Abstract Sample Selection Observations Reduction Depth and Flatness of the Field
 Abstract We present a deep broadband optical imaging study of a complete sample of luminous elliptical galaxies (MB-20) at distances 15 Mpc - 50 Mpc, selected from the Tully catalog of nearby galaxies. The images are flat to ~0.35% across the 20' field and reach a depth of 27.7 mag arcsec-2. We derive an objective tidal interaction parameter for all galaxies and find that 74% of them show tidal disturbance signatures in their stellar bodies. This is the first time that such an analysis is done on a statistically complete sample and it confirms that tidal features in ellipticals are common even in the local Universe. Sample Selection Candidates for inclusion in this study were initially selected from the Nearby Galaxies Catalog (Tully, 1988), consisting of all elliptical galaxies at a declination between -85 and +10. A distance cut was applied to the initial sample to exclude galaxies that are farther than 50 Mpc or closer than 15 Mpc. The distance threshold ensures that the outer parts of all galaxies fit in the instrument's field-of-view and that a sufficient signal-to-noise ratio is achieved for all targets. A luminosity cut of MB<-20.15 was used, with the magnitudes taken from Tully (1988) and converted to our cosmology. This limit corresponds to MB<-20.0 in the Tully (1988) atlas, as he used H0=75 km s-1 Mpc-1, and to L ≥ L∗ (Blanton et al., 2003). The last selection criterion that we used rejected galaxies with Galactic latitude of less than 17 due to difficulties of constraining a good model fit in a crowded stellar field. Two galaxies (NGC 4645 and NGC 5796) were excluded due to poor observing conditions that resulted in noise levels over 5 times worse than the sample mean. The final catalog therefore consists of 54 giant ellipticals and it includes members of four nearby clusters (Virgo, Fornax, Centaurus and Antlia). Galaxy environments were determined from the literature using NASA's Astrophysics Data System Bibliographic Services to find references for previous studies of the objects. The methods used to derive these assumed environments are therefore inconsistent throughout the sample and they vary in accuracy. Observations Broadband optical observations of the sample galaxies had been acquired by multiple authors, showing some of the tidal features that we discuss in this work. However, the standard techniques used to obtain and reduce these previous data were not aimed at revealing the faint gravitational interaction signatures and they were washed out by residual background level variations across the field. In order to overcome this we supplemented each set of galaxy observations with a sequence of dark sky exposures of equivalent depth. The galaxies were observed in a sequence of 33 pointings, including both object and dark sky frames. We used the V band as it provides the highest signal-to-noise ratio in a given exposure time. The observing pattern included seventeen 300 sec exposures of the target galaxy and sixteen 300 sec exposures of background sky, acquired in alternating order. All object frames were shifted from each other by 1-3' in order to correct for cosmetic defects in the CCD. All galaxies were observed with total exposure times between 4200 and 7200 seconds. In order to improve the signal-to-noise ratio the data were binned by a factor 2x2 at the telescope, producing a pixel size of 0.578''. To further increase the sensitivity to tidal features we also binned in software, resulting in a final pixel size of 1.156''. The typical stellar FWHM of the images used for analysis is ~1.7''. Reduction Initial reduction of the data followed standard techniques and consisted of zero level subtraction and first order field variation corrections using dome flat frames. In order to flatten the field to a higher degree dark-sky flat-field frames were prepared for each of the sample galaxies. All sources were masked in the individual dark-sky exposures which were then averaged and applied to the object frames. Lastly, the reduced object exposures were aligned and combined to create the final data products. Apparent magnitudes were calibrated using aperture photometry of Prugniel et al. (1998) and were corrected for Galactic reddening using infrared dust maps from Schlegel et al. (1998). We assume distance measurements from the Tully catalog (corrected to our cosmology) to convert the luminosity profiles to physical units. The data were reduced using the NOAO/IRAF software. Depth and Flatness of the Field We used two methods to determine the depths of the sky flat field images. First, we reduced and stacked a dark sky image in the same way that was used to produce the target frames and smooth it to the scale of a typical tidal feature (~20''). Although the resultant image was inherently flat, it preserves the photon noise level as limited by the telescope, instrument and the observing program. Second, we aggressively masked a target frame for bright objects and subsequently smooth it using a median kernel. This frame is insensitive to any pixel-to-pixel variation but it reflects the large-scale variation due to residual flattening issues or sky conditions at the time of observation. We then measured the standard deviation across both frames to obtain the detection threshold of faint tidal features. For a 20''x20'' box (corresponding to a typical tidal feature scale) we derived a 1$\sigma$ photon-noise detection threshold of ~29 mag. From the flatness limited frame we measured a 1$\sigma$ detection threshold of ~27.7 mag. This result implies that a similar program carried on a larger telescope will not necessarily yield a lower detection threshold as the data are dominated by residual flatness variations rather than photon noise.