Journal Article

Hydrostatic equilibrium profiles for gas in elliptical galaxies

Pedro R. Capelo, Priyamvada Natarajan and Paolo S. Coppi

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 407, issue 2, pages 1148-1156
Published in print September 2010 | ISSN: 0035-8711
Published online September 2010 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2010.16962.x
Hydrostatic equilibrium profiles for gas in elliptical galaxies

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We present an analytic formulation for the equilibrium gas density profile of early-type galaxies that explicitly includes the contribution of stars in the gravitational potential. We build a realistic model for an isolated elliptical galaxy and explore the equilibrium gas configurations as a function of multiple parameters. For an assumed central gas temperature kBT0= 0.6 keV, we find that neglecting the gravitational effects of stars, which can contribute substantially in the innermost regions, leads to an underestimate of the enclosed baryonic gas mass by up to ∼65 per cent at the effective radius and by up to ∼15 per cent at the Navarro–Frenk–White (NFW) scale radius, depending on the stellar baryon fraction. This formula is therefore important for estimating the baryon fraction in an unbiased fashion. These new hydrostatic equilibrium solutions, derived for the isothermal and polytropic cases, can also be used to generate more realistic initial conditions for simulations of elliptical galaxies. Moreover, the new formulation is relevant when interpreting X-ray data. We compare our composite isothermal model to the standard β-model used to fit X-ray observations of early-type galaxies, to determine the value of the NFW scale radius rs. Assuming a 10 per cent stellar baryon fraction, we find that the exclusion of stars from the gravitational potential leads to (i) an underestimate of rs by ∼80 per cent and (ii) an overestimate of the enclosed dark matter at rs by a factor of ∼2, compared to the equivalent β-model fit results when stars are not taken into account. For higher stellar mass fractions, a β-model is unable to accurately reproduce our solution, indicating that when the observed surface brightness profile of an isolated elliptical galaxy is found to be well fitted by a β-model, the stellar mass fraction cannot be much greater than ∼10 per cent.

Keywords: galaxies: elliptical and lenticular, cD; galaxies: haloes; galaxies: ISM; galaxies: stellar content; galaxies: structure; dark matter

Journal Article.  5812 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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