The Stellar Number Density Distribution in the Local Solar Neighborhood is North-South Asymmetric

TL;DR

This study analyzes the distribution of K and M dwarf stars near the Sun, confirming a significant North-South asymmetry in stellar density, and assesses potential systematic effects that could influence this finding.

Contribution

The paper provides a refined analysis of stellar density asymmetry using a larger SDSS DR9 sample, confirming earlier findings and evaluating systematic uncertainties.

Findings

Confirmed North-South asymmetry in stellar density

Systematic effects have negligible impact on asymmetry detection

Refined the measurement with a larger star sample

Abstract

We study the number density distribution of a sample of K and M dwarf stars, matched North and South of the Galactic plane within a distance of 2 kpc from the sun, using observations from the Ninth Data Release of the Sloan Digital Sky Survey. We determine distances using the photometric parallax method, and in this context systematic effects exist which could potentially impact the determination of the number density profile with height from the Galactic plane --- and ultimately affect a number density North-South asymmetry. They include: (i) the calibration of the various photometric parallax relations, (ii) the ability to separate dwarfs from giants in our sample, (iii) the role of stellar population differences such as age and metallicity, (iv) the ability to determine the offset of the sun from the Galactic plane, and (v) the correction for reddening from dust in the Galactic plane, though our stars are at high Galactic latitudes. We find the various analyzed systematic effects to have a negligible impact on our observed asymmetry, and using a new and larger sample of stars we confirm and refine the earlier discovery of Widrow et al. of a significant Galactic North-South asymmetry in the stellar number density distribution.