Discovery of a slow bar in a spiral galaxy
1er avril 2009
Since more than a decade, kinematical observations of spiral galaxies have shown that the stellar bars seen in their centre are fast rotators. These measurements have been mostly obtained for bright early-type spirals, such as lenticular galaxies (S0). New observations perfomed by an international team, led by an astronomer from the Observatoire de Paris, report the discovery of a slow stellar bar in another type of galaxy, a late-type spiral galaxy (Sm). The observed target (UGC 628) has the particularity to be also a low surface brightness disc whose mass distribution is fully dominated by the dark matter.
Bars are density waves which rotate at constant angular speed in a disk galaxy, whereas the speed of stars varies as a function of the distance to the center : very fast in the center, and slower at the edge. The bar thus rotates more slowly that the stars at the same radius, and cannot extend beyond the radius of corotation, where it turns at the same speed as the stars. On the other hand, simulations predict that the bar could stop much before corotation, either because of a late galaxy morphology without bulge, or because of the braking of the dark matter. But rare are these cases of slow bars in the observations, and the theory could not yet be entirely confirmed.
UGC 628 (Figure 1) has been observed by means of optical Fabry-Perot interferometry in the Halpha emission line of the ionized gas with the instrument FaNTOmM, which was mounted on the Canada-France-Hawaii Telescope. The angular velocity of the bar (Omega-p) has been derived by applying the Tremaine-Weinberg method (TW), which is the only observational method that enables to directly measure a bar pattern speed, with no need of modeling the observations.
- Figure 1 : Galaxie UGC 628 : image en bande large (à gauche), carte d’émission Halpha (au centre), champ de vitesse Halpha (à droite). Sur l’image de gauche, la barre est facilement identifiable au centre de la galaxie par une surbrillance du disque stellaire. Cliquer sur l’image pour l’agrandir
A bar is fast when the ratio of its corotation radius (Rc, where its speeds equals that of stars in the disk) to its semi-length (Rb, or bar radius) is within 1.0 and 1.4 : 1.0 < Rc/Rb 1.4 is slow. Until now, the TW method has only identified pattern speeds consistent with bars being fast (in bright early-type galaxies). It is measured Omega-p = 11.3 km/s/kpc for UGC 628 (Figure 2). This amplitude is the lowest observed for a stellar bar up to now. It implies a ratio Rc/Rb = 2.0, and thus a slow bar in UGC 628.
- Figure 2 : Mesure de la vitesse angulaire de rotation de la barre stellaire de UGC 628 par la méthode TW. La vitesse angulaire est la pente de la relation (droite tracée en trait continu) ajustée à partir des points de mesures cinématiques dans la région de la barre (symbolisés par des carrés pleins). Les symboles ouverts sont les points de mesure en dehors de la barre. Ils sont ajustés par la droite en pointillés, dont la pente correspondrait à la vitesse de la spirale, en dehors de la barre. Cliquer sur l’image pour l’agrandir
Though it still remains an isolated case, the discovery of a slow bar in a Sm galaxy seems to point out a correlation between the bar pattern speed and the morphological type of galaxies : the angular velocity decreases from early types towards late types of galaxies. Moreover, the dominating mass of the dark matter
over the mass distribution of UGC 628 likely contributes to slow down
more efficiently the bar rotation.
That discovery is in full agreement with results from many numerical simulations
showing the evolution of barred galaxies.
Reference A slow bar in a dark matter dominated galaxy Laurent Chemin & Olivier Hernandez A&A Letter, in press
Contact
Laurent Chemin (Observatoire de Paris, GEPI et CNRS)
Dernière modification le 4 mars 2013