Photometric study of a sample of southern binary galaxies

Abstract

Aims. This work exhibits the basic optical photometric data for a sample of 50 probable southern binary galaxies. Our sample covers a broad range of pair separations, stages of interaction, and morphologies. From the initial list of selected pairs, using spectroscopic data from the literature and our own data, we conclude that 84% of these systems are true binary galaxies. Methods. We present residual and asymmetric maps, R major semi-axis profiles of surface brightness, ellipticity, position-angle, harmonic Fourier coefficients of third and fourth order (b 3 and b 4 ) for 50 probable pairs, and B − R color maps for 47 of these pairs. For most galaxies, we present the profiles in two different ways, aiming to verify the influence of random errors on them. Results. We note that random errors in position-angle profiles are at least 2• , but a more significant result must take into account a variation larger than 11• for this distribution. Barred galaxies usually show a typical behavior in ellipticity and position-angle profiles: these profiles display variations when changing from a bar to a disk region. In some cases, the variations also occur along the bar. Some galaxies show distribution profiles that are common for their morphological type, and the interaction signature is only evidenced by their residual maps. Bars are usually redder and rings are bluer, when compared with the galaxy outskirts. Conclusions. Our data indicates that there is a connection between interaction strength and morphological distortions in binary galaxies. If we consider the projected separation of a pair as an indication of interaction strength, distortions such as displaced centers, anomalous shapes of spiral arms, and twistings of external regions are easily detected in some close pairs, although not all components of close pairs show this behavior. Our data suggests that besides interaction, other parameters, like orbital geometry and internal properties of galaxies, can stimulate binary galaxies' peculiarities.

Optical spectroscopy for a sample of southern binary galaxies

“…The number of spirals with a morphological type later than Sc is very small, only two, and both are barred; no spectroscopic data was obtained for them in this work. There is a small difference between pairs studied in this sample and the one studied in photometry by Couto da Silva & de Souza (2006), but the results are nearly the same. Donzelli & Pastoriza (1997) studied a sample of pairs consisting of a main galaxy (component A) and a companion with half or less the diameter of the main component (B component), and they noted that A components show a mean strength of EW (H α + [N II ]) ∼ 37 Å, a slightly higher strength than the 29 Å obtained by KK 83 for normal spiral galaxies (the mean strength for EW (H α + [N II ]) for our sample of spiral components of optical pairs is ∼27 Å).…”

“…It presents a very blue central region, a boxy-shaped bulge (Couto da Silva & de Souza 2006), and a strong central HII emission, but it is not a pair component. Perhaps it had an unbound encounter with P402a sometime in the past.…”

“…CS* indicates that this galaxy has a radial velocity determined in both runs; MF, Mathewson & Ford (1996); and SWb, Sekiguchi & Wolstencroft (1993). In fact, P221 is not a pair but a compact group; this group image and the galaxies associated with the letters a, b, c, and are shown in Couto da Silva & de Souza (2006). P268 is also a compact group (see more details in our previous work).…”

“…In this table, the first column presents the object identification as it is described by Soares et al (1995). A * symbol indicates that this galaxy was also studied in photometry (Couto da Silva & de Souza 2006). Columns 2 and 3 present the object identification and the radial velocity from ESO-LV catalogue (Lauberts & Valentijn 1989).…”

“…For the majority of these pairs, we obtained photometric data as well (Couto da Silva & de Souza 2006). For all but two galaxies belonging to this sample, the morphological classification was obtained in a very accurate way in our photometric work, taking into account the inner structure of galaxies.…”