The ACS Nearby Galaxy Survey Treasury VIII. The Global Star Formation Histories of 60 Dwarf Galaxies in the Local Volume (1101.1093v1)

Abstract: We present uniformly measured star formation histories (SFHs) of 60 nearby (D~4Mpc) dwarf galaxies based on CMDs of resolved stellar populations from images taken with HST as part of the ACS Nearby Galaxy Survey Treasury program (ANGST). This volume-limited sample contains 12 dSph/dE, 5 dwarf spiral, 28 dI, 12 transition, and 3 tidal dwarf galaxies. From the best fit SFHs we find three significant results: (1) the average dwarf galaxy formed >50% of its stars by z~2 and 60% of its stars by z~1, regardless of current morphological type; (2) the mean SFHs of dIs, dTrans, and dSphs are similar over most of cosmic time, with the clearest differences appearing during the most recent 1 Gyr; and (3) the mean values are inconsistent with simple SFH models, e.g., exponentially declining SFRs. The mean SFHs are in general agreement with the cosmic SFH, although we observe offsets near z~1 that could be evidence that low mass systems experienced delayed star formation relative to more massive galaxies. The sample shows a strong density-morphology relationship, such that the dSphs are less isolated than dIs. We find that the transition from a gas-rich to gas-poor galaxy cannot be solely due to internal mechanisms such as stellar feedback, and instead is likely the result of external mechanisms, e.g., ram pressure and tidal forces. The average transition dwarf is slightly less isolated and less gas-rich than the typical dI. Further, the transition dwarfs can be divided into two groups: interacting and gas-poor or isolated and gas-rich, suggesting two possible evolutionary pathways.

• The paper shows that over half of each dwarf galaxy's stars formed by z ~2, indicating broadly consistent early star formation across various types.
• It employs HST color-magnitude diagrams to analyze complex, multi-episode star formation histories influenced by internal feedback and external interactions.
• The findings highlight a strong morphology-density relation, with recent star formation diverging in isolated versus clustered environments, prompting refined evolutionary models.

Overview of "The ACS Nearby Galaxy Survey Treasury VIII: The Global Star Formation Histories of 60 Dwarf Galaxies in the Local Volume"

This paper presents a detailed investigation into the star formation histories (SFHs) of 60 dwarf galaxies in the Local Volume (D < 4 Mpc), analyzed using color-magnitude diagrams (CMDs) of resolved stellar populations obtained from the Hubble Space Telescope. This research is conducted under the Advanced Camera for Surveys Nearby Galaxy Survey Treasury (ANGST) program. The authors aim to provide insights into the evolution and formation of dwarf galaxies by considering a volume-limited sample that spans a range of morphological types and environments. The sample includes dwarf spheroidals/dwarf ellipticals (dSph/dE), dwarf irregulars (dIrr), transition dwarfs (dSph/dIrr), dwarf spirals, and tidal dwarf galaxies.

Key Findings

1. Star Formation Histories Across Morphological Types:
   • Across the studied sample, the average dwarf galaxy formed more than half of its stars by redshift z ~ 2, and 60% by z ~ 1, regardless of its current morphological classification. This suggests a broadly similar SF pattern among different dwarf galaxy types throughout much of cosmic history.
   • Divergence in SFHs becomes pronounced only within the last few billion years. Particularly in the past 1 Gyr, the SF patterns among dSphs, dIs, and other dwarf types start to differentiate significantly.
2. Complex SFHs and Model Inconsistencies:
   • The SFHs demonstrated by these galaxies are complex, rendering simple model assumptions (such as single bursts or constant SF rates) inadequate to describe their evolutionary narratives.
   • This complexity is reflected in the interplay between internal processes like stellar feedback and external mechanisms such as intergalactic interactions.
3. Morphology-Density Relation:
   • The distribution of morphological types is strongly correlated with environmental density. Gas-poor dSphs are typically found in less isolated environments compared to the gas-rich dIs, reinforcing the significance of environmental factors in dwarf galaxy evolution.
4. Transition Dwarf Galaxies:
   • Transition dwarfs suggest dual evolutionary pathways: one group exhibits interactions and gas depletion, aligning towards dSph characteristics, while another remains more isolated and gas-rich, possibly representing an evolutionary state between SF episodes.
5. Quantitative SF Complements Cosmic SFH:
   • The cumulative SFHs derived from this sample show broad consistency with the cosmic SFH, though with notable deviations during intermediate epochs (z ~ 1). These offsets might indicate delayed SF in lower-mass systems compared to their more massive counterparts.

Implications and Prospects

The insights garnered from this paper highlight the heterogeneity in the evolutionary pathways of dwarf galaxies, dictated by their star formation histories and influenced by both intrinsic properties and environmental pressures. The complex SF patterns underscore the necessity for nuanced models that accommodate multi-component processes over simplistic assumptions.

The results have broad implications for understanding galaxy formation in a cosmological context, particularly as they suggest that dwarf galaxies might not align with traditional evolutionary scenarios like those proposed for larger galaxies. The clear morphology-density relationship underscores the importance of environment in shaping galactic evolution, potentially corroborating models that involve a blend of interaction-induced and feedback-driven processes.

For future developments, this research paves the way for more targeted studies on individual galaxies within specific environments to decipher the precise mechanisms underpinning dwarf galaxy evolution. Additionally, advancements in stellar evolution models, particularly AGB star modeling, will refine SFH determinations and contribute to resolving discrepancies observed in this paper. As simulations and models improve, the role of dwarf galaxies in scenarios such as galaxy downsizing and hierarchical assembly will become clearer, potentially offering new insights into the early universe's building blocks.