Lost and found dark matter in elliptical galaxies

Area of Science:

• Cosmology
• Astrophysics
• Galaxy Formation

Background:

• The universe's mass is largely attributed to dark matter, which forms galactic halos.
• Recent observations of low stellar velocities in elliptical galaxies challenge the dark matter hypothesis.
• These low velocities were interpreted as a lack of dark matter in galactic outskirts.

Purpose of the Study:

• To investigate if low stellar velocities in elliptical galaxies are compatible with dark matter halo formation.
• To reconcile observational data with the prevailing dark matter paradigm in cosmology.

Main Methods:

• Numerical simulations of disk-galaxy mergers were employed.
• Analysis focused on the orbital dynamics of stars in the outer regions of simulated elliptical galaxies.

Main Results:

• Simulations revealed highly elongated stellar orbits in the outer regions of resulting elliptical galaxies.
• These elongated orbits, caused by tidal forces during galaxy mergers, explain low observed velocities.
• The results are consistent with dark matter presence, even with steeply falling density profiles.

Conclusions:

• Low stellar velocities in elliptical galaxies do not contradict the existence of dark matter halos.
• Galaxy formation models within dark matter halos can explain these observations.
• Projection effects in triaxial galaxies can further reduce observed velocities, supporting the dark matter model.