Gas/star coupling in early-type galaxies

Eric Emsellem, Roland Bacon, Guy Monnet.

We have successfully observed the centre of a number of early-type galaxies using the two-dimensional spectrograph TIGER in April 1996. Two spectral domains were covered in order to derive the 2D distribution and kinematics of the stellar and gaseous components. The aim of this project is to constrain the formation and evolution processes of the nuclei of ellipticals by a detailed study of the gas behaviour in the potentiel well of the galaxy.

The observing run was a success: the 4 allocated nights were clear with an average seeing of 0.75 arcsec FWHM (and 0.55 during the fourth night). We obtained scientific exposures of 6 early-type galaxies with an average of 2 hours for the blue spectral domain (around the Mg triplet) and 1.5 hour for the red domain (including [NII], H and [SII]). This completed our sample of 10 galaxies started in Nov. 1993.

All exposures have been reduced. A proper stellar subtraction was required before any analysis of the emission lines could be achieved, particularly in the case of H which is strong in absorption in all the galaxies of our sample. We have derived the two-dimensional maps of the stellar and gas distributions/kinematics as well as of the emission line ratios (e.g. H/ [NII]) and stellar line strengths (e.g. Mgb). The galaxies in our sample show a wide variety of behaviours including evidence for triaxiality, rapidly rotating gas disks, multiple decoupled gaseous components, Broad Line Regions (BLRs), bars ... The case of NGC 4278 is illustrated in Fig 10: the gas distribution exhibits a central spiral arm structure tilted by about 45 degres with respect to the stellar one. Line ratios show evidence for shocks along the arm where the line widths are also high. The stellar component has a mild rotation. An other (rather extreme) case which was included in the Nov. 1993 sample is presented in Fig 11: we revealed the presence of a counter-rotating tilted gas disc in the peanut galaxy NGC 128. This demonstrates that the peanut corresponds to a bar viewed nearly edge-on (Emsellem & Arsenault, submitted).

In the light of these TIGER observations, it seems that each galaxy in our sample has a very distinct and complex morphology. We can still draw a few general results - the 2D maps are definitely required to reveal the morphology/kinematics of the stellar and gaseous components. - the gas is almost systematically kinematically decoupled from the stellar component: this suggests an external origin (e.g. accretion). - the emission lines in the centre of these early-type galaxies are generally of LINER type. Some of them include a BLR redhifted with respect to the narrow line system. A full account of these results will be presented in a forthcoming paper (Emsellem et al., in prep.).

Following the April 1996 run, the TIGER spectrograph was dismounted. This ended a period of nearly 10 years for this prototype instrument. TIGER-like spectrographs are now being built for a number of 4 to 8 meter class telescopes. We wish here to sincerely thank all the individuals who have contributed to this success. E. Emsellem also wish to personally thank Barney Magrath who has been of invaluable assistance for the TIGER observing runs.

Following pages: Page 12: Figure 10: TIGER observations of NGC 4278. The stellar (left panels) and gas (right panels) reconstructed distributions (top), velocity fields (middle) and dispersion fields (bottom). Page 13: Figure 11: HRCAM (top panels) and TIGER (middle and bottom panels ) observations of the peanut galaxy NGC 128. The ionized gas is distributed in a tilted disc-like structure and counter-rotates with respect to the stellar component.
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