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Abstract
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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.
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Sample Selection
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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.
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Observations
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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''.
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Reduction
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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.
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Depth and Flatness of the Field
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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.
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