2 Sample and observations

Through the main optical observatories in México (Observatorio Astronómico Nacional at San Pedro Mártir in Baja California and Observatorio Astrofísico Guillermo Haro in Cananea) we have started a joint observational programme devoted to obtain uniform photometric data for one of the most complete and homogeneous pair samples currently available. The sample of disk-disk (S+S) pairs amounts to more than 300 pairs from a total of 602 pairs in the KPG catalogue. The observations were begun since January 1999. The CCD BVRI images reported here (in the Cousins system) were obtained with a LFOSC detector attached to the 2.1 m telescope, at Observatorio Guillermo Haro, Cananea, Sonora, México, covering an area of about $6\hbox{$^\prime$ }\times 4\hbox{$^\prime$ }$, with a scale of 1 $\hbox{$^{\prime\prime}$ }$/pixel.

Since our goal is to observe all or most of the KPG (S+S) sample, we have applied no special strategy in selecting the current subset of 33 (S+S) pairs. Available observing time and weather conditions were the main factors limiting the number of observed pairs. This will be the observational strategy for the next reports up to the point where most of the (S+S) sample is observed. The selection criteria and statistical properties for the (S+S) sample that are most relevant to the present and further photometric analysis are stated here.

2.1 Statistical properties of the (S+S) sample

The isolated (S+S) pairs in the KPG sample were selected from a visual search of the Palomar Sky Survey. The catalogue samples the sky north of $\delta \geq -3\hbox{$^\circ$ }$. The vast majority of objects are found in high Galactic latitude regions ( $b \geq 20\hbox{$^\circ$ }$) and as a sample, they are reasonably complete (${\sim} 90$%) in the magnitude range $13.5 \leq m_{zw} \leq 15.7$. All galaxies in the (S+S) sample have measured redshifts. We next summarize possible limitations and sources of bias (mainly due to the optical selection criteria) that may affect the interpretation of the photometric analysis.

(1)

Although the projected physical separation (H₀= 75 km s^-1 Mpc^-1) for the whole (S+S) sample is small (<x> = 42.1 kpc), there is still a source of contamination from accordant redshift optical pairs. This is a difficult source to evaluate because most such optical pairs are members of loose group structures with magnitude and redshift properties completely within the domain of expectation for physical binary systems. Our photometric study, however, is most useful for solving the problem if we observe as much as possible the fraction of the (S+S) sample that shows direct/indirect signs of interactions. That is, by observing the most probable physical pairs, on an individual basis;

(2)

The (S+S) sample will reject highly evolved pairs such as mergers in the final stages of coalescence. This stems from the selection requirement that the galaxies have a discernible diameter. This excludes, for example, ultra-luminous infrared galaxies (ULIRGs) from our photometric study of (S+S) pairs;

(3)

The maximum size ratio between components (from the basic selection criteria) is $a_{2}/a_{1} \sim 8$. This means that the (S+S) sample favours magnitude and thus size concordance, biasing this sample against hierarchical binaries. However, an important difference in visual apparent magnitudes between member pairs ( $\Delta m \sim 3^{\rm m}$) can be found;

(4)

(S+S) pairs with the faintest apparent magnitudes must have smaller apparent separations in order to survive the isolation criterion and enter the Karachentsev catalogue. This means that apparently close pairs are biased towards physically close binaries - pairs near pericenter (van Moorsel 1982), or with smaller mean physical separations. By selecting the brightest (S+S) pairs, we therefore sample a wider range of physical separations in our photometric study;

(5)

The (S+S) pairs were selected whith a strong isolation criteria. Thus, we expect that only intrinsic properties of the individual galaxies and the effects of their mutual interactions should affect the observed morphological and photometrical properties.