Structure and Evolution of Dwarf Galaxies

TL;DR

This paper reviews the structural properties and evolutionary links among various galaxy types, emphasizing that dwarf galaxies are the extreme, dark matter-dominated outcomes of transformation processes affecting larger galaxies.

Contribution

It proposes a unified classification scheme and demonstrates the continuous structural parameter sequences from giant to dwarf galaxies, highlighting their evolutionary connections.

Findings

Dwarf galaxies are the extreme products of galaxy transformation processes.

Parameter correlations show a complete continuity from giant to dwarf galaxies.

Extreme dwarfs are predominantly dark matter dominated.

Abstract

Different structural parameter correlations show how classical bulge components and elliptical galaxies are different from spiral and S0 galaxy disks, irregular (Im) galaxies, and spheroidal (Sph) galaxies. In contrast, the latter, apparently diverse galaxies or galaxy components have almost identical parameter correlations. This shows that they are related. A review of galaxy transformation processes suggests that S0 and spheroidal galaxies are star-formation-quenched, "red and dead" versions of spiral and Im galaxies. In particular, Sph galaxies are bulgeless S0s. This motivates a parallel sequence galaxy classification in which an S0a-S0b-S0c-Sph sequence of decreasing bulge-to-total ratios is juxtaposed to an Sa-Sb-Sc-Im sequence of star-forming galaxies. All parameter sequences show a complete continuity from giant galaxies to the tiniest dwarfs. Dwarfs are not a new or different class of galaxies. Rather, they are the extreme products of transformation processes that get more important as gravitational potential wells get more shallow. Smaller Sph and S+Im galaxies have lower stellar densities because they retain fewer baryons. Comparison of the baryonic parameter correlations with those for dark matter halos allows us to estimate baryon loss as a function of galaxy mass. Extreme dwarfs are almost completely dominated by dark matter.