Mass modelling with minimum kinematic information
Abstract
Mass modelling of earlytype systems is a thorny issue; even for the few close by galaxies for which kinematic data are available, the implementation of this data can get embroiled in problems that are hard to overcome, unless more complete data sets are available. The massanisotropy degeneracy is a typical example of this. In this paper, we present a new mass modelling formalism for ellipticals that invokes no other observations other than the central velocity dispersion (σ_{0}) and photometry. The essence of the method lies in choosing a local masstolight ratio (M/L) profile for a galaxy, with which the deprojected luminosity density distribution (along the major axis coordinate x) is scaled. The resulting discontinuous mass density profile is then smoothed, according to a laid out prescription; the local M/L profile that stems from this smoothed mass density, is found to be significantly different from the raw M/L distribution. A suite of model galaxies (both Sersic and cored in nature) is used for intensive experimentation in order to characterize this raw M/L profile and in each case, the mass density recovered from this mass modelling technique is compared to the known mass distribution. We opt to work with a raw M/L profile that is a simple twostep function of x, with a low inner and higher outer value of M/L  Υ_{in} and Υ_{out}, respectively. The only constraint that we have on this profile is in the centre of the galaxy, via σ_{0}. This value of σ_{0} is implemented in the virial theorem to obtain an estimate of the central M/L ratio of the galaxy. The fallibility of the virial mass estimate is taken care of, by allowing for a range in the values of Υ_{in} that can be used for a given galaxy model. Moreover, our experiments indicate that Υ_{out} is uniquely known, for a given Υ_{in} for cored galaxies, this functional form is found uniquely dependant on the core radius. The physical basis for such a connection to exist between the inner and outer M/L is discussed. The jump radius of the raw M/L profile is chosen to be about thrice the effective radius of the galaxy. In this way, the local M/L distribution is completely specified and mass profiles of ellipticals can be constructed till about three times the effective radius. The proposed technique is extended to predict a mass profile for M87 which is then successfully compared to distributions reported earlier from kinematic analyses.
 Publication:

Monthly Notices of the Royal Astronomical Society
 Pub Date:
 May 2007
 DOI:
 10.1111/j.13652966.2007.11583.x
 arXiv:
 arXiv:astroph/0702065
 Bibcode:
 2007MNRAS.377...30C
 Keywords:

 galaxies: kinematics and dynamics;
 galaxies: structure;
 stellar dynamics;
 celestial mechanics;
 Astrophysics
 EPrint:
 9 figures, accepted for publication in MNRAS