VLT observations of NGC 1097's “dog-leg” tidal stream

Abstract

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.

Filamentary accretion cannot explain the orbital poles of the Milky Way satellites

“…Indeed, other extragalactic post‐merger cases of aligned dSph satellites with tidal arms have emerged (Malphrus et al 1997; Bournaud et al 2007; Galianni et al 2010; Duc et al 2011), strengthening the notion that dSph and dwarf elliptical (dE) satellites are old TDGs (Okazaki & Taniguchi 2000). The implications of this for cosmological theory and fundamental physics are discussed in Kroupa et al (2010) and Kroupa (2012).…”

“…TDGs are found to form both in simulated (Bournaud & Duc 2006;Wetzstein, Naab & Burkert 2007;Bournaud, Duc & Emsellem 2008) and observed (Mirabel, Dottori & Lutz 1992;Hunsberger, Charlton & Zaritsky 1996;Weilbacher, Duc & Fritze-v. Alvensleben 2003) galaxy collisions. They seem to be long-lived objects (Kroupa 1997;Recchi et al 2007;Galianni et al 2010;Duc et al 2011) and share the same properties as dwarf elliptical (dE) galaxies (Dabringhausen et al, in preparation). This makes them a fundamental addition, if not even an alternative, to cosmologically formed dwarf galaxies.…”

The VPOS: a vast polar structure of satellite galaxies, globular clusters and streams around the Milky Way

“…Their age estimate of these dwarf galaxies (at least 2 Gyr, but maybe up to 5 Gyr) demonstrates that TDGs are long‐lived objects which contribute to the population of satellite galaxies. Galianni et al (2010) discuss that the satellite galaxies of NGC 1097, embedded in a tidal stream, might be old TDGs. The stability of TDGs has also been investigated numerically.…”

“…This spatial alignment can be conserved over time. Galianni et al (2010) discuss that the satellite galaxies of NGC 1097, embedded in a tidal stream, might be old TDGs. Recently, Duc et al (2011) found three old TDG candidates, aligned along a tidal tail around the galaxy NGC 5557.…”

The Dark Matter Crisis: Falsification of the Current Standard Model of Cosmology

“…In the local universe there are a number of known host galaxies with stream-TDG satellite alignments: (1) The NGC 1097 'dog leg' stellar stream with two typical dSph satellite galaxies (Galianni et al 2010). (2) The about 300 kpc long tidal stream with at least three dSph satellite galaxies in the 1-2 Gyr old post-interaction NGC 5557 (Duc et al 2011).…”