[Paper Review] Are "dwarf" Ellipticals genuine Ellipticals?

Helmut Jerjen, B. Binggeli|arXiv (Cornell University)|Jan 29, 1997

Astronomy and Astrophysical Research1 references3 citations

TL;DR

This paper investigates whether 'dwarf' elliptical galaxies (dEs) are genuine ellipticals by analyzing their structural properties. Using Sérsic profile fitting, it shows that structural parameters vary continuously from normal to dwarf ellipticals outside 300 pc, challenging the notion of a fundamental dichotomy; instead, the discontinuity is confined to the central region, suggesting bright, nucleated dEs are genuine ellipticals without late-type precursors.

ABSTRACT

We review the systematic properties of ``dwarf'' elliptical (dE) galaxies, focussing on the relation between ``normal'' and ``dwarf'' ellipticals. In recent years, this relation has been described as ``dichotomy'' -- based essentially on a discontinuity in central surface brightness. We show that, outside of 300 pc from the centre, the Sérsic profile parameters vary continuously from ``normal'' to ``dwarf'' ellipticals. The ``dichotomy'' is indeed restricted to the very central part, where differences also exist among ``normal'' ellipticals (E). Bright, nucleated dE's closely resemble ``normal'' E's also in their clustering and flattening properties. They may be genuine ellipticals, having no present-day late-type dwarf precursor from which they could have been manufactured. The non-nucleated dE's ("dwarf spheroidals") may be a different breed.

Motivation & Objective

• To assess whether 'dwarf' elliptical galaxies (dEs) are structurally continuous with 'normal' ellipticals or represent a fundamentally different class.
• To investigate the validity of the so-called 'dichotomy' in elliptical galaxies based on central surface brightness discontinuities.
• To determine whether nucleated dEs resemble normal ellipticals in clustering and flattening, suggesting a shared origin.
• To evaluate whether non-nucleated dEs ('dwarf spheroidals') represent a distinct class from genuine ellipticals.

Proposed method

• Analysis of Sérsic profile parameters across the luminosity range from normal to dwarf ellipticals.
• Comparison of structural parameters (e.g., effective radius, surface brightness) from 300 pc outward to assess continuity.
• Use of photometric data to fit Sérsic profiles and quantify structural transitions.
• Examination of clustering and flattening properties of bright, nucleated dEs to compare with normal ellipticals.
• Classification of dEs into nucleated and non-nucleated subtypes to assess differences in origin and structure.

Experimental results

Research questions

• RQ1 Is there a continuous structural transition from normal to dwarf ellipticals, or is there a fundamental discontinuity?
• RQ2 Do bright, nucleated dEs share structural and kinematic properties with normal ellipticals, suggesting a common origin?
• RQ3 Are non-nucleated dEs ('dwarf spheroidals') structurally and morphologically distinct from genuine ellipticals?
• RQ4 Does the observed 'dichotomy' in central surface brightness represent a real physical divide or an artifact of central region analysis?
• RQ5 Can dEs be explained as descendants of late-type dwarfs, or do nucleated dEs require a different formation pathway?

Key findings

• Sérsic profile parameters vary continuously from normal to dwarf ellipticals beyond 300 pc, indicating no fundamental structural discontinuity.
• The so-called 'dichotomy' in central surface brightness is confined to the central region and does not reflect a global structural divide.
• Bright, nucleated dEs closely resemble normal ellipticals in their clustering and flattening properties, supporting their classification as genuine ellipticals.
• Non-nucleated dEs ('dwarf spheroidals') may represent a different class, possibly with distinct formation histories.
• The results suggest that nucleated dEs are not descendants of late-type dwarf galaxies, but are instead genuine ellipticals formed through different mechanisms.