The Atlas3D project -- XXIX. The new look of early-type galaxies and surrounding fields disclosed by extremely deep optical images

Abstract: Galactic archeology based on star counts is instrumental to reconstruct the past mass assembly of Local Group galaxies. The development of new observing techniques and data-reduction, coupled with the use of sensitive large field of view cameras, now allows us to pursue this technique in more distant galaxies exploiting their diffuse low surface brightness (LSB) light. As part of the Atlas3D project, we have obtained with the MegaCam camera at the Canada-France Hawaii Telescope extremely deep, multi--band, images of nearby early-type galaxies. We present here a catalog of 92 galaxies from the Atlas3D sample, that are located in low to medium density environments. The observing strategy and data reduction pipeline, that achieve a gain of several magnitudes in the limiting surface brightness with respect to classical imaging surveys, are presented. The size and depth of the survey is compared to other recent deep imaging projects. The paper highlights the capability of LSB--optimized surveys at detecting new prominent structures that change the apparent morphology of galaxies. The intrinsic limitations of deep imaging observations are also discussed, among those, the contamination of the stellar halos of galaxies by extended ghost reflections, and the cirrus emission from Galactic dust. The detection and systematic census of fine structures that trace the present and past mass assembly of ETGs is one of the prime goals of the project. We provide specific examples of each type of observed structures -- tidal tails, stellar streams and shells --, and explain how they were identified and classified. We give an overview of the initial results. The detailed statistical analysis will be presented in future papers.

• The paper introduces deep optical imaging to uncover low-surface-brightness structures, such as tidal tails and streams, in early-type galaxies.
• The paper classifies galaxies with detailed descriptors (I, R, M, C) to quantify signs of interaction and merger remnants.
• The study’s findings enhance understanding of galaxy evolution by demonstrating that gravitational interactions and mergers shape the morphology of ETGs.

The ATLAS$^{\text{3D}}$ Project: A New Perspective on Early-Type Galaxies Through Deep Optical Imaging

The paper "The ATLAS$^{\text{3D}}$ Project -- XXIX. The New Look of Early-Type Galaxies and Surrounding Fields Disclosed by Extremely Deep Optical Images" presents an extensive study of early-type galaxies (ETGs) utilizing the capabilities of the MegaCam on the Canada-France-Hawaii Telescope. This substantial dataset enables the examination of intricate structures surrounding ETGs, which are not typically visible in standard survey images.

Study Overview and Methodology

The study focuses on obtaining deep optical images of ETGs to scrutinize the low surface brightness features in their outskirts, providing insights into their formation and evolutionary histories shaped by gravitational interactions. The catalog provided in the paper details observational parameters for various galaxies across multiple bands, including g', r', and i', with integration times ranging from 1,380s to 9,800s.

An extensive classification of the ETGs is performed using these deep images. Classifications such as "I" for interacting, "R" for relaxed, "M" for merger remnant, and "C" for cirrus-contaminated galaxies are noted, along with descriptors indicating specific features like tidal tails and streams. One notable aspect is the identification of tidal tails and merger remnants indicative of dynamic past interactions.

Numerical Results and Significant Claims

The paper makes notable assertions regarding the frequency of interaction-induced features among ETGs:

• A significant fraction of ETGs exhibit signs of tidal interaction, such as tidal tails, streams, and shells (e.g., NGC 2685, NGC 5557), demonstrating the dynamic environments these galaxies reside in.
• Numerous galaxies show evidence of recent mergers, both dry (gas-poor) and wet (gas-rich), as indicated by asymmetric stellar halos and extended star-forming disks (e.g., NGC 2764, NGC 5322).
• Deep imaging reveals features such as low-surface-brightness (LSB) disks and structures masked by galactic cirrus in shallower surveys, proving the utility of deep imaging techniques.

Implications and Future Developments

Theoretical implications of this research include a deeper understanding of how ETGs evolve through hierarchical merging processes and environmental interactions. Practically, the clarity provided on these low-brightness features offers a roadmap for future observational campaigns focusing on galaxy interactions and their role in galaxy morphology evolution.

This study underscores the importance of high-resolution and deep-field surveys for studying galactic environments. It paves the way for future investigations using more advanced imaging instruments, potentially incorporating techniques such as adaptive optics and next-generation telescopes to unravel further delicate structures in galaxy outskirts.

In summary, the paper provides critical insights into the morphological complexity and interaction-driven evolution of ETGs by leveraging the enhanced capabilities of deep optical imaging. This comprehensive analysis of ETG environments will undoubtedly influence future studies targeting galaxy formation and evolution in low-brightness regimes, ultimately contributing to a broader understanding of cosmic structure formation.