Weak Galactic halo--dwarf spheroidal connection from RR Lyrae stars

TL;DR

This study uses RR Lyrae stars to investigate the connection between dwarf galaxies and the Galactic halo, finding that metal-rich dwarfs may have contributed more to halo formation than metal-poor ones.

Contribution

It provides new evidence that metal-rich dwarf galaxies are more likely to have contributed to the Galactic halo than metal-poor dwarf spheroidals.

Findings

Dwarf spheroidals lack high amplitude short period RR Lyrae variables common in the halo.

Metallicity is the main parameter influencing RR Lyrae properties.

Massive, metal-rich dwarfs like the LMC and Sagittarius likely contributed significantly to the halo.

Abstract

We discuss the role that dwarf galaxies may have played in the formation of the Galactic halo (Halo) using RR Lyrae stars (RRL) as tracers of their ancient stellar component. The comparison is performed using two observables (periods, luminosity amplitudes) that are reddening and distance independent. Fundamental mode RRL in six dwarf spheroidals and eleven ultra faint dwarf galaxies (1,300) show a Gaussian period distribution well peaked around a mean period of <Pab>=0.610+-0.001 days (sigma=0.03). The Halo RRL (15,000) are characterized by a broader period distribution. The fundamental mode RRL in all the dwarf spheroidals apart from Sagittarius are completely lacking in High Amplitude Short Period (HASP) variables, defined as those having P< 0.48 days and Av> 0.75mag. Such variables are not uncommon in the Halo and among the globular clusters and massive dwarf irregulars. To further interpret this evidence, we considered eighteen globulars covering a broad range in metallicity (-2.3< [Fe/H]< -1.1) and hosting more than 35 RRL each. The metallicity turns out to be the main parameter, since only globulars more metal--rich than [Fe/H] -1.5 host RRL in the HASP region. This finding suggests that dSphs similar to the surviving ones do not appear to be the major building-blocks of the Halo. Leading physical arguments suggest an extreme upper limit of 50% to their contribution. On the other hand, massive dwarfs hosting an old population with a broad metallicity distribution (Large Magellanic Cloud, Sagittarius) may have played a primary role in the formation of the Halo.