F. J. Sánchez-Salcedo,1 A. M. Hidalgo-Gámez,2 and Eric E. Martínez-García3
1 Instituto de Astronomía, Universidad Nacional Autónoma de México, Apdo. Postal 70-264, C.P. 04510, México, D.F., México (jsanchez@astro.unam.mx).
2 Departamento de Física, Escuela Superior de Física y Matemáticas, Instituto Politécnico Nacional, U.P. Adolfo López Mateos, C.P. 07738, México, D.F., México (ahidalgo@esfm.ipn.mx).
3 Instituto Nacional de Astrofísica, Óptica y Electrónica, Apdo. Postal 51 y 216, C.P. 72000, Puebla, México (ericmartinez@inaoep.mx).
Received 2014 February 6. ]]> Accepted 2014 March 31.
RESUMEN
Damos restricciones al ángulo de inclinación del disco de H I de la galaxia irregular enana Holmberg II (Ho II) a partir de un análisis de la estabilidad gravitacional del disco de gas en las partes externas. Encontramos que se requiere un ángulo de inclinación medio de 27° y, por lo tanto, una velocidad circular en su parte plana de ≈60 km s-1, para tener un grado de estabilidad similar al que tienen otras galaxias. Para esa inclinación, Ho II cae en la posición correcta en la relación de Tully-Fisher bariónica y, además, su curva de rotación es congruente con MOND. Sin embargo, el análisis de estabilidad correspondiente indica que esta galaxia podría ser problemática para MOND porque las partes externas de esta galaxia serían marginalmente inestables bajo esa teoría de gravedad. Se requieren simulaciones numéricas de las galaxias enanas ricas en gas para ver la factibilidad de MOND.
ABSTRACT
We provide constraints on the inclination angle of the H I disk of the dwarf irregular galaxy Holmberg II (Ho II) from a stability analysis of the outer gaseous disk. We point out that a mean inclination angle of 27° and thus a flat circular velocity of ≈60 km s-1, is required to have a level of gravitational stability similar to that found in other galaxies. Adopting this inclination angle, we find that Ho II lies on the right location in the baryonic Tully-Fisher relation. Moreover, for this inclination, its rotation curve is consistent with MOND. However, the corresponding analysis of the stability under MOND indicates that this galaxy could be problematic for MOND because its outer parts are marginally unstable in this gravity theory. We urge MOND simulators to study numerically the non-linear stability of gas-rich dwarf galaxies since this may provide a new key test for MOND.
Key Words: dark matter — galaxies: individual (Holmberg II) — galaxies: kinematics and dynamics — gravitation.
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]]>ACKNOWLEDGMENTS
The authors are indebted to the referee for a thoughtful report. The authors made use of THINGS 'The Hi Nearby Galaxy Survey' (Walter et al. 2008). This work was supported by the following projects: CONACyT 165584, PAPIIT IN106212 and SIP-20141379.
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