Disentangling the Galaxy at low Galactic latitudes

TL;DR

This study uses deep photometry of low-latitude fields to evaluate if a canonical Galactic model with thin and thick discs can explain observed star counts, finding the classical halo-thin/thick disc decomposition sufficient.

Contribution

It demonstrates that a standard Galactic structure model with double exponential discs and a De Vaucouleurs spheroid explains low-latitude star counts without needing additional populations.

Findings

Classical halo-thin/thick disc model fits observations well.

Thick disc scale length is comparable to or slightly longer than thin disc.

No extra stellar populations are required to match the data.

Abstract

We have used the field stars from the open cluster survey BOCCE, to study three low-latitude fields imaged with the Canada-France-Hawaii telescope (CFHT), with the aim of better understanding the Galactic structure in those directions. Due to the deep and accurate photometry in these fields, they provide a powerful discriminant among Galactic structure models. In the present paper we discuss if a canonical star count model, expressed in terms of thin and thick disc radial scales, thick disc normalization and reddening distribution, can explain the observed CMDs. Disc and thick disc are described with double exponentials, the spheroid is represented with a De Vaucouleurs density law. In order to assess the fit quality of a particular set of parameters, the colour distribution and luminosity function of synthetic photometry is compared to that of target stars selected from the blue sequence of the observed colour-magnitude diagrams. Through a Kolmogorov-Smirnov test we find that the classical decomposition halo-thin/thick disc is sufficient to reproduce the observations--no additional population is strictly necessary. In terms of solutions common to all three fields, we have found a thick disc scale length that is equal to (or slightly longer than) the thin disc scale.