EDP Sciences
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Volume 394, Number 3, November II 2002
Page(s) 883 - 899
Section Galactic structure and dynamics
DOI http://dx.doi.org/10.1051/0004-6361:20021175

A&A 394, 883-899 (2002)
DOI: 10.1051/0004-6361:20021175

Old stellar Galactic disc in near-plane regions according to 2MASS: Scales, cut-off, flare and warp

M. López-Corredoira1, 2, A. Cabrera-Lavers2, F. Garzón2, 3 and P. L. Hammersley2

1  Astronomisches Institut der Universität Basel, Venusstrasse 7, 4102 Binningen, Switzerland
2  Instituto de Astrofísica de Canarias, 38200 La Laguna, Tenerife, Spain
3  Departamento de Astrofísica, Universidad de La Laguna, Tenerife, Spain

(Received 21 December 2001 / Accepted 7 August 2002)

We have pursued two different methods to analyze the old stellar population near the Galactic plane, using data from the 2MASS survey. The first method is based on the isolation of the red clump giant population in the color-magnitude diagrams and the inversion of its star counts to obtain directly the density distribution along the line of sight. The second method fits the parameters of a disc model to the star counts in 820 regions. Results from both independent methods are consistent with each other. The qualitative conclusions are that the disc is well fitted by an exponential distribution in both the galactocentric distance and height. There is not an abrupt cut-off in the stellar disc (at least within R<15 kpc). There is a strong flare (i.e. an increase of scale-height towards the outer Galaxy) which begins well inside the solar circle, and hence there is a decrease of the scale-height towards the inner Galaxy. Another notable feature is the existence of a warp in the old stellar population whose amplitude is coincident with the amplitude of the gas warp. It is shown for low latitude stars (mean height: $\vert z\vert\sim 300$ pc) in the outer disc (galactocentric radius R> 6 kpc) that: the scale-height in the solar circle is $h_z(R_\odot)=3.6\times 10^{-2}~R_\odot$, the scale-length of the surface density is $h_R=0.42~R_{\odot}$ and the scale-length of the space density in the plane (i.e. including the effect of the flare) is $H=0.25~R_{\odot}$. The variation of the scale-height due to the flare follows roughly a law $h_z(R)\approx h_z(R_\odot)
\exp \left(\frac{R-R_\odot}{[12-0.6R({\rm kpc})]\ {\rm kpc}}\right)$ (for $R\la 15$ kpc; $R_\odot=7.9$ kpc). The warp moves the mean position of the disc to a height $z_w=1.2\times 10^{-3} R({\rm kpc})^{5.25}\sin (\phi +5^\circ )$ pc (for $R\la 13$ kpc; $R_\odot=7.9$ kpc).

Key words: Galaxy: general -- Galaxy: stellar content -- Galaxy: structure -- infrared: stars

Offprint request: M. López-Corredoira, martinlc@astro.unibas.ch

© ESO 2002