Forming Realistic Late-Type Spirals in a LCDM Universe: The Eris Simulation (1103.6030v2)

Abstract: Simulations of the formation of late-type spiral galaxies in a cold dark matter LCDM universe have traditionally failed to yield realistic candidates. Here we report a new cosmological N-body/SPH simulation of extreme dynamic range in which a close analog of a Milky Way disk galaxy arises naturally. Termed Eris, the simulation follows the assembly of a galaxy halo of mass Mvir=7.9x10^11 Msun with a total of N=18.6 million particles (gas + dark matter + stars) within the final virial radius, and a force resolution of 120 pc. It includes radiative cooling, heating from a cosmic UV field and supernova explosions, a star formation recipe based on a high gas density threshold (nSF=5 atoms cm^-3 rather than the canonical nSF=0.1 atoms cm^-3), and neglects AGN feedback. At the present epoch, the simulated galaxy has an extended rotationally-supported disk with a radial scale length Rd=2.5 kpc, a gently falling rotation curve with circular velocity at 2.2 disk scale lenghts of V2.2=214 km/s, a bulge-to-disk ratio B/D=0.35, and a baryonic mass fraction that is 30% below the cosmic value. The disk is thin, is forming stars in the region of the Sigma_SFR - Sigma_HI plane occupied by spiral galaxies, and falls on the photometric Tully-Fisher and the stellar mass-halo virial mass relations. Hot (T>3x10^5 K), X-ray luminous halo gas makes only 26% of the universal baryon fraction and follows a flattened density profile proportional to r^-1.13 out to r=100 kpc. Eris appears then to be the first cosmological hydrodynamic simulation in which the galaxy structural properties, the mass budget in the various components, and the scaling relations between mass and luminosity are all consistent with a host of observational constraints. (Abridged)

• The paper demonstrates that using a high-density star formation threshold in advanced cosmological simulations can form realistic late-type spiral galaxies.
• The Eris Simulation employs an SPH approach with 18.6 million particles to accurately replicate key Milky Way properties, including rotation curves and disk structure.
• The study highlights the role of early strong feedback and clustered star formation in removing low angular momentum gas, refining galaxy formation models.

The Eris Simulation: Realistic Late-Type Spiral Formation in a $\Lambda$CDM Universe

The paper by Guedes et al., titled "Forming Realistic Late-Type Spirals in a $\Lambda$CDM Universe: The Eris Simulation," addresses a longstanding challenge in galaxy formation modeling within the $\Lambda$ Cold Dark Matter ($\Lambda$CDM) framework: the inability to accurately replicate realistic late-type spiral galaxies. Utilizing an advanced cosmological $N$-body/smooth particle hydrodynamical (SPH) approach, the authors present "Eris," a simulation that successfully forms a galaxy closely akin to the Milky Way without resorting to explicitly angular momentum altering processes.

Simulation Methodology and Parameters

Eris follows the evolution of a Milky Way-mass galaxy halo, with a total particle count of 18.6 million, capturing gas, dark matter, and stars, inside the final virial radius. The simulation achieves a fine force resolution of 120 pc, incorporating radiative cooling, supernova feedback, and a cosmic UV field. Notably, the authors employ a high gas density threshold for star formation ($n_{\rm SF}=5$ atoms cm$^{-3}$), diverging from the more commonly used $n_{\rm SF}=0.1$ atoms cm$^{-3}$, helping to capture the clumpy interstellar medium (ISM) critical for producing late-type spirals.

Results and Interpretation

The Eris simulation produces a galaxy with a range of properties in agreement with observational data. At redshift zero, the simulated galaxy features:

• A rotation curve with a peak circular velocity of 238 km/s and a decreasing profile beyond the solar radius, aligning well with the Milky Way data.
• A well-defined thin disk with a scale length $R_d=2.5$ kpc and a stellar mass in the disk conforming to empirical Tully-Fisher and stellar mass-halo mass relations.
• A baryon fraction 30% below the cosmic value, suggesting effective early feedback processes moderated star formation and gas accretion.
• A pseudobulge rather than a classical bulge, supported by a Sersic index $n_s=1.4$.
• Hot halo gas profiles incompatible with standard $\Lambda$CDM expectations but consistent with observations of diminished X-ray coronas.

Significance and Implications

The key to these results lies in the high-density threshold for star formation. By facilitating a more inhomogeneous ISM, Eris allows star formation and supernova heating to occur in a regionally clustered manner. This results in strong high-redshift outflows that decrease central baryonic content, reducing the mass of the bulge and fostering a more extended disk—a result aligned with evidence for preferential removal of low angular momentum gas through feedback processes.

The Eris experiment underscores the importance of star formation threshold parameters in simulations and suggests that a higher threshold may be necessary to produce realistic late-type spirals. The findings bolster the argument that galactic feedback mechanisms and ISM inhomogeneity are decisive in regulating baryon retention and angular momentum distribution.

Future Directions

Eris sets a new standard for the simulation of disk galaxies, but as acknowledged, there is room for refinement. Future higher-resolution simulations could enhance the resolution of the star-forming ISM, integrate molecular gas physics more accurately, and explore variations in merger histories and halo environments to refine these results further. Incorporating additional elements like metal cooling and exploring the impacts of AGN feedback could also provide more comprehensive insights.

Overall, this work paves the way for more accurately modeling the complex processes that shape galaxy formation and its diverse morphologies, presenting an improved framework for testing theoretical predictions against observations in astrophysics.