VLT observations of NGC 1097's dog-leg tidal stream Dwarf spheroidals 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. Spectra were obtained for eleven sources in the field surrounding the tidal stream. The great majority of them turned out to be background or foreground sources, but the redshift of knot A (and perhaps of knot B) is consistent with that of NGC 1097. Using the V-band image of the dog-leg tidal feature we find that the two knots match the photometric scaling relations of canonical dwarf spheroidal galaxies (dSph) very well. Spectral analysis shows that knot A is mainly composed of stars near G-type, with no signs of ongoing star formation. Comparing its spectrum with a library of high resolution spectra of galactic globular clusters (GCs), we find that the stellar population of this dSph-like object is most similar to intermediate to metal rich galactic 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 populating 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 arises 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.