[Paper Review] The AMIGA sample of isolated galaxies XII. Revision of the isolation degree for AMIGA galaxies using the SDSS

M. Argudo–Fernández, S. Verley+10 more|DIGITAL.CSIC (Spanish National Research Council (CSIC))|Nov 4, 2013

Galaxies: Formation, Evolution, Phenomena68 references20 citations

TL;DR

This study revises the isolation criteria for 636 galaxies from the Catalogue of Isolated Galaxies (CIG) using SDSS-DR9 photometry and spectroscopy, improving environmental quantification by identifying fainter and more distant neighbours. It finds that ~20% of the original AMIGA sample should be excluded due to hidden background contaminants, and that ~92% of velocity-matched neighbours are missed by the CIG criteria, significantly affecting isolation assessment.

ABSTRACT

In the framework of the AMIGA project we present a revision of the environment for galaxies in the Catalogue of Isolated Galaxies (CIG, Karachentseva 1973) using the ninth data release of the Sloan Digital Sky Survey (SDSS-DR9). The aims of this study are to refine the photometric-based AMIGA sample of isolated galaxies and to provide an improvement of the quantification of the isolation degree with respect to previous works, using both photometry and spectroscopy. We developed an automatic method to search for neighbours within a projected area of 1 Mpc radius centred on each primary galaxy to revise the CIG isolation criteria. Of the 636 CIG galaxies considered in the photometric study, 426 galaxies fulfil the CIG isolation criteria within 1 Mpc, taking into account projected neighbours. Of the 411 CIG galaxies considered in the spectroscopic study, 347 galaxies fulfil the CIG isolation criteria when a criterion about redshift difference is added. The available redshifts allow us to reject background neighbours and thus improve the photometric assessment. For the first time, the environment and the isolation degree of AMIGA galaxies are quantified using digital data. The use of the SDSS database permits one to identify fainter and smaller-size satellites than in previous AMIGA works. About 50% of the neighbours considered as potential companions in the photometric study are in fact background objects. We also find that about 92% of the neighbour galaxies that show recession velocities similar to the corresponding CIG galaxy are not considered by the CIG isolation criteria as potential companions, which may have a considerable influence on the evolution of the central CIG galaxy.

Motivation & Objective

• To improve the quantification of isolation for galaxies in the AMIGA sample by revising the CIG isolation criteria using modern SDSS data.
• To assess the validity of the original CIG isolation criteria by comparing photometric and spectroscopic neighbour detections.
• To reduce contamination from background galaxies in the isolation sample using redshift information.
• To quantify local number density and tidal strength for isolated galaxies to better characterize their environment.
• To refine the AMIGA sample by removing galaxies affected by unseen companions, improving its use as a reference for galaxy evolution studies.

Proposed method

• Applied an automated method to search for neighbours within a 1 Mpc projected radius around each CIG galaxy using SDSS-DR9 photometric data.
• Calculated local number density and tidal strength parameters to quantify environmental influence on each CIG galaxy.
• Incorporated spectroscopic redshifts to filter out background galaxies, applying a velocity difference threshold of |Δv| ≤ 500 km s⁻¹.
• Used photometric redshifts to estimate upper limits for isolation parameters in fields with incomplete redshift coverage.
• Compared photometric and spectroscopic results to assess the reliability of the original CIG isolation criteria.
• Employed statistical tools (STILTS, TOPCAT, Python, Matplotlib) and databases (NED, HyperLeda) for data processing and visualization.

Experimental results

Research questions

• RQ1 To what extent do the original CIG isolation criteria fail to exclude background galaxies when relying solely on photometry?
• RQ2 How many velocity-matched neighbours are missed by the CIG criteria, and what is their potential impact on galaxy evolution?
• RQ3 What is the true level of environmental influence (via number density and tidal strength) on galaxies classified as isolated in the CIG?
• RQ4 How does the inclusion of spectroscopic redshifts improve the accuracy of isolation assessment compared to photometry alone?
• RQ5 To what extent does the revised sample differ from the original AMIGA sample in terms of size and environmental purity?

Key findings

• Of the 636 CIG galaxies, 426 satisfy the CIG isolation criteria when using photometry within 1 Mpc, but 50% of the detected neighbours are background objects.
• Among the 411 galaxies with redshift data, 347 are isolated under the revised criteria (|Δv| ≤ 500 km s⁻¹), reducing the original AMIGA sample by approximately 20%.
• Approximately 92% of neighbour galaxies with radial velocities similar to the central CIG galaxy are not considered as companions by the CIG criteria, indicating a major flaw in the original isolation definition.
• The revised sample shows significantly lower local number density and tidal strength values than galaxies in denser environments such as pairs, triplets, compact groups, and clusters.
• The use of spectroscopic data improves isolation quantification by filtering out non-physical, background companions that were undetected in photometric-only analysis.
• The upper-limit estimate for isolation, based on photometric redshifts, identifies 103 CIG galaxies with no potential companions within 1 Mpc and |Δv| ≤ 500 km s⁻¹, enhancing confidence in their isolation status.