A dwarf galaxy's transformation and a massive galaxy's edge: autopsy of kill and killer in NGC 1097

Abstract

(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.