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Astron. Astrophys. 347, 55-62 (1999)

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3. Absorption line spectra

The characterisation of the stellar population of the interacting galaxies is based on the equivalent widths (EW s) of the more prominent absorption features, i.e., [FORMULA], [FORMULA], CH G band, and [FORMULA]. We have also measured additional absorption features and the flux contribution at [FORMULA] Å and [FORMULA] Å for those galaxies in which the population template selection was difficult. These data are presented in Table 1 for both components of the interacting pairs. Previous to the measurements, all spectra were normalised to the continuum at [FORMULA] Å where the flux contribution from young stellar populations is less than 2% (Bica 1988). Typical errors are [FORMULA] for [FORMULA] and somewhat higher ([FORMULA]) for [FORMULA]. Continuum tracing and spectral windows are those defined by Bica & Alloin (1986). The EW s were used to classify the spectrum of each galaxy in terms of Bica's (1988) templates, which span the properties of the more usual nuclear stellar populations observed in normal galaxies. The basic characteristics of the templates can be summarised as follows: E1-E3 and S1-S3 are red stellar populations with metallicity decreasing from 4 to 1 times solar. S4-S7 is a sequence of increasing contributions from blue components: S4 contains 10% of flux contribution at [FORMULA] Å from populations younger than 1 Gyr, while S7 contains 80%.


Table 1. Equivalent widths for the galaxy sample.
Table Notes:
Column (7) gives the stellar population template derived from the EW s.

The resulting frequency of stellar population templates for each component of our sample pairs is shown in Fig. 3. The histogram shows an excess of blue stellar population templates for the B components with respect to the A components. In fact 55% of the B components have S5 to S7 stellar population types, while [FORMULA]% of the A components have blue stellar population types. This result reveals intrinsic differences between the stellar populations of both components of the galaxy pairs, but it might also be related to the size of the observed region. In fact, the average major diameter of the B components is [FORMULA], and since the spectra have been extracted from a rectangular aperture of [FORMULA], the bulge and part of the disk of the B components have been observed. On the other hand, for the A components, which have an average major diameter larger than 1.5', only the central region was observed.

[FIGURE] Fig. 3. Frequency of stellar population templates in the sample galaxies

The stellar population analysis shows that: (i) nearly half of the A component bulges have a red stellar population (E1, E2 and S2 templates), and thus typical young stars are absent in the nuclear region of these galaxies. In Fig. 4 (top panel) we show the observed spectrum of AM 2238-575 A along its E2 stellar population template. Notice that this is a case of an Sc galaxy (Fig. 1) with a very red bulge. The remaining A components exhibit an appreciable flux contribution from young stars (S4-S7 templates) at the normalisation wavelength [FORMULA]. In the bottom panel of Fig. 4 we show the spectrum of AM 2322-821 B together with its blue S5 stellar population template; (ii) more than 70% of the B components have a strong flux contribution from stellar populations younger than [FORMULA] years (S4-S7 templates). Intense star formation occurs on the main body of the weaker component of the pairs; (iii) due to the small apparent diameter of these B components and the tidal distortion of their main body, it is very difficult to classify them according to morphological type. Thus, from the stellar population analysis we infer that most of these galaxies are gas-rich irregulars or distorted late-type spirals, except AM 0821-783B, AM 1254-321B and AM 1256-433 which probably are early-type galaxies.

[FIGURE] Fig. 4. Spectra of the galaxies AM 2238-575A (top) and AM 2322-821B (bottom). The corresponding templates and the resulting pure-emission spectra are also shown. Constants have been added to the spectra for ease of visualisation.

The frequency of stellar population templates in the sample galaxies (Fig. 3) shows that most of these galaxies have an important flux contribution from stars younger than [FORMULA] years. V-band luminosity and the corresponding mass fraction for a selection of E1-S7 galaxy stellar population models have been computed by Bica, Arimoto & Alloin (1988) which show that recent star formation (S1-S6 templates), although very important in V flux, has a negligible contribution (less than 5%) in mass. For the S7 template, the contribution from young stars to the observed light at [FORMULA] amounts to more than 80%, and its mass contribution (about 15%) becomes important.

As remarked by Bica, Arimoto & Alloin (1988), early-type galaxies with recent star-formation (E7 and E8 templates) require some mechanism of external gas injection such as, for example, interaction and/or merger. In our sample there are examples of early-type galaxies interacting with star-forming galaxies (e.g. AM 1224-331, AM 2330-451 and AM 2105-332) whose stellar population is described by the E1, E2 or E3 templates - which do not present recent star formation. This could mean that there was no gas transfer between both components.

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© European Southern Observatory (ESO) 1999

Online publication: June 18, 1999