Galaxy evolution in groups. NGC 3447/NGC 3447A: the odd couple in LGG 225

TL;DR

This study investigates the galaxy pair NGC 3447/NGC 3447A in a low-density environment, combining multi-wavelength observations and simulations to understand their interaction, structure, and evolution, revealing a shared halo and potential misclassification as a true pair.

Contribution

It provides a detailed multi-wavelength analysis and hydrodynamic simulations showing that NGC 3447/NGC 3447A are part of the same halo, challenging traditional pair identification methods.

Findings

NGC 3447/NGC 3447A share a common halo.

The velocity field shows no significant rotation, indicating a shared structure.

Simulations suggest the system results from an encounter between equal-mass halos.

Abstract

Local Group Analogs (LGA) are galaxy associations dominated by few bright Spirals, reminiscent of the LG. The NGC3447/NGC3447A system, member of the LGG 225 group, a nearby LGA, is considered a physical pair: an intermediate luminosity late type spiral, NGC3447, and an irregular companion, NGC3447A, linked by a faint filament of matter. A ring-like structure in the NGC3447 outskirts is emphasised by UV observations. This work aims to contribute to the understanding of galaxy evolution in low density environments, favourable habitat to highly effective encounters. We performed a multi-wavelength analysis of the surface photometry of this system to derive spectral energy distribution and structural properties using UV and optical images. We also characterised the velocity field of the pair using new kinematic observations. All these data are used to constrain smooth particle hydrodynamic simulations with chemo-photometric implementation to shed light on the evolution of this system. Luminosity profiles are all consistent with the presence of a disc extending and including NGC3447A. The overall velocity field does not emphasise any significant rotation pattern, rather a small velocity gradient between NGC3447 and NGC3447A. Our simulation, detached from a large grid explored to best-fit the global properties of the system, suggests that this arises from an encounter between two halos of equal mass. NGC3447 and NGC3447A belong to the same halo, NGC3447A being a substructure of the same disk as NGC3447. The halo gravitational instability, enhanced by the encounter, fuels a long lived instability in this dark matter dominated disk, driving its morphology. This system may warn about a new class of "false pairs" and the potential danger of a misunderstanding of such objects in pair surveys that could produce a severe underestimate of the total mass of the system. (abridged)