NIHAO XI: Formation of Ultra-Diffuse Galaxies by outflows (1608.01327v2)

Abstract: We address the origin of Ultra-Diffuse Galaxies (UDGs), which have stellar masses typical of dwarf galaxies but effective radii of Milky Way-sized objects. Their formation mechanism, and whether they are failed $\rm L_{\star}$ galaxies or diffuse dwarfs, are challenging issues. Using zoom-in cosmological simulations from the NIHAO project, we show that UDG analogues form naturally in medium-mass haloes due to episodes of gas outflows associated with star formation. The simulated UDGs live in isolated haloes of masses $10^{10-11}\rm M_{\odot}$, have stellar masses of $10^{7-8.5}\rm M_{\odot}$, effective radii larger than 1 kpc and dark matter cores. They show a broad range of colors, an average S\'ersic index of 0.83, a typical distribution of halo spin and concentration, and a non-negligible HI gas mass of $10^{7-9}\rm M_{\odot}$, which correlates with the extent of the galaxy. Gas availability is crucial to the internal processes that form UDGs: feedback driven gas outflows, and subsequent dark matter and stellar expansion, are the key to reproduce faint, yet unusually extended, galaxies. This scenario implies that UDGs represent a dwarf population of low surface brightness galaxies and should exist in the field. The largest isolated UDGs should contain more HI gas than less extended dwarfs of similar $\rm M_{\star}$.

Formation of Ultra-Diffuse Galaxies Through Feedback-Driven Outflows

The paper investigates the origin of Ultra-Diffuse Galaxies (UDGs), a class of galaxies that possess the stellar masses typical of dwarf galaxies yet exhibit effective radii comparable to larger systems like the Milky Way. The research utilizes simulations from the Numerical Investigation of a Hundred Astrophysical Objects (NIHAO) project, exploring the hypothesis that UDGs can form naturally in dwarf-sized halos as a result of feedback-driven outflows associated with star formation.

Formation Mechanism

The authors present simulations indicating that UDG analogues arise naturally in isolated halos with masses ranging from 10<sup\>10</sup> to 10<sup\>11</sup> M_☉. The simulated UDGs exhibit stellar masses between 10<sup\>7</sup> and 10<sup\>8.5</sup> M_☉, effective radii greater than 1 kpc, dark matter cores, and varied colors. These findings suggest that episodes of gas outflows, driven by star formation feedback, prompt the expansion of both dark matter and stellar components, thereby forming sparse, extended galaxies. Contrary to some hypotheses categorizing UDGs as failed L_★ galaxies, this paper supports their classification as a form of diffuse dwarf galaxy.

Key Findings

1. Gas Availability: A significant finding is the role of available gas supply in forming UDGs. Feedback-driven outflows require sufficient gas mass within the halo to exert the potential to expand stellar and dark matter distributions.
2. Stellar and Dark Matter Distribution: The formation scenario highlights that the stellar and dark matter components both experience expansion due to repeated star formation and associated feedback. This aligns with broader theoretical understandings of feedback’s impacts on dark matter density profiles.
3. Halo Spin and Concentration: UDGs have spin and concentration parameters consistent with other dwarfs, suggesting that they do not necessarily reside in high-spin halos as proposed by other models.

Implications and Future Directions

The simulations suggest UDGs should be discernible in the isolated field, not exclusively within clusters, reaffirming that their formation is largely internally driven rather than environmentally dependent. The abundance of HI gas in simulated UDGs suggests that some might correspond to "dark" galaxies detected in HI surveys. Further observational work could aim to validate simulated predictions, particularly those regarding gas mass and distribution in isolated UDGs.

The paper also implies a potential observational overlap with existing HI surveys, indicating possible misclassifications of UDGs as gas-rich systems without optical counterparts. Future research could explore these connections further, potentially revising understandings of both UDG and LSB galaxy populations.

In summary, the paper advances a feedback-driven model of UDG formation, integrating cosmological simulations to demonstrate their emergence naturally in dwarf-sized halos under sufficient gas availability. This contributes to a nuanced understanding of UDGs, offering a framework for interpreting their observable properties and distribution, while suggesting considerations for forthcoming observational campaigns.