A dwarf galaxy's transformation and a massive galaxy's edge: autopsy of
  kill and killer in NGC 1097
  (abridged) We present a dynamical analysis of the extended stellar stream
encircling NGC 1097. Within a statistical framework, we model its surface
brightness using mock streams as in Amorisco (2015) and deep imaging data from
the CHART32 telescope (Stellar Tidal Stream Survey). We reconstruct the
post-infall evolution of the progenitor, which has experienced 3 pericentric
passages and lost more than 2 orders of magnitude in mass. At infall,
$5.4\pm0.6$ Gyr ago, the progenitor was a disky dwarf with mass of
$\log_{10}[m(<3.4\pm1 {\rm kpc})/ M_\odot]=10.35\pm0.25$. We illustrate how the
90$^\circ$ turn in the stream, identifying the `dog leg', is the signature of
the progenitor's prograde rotation. Today, the remnant is a nucleated dwarf,
with a LOS velocity of $v_{\rm p, los}^{\rm obs}=-30\pm 30$ kms$^{-1}$, and a
luminosity of $3.3\times 10^7 L_{V,\odot}$ (Galianni et al. 2010). Our
independent analysis predicts $v_{\rm p, los}=-51^{-17}_{+14}$ kms$^{-1}$, and
measures $\log_{10}(m/ M_\odot)=7.4^{+0.6}_{-0.8}$, so that the compact nucleus
is soon becoming a low-luminosity UCD. We find that NGC 1097 has a mass of
$M_{200}=1.8^{+0.5}_{-0.4} \times 10^{12}\; M_{\odot}$, and its concentration
$c_{200}=6.7^{+2.4}_{-1.3}$ is in agreement with LCDM. The stream is described
almost down to the noise in a spherical host potential, we find this would not
be possible if the halo was substantially triaxial at large radii. Its
morphology shows that the slope of the total density profile bends from an
inner $\gamma(r_{\rm peri})=1.5\pm0.15$. The progenitor's orbit reaches $r_{\rm
apo}=150\pm 15$ kpc, more than a half of the virial radius of the host, so
that, for the first time on an individual extragalactic halo, we measure the
outer density slope, $\gamma(0.6r_{200,c})=3.9\pm0.5$. This demonstrates the
promise of the newborn field of detailed, statistical modelling of
extragalactic tidal streams.
