Optical spectroscopy for a sample of southern binary galaxies

Free access article

Issue		A&A Volume 457, Number 2, October II 2006


Page(s)		425 - 435
Section		Extragalactic astronomy
DOI		http://dx.doi.org/10.1051/0004-6361:20054444

A&A 457, 425-435 (2006)
DOI: 10.1051/0004-6361:20054444

Optical spectroscopy for a sample of southern binary galaxies

T. C. Couto da Silva¹ and R. E. de Souza²

¹ Departamento de Física - ICET, UFMT, Av. Fernando Correa s/n, Cuiabá, MT, Brazil
e-mail: tcsp@terra.com.br
² Instituto de Astronomia, Geofísica e Ciências Atmosféricas - USP C.Postal 9638, 01065 - 970 São Paulo, Brazil
e-mail: ronaldo@astro.iag.usp.br

(Received 30 October 2005 / Accepted 31 March 2006 )

Abstract
Aims.This work is part of a joint observational program aiming to get photometric and spectroscopic information on southern pairs of galaxies. We present optical long-slit spectroscopic data on 80 probable components of pairs, 61 of them collected with a spectral resolution of $3.4~\hbox{\rm\AA}$ , and 19 with $12~\hbox{\rm\AA}$ . Nevertheless, our analysis takes into account 53 components of pairs with better spectral resolution, as 8 of these target galaxies actually belong to optical pairs. For the sample with better resolution, the covered wavelength range is $5724 \leq \lambda \leq 7036~\hbox{\rm\AA}$ . The spectroscopic and photometric information is gathered for an analysis relating galaxy morphologies to their spectra.
Methods.We use $\rm H_{\alpha} + [N_{II}]$ and H $_{\alpha}$ equivalent widths as star formation tracers for the central region of our sample galaxies, and we classify the spectra according to the emission lines' relative strength by looking at their behavior.
Results. Some of our sample galaxies exhibit high central star formation, most of them belonging to close pairs. However, not all galaxies' components of close pairs show this behavior. This may be a clue that besides interaction, other agents can stimulate central emission in binary galaxies. We suggest an enhancement in the number of galaxies with peculiar spectra (probably Seyferts) in our binary sample, when compared to isolated galaxies. Our data indicates that the morphological types of interacting galaxies are related to their spectral characteristics, as almost all early-type galaxies of our sample do not exhibit central optical emission. We note that the star formation activity is most likely to take place in both pairs' components, with a slightly higher mean strength for the less bright component of the pair. It is interesting to point out that most spirals exhibiting a strong HII emission line spectra present either a bar or a peculiarity, but on a general basis we do not find an enhancement of star formation in our interaction sample.

Key words: galaxies: interactions

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