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5 Summary

We present the first of a series of papers (Longhetti et al. 1997a,b,c) that will study the star formation history in early-type galaxies. In the present paper, we present a sample of 20 shell galaxies and 32 pair members in low density environments that is studied for this purpose. Most of the objects show present/past interaction signatures.

The paper describes the observations and the reduction of the spectrophotometric data obtained from spectra in the wavelength range of 3700 Å$< \lambda < $ 5700 Å at 2.1 Å FWHM resolution.

A set of 16 red ( $\lambda \gt$ 4200 Å) nuclear (5$^{\prime\prime}$)line-strength indices defined by the Lick Group has been obtained and converted into the Lick-IDS "standard'' system. 5 reference galaxies, selected from the G93 sample, are also included to test the results of the procedure.

Three blue ($\lambda <$ 4200) nuclear line-strength indices, namely $\Delta$(4000 Å), H+K(CaII) and H$\delta$/FeI, have been also determined with the purpose of tracing recent starbursts in the galaxies history.

The kinematics of the gas and of the stellar components are the subject of the Paper II. Stellar kinematical data (organized and discussed in the Paper II) have been used to correct present line-strength indices for velocity dispersion.

  
\begin{figure}
\centering

\includegraphics [width=8cm,clip=]{ds1429f5.eps}\end{figure} Figure 5: Atlas of the spectra of the observed galaxies: galaxies from the G93 sample used as spectrophotometric templates. Plots are in the rest frame, and the fluxes are in arbitrary units. The most remarkable features are labelled on top
  
\begin{figure*}
\centering

\includegraphics [width=7.5cm]{1429f6a.eps}

\includegraphics [width=7.5cm]{1429f6b.eps}

a)\hspace{8cm} b)\end{figure*} Figure 6: Same as Fig. 5 for shell galaxies
  
\begin{figure*}
\includegraphics [width=6.8cm]{1429f7a.eps}

\includegraphics [width=6.8cm]{1429f7b.eps}

\includegraphics [width=6.8cm]{1429f7c.eps}\end{figure*} Figure 7: Same as Fig. 5 for pair members
A fraction of the galaxies of the present study shows detectable emission lines, as can be deduced from the Atlas of the spectra (Figs. 5 to 7). A discussion about the physical conditions of the (e.g. LINER properties of the nucleus, ect.) is also given in the Paper II. Line-strength indices suffer the contamination caused by emission lines. G93 corrected the H$\beta$ absorption line-strength for the filling due to the H$\beta$ emission by means of a relation between the H$\beta$ line emission strength and that of the [OIII] line at 5007 Å. Namely $\Delta$ H$\beta$=0.7[OIII]. The validity of this correction has been questioned by Carrasco et al. (1995). We discuss the correction of the line-strength indices for the emission line filling in a forthcoming papera forthcoming paper.


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