The remnants of galaxy formation from a panoramic survey of the region around M31 (0909.0398v1)

Abstract: In hierarchical cosmological models, galaxies grow in mass through the continual accretion of smaller ones. The tidal disruption of these systems is expected to result in loosely bound stars surrounding the galaxy, at distances that reach $10 - 100$ times the radius of the central disk. The number, luminosity and morphology of the relics of this process provide significant clues to galaxy formation history, but obtaining a comprehensive survey of these components is difficult because of their intrinsic faintness and vast extent. Here we report a panoramic survey of the Andromeda galaxy (M31). We detect stars and coherent structures that are almost certainly remnants of dwarf galaxies destroyed by the tidal field of M31. An improved census of their surviving counterparts implies that three-quarters of M31's satellites brighter than $M_V < -6$ await discovery. The brightest companion, Triangulum (M33), is surrounded by a stellar structure that provides persuasive evidence for a recent encounter with M31. This panorama of galaxy structure directly confirms the basic tenets of the hierarchical galaxy formation model and reveals the shared history of M31 and M33 in the unceasing build-up of galaxies.

• The paper reveals clear remnants of disrupted dwarf galaxies around M31, supporting hierarchical galaxy formation models.
• The paper employs panoramic imaging with the Canada-France-Hawaii Telescope to identify coherent stellar substructures extending up to 100 kpc.
• The paper finds fewer detectable satellite galaxies than predicted, prompting a reevaluation of dark matter halo baryon content.

Insights into Galaxy Formation from the Pan-Andromeda Archaeological Survey

This paper presents a comprehensive analysis of the cosmic history and structure formation around the Andromeda Galaxy (M31) through an extensive panoramic survey. Utilizing the Canada-France-Hawaii Telescope, the Pan-Andromeda Archaeological Survey (PAndAS) offers insights into the remnants of galaxy formation that date back to hierarchical cosmic accretion processes.

Study Overview

The paper primarily examines the evidence supporting hierarchical galaxy formation models, which propose that galaxies accumulate mass over time via accretion and mergers of smaller systems. Crucial to these models is the idea that tidal disruptions during accretions lead to loosely bound stars forming extensive structures around the central galaxy. The paper provides a thorough survey of M31 and its surrounding satellites, contributing crucial data to understand the accretion history and structure of one of our nearest astronomical neighbors.

Key Findings

1. Visible Remnants: The paper discovered numerous stellar structures that are remnants of now-disrupted dwarf galaxies previously part of or interacting with M31. These findings substantiate models that predict galaxy stellar halos would consist of such accreted debris.
2. Substructures and their Coherence: New stellar substructures have been identified, including extensions up to 100 kpc from M31. Some of these were coherent over large distances, implying that they have a long-term historical presence, potentially persisting for several gigayears.
3. Satellite Dwarfs: The research suggests that M31 hosts far fewer detectable satellite galaxies than predicted by cosmological simulations of dark matter substructure. The paper concludes that only about a quarter of the expected satellites, based on dark matter theories, have so far been discovered.
4. Interaction Hypothesis: The investigation into M33, a major companion of M31, reveals a significant stellar structure extending considerably from its classical disk. Simulation and analysis suggest this might be attributed to a past tidal interaction with M31, providing a physical model consistent with observed asymmetric features in their respective halos.
5. Luminosity Function and Schechter Parameters: The cumulative luminosity distribution is characterized by a Schechter function with a faint-end slope of $\alpha = 0.98 \pm 0.07$. The authors suggest that up to $88 \pm 20$ satellite galaxies brighter than $M_V \approx -6$ may exist within 300 kpc of M31.

Implications and Future Research

The implications of this paper are significant both theoretically and practically. The confirmation of hierarchical formation models enhances our understanding of galaxy evolution dynamics. However, the apparent shortfall in the number of observed satellites also prompts a reevaluation of assumptions regarding the baryon content in dark matter halos.

Practically, the survey techniques and data offer templates for further extensive sky surveys, potentially improving our understanding of cosmic structure on a grander scale. Looking forward, additional targeted searches around M31 could yield further discoveries, helping close the gap between theory and observations and addressing the "missing satellites problem" inherent in current cosmological models.

Overall, the presented paper enriches the existing narrative of galactic formation and the role of satellite interactions, urging future investigations to refine simulations and observational strategies. Continued paper of such interactions and substructure coherence in other galaxies could unveil universal trends and irregularities, advancing astrophysics and cosmology as fields.