Dark matter cores all the way down (1508.04143v3)

Abstract: We use high resolution simulations of isolated dwarf galaxies to study the physics of dark matter cusp-core transformations at the edge of galaxy formation: M200 = 10^7 - 10^9 Msun. We work at a resolution (~4 pc minimum cell size; ~250 Msun per particle) at which the impact from individual supernovae explosions can be resolved, becoming insensitive to even large changes in our numerical 'sub-grid' parameters. We find that our dwarf galaxies give a remarkable match to the stellar light profile; star formation history; metallicity distribution function; and star/gas kinematics of isolated dwarf irregular galaxies. Our key result is that dark matter cores of size comparable to the stellar half mass radius (r_1/2) always form if star formation proceeds for long enough. Cores fully form in less than 4 Gyrs for the M200 = 10^8 Msun and 14 Gyrs for the 10^9 Msun dwarf. We provide a convenient two parameter 'coreNFW' fitting function that captures this dark matter core growth as a function of star formation time and the projected stellar half mass radius. Our results have several implications: (i) we make a strong prediction that if LCDM is correct, then 'pristine' dark matter cusps will be found either in systems that have truncated star formation and/or at radii r > r_1/2; (ii) complete core formation lowers the projected velocity dispersion at r_1/2 by a factor ~2, which is sufficient to fully explain the 'too big to fail problem'; and (iii) cored dwarfs will be much more susceptible to tides, leading to a dramatic scouring of the subhalo mass function inside galaxies and groups.

• The paper demonstrates that persistent star formation drives the formation of dark matter cores in dwarf galaxies using detailed high-resolution simulations.
• It employs a two-parameter fitting function based on a modified NFW profile to connect core size with the stellar half-mass radius and the duration of star formation.
• The results offer insights into resolving the 'too big to fail' dilemma and refining the subhalo mass function by highlighting stellar feedback's impact on dark matter structures.

Analyzing Dark Matter Cores in Dwarf Galaxies

This paper explores the phenomenon of dark matter (DM) cusp-core transformations in isolated dwarf galaxies, specifically focusing on the mass range $M_{200} = 10^7 - 10^9\,M_\odot$. Through high-resolution simulations, the paper investigates the impact of stellar feedback on DM halos and its implications for theories of galaxy formation and cosmology.

Dwarf galaxies serve as unique testbeds for studying galaxy formation and DM physics due to their detailed star formation histories, orbits, and mass profiles. The authors conduct simulations at a resolution of approximately 4 pc, with individual particles having a mass of about 250 M$_\odot$. This allows them to resolve the effects of individual supernovae on the interstellar medium and the surrounding DM.

Key Results and Methodology

The authors establish that DM cores of sizes comparable to the stellar half-mass radius consistently form within these simulations if star formation persists for an extended period. Specifically, cores complete formation within a timescale of less than 4 Gyr for galaxies with $M_{200} = 10^8\,M_\odot$ and about 14 Gyr for those with $M_{200} = 10^9\,M_\odot$.

To capture the processes of DM core growth, the paper introduces a two-parameter fitting function that relates core formation to star formation duration and the projected stellar half-mass radius. This function is grounded in a modified NFW profile termed the \ profile, which incorporates parameters that handle the degree and timescale of core flattening.

The simulations reveal that the resulting dwarf galaxies closely match observed isolated dwarf irregular galaxies in terms of their stellar light profiles, star formation histories, metallicity distributions, and kinematics. Notably, this suggests a fundamental understanding of DM interactions with stellar feedback.

Implications for Cosmology

The findings have several implications:

1. Pristine Cusps: If $\Lambda$CDM is accurate, DM cusps might be detected in systems with truncated star formation. Furthermore, cusps could persist beyond the stellar half-mass radius.
2. Addressing the 'Too Big to Fail' Problem: Core formation significantly reduces the projected velocity dispersion observed at the stellar half-mass radius, potentially resolving discrepancies in DM distributions in the Local Group's dwarf galaxies.
3. Remodeled Subhalo Mass Function: Cored dwarfs are more vulnerable to tidal forces, implying a refined evolution of the subhalo mass function in galaxies and groups, addressing the missing satellites problem.

Future Directions

The paper opens avenues for further exploration regarding mass scale limits where core formation might cease, particularly below $10^8\,M_\odot$. Moreover, it highlights the necessity of considering environmental factors such as tides and ram pressure in future simulations to capture a more comprehensive picture of galactic evolution.

This research advances our understanding of DM dynamics in dwarf galaxies and strengthens the predictive capacity of simulations modeling these complex interactions. The insights derived from such studies are crucial for refining the theoretical models of galaxy formation and the broader cosmological framework they inhabit.