2D stellar kinematics of nuclear bars
We just started an observational program to obtain 2D kinematics of nuclear bars, to be compared to N body simulations of single and double-barred galaxies.
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arXiv subjects
Eric Emsellem
Publications and source records attributed to Eric Emsellem.
Gas and stellar 2D kinematics in early-type galaxies
We have obtained integral field spectroscopy of a small sample of early-type galaxies to study the kinematical coupling between the stellar and gaseous components in their central regions.
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Dynamical models of NGC 3115
We present new dynamical models of the S0 galaxy N3115, making use of the available published photometry and kinematics as well as of two-dimensional TIGER spectrography. We first examined the kinematics in the central 40 arcsec in the light of two integral f(E,J) models. Jeans equations were used to constrain the mass to light ratio, and the central dark mass whose existence was suggested by previous studies. The even part of the distribution function was then retrieved via the Hunter & Qian formalism. We thus confirmed that the velocity and dispersion profiles in the central region could be well fit with a two-integral model, given the presence of a central dark mass of ~10^9 Msun. However, no two integral model could fit the h_3 profile around a radius of 25 arcsec where the outer disc dominates the surface brightness distribution. Three integral analytical models were therefore built using a Quadratic Programming technique. These models showed that three integral components do indeed provide a reasonable fit to the kinematics, including the higher Gauss-Hermite moments. Again, models without a central dark mass failed to reproduce the observed kinematics in the central arcseconds. This clearly supports the presence of a nuclear black hole of at least 6.5 10^8 Msun in the centre of NGC 3115. These models were finally used to estimate the importance of the dark matter in the outer part of NGC 3115, suggested by the flat stellar rotation curve observed by Capaccioli et al. (1993).
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Bar driven evolution of S0s: the edge-on galaxy NGC 4570
We present circumstantial evidence that the central region of the edge-on S0 galaxy NGC 4570, which harbors a 150 pc scale nuclear disc in addition to its main outer disc, has been shaped under the influence of a small (approx. 500pc) bar. This is based on the finding of two edge-on rings, whose locations are consistent with the Inner Lindblad and Ultra Harmonic Resonances of a rapidly tumbling triaxial potential. Observed features in the photometry and rotation curve nicely correspond with the positions of the main resonances, strengthening the case for a tumbling bar potential. The relative blue colour of the ILR ring, and the complete absence of any detected ISM, indicates that the nuclear ring is made of relatively young (< 2 Gyr) stars. We discuss a possible secular-evolution scenario for this complex multi-component galaxy, which may also apply to many other S0-galaxies with observed rings and/or multiple disc-components.
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N body simulations of the nucleus of M 31
We test through stellar N-body simulations some scenarios to explain the dynamics of the peculiar nucleus of the Andromeda galaxy (M~31): although HST observations reveal a double nucleus morphology, the rotation field is almost symmetric around the bulge gravity centre and the velocity dispersion is off-centred. We show that any m=1 perturbation has a very short life-time (a few 10$^5$ yr). Assuming that the bright peak (P1) is a cold stellar cluster infalling into the nucleus, and that the large central velocity gradient is due to a central dark mass (in the range 7~$10^7$--$10^8$~\Msun), we obtain a reasonably good fit to the observations. However, if this cluster lies in the central 20 pc, we estimate the life-time of the cluster to be less than 0.5~Myr. The dynamical friction is more efficient than estimated by analytic formulae, and is essentially due to the deformation of the stellar cluster through the huge tidal forces provided by the black hole. We show that the cluster cannot be on a circular orbit around the centre if the nucleus hosts a massive black hole of a few $10^7$~\Msun, and finally provide some estimates of the kinematics as observed with HST.
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A counter-rotating tilted gas disc in the peanut galaxy NGC~128
We have obtained $V$, $R_c$, $I_c$ HRCAM images and TIGER spectrography of the central region of the peanut galaxy NGC~128. The colour images reveal the presence of a red disc tilted by about 26 degres with respect to the major-axis of the galaxy. This tilted disc is made of dust and gas, as revealed by the 2D TIGER map of the ionized gas distribution. The TIGER stellar and gas velocity fields show that the angular momentum vectors of the stellar and gaseous components are reversed. We therefore suggest that the gas orbits belong to the so-called anomalous family, which is evidence for a tumbling triaxial potential (a bar) associated with the peanut morphology. The bar formation has very probably been triggered through the interaction with its nearby companion NGC~127, from which the dissipative component is being accreted along retrograde orbits.
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Ionized gas in early-type galaxies: Its effect on Mgb and other stellar line-strength indices
In the light of the recent finding that nebular emission lines are commonly found in the inner regions of early-type galaxies, we evaluate their effect to stellar absorption-line indices. We derive analytical expressions for changes induced by the presence of nebular emission lines, both for atomic and molecular absorption-line indices. We find that the [NI] emission-line doublet at 5199 A can significantly affect Mgb line-strength measurements. For typical equivalent widths of the [NI] doublet in nuclei of early-type galaxies featuring nebular emission, the Mgb index is artificially enhanced by 0.4 - 2 A, which represents a significant fraction of the typical equivalent width of the Mgb line in early-type galaxies. We illustrate this effect in the case of NGC 2974.
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