Deep Imaging of M51: a New View of the Whirlpool's Extended Tidal Debris
  We present deep, wide-field imaging of the M51 system using CWRU's Burrell
Schmidt telescope at KPNO to study the faint tidal features that constrain its
interaction history. Our images trace M51's tidal morphology down to a limiting
surface brightness of $\mu_{B,lim}\sim $30 mag arcsec$^{-2}$, and provide
accurate colors ($\sigma_{B-V} < 0.1$) down to $\mu_B\sim 28$. We identify two
new tidal streams in the system (the South and Northeast Plumes) with surface
brightnesses of $\mu_B =29$ and luminosities of $\sim 10^6 L_{\odot,B}$. While
the Northeast Plume may be a faint outer extension of the tidal "crown" north
of NGC 5195 (M51b), the South Plume has no analogue in any existing M51
simulation and may represent a distinct tidal stream or disrupted dwarf galaxy.
We also trace the extremely diffuse Northwest Plume out to a total extent of
20' (43 kpc) from NGC 5194(M51a), and show it to be physically distinct from
the overlapping bright tidal streams from M51b. The Northwest Plume's
morphology and red color ($B-V=0.8$) instead argue that it originated from
tidal stripping of M51a's extreme outer disk. Finally, we confirm the strong
segregation of gas and stars in the Southeast Tail, and do not detect any
diffuse stellar component in the HI portion of the tail. Extant simulations of
M51 have difficulty matching both the wealth of tidal structure in the system
and the lack of stars in the HI tail, motivating new modeling campaigns to
study the dynamical evolution of this classic interacting system.
