[Paper Review] The ESO Nearby Abell Cluster Survey VIII. Morphological and spectral classification of galaxies

T. Thomas, P. Katgert|University of Twente Research Information|Oct 10, 2005

Galaxies: Formation, Evolution, Phenomena46 references3 citations

TL;DR

This study presents a morphological and spectral classification of 2,295 galaxies from the ESO Nearby Abell Cluster Survey using CCD imaging and spectroscopy. It finds that spectral types distinguish early- and late-type galaxies with 83% reliability, and reveals a strong correlation between Hubble type, color, and metallicity—extending the color-magnitude relation to spirals and showing fainter galaxies are bluer and less metal-rich than brighter ones.

ABSTRACT

We have determined the morphological types of 2295 galaxies from the ESO Nearby Abell Cluster Survey (ENACS) from CCD images made with the Dutch telescope on La Silla. The reliability of our classification appears to be quite comparable to that of other classifiers. Recalibration of the ENACS spectral classification shows that early- and late- type galaxies can be distinguished from their spectra with 83% reliability. Ellipticals and S0 galaxies cannot be distinguished spectrally, while spectral classification of late spirals has a reliability of 70%. We derive average pseudo-colors and linestrengths from the ENACS spectra for the galaxies of different morphological types, considering bright (M <= -20) and faint (M > -20) subsets of the galaxies without emission lines (non-ELG) separately. We find a strong and significant correlation between the average color and the average strength of the metal absorption lines. The average metallicity decreases and the average color gets bluer towards later Hubble type. In each morphological class the faint galaxies are bluer and less metal-rich than the bright ones. This extends the well-established color-magnitude relation of early-type galaxies to (late) spirals. Bright early spirals may, on average, have somewhat stronger H-delta absorption than the other galaxies, which could be due to recent starformation. The galaxies with emission lines (ELG) have a bluer spectral continuum than the non-ELG, and the amount of blueing hardly depends on morphological type. The fraction of ELG depends strongly on morphological type but it varies very little with projected distance from the cluster center.

Motivation & Objective

• To determine accurate morphological types for 2,295 galaxies in the ESO Nearby Abell Cluster Survey using CCD images from the Dutch telescope at La Silla.
• To recalibrate the existing spectral classification of ENACS galaxies using the new morphological types to improve classification reliability.
• To investigate the correlation between morphological type, spectral properties, color, and metallicity in cluster galaxies.
• To examine how spectral characteristics—such as continuum slope and line strengths—vary with luminosity and morphological type, especially for galaxies with and without emission lines.
• To assess the role of recent star formation and environmental effects on galaxy evolution by analyzing Hδ absorption and emission-line fractions across morphological types and cluster radii.

Proposed method

• Morphological classification of 2,295 galaxies was performed via visual inspection of high-resolution CCD images taken with the 1.5-m Dutch telescope at La Silla.
• The reliability of the new morphological types was validated by comparing them with literature classifications for 450 galaxies, showing consistency comparable to other expert classifiers.
• Spectral classification was recalibrated using the new morphological types, enabling improved distinction between early- and late-type galaxies based on spectra.
• Pseudo-colors and linestrengths (e.g., metal absorption lines) were derived from ENACS spectra for galaxies grouped by morphological type and luminosity (MR ≤ -20 vs. MR > -20).
• Statistical analysis was applied to examine correlations between average color, metallicity, Hubble type, and luminosity, including sub-samples of non-emission-line (non-ELG) and emission-line (ELG) galaxies.
• The dependence of emission-line fraction on morphological type and projected cluster radius was quantified to assess environmental versus intrinsic drivers of gas content and star formation.

Experimental results

Research questions

• RQ1 To what extent can spectral classification reliably distinguish early-type from late-type galaxies when combined with morphological types?
• RQ2 How do average spectral colors and metallicity (via absorption line strengths) correlate with Hubble type and luminosity in cluster galaxies?
• RQ3 To what extent does the color-magnitude relation extend from early-type galaxies to late-type spirals?
• RQ4 What is the relationship between emission-line fraction and morphological type, and does this fraction vary significantly with projected distance from the cluster center?
• RQ5 Are there spectral indicators—such as enhanced Hδ absorption—suggesting recent star formation in bright early spirals or other morphological types?

Key findings

• Spectral classification distinguishes early- and late-type galaxies with 83% reliability when combined with morphological types, and late spirals can be classified from spectra alone with over 70% reliability.
• There is a strong, significant correlation between Hubble type and both average color and metallicity: metallicity decreases and color becomes bluer toward later Hubble types.
• The color-magnitude relation extends to late-type spirals: fainter galaxies of each morphological type are bluer and less metal-rich than their brighter counterparts.
• Faint S0 galaxies and bright early spirals have remarkably similar colors and metallicities, suggesting a physical similarity despite morphological differences.
• Bright early spirals show slightly stronger Hδ absorption (at ~2σ significance), indicating possible recent star formation, though the effect is weak.
• Galaxies with emission lines (ELG) have a bluer continuum than non-ELG, and the degree of blueing is nearly independent of morphological type; the ELG fraction ranges from 4±1% in ellipticals to 59±4% in late spirals, with minimal variation across cluster radii.