Editing Dwarf elliptical galaxy (section)

== Origins ==
Dwarf ellipticals may be primordial objects. Within the currently favoured cosmological [[Lambda-CDM model]], small objects (consisting of dark matter and gas) were the first to form. Because of their mutual gravitational attraction, some of these will coalesce and merge, forming more massive objects. Further mergers lead to ever more massive objects.
The process of coalescence could lead to the present-day galaxies, and has been called "hierarchical merging". If this hypothesis is correct, dwarf galaxies may be the building blocks of today's [[spiral galaxy|large spiral]] galaxies, which in turn are thought to merge to form [[Elliptical galaxy#giant_elliptical_formation_scenario|giant ellipticals]].  

An alternative suggestion<ref>{{cite journal |last1=Moore |first1=B. |display-authors=etal |year=1996 |title=Galaxy harassment and the evolution of clusters of galaxies |journal=Nature |volume=379 |issue=6566 |pages=613–616 |doi=10.1038/379613a0 |arxiv=astro-ph/9510034 |bibcode=1996Natur.379..613M |s2cid=4303659 |url=http://adsabs.harvard.edu/abs/1996Natur.379..613M}}</ref> is that dEs could be the remnants of low-mass [[spiral galaxy|spiral galaxies]] that obtained a rounder shape through the action of repeated gravitational interactions with ordinary galaxies within a cluster. This process of changing a galaxy's morphology by interactions, and the removal of much of its stellar disk, has been called "[[Interacting galaxy#Galaxy harassment|galaxy harassment]]". Evidence for this latter hypothesis has been claimed due to stellar disks and weak spiral arms seen in some dEs. Under this alternative hypothesis, the anaemic spiral arms and disk are a modified version of the original stellar disk of the now transformed spiral galaxy.

At the same time, the galaxy harassment scenario can not be the full picture.<ref>{{cite journal |last1=Janz |first1=J. |display-authors=etal |year=2017 |title=Implications for the origin of early-type dwarf galaxies – the discovery of rotation in isolated, low-mass early-type galaxies |journal=Monthly Notices of the Royal Astronomical Society |volume=468 |issue=3 |pages=2850–2864 |doi=10.1093/mnras/stx634 |doi-access=free |arxiv=1703.04975 |bibcode=2017MNRAS.468.2850J }}</ref> The highly isolated dwarf elliptical galaxy [[LEDA 2108986|CG&nbsp;611]] possesses the same physical attributes as dE galaxies in clusters – such as coherent rotation and faint spiral arms – attributes that were previously assumed to provide evidence that dE galaxies were once spiral galaxies prior to a transformation process requiring immersion with a cluster of galaxies.  CG&nbsp;611 has a gas disk which counter-rotates to its stellar disk,<ref>{{cite journal |last1=Graham |first1=A.W. |display-authors=etal 
|year=2017 |title=Implications for the origin of early-type dwarf galaxies: A detailed look at the isolated, rotating, early-type dwarf galaxy LEDA&nbsp;2108986 (CG&nbsp;611), ramifications for the fundamental plane's {{mvar|S}}{{su|b=K|p=2}} kinematic scaling, and the spin-ellipticity diagram |journal=Astrophysical Journal |volume=840 |issue=2 |page=68 |doi=10.3847/1538-4357/aa6e56 |arxiv=1705.03587 |bibcode=2017ApJ...840...68G |s2cid=54018338 |doi-access=free }}</ref> clearly revealing that this dE galaxy's disk is growing via accretion events. If CG&nbsp;611 was to fall into a galaxy cluster, [[Ram pressure|ram-pressure stripping]] by the cluster's halo of hot X-ray gas would strip away CG&nbsp;611's gas disk and leave a gas-poor dE galaxy that immediately resembles the other dEs in the cluster. That is, no removal of stars nor re-shaping of the galaxy within the dense galaxy cluster environment would be required, undermining the idea that dE galaxies were once spiral galaxies.