Deep Imaging of M51: a New View of the Whirlpool's Extended Tidal Debris

TL;DR

This study uses deep wide-field imaging to reveal faint tidal features in M51, identifying new streams and providing insights into its interaction history and the origin of its tidal debris.

Contribution

It presents the first detailed imaging of M51's faint tidal structures down to very low surface brightness levels, discovering new tidal streams and analyzing their origins.

Findings

Identified two new tidal streams with surface brightness of 29 mag/arcsec^2.

Mapped the extent of the Northwest Plume and linked it to M51a's outer disk.

Confirmed the segregation of gas and stars in the Southeast Tail.

Abstract

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.