VLT Observations of NGC 1097's "dog-leg" tidal stream
  CONTEXT: Dwarf Spheroidal Galaxies and tidal streams. AIMS: We investigate
the structure and stellar population of two large stellar condensations (knots
A & B) along one of the faint optical "jet-like" tidal streams associated with
the spiral NGC 1097, with the goal of establishing their physical association
with the galaxy and their origin. METHODS: We use the VLT/FORS2 to get deep
V-band imaging and low-resolution optical spectra of two knots along NGC 1097's
northeast "dog-leg" tidal stream. With this data, we explore their morphology
and stellar populations. RESULTS: The FORS2 spectra show that the redshift of
knot A (and perhaps of knot B) is consistent with that of NGC 1097. The FORS2
photometry shows that the two knots match very well the photometric scaling
relations of canonical dwarf spheroidal galaxies (dSph). From the spectral
analysis we find that knot A is mainly composed of stars near G-type, with no
signs of ongoing star formation. Comparing its spectrum to a library of
Galactic GC spectra, we find that the stellar population of this dSph-like
object is most similar to intermediate to metal rich GCs. We find moreover,
that the tidal stream shows an "S" shaped inflection as well as a pronounced
stellar overdensity at knot A's position. This suggests that knot A is being
tidally stripped, and populates the stellar stream with its stars. CONCLUSIONS:
We have discovered that two knots along NGC 1097's northeast tidal stream share
most of their spectral and photometric properties with ordinary dwarf
spheroidal galaxies (dSph). Moreover, we find strong indications that the
"dog-leg" tidal stream arise from the tidal disruption of knot A. Since it has
been demonstrated that tidally stripping dSph galaxies need to loose most of
their dark matter before starting to loose stars, we suggest that knot A is at
present a CDM-poor object.
