Total Mass Of NGC Research Articles

KINEMATICAL FOCUS ON NGC 7086

The main physical parameters; the cluster center, distance, radius, age, reddening, and visual absorbtion; have been re-estimated and improved for the open cluster NGC 7086. The metal abundance, galactic distances, membership richness, luminosity function, mass function, and the total mass of NGC 7086 have been examined for the first time here using Monet et al. (2003) catalog.

Spatial dependence of 2MASS luminosity and mass functions in the old open cluster NGC 188

Luminosity and mass functions in the old open cluster NGC 188 are analysed by means of J and H 2MASS pho- tometry, which provides uniformity and spatial coverage for a proper background subtraction. With an age of about 6-8 Gyr, NGC 188 is expected to be suffering the effects of advanced dynamical evolution. Indeed, previous works in optical bands have suggested the presence of mass segregation. Within the uncertainties, the observed projected radial density profile of NGC 188 departs from the two-parameter King model in two inner regions, which reflects the non-virialized dynamical state and possibly, some degree of non-sphericity in the spatial shape of this old open cluster. Fits with two and three-parameter King models to the radial distribution of stars resulted in a core radius Rcore = 1.3 ± 0.1 pc and a tidal radius Rtidal = 21 ± 4 pc, about twice as large as the visual limiting radius. The concentration parameter c = 1.2 ± 0.1 of NGC 188 makes this open cluster structurally comparable to the loose globular clusters. The present 2MASS analysis resulted in significant slope variations with distance in the mass function φ(m) ∝ m −(1+χ) , being flat in the central parts (χ = 0.6 ± 0.7) and steep in the cluster outskirts (χ = 7.2 ± 0.6). The overall mass function has a slope χ = 1.9 ± 0.7, slightly steeper than a standard Salpeter mass function. In this context, NGC 188 is similar to the 3.2 Gyr, dynamically evolved open cluster M 67. Solar metallicity Padova isochrone fits to the near- infrared colour-magnitude diagram of NGC 188 resulted in an age of 7.0 ± 1.0 Gyr. The best fit, obtained with the 7.1 Gyr isochrone, produced a distance modulus (m − M)0 = 11.1 ± 0.1, E(B − V) = 0.0, and a distance to the Sun d� = 1.66 ± 0.08 kpc. The observed stellar mass (in the range 0.98 M� −1.08 M� ) in NGC 188 is mobs = 380 ± 12 M� . A simple extrapolation of the observed overall mass function to stars with 0.08 Mresulted in a total present mass of mtot ∼ (1.8 ± 0.7) × 10 4 M� .O n the other hand, for a more realistic initial mass function which flattens in the low-mass range, the total mass in NGC 188 drops to mtot ∼ (3.8 ± 1.6) × 10 3 M� . Since mass-loss processes such as evaporation and tidal stripping have been occurring in this old open cluster for about 7 Gyr, the primordial mass in NGC 188 must have been significantly larger than ∼4 × 10 3 M� .W e also examined the consequences of the presence of unresolved binaries and concluded that, even if dominant in the CMD, binaries alone are not responsible for the flat central mass function, which supports the mass-segregation scenario.

Breaking the degeneracy between anisotropy and mass: the dark halo of the E0 galaxy NGC 6703

(abridged) We have measured line-of-sight velocity profiles (VPs) in the E0 galaxy NGC 6703 out to 2.6 R_e. From these data we constrain the mass distribution and the anisotropy of the stellar orbits in this galaxy. We have developed a non-parametric technique to determine the DF f(E,L^2) directly from the kinematic data. From Monte Carlo tests using the spatial extent, sampling, and error bars of the NGC 6703 data we find that smooth underlying DFs can be recovered to an rms accuracy of 12%, and the anisotropy parameter beta(r) to an accuracy of 0.1, in a given potential. An asymptotically constant halo circular velocity v_0 can be determined with an accuracy of +- \lta 50km/s. For NGC 6703 we determine the true circular velocity at 2.6 R_e to be 250 +- 40km/s at 95% c.l., corresponding to a total mass in NGC 6703 inside 78'' (13.5 h_50^-1 kpc), of 1.6-2.6 x 10^11 h_50^-1 Msun. No model without dark matter will fit the data; however, a maximum stellar mass model in which the luminous component provides nearly all the mass in the centre does. In such a model, the total luminous mass inside 78'' is 9 x 10^10 Msun and the integrated M/L_B=5.3-10, corresponding to a rise from the center by at least a factor of 1.6. The anisotropy of the stellar distribution function in NGC 6703 changes from near-isotropic at the centre to slightly radially anisotropic (beta=0.3-0.4 at 30'', beta=0.2-0.4 at 60'') and is not well-constrained at the outer edge of the data. Our results suggest that also elliptical galaxies begin to be dominated by dark matter at radii of \sim 10kpc.

X-Ray Observations and the Structure of Elliptical Galaxies

We compare optical and high-quality X-ray data for three bright elliptical galaxies in the Virgo Cluster, NGC 4472, 4649, and 4636. The distribution of total mass in NGC 4472 and 4649 determined from X-ray data is sensitive to the stellar mass over a considerable range in galactic radius extending to r ≈ re, the effective radius. The agreement of X-ray and optically determined stellar masses provides a unique verification of the stellar mass-to-light ratio, which is essentially constant over the range 0.1 r/re 1. However, for NGC 4636 the dark matter is important at all radii 0.35re. Evidently, the dark to stellar mass ratio varies in quite different ways in elliptical galaxies of comparable optical luminosity, implying that the radial structure of dark halos may not be universal. There is some evidence in NGC 4636 for additional support of the hot interstellar gas at r 0.35re; either a field B ~ 10-4 G or a small (mechanically unstable) central region of high gas temperature (T ~ 107 K) is required. The global temperature structure in the hot interstellar medium of many recently observed elliptical galaxies is very similar, reaching a maximum near 3re-4re. This feature, which may suggest a new structural scale in these galaxies, is inconsistent with current theoretical gasdynamical models.