The faint outer regions of the Pegasus Dwarf Irregular galaxy: a much
  larger and undisturbed galaxy
  We investigate the spatial extent and structure of the Pegasus dwarf
irregular galaxy using deep, wide-field, multicolour CCD photometry from the
Sloan Digital Sky Survey (SDSS) and new deep HI observations. We study an area
of ~0.6 square degrees centred on the Pegasus dwarf that was imaged by SDSS.
Using effective filtering in colour-magnitude space we reduce the contamination
by foreground Galactic field stars and increase significantly the contrast in
the outer regions of the Pegasus dwarf. Our extended surface photometry,
reaches down to a surface brightness magnitude mu_r~32 mag/sq.arcsec. It
reveals a stellar body with a diameter of ~8 kpc that follows a Sersic surface
brightness distribution law, which is composed of a significantly older stellar
population than that observed in the ~2 kpc main body. The galaxy is at least
five times more extended than listed in NED. The faint extensions of the galaxy
are not equally distributed around its circumference; the north-west end is
more jagged than the south-east end. We also identified a number of stellar
concentrations, possibly stellar associations, arranged in a ring around the
main luminous body. New HI observations were collected at the Arecibo
Observatory as part of the ALFALFA survey. They reveal an HI distribution
somewhat elongated in RA and about 0.3 deg. wide, with the region of highest
column density coincident with the luminous galaxy. The HI rotation curve shows
a solid-body rotation behaviour, with opposite ends differing by 15 km/s. There
is a stream to lower velocities about 5 arcmin from the centre of the galaxy.
We were able to measure ugriz colours in a number of apertures using the SDSS
data and compared these with predictions of evolutionary synthesis models.
(abridged)
