The unabridged satellite luminosity function of Milky Way-like galaxies in $\Lambda$CDM: the contribution of "orphan" satellites

TL;DR

This study uses simulations and semi-analytic modeling to analyze the abundance, distribution, and orbits of Milky Way-like satellite galaxies, emphasizing the significant role of 'orphan' satellites in the overall population.

Contribution

It introduces a detailed analysis of 'orphan' satellites, showing their importance in satellite populations and resolving previous modeling issues without populating subhalos below the critical threshold.

Findings

Orphans constitute about half of all satellites in high-resolution simulations.

Including orphans aligns models with observed satellite distributions without extra assumptions.

Most orphans were accreted early and are located near the galaxy center.

Abstract

We study the abundance, radial distribution, and orbits of luminous satellites in simulations of MW-mass dark halos in the LCDM cosmology. We follow the evolution of a halo from the Aquarius project and the formation of its maximal satellite population with the GALFORM semi-analytic model of galaxy formation. This population consists of all subhalos able to form stars through efficient gas cooling before or after reionization, which effectively selects systems with peak circular velocities exceeding a critical threshold of roughly 15-20 km/s. The total number of luminous satellites is sensitive to the assumed redshift of reionization, but the shape of the GALFORM satellite stellar mass function is robust, peaking at the stellar mass (10^3Msun) of a halo just above the critical threshold. Subhalos are prone to artificial disruption in the tidal field of the main halo, with the number of surviving satellites increasing with resolution. Even in the highest resolution simulation (Aq-L1, with particle mass of 10^3Msun), a substantial number of satellite subhalos are disrupted, leaving behind ``orphan'' galaxies tracked in GALFORM by the subhalo's most-bound particle before disruption. When orphans are included (and the effects of tidal stripping on stars neglected), all simulations that adequately resolve the critical threshold yield a converged maximal satellite stellar mass function. Most orphans were accreted early, are found in the central regions of the main halo, and make up roughly half of all satellites in Aq-L1. Taking orphans into account there is no need to populate subhalos below the critical threshold with satellites to fit the radial distribution of MW satellites, as has been argued in recent work. Our model predicts that orphans dominate the ultra-faint population and that many more satellites with small apocentric radii should be detected in upcoming deep wide-field surveys.