Unveiling the nature of M94's (NGC4736) outer region: a panchromatic perspective

TL;DR

This study uses multi-wavelength data to reveal that M94's outer region is a bright, active spiral arm structure, not a closed stellar ring, with significant star formation and a dynamic origin linked to the inner disk.

Contribution

It provides the first detailed multi-wavelength analysis showing the outer region of M94 is a spiral arm structure, challenging previous ring-based models.

Findings

Outer disk contains ~23% of stellar mass

Outer disk contributes ~10% of new stars

Star formation rate in outer disk is twice that of inner disk

Abstract

We have conducted a deep multi-wavelength analysis (0.15-160 mum) to study the outer region of the nearby galaxy M94. We show that the non-optical data support the idea that the outskirts of this galaxy is not formed by a closed stellar ring (as traditionally claimed in the literature) but by a spiral arm structure. In this sense, M94 is a good example of a Type III (anti-truncated) disk galaxy having a very bright outer disk. The outer disk of this galaxy contains ~23% of the total stellar mass budget of the galaxy and contributes ~10% of the new stars created showing that this region of the galaxy is active. In fact, the specific star formation rate of the outer disk (~0.012 Gyr^{-1}) is a factor of ~2 larger (i.e. the star formation is more efficient per unit stellar mass) than in the inner disk. We have explored different scenarios to explain the enhanced star formation in the outer disk. We find that the inner disk (if considered as an oval distortion) can dynamically create a spiral arm structure in the outer disk which triggers the observed relatively high star formation rate as well as an inner ring similar to what is found in this galaxy.