Hubble Space Telescope Pixel Analysis of the Interacting S0 Galaxy NGC 5195 (M51B)

TL;DR

This study uses pixel analysis of HST images to explore the stellar populations and dust properties of the interacting galaxy NGC 5195, revealing transient star formation features linked to galaxy interaction.

Contribution

It introduces a pixel-based analysis of NGC 5195's properties, highlighting the impact of galaxy interaction on star formation and dust distribution, which is a novel approach for early-type galaxies.

Findings

Red pixel sequence is redder than NGC 5194, indicating higher dust optical depth.

Blue pixel sequence is weak and associated with the tidal bridge, suggesting transient star formation.

Young stellar populations are concentrated in the tidal bridge, especially at its midpoint.

Abstract

We report the properties of the interacting S0 galaxy NGC 5195 (M51B), revealed in a pixel analysis using the HST/ACS images in the F435W, F555W and F814W (BVI) bands. We analyze the pixel color-magnitude diagram (pCMD) of NGC 5195, focusing on the properties of its red and blue pixel sequences and the difference from the pCMD of NGC 5194 (M51A; the spiral galaxy interacting with NGC 5195). The red pixel sequence of NGC 5195 is redder than that of NGC 5194, which corresponds to the difference in the dust optical depth of 2<\Delta\tau_V<4 at fixed age and metallicity. The blue pixel sequence of NGC 5195 is very weak and spatially corresponds to the tidal bridge between the two interacting galaxies. This implies that the blue pixel sequence is not an ordinary feature in the pCMD of an early-type galaxy, but that it is a transient feature of star formation caused by the galaxy-galaxy interaction. We also find a difference in the shapes of the red pixel sequences on the pixel color-color diagrams (pCCDs) of NGC 5194 and NGC 5195. We investigate the spatial distributions of the pCCD-based pixel stellar populations. The young population fraction in the tidal bridge area is larger than that in other areas by a factor > 15. Along the tidal bridge, young populations seem to be clumped particularly at the middle point of the bridge. On the other hand, the dusty population shows a relatively wide distribution between the tidal bridge and the NGC 5195 center.