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Post Info TOPIC: Fornax Dwarf Spheroidal Galaxy


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RE: Fornax Dwarf Spheroidal Galaxy
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Title: An XMM-Newton search for X-ray sources in the Fornax dwarf galaxy
Authors: A. A. Nucita, L. Manni, F. De Paolis, D. Vetrugno, G. Ingrosso

We report the results of a deep archive XMM-Newton observation of the Fornax spheroidal galaxy that we analysed with the aim of fully characterising the X-ray source population (in most of the cases likely to be background active galactic nuclei) detected towards the target. A cross correlation with the available databases allowed us to find a source that may be associated with a variable star belonging to the galaxy. We also searched for X-ray sources in the vicinity of the Fornax globular clusters GC 3 and GC 4 and found two sources probably associated with the respective clusters. The deep X-ray observation was also suitable for the search of the intermediate-mass black hole (of mass \simeq 10^{4} solar masses expected to be hosted in the center of the galaxy. In the case of Fornax, this search is extremely difficult since the galaxy centroid of gravity is poorly constrained because of the large asymmetry observed in the optical surface brightness. Since we cannot firmly establish the existence of an X-ray counterpart of the putative black hole, we put constraints only on the accretion parameters. In particular, we found that the corresponding upper limit on the accretion efficiency, with respect to the Eddington luminosity, is as low as a few 10^{-5}.

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Title: The core size of the Fornax dwarf Spheroidal
Authors: N.C. Amorisco, A. Agnello, N.W. Evans (Cambridge)

We exploit the detection of three distinct stellar subpopulations in the red giant branch of the Fornax dwarf Spheroidal to probe its density distribution. This allows us to resolve directly the evolution with radius of the dark matter mass profile. We find that a cored dark matter halo provides a perfect fit to the data, being consistent with all three stellar populations well within 1-sigma, and for the first time we are able to put constraints on the core size of such a halo. With respect to previous work, we do not strengthen the statistical exclusion of a dark matter cusp in Fornax, but we find that Navarro-Frenk-White haloes would be required to have unrealistically large scale radii in order to be compatible with the data, hence low values of the concentration parameter. We are then forced to conclude that the Fornax dwarf Spheroidal sits within a dark matter halo having a constant density core, with a core size of between 0.6 and 1.8 kpc.

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Title: A Troublesome Past: Chemodynamics of the Fornax dwarf spheroidal
Authors: N. C. Amorisco, N. W. Evans

We present compelling evidence for the complexity of the Fornax dwarf spheroidal. By disentangling three different stellar subpopulations in its red giant branch, we are able to study in detail the dependence between kinematics and metallicity. A well-defined ordering in velocity dispersion, spatial concentration, and metallicity is evident in the subpopulations. We also present evidence for a significant misalignment between the angular momentum vectors of the old and intermediate-age populations. According to the HST measurement of Fornax's proper motion, this corresponds to counter-rotation. These ingredients are used to construct a novel evolutionary history of the Fornax dwarf spheroidal, characterised as a late merger of a bound pair.

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Title: The Dark Matter Density Profile of the Fornax Dwarf
Authors: John Jardel, Karl Gebhardt

We construct axisymmetric Schwarzschild models to measure the mass profile of the local group dwarf galaxy Fornax. These models require no assumptions to be made about the orbital anisotropy of the stars, as is the case for commonly used Jeans models. We test a variety of parameterisations of dark matter density profiles and find cored models with uniform density rho_c = (1.6 ± 0.1) x 10^-2 M_sun pc^-3 fit significantly better than the cuspy halos predicted by cold dark matter simulations. We also construct models with an intermediate-mass black hole, but are unable to make a detection. We place a 1-sigma upper limit on the mass of a potential intermediate-mass black hole at M_BH < 3.2 x 10^4 M_sun.

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Title: AGB Stars in the Fornax Dwarf Spheroidal Galaxy
Authors: Patricia A. Whitelock, John W. Menzies, Michael W. Feast, Noriyuki Matsunaga, Toshihiko Tanabe, Yoshifusa Ita

We report on a multi-epoch study of the Fornax dwarf spheroidal galaxy, made with the Infrared Survey Facility, over an area of about 42'x42'. The colour-magnitude diagram shows a broad well-populated giant branch with a tip that slopes down-wards from red to blue, as might be expected given Fornax's known range of age and metallicity. The extensive AGB includes seven Mira variables and ten periodic semi-regular variables. Five of the seven Miras are known to be carbon rich. Their pulsation periods range from 215 to 470 days, indicating a range of initial masses. Three of the Fornax Miras are redder than typical LMC Miras of similar period, probably indicating particularly heavy mass-loss rates. Many, but not all, of the characteristics of the AGB are reproduced by isochrones from Marigo et al. for a 2 Gyr population with a metallicity of Z=0.0025.
 An application of the Mira period-luminosity relation to these stars yields a distance modulus for Fornax of 20.69±0.04 (internal), ±0.08 (total) (on a scale that puts the LMC at 18.39 mag) in good agreement with other determinations. Various estimates of the distance to Fornax are reviewed.

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