Scaling Relations and the Fundamental Line of the Local Group Dwarf Galaxies

TL;DR

This study reveals that Local Group dwarf galaxies primarily follow a one-parameter 'Fundamental Line' driven by stellar mass, with notable differences between dwarf irregulars and ellipticals in their metallicity and star formation properties.

Contribution

It introduces a detailed analysis of the scaling relations of dwarf galaxies, extending PCA to a 4-D space including metallicity or SFR, and highlights differences between galaxy types.

Findings

Dwarfs define a one-parameter 'Fundamental Line' mainly driven by stellar mass.

Dwarf irregulars have lower metallicities and different star formation rates compared to dwarf ellipticals.

The scaling relations support supernova feedback predictions but also reveal differences between galaxy types.

Abstract

We study the scaling relations between global properties of dwarf galaxies in the Local Group. In addition to quantifying the correlations between pairs of variables, we explore the "shape" of the distribution of galaxies in log parameter space using standardised Principal Component Analysis (PCA), the analysis is performed first in the 3-D structural parameter space of stellar mass M*, internal velocity V and characteristic radius R* (or surface brightness mu*). It is then extended to a 4-D space that includes a stellar-population parameter such as metallicity Z or star formation rate SFR. We find that the Local-Group dwarfs basically define a one-parameter "Fundamental Line" (FL), primarily driven by stellar mass, M*. A more detailed inspection reveals differences between the star-formation properties of dwarf irregulars (dI's) and dwarf ellipticals (dE's), beyond the tendency of the latter to be more massive. In particular, the metallicities of dI's are typically lower by a factor of 3 at a given M* and they grow faster with increasing M*, showing a tighter FL in the 4-D space for the dE's. The structural scaling relations of dI's resemble those of the more massive spirals, but the dI's have lower star formation rates for a given M* which also grow faster with increasing M*. On the other hand, the FL of the dE's departs from the fundamental plane of bigger ellipticals. While the one-parameter nature of the FL and the associated slopes of the scaling relations are consistent with the general predictions of supernova feedback (Dekel & Woo 2003), the differences between the FL's of the dE's and the dI's remain a challenge and should serve as a guide for the secondary physical processes responsible for these two types.