On the Dichotomy between Normal and Dwarf Ellipticals

Abstract

Using images from the SDSS DR13 library, we examine the structural properties of 374 bright (classed E0 to E6) and dwarf ellipticals (classed dE(nN) to dE(N)). The sample combines a multicolor sample of bright ellipticals (252 galaxies with M g < −20) with a new sample of faint ellipticals (60 galaxies with M g > −20) which overlaps the dwarf elliptical sample (62 galaxies) in luminosity and size. The faint ellipticals extend the linear structural correlations found for bright ellipticals into parameter space not occupied by dwarf ellipticals indicating a dichotomy exists between the two types. In particular, many faint ellipticals have significantly higher effective surface brightnesses compared to dE's which eliminates any connection at a set stellar mass. Template analysis of the three subsets of ellipticals demonstrates that the bright and faint ellipticals follow the same trends of profile shape (weak homology), but that dwarf ellipticals form a separate and distinct structural class with lower central surface brightnesses and extended isophotal radii.

Colors of Dwarf Ellipticals from GALEX to WISE

“…There is no evidence that the new sample of faint ellipticals deviates from the CMR slopes fit to the bright ellipticals (shown in each Figure as green symbols). This makes a solid statement that power-law shaped ellipticals, that define the bright and faint ellipticals sample (Schombert 2017), also obey the same structural and stellar population relationships. This will be somewhat of a challenge to many hierarchical models of galaxy formation which predict extended epochs of star formation that varies significantly with mass (Naab et al 2007).…”

“…In addition, there is no strong motivation to presume the SFH of dwarf ellipticals resembles normal ellipticals. Aside from their lack of visible structure (i.e., spiral arms) and no SF features (i.e., HII regions), the family of dE's has little else in common with normal ellipticals as they are structurally and kinematically distinct from the normal elliptical sequence (Schombert 2017). It also seems difficult to understand how normal ellipticals can be built from small objects like dE's or dIrr's given the differences in their stellar populations, unless this construction occurs at redshifts before the initial SF epoch.…”

“…This study is an extension of the color analysis of bright ellipticals (M g < −20) from Schombert (2016) that focuses on presenting a large sample of morphologically classified dwarf ellipticals (class dE) and bridging the gap between normal and dwarf ellipticals (the luminosity range between −18 and −20). As demonstrated in Schombert (2017), this gap is populated by the rare class of faint ellipticals with power-law surface brightness profiles, in the same structural family as normal ellipticals. The family of dE's have structurally distinct profiles from normal ellipticals, yet appear to transition coherently in color with normal ellipticals (Caldwell 1983).…”