Dwarfs Gobbling Dwarfs: A Stellar Tidal Stream Around NGC 4449 and Hierarchical Galaxy Formation on Small Scales (1112.2154v2)

Abstract: A candidate diffuse stellar substructure was previously reported in the halo of the nearby dwarf starburst galaxy NGC 4449 by Karachentsev et al. We map and analyze this feature using a unique combination of deep integrated-light images from the Black Bird 0.5-meter telescope, and high-resolution wide-field images from the 8-meter Subaru telescope, which resolve the nebulosity into a stream of red giant branch stars, and confirm its physical association with NGC 4449. The properties of the stream imply a massive dwarf spheroidal progenitor, which after complete disruption will deposit an amount of stellar mass that is comparable to the existing stellar halo of the main galaxy. The ratio between luminosity or stellar-mass between the two galaxies is ~1:50, while the indirectly measured dynamical mass-ratio, when including dark matter, may be ~1:10-1:5. This system may thus represent a "stealth" merger, where an infalling satellite galaxy is nearly undetectable by conventional means, yet has a substantial dynamical influence on its host galaxy. This singular discovery also suggests that satellite accretion can play a significant role in building up the stellar halos of low-mass galaxies, and possibly in triggering their starbursts.

Analysis of Dwarf Galaxy Mergers through Tidal Streams: The Case of NGC 4449

This paper, authored by David Mart{\'i}nez-Delgado et al., investigates the phenomena of dwarf galaxy mergers through the paper of stellar tidal streams, focusing on the nearby dwarf starburst galaxy NGC 4449. Utilizing a combination of deep integrated-light imaging from small telescopes and high-resolution observations from the 8-meter Subaru telescope, the authors provide compelling evidence for a massive dwarf spheroidal progenitor currently undergoing disruption by NGC 4449. The findings present challenges to the prevailing theories of hierarchical galaxy formation and have implications for understanding starburst triggers in dwarf galaxies.

Observations and Methods

The research employs integrated-light images from the 0.5-meter Black Bird Robotic Observatory and resolved stellar population images from the Subaru telescope to map a diffuse stellar substructure in the halo of NGC 4449. The stream of red giant branch (RGB) stars, confirmed to be physically associated with NGC 4449, indicates a hierarchical assembly process at dwarf galaxy scales. The team conducted detailed analysis through stellar density and color-magnitude diagrams (CMDs), providing evidence of the progenitor's mass and star formation history.

Key Findings

1. Stellar Tidal Stream Characteristics: The paper identifies an S-shaped stellar tidal stream, suggesting the presence of a disrupted dwarf spheroidal galaxy. This stream, characterized by RGB stars, has dimensions suggestive of extensive tidal effects exerted by NGC 4449.
2. Luminosity and Mass Estimation: The paper estimates a luminosity ratio of approximately 1:50 between NGC 4449 and the stream. Using multi-method approaches to derive stellar mass, including CMD analysis and mass-to-light ratio computations, the inferred stellar mass ratio remains consistent, suggesting a significant contribution of the stream to the stellar halo of NGC 4449.
3. Dynamical Implications: The dynamic mass ratio, incorporating dark matter, suggests the encounter could be characterized as a stealth merger, where the infalling dwarf galaxy's impact is substantial despite low visibility. The inferred mass ratios align with predictions from $\Lambda$CDM models but challenge the simplicity of current small-scale substructure formation theories.
4. Starburst Triggers and Accretion Events: The paper speculates that the accretion of this dwarf galaxy may be linked to the starburst nature of NGC 4449, proposing that such events may frequently instigate periods of intense star formation in dwarf galaxies.

Implications and Future Directions

The discovery of the stellar tidal stream in NGC 4449 has several implications for both theoretical and observational astrophysics:

• Galaxy Evolution: This finding supports the idea that dwarf galaxies can play a significant role in the development and evolution of larger galaxies through accretion processes, challenging previous assumptions about the preservation of low-mass galactic halos.
• Starburst Mechanisms: The correlation between accretion events and starbursts in dwarf galaxies invites reevaluation of star formation triggers, particularly in low-density environments.
• Cosmological Models: The observed stream and interactions support some aspects of the $\Lambda$CDM cosmology but also highlight its inadequacies in accurately predicting the complex interactions at play in dwarf galaxy mergers.

Future research is needed to explore the prevalence of such streams in dwarf galaxies and further test their implications for galaxy formation paradigms. Enhanced surveys using large telescopes could help in identifying additional low-surface-brightness features to better understand the role of low-mass mergers in galaxy evolution.