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Modeling gas and stellar kinematics in disc galaxies

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 Added by Ezio Pignatelli
 Publication date 2000
  fields Physics
and research's language is English
 Authors E. Pignatelli




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We present V-band surface photometry and major-axis kinematics of stars and ionized gas of three early-type spiral galaxies, namely NGC 772, NGC 3898 and NGC 7782. For each galaxy we built a self-consistent Jeans model for the stellar kinematics, adopting the light distribution of bulge and disc derived by means of a two-dimensional parametric photometric decomposition. This allowed us to investigate the presence of non-circular gas motions, and derive the mass distribution of luminous and dark matter in these objects. We found that the observed gas rotation corresponds to the circular velocity except for the innermost region (|r|<8) of NGC 3898. This behaviour is quite common, although not ubiquitous, in the few bulge-dominated galaxies, for which dynamical modeling allows the comparison between the gas velocity and the circular speed.



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(abridged) Photometry and long-slit spectroscopy are presented for a sample of 6 galaxies with a low surface brightness stellar disc and a bulge. The stellar and ionised-gas kinematics were measured along the major and minor axis in half of the sample galaxies, whereas the other half was observed only along two diagonal axes. Spectra along two diagonal axes were obtained also for one of the objects with major and minor axis spectra. The kinematic measurements extend in the disc region out to a surface-brightness level mu_R~24mag/arcsec^2 reaching in all cases the flat part of the rotation curve. The stellar kinematics turns out to be more regular and symmetric than the ionised-gas kinematics, which often shows the presence of non-circular, off-plane, and non-ordered motions. This raises the question about the reliability of the use of the ionised gas as the tracer of the circular velocity in the modeling of the mass distribution, in particular in the central regions of low surface brightness galaxies.
The metallicity of star-forming gas in galaxies from the EAGLE simulations increases with stellar mass. Here we investigate whether the scatter around this relation correlates with morphology and/or stellar kinematics. At redshift $z=0$, galaxies with more rotational support have lower metallicities on average when the stellar mass is below $M_starapprox 10^{10}~{rm M}_odot$. This trend inverts at higher values of $M_star$, when prolate galaxies show typically lower metallicity. At increasing redshifts, the trend between rotational support and metallicity becomes weaker at low stellar mass but more pronounced at high stellar mass. We argue that the secondary dependence of metallicity on stellar kinematics is another manifestation of the observed anti-correlation between metallicity and star formation rate at a given stellar mass. At low masses, such trends seem to be driven by the different star-formation histories of galaxies and stellar feedback. At high masses, feedback from active galactic nuclei and galaxy mergers play a dominant role.
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We present V-band surface photometry and major-axis kinematics of stars and ionized gas of three early-type spiral galaxies, namely NGC 772, NGC 3898 and NGC 7782. For each galaxy we present a self-consistent Jeans model for the stellar kinematics, adopting the light distribution of bulge and disc derived by means of a two-dimensional parametric photometric decomposition. This allowed us to investigate the presence of non-circular gas motions, and derive the mass distribution of luminous and dark matter in these objects. NGC 772 and NGC 7782 have apparently normal kinematics with the ionized gas tracing the gravitational equilibrium circular speed. This is not true in the innermost region (r < 8) of NGC 3898 where the ionized gas is rotating more slowly than the circular velocity predicted by dynamical modelling. This phenomenon is common in the bulge-dominated galaxies for which dynamical modelling enables us to make the direct comparison between the gas velocity and the circular speed, and it poses questions about the reliability of galaxy mass distributions derived by the direct decomposition of the observed ionized-gas rotation curve into the contributions of luminous and dark matter.
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