dark matter halos. Galaxies other than spirals and late-type dwarfs may also possess great amounts of dark matter, and therefore ellipticals, dwarf spirals, lenticulars and polar ring galaxies are also considered. Furthermore, other methods for determining galactic dark matter, such as those provided by binaries, satellites or globular clusters, have to be included. Cold dark matter hierarchical models constitute the standard way to explain rotation curves, and thus the problem becomes just one aspect of a more general theory explaining structure and galaxy formation. Alternative theories also are included. In themagnetic model, rotation curves could also be a particular aspect of the whole history of cosmic magnetism during different epochs of the Universe."

Yet on the other hand, perhaps the differing rotations were caused by the merger itself - with no dark matter involved. "Quite a point has been made about deviations of some galaxies from flat rotation curves, specifically the decreased velocity in outer parts of the curves. Such cases can be explained under the diffusion model by considering collisions and tidal interactions between galaxies. In this explanation, the excess gravitational force is considered to be caused by a “cloud” of the agent that carries gravitational force that always is diffusing freely, although more concentrated in some regions than others as a result of the time required for the diffusion process and the size of the regions involved." says Roy J. Britten. "When tidal interactions have occurred between galaxies, some momentum could be transferred between stars, gas, and dust that would not be shared by the diffusing clouds, and therefore, asymmetries in the gravitational forces would result. For example, the cloud and galaxies could separate if the two galaxies merged because the galaxies would share their momentum and the clouds would remain independent and continue to diffuse. Then, new gravitational clouds would be built slowly by diffusion from the merged galaxy."

Dark matter or no dark matter, NGC 2403 (07h 36m 51.4s, +65° 36′ 09″) is a pleasure to observe. Located in the northern constellation of Camelopardalis, this 8.4 magnitude spiral galaxy can be spotted under dark sky conditions with ordinary 10X50 binoculars. In 1954 Fritz Zwicky reported a supernova event and 50 years later it happened again, keeping astronomers wondering about this galaxy with the low-luminosity "dwarf" Seyfert nucleus. SN2004 is the bright yellow "star" in this portrait and it is the closest - and brightest - stellar explosion discovered in more than a decade...

As close as your eyepiece on the next dark night!

Many thanks to Warren Keller of Billions and Billions for sharing his incredible work!