The Cosmic Abundance of Classical Milky Way Satellites

TL;DR

This study analyzes the abundance of classical dwarf spheroidal satellites around Milky Way-like galaxies using SDSS data, finding the Milky Way's satellite population typical and setting limits on fainter satellites, with implications for dark matter models.

Contribution

It provides new observational constraints on satellite galaxy counts around Milky Way analogs, highlighting the discrepancy with Lambda-CDM predictions at faint luminosities.

Findings

Milky Way's satellite population is statistically typical.

Upper limit of 13 satellites brighter than Fornax.

Current methods limited by redshift incompleteness.

Abstract

We study the abundance of satellites akin to the brightest, classical dwarf spheroidals around galaxies similar in magnitude and isolation to the Milky Way and M31 in the Sloan Digital Sky Survey. From a combination of photometric and spectroscopic redshifts, we bound the mean and the intrinsic scatter in the number of satellites down to ten magnitudes fainter than the Milky Way. Restricting to magnitudes brighter than Sagittarius, we show that the Milky Way is not a significant statistical outlier in its population of classical dwarf spheroidals. At fainter magnitudes, we find an upper limit of 13 on the mean number of satellites brighter than the Fornax dwarf spheroidal. Methods to improve these limits that utilize full photometric redshift distributions hold promise, but are currently limited by incompleteness at the very lowest redshifts. Theoretical models are left to explain why the majority of dark matter subhalos that orbit Milky Way-like galaxies are inefficient at making galaxies at the luminosity scale of the brightest dwarf spheroidals, or why these subhalos predicted by Lambda-CDM do not exist.