We present a detailed dynamical analysis of the thin , extended stellar stream that encircles the disk galaxy NGC 1097 .
Within a fully statistical framework , we model the surface brightness map of the so called ‘ dog leg stream ’ , using mock streams generated as in Amorisco ( 4 ) , and new deep imaging data obtained with the CHART32 telescope for the Stellar Tidal Stream Survey .
We reconstruct the post-infall evolution of the progenitor galaxy , which , we find , has experienced three pericentric passages , and lost more than two orders of magnitude in mass .
At infall , 5.4 \pm 0.6 Gyr ago , the progenitor was a disky dwarf with a mass of \log _ { 10 } ( m / M _ { \odot } ) = 10.35 \pm 0.25 within its tidal radius , r _ { t } = 3.4 \pm 1 kpc .
We illustrate how the sharp 90 degree turn in the stream , identifying the ‘ dog leg ’ , is the signature of the progenitor ’ s internal rotation , inclined with respect to the orbital plane and imprinted in the escape conditions of the stream members .
Today , the remnant is observed to be a nucleated dwarf galaxy , with a line-of-sight velocity of v _ { p,los } ^ { obs } = -30 \pm 30 kms ^ { -1 } with respect to NGC 1097 , and a total luminosity of 3.3 \times 10 ^ { 7 } L _ { V, \odot } ( 31 ) .
Our independent analysis predicts a line-of-sight velocity of v _ { p,los } = -51 ^ { -17 } _ { +14 } kms ^ { -1 } and measures its current total mass at \log _ { 10 } ( m / M _ { \odot } ) = 7.4 ^ { +0.6 } _ { -0.8 } , implying that the compact nucleus ( L \approx 6.9 \times 10 ^ { 5 } L _ { V, \odot } , 31 ) is soon destined to become a low-luminosity Ultra Compact Dwarf galaxy .
The progenitor ’ s orbit is strongly radial , with a pericenter of a few kpc , and an apocenter reaching r _ { apo } = 150 \pm 15 kpc , more than a half of the inferred virial radius of the host , r _ { 200 } = 250 \pm 20 kpc .
We find that NGC 1097 has a mass of M _ { 200 } = 1.8 ^ { +0.5 } _ { -0.4 } \times 10 ^ { 12 } M _ { \odot } , and its inferred concentration c _ { 200 } = 6.7 ^ { +2.4 } _ { -1.3 } is in very good agreement with the expectation of the cold dark matter model .
We can describe the stream almost down to the noise using a spherical host potential , and find that this would not be possible in case of a halo that is substantially triaxial at large radii .
In the morphology of the stream , we can see the logarithmic density slope of the total density profile , \gamma , bending from its inner value , \gamma ( r _ { peri } ) = 1.5 \pm 0.15 , and steepening towards large radii .
For the first time on an individual extragalactic halo , we measure the outer slope of the density profile , \gamma ( 0.6 r _ { 200 } ) = 3.9 \pm 0.5 .
This demonstrates the promise of the newborn field of detailed , statistical modelling of extragalactic tidal streams .